f

Toxicological Profile for

Molybdenum

May 2020
MOLYBDENUM ii

DISCLAIMER

Use of trade names is for identification only and does not imply endorsement by the Agency for Toxic 

Substances and Disease Registry, the Public Health Service, or the U.S. Department of Health and Human 

Services.
MOLYBDENUM iii

FOREWORD

This toxicological profile is prepared in accordance with guidelines* developed by the Agency for Toxic 

Substances and Disease Registry (ATSDR) and the Environmental Protection Agency (EPA). The 

original guidelines were published in the Federal Register on April 17, 1987. Each profile will be revised 

and republished as necessary.

The ATSDR toxicological profile succinctly characterizes the toxicologic and adverse health effects 

information for these toxic substances described therein. Each peer-reviewed profile identifies and 

reviews the key literature that describes a substance's toxicologic properties. Other pertinent literature is 

also presented, but is described in less detail than the key studies. The profile is not intended to be an 

exhaustive document; however, more comprehensive sources of specialty information are referenced.

The focus of the profiles is on health and toxicologic information; therefore, each toxicological profile 

begins with a relevance to public health discussion which would allow a public health professional to 

make a real-time determination of whether the presence of a particular substance in the environment 

poses a potential threat to human health. The adequacy of information to determine a substance's health 

effects is described in a health effects summary. Data needs that are of significance to the protection of 

public health are identified by ATSDR.

Each profile includes the following:

(A) The examination, summary, and interpretation of available toxicologic information and 

epidemiologic evaluations on a toxic substance to ascertain the levels of significant 

human exposure for the substance due to associated acute, intermediate, and chronic 

exposures;

(B) A determination of whether adequate information on the health effects of each substance 

is available or in the process of development to determine levels of exposure that present 

a significant risk to human health of acute, intermediate, and chronic health effects; and

(C) Where appropriate, identification of toxicologic testing needed to identify the types or 

levels of exposure that may present significant risk of adverse health effects in humans.

The principal audiences for the toxicological profiles are health professionals at the Federal, State, and 

local levels; interested private sector organizations and groups; and members of the public.

This profile reflects ATSDR’s assessment of all relevant toxicologic testing and information that has been 

peer-reviewed. Staffs of the Centers for Disease Control and Prevention and other Federal scientists have 

also reviewed the profile. In addition, this profile has been peer-reviewed by a nongovernmental panel 

and was made available for public review. Final responsibility for the contents and views expressed in 

this toxicological profile resides with ATSDR.

Patrick N. Breysse, Ph.D., CIH

Director, National Center for Environmental Health and

Agency for Toxic Substances and Disease Registry

Centers for Disease Control and Prevention
MOLYBDENUM iv

*Legislative Background

The toxicological profiles are developed under the Comprehensive Environmental Response, 

Compensation, and Liability Act of 1980, as amended (CERCLA or Superfund). CERCLA section 

104(i)(1) directs the Administrator of ATSDR to “...effectuate and implement the health related 

authorities” of the statute. This includes the preparation of toxicological profiles for hazardous 

substances most commonly found at facilities on the CERCLA National Priorities List (NPL) and that 

pose the most significant potential threat to human health, as determined by ATSDR and the EPA. 

Section 104(i)(3) of CERCLA, as amended, directs the Administrator of ATSDR to prepare a 

toxicological profile for each substance on the list. In addition, ATSDR has the authority to prepare 

toxicological profiles for substances not found at sites on the NPL, in an effort to “...establish and 

maintain inventory of literature, research, and studies on the health effects of toxic substances” under 

CERCLA Section 104(i)(1)(B), to respond to requests for consultation under section 104(i)(4), and as 

otherwise necessary to support the site-specific response actions conducted by ATSDR.
MOLYBDENUM v

VERSION HISTORY

Date Description

May 2020 Final toxicological profile released

April 2017 Draft for public comment toxicological profile released
MOLYBDENUM vi

CONTRIBUTORS & REVIEWERS 

CHEMICAL MANAGER TEAM

G. Daniel Todd, Ph.D. (Lead) Lisa Ingerman, Ph.D., D.A.B.T.

Sam Keith, M.S., C.H.P. Mario Citra, Ph.D.

Obaid Faroon, D.V.M., Ph.D. Gary L. Diamond, Ph.D.

Melanie Buser, M.P.H. Courtney Hard, B.A.

Julie M. Klotzbach, Ph.D.

Amy Nguyen, Ph.D.

ATSDR, Division of Toxicology and Human Health 

Sciences, Atlanta, GA

SRC, Inc., North Syracuse, NY

REVIEWERS

Interagency Minimal Risk Level Workgroup: 

Includes ATSDR; National Center for Environmental Health (NCEH); National Institute for

Occupational Health and Safety (NIOSH); U.S. Environmental Protection Agency (EPA); National 

Toxicology Program (NTP). 

Additional reviews for science and/or policy: 

ATSDR, Division of Community Health Investigations; ATSDR, Office of Science; NCEH, Division 

of Laboratory Science; NCEH, Division of Environmental Health Science and Practice; EPA, Office of 

Water.

PEER REVIEWERS

1. David C. Dorman, DVM, Ph.D., DABT, DABVT, Professor, Toxicology, North Carolina State

University, Raleigh, North Carolina

2. Jonathan H Freedman, Ph.D., Professor, Department of Pharmacology and Toxicology,

University of Louisville School of Medicine, Louisville, Kentucky

3. Michael Aschner, Ph.D., Department of Molecular Pharmacology, Albert Einstein College of

Medicine, Bronx, New York

4. John Meeker, Sc.D., C.I.H., Professor, Environmental Health Sciences, Associate Dean for

Research, School of Public Health, University of Michigan, Ann Arbor, Michigan

5. Alexander V. Lyubimov, M.D., Ph.D., D.A.B.T., Toxicology Research Laboratory, Chicago,

Illinois

6. Dagobert Heijerick, ARCHE consulting, Gent, Belgium

These experts collectively have knowledge of toxicology, chemistry, and/or health effects. All reviewers 

were selected in conformity with Section 104(I)(13) of the Comprehensive Environmental Response, 

Compensation, and Liability Act, as amended.

ATSDR scientists review peer reviewers’ comments and determine whether changes will be made to the 

profile based on comments. The peer reviewers’ comments and responses to these comments are part of 

the administrative record for this compound.

The listing of peer reviewers should not be understood to imply their approval of the profile's final 

content. The responsibility for the content of this profile lies with ATSDR.
MOLYBDENUM vii

CONTENTS

DISCLAIMER........................................................................................................................................ii

FOREWORD .......................................................................................................................................iii

VERSION HISTORY .............................................................................................................................v

CONTRIBUTORS & REVIEWERS.......................................................................................................vi

CONTENTS ......................................................................................................................................vii

LIST OF FIGURES................................................................................................................................ix

LIST OF TABLES ..................................................................................................................................x

CHAPTER 1. RELEVANCE TO PUBLIC HEALTH.............................................................................1

1.1 OVERVIEW AND U.S. EXPOSURES .....................................................................................1

1.2 SUMMARY OF HEALTH EFFECTS.......................................................................................2

1.3 MINIMAL RISK LEVELS (MRLs)..........................................................................................4

CHAPTER 2. HEALTH EFFECTS ........................................................................................................6

2.1 INTRODUCTION.....................................................................................................................6

2.2 DEATH................................................................................................................................... 33

2.3 BODY WEIGHT..................................................................................................................... 33

2.4 RESPIRATORY ..................................................................................................................... 35

2.5 CARDIOVASCULAR............................................................................................................ 37

2.6 GASTROINTESTINAL.......................................................................................................... 37

2.7 HEMATOLOGICAL .............................................................................................................. 38

2.8 MUSCULOSKELETAL ......................................................................................................... 39

2.9 HEPATIC ...............................................................................................................................41

2.10 RENAL...................................................................................................................................42

2.11 DERMAL ...............................................................................................................................44

2.12 OCULAR................................................................................................................................44

2.13 ENDOCRINE ......................................................................................................................... 45

2.14 IMMUNOLOGICAL .............................................................................................................. 46

2.15 NEUROLOGICAL.................................................................................................................. 46

2.16 REPRODUCTIVE .................................................................................................................. 47

2.17 DEVELOPMENTAL.............................................................................................................. 50

2.18 OTHER NONCANCER.......................................................................................................... 51

2.19 CANCER................................................................................................................................53

2.20 GENOTOXICITY................................................................................................................... 54

2.21 MECHANISMS OF ACTION................................................................................................. 58

CHAPTER 3. TOXICOKINETICS, SUSCEPTIBLE POPULATIONS, BIOMARKERS, CHEMICAL 

INTERACTIONS .......................................................................................................... 61

3.1 TOXICOKINETICS................................................................................................................ 61

3.1.1 Absorption........................................................................................................................ 61

3.1.2 Distribution ...................................................................................................................... 64

3.1.3 Metabolism ...................................................................................................................... 66

3.1.4 Excretion.......................................................................................................................... 66

3.1.5 Physiologically Based Pharmacokinetic (PBPK)/Pharmacodynamic (PD) Models............. 68

3.1.6 Animal-to-Human Extrapolations..................................................................................... 72
MOLYBDENUM viii

3.2 CHILDREN AND OTHER POPULATIONS THAT ARE UNUSUALLY SUSCEPTIBLE .... 72

3.3 BIOMARKERS OF EXPOSURE AND EFFECT.................................................................... 74

3.3.1 Biomarkers of Exposure ................................................................................................... 75

3.3.2 Biomarkers of Effect ........................................................................................................ 75

3.4 INTERACTIONS WITH OTHER CHEMICALS.................................................................... 75

CHAPTER 4. CHEMICAL AND PHYSICAL INFORMATION.......................................................... 77

4.1 CHEMICAL IDENTITY......................................................................................................... 77

4.2 PHYSICAL AND CHEMICAL PROPERTIES ....................................................................... 79

CHAPTER 5. POTENTIAL FOR HUMAN EXPOSURE..................................................................... 83

5.1 OVERVIEW........................................................................................................................... 83

5.2 PRODUCTION, IMPORT/EXPORT, USE, AND DISPOSAL................................................ 85

5.2.1 Production ........................................................................................................................ 85

5.2.2 Import/Export ................................................................................................................... 88

5.2.3 Use................................................................................................................................... 89

5.2.4 Disposal ........................................................................................................................... 90

5.3 RELEASES TO THE ENVIRONMENT................................................................................. 90

5.3.1 Air.................................................................................................................................... 91

5.3.2 Water ............................................................................................................................... 93

5.3.3 Soil................................................................................................................................... 94

5.4 ENVIRONMENTAL FATE.................................................................................................... 94

5.4.1 Transport and Partitioning ................................................................................................ 94

5.4.2 Transformation and Degradation....................................................................................... 96

5.5 LEVELS IN THE ENVIRONMENT....................................................................................... 97

5.5.1 Air.................................................................................................................................... 98

5.5.2 Water ............................................................................................................................. 100

5.5.3 Sediment and Soil........................................................................................................... 103

5.5.4 Other Media ................................................................................................................... 104

5.6 GENERAL POPULATION EXPOSURE.............................................................................. 104

5.7 POPULATIONS WITH POTENTIALLY HIGH EXPOSURES............................................ 112

CHAPTER 6. ADEQUACY OF THE DATABASE ........................................................................... 113

6.1 INFORMATION ON HEALTH EFFECTS ........................................................................... 113

6.2 IDENTIFICATION OF DATA NEEDS ................................................................................ 113

6.3 ONGOING STUDIES........................................................................................................... 121

CHAPTER 7. REGULATIONS AND GUIDELINES......................................................................... 122

CHAPTER 8. REFERENCES ............................................................................................................ 125

APPENDICES

APPENDIX A. ATSDR MINIMAL RISK LEVEL WORKSHEETS.................................................. A-1 

APPENDIX B. LITERATURE REVIEW SEARCH FRAMEWORK FOR MOLYBDENUM............ B-1 

APPENDIX C. FRAMEWORK FOR ATSDR’S SYSTEMATIC REVIEW OF HEALTH EFFECTS 

DATA FOR MOLYBDENUM................................................................................... C-1 

APPENDIX D. USER’S GUIDE........................................................................................................ D-1 

APPENDIX E. QUICK REFERENCE FOR HEALTH CARE PROVIDERS....................................... E-1 

APPENDIX F. GLOSSARY................................................................................................................F-1 

APPENDIX G. ACRONYMS, ABBREVIATIONS, AND SYMBOLS................................................G-1
MOLYBDENUM ix

LIST OF FIGURES

1-1. Health Effects Found in Animals Following Oral Exposure to Molybdenum....................................3

1-2. Summary of Sensitive Targets of Molybdenum – Oral.....................................................................5

2-1. Overview of the Number of Studies Examining Molybdenum Health Effects................................. 11

2-2. Levels of Significant Exposure to Molybdenum – Inhalation ......................................................... 16

2-3. Levels of Significant Exposure to Molybdenum – Oral .................................................................. 26

3-1. The Proposed Systemic Model for Molybdenum Radionuclides..................................................... 69

3-2. Diagram of the Compartment Molybdenum Model........................................................................ 71

5-1. Number of NPL Sites with Molybdenum Contamination ............................................................... 83

6-1. Summary of Existing Health Effects Studies on Molybdenum By Route and Endpoint................. 114
MOLYBDENUM x

LIST OF TABLES

1-1. Minimal Risk Levels (MRLs) for Molybdenum...............................................................................5

2-1. Levels of Significant Exposure to Molybdenum – Inhalation ......................................................... 12

2-2. Levels of Significant Exposure to Molybdenum – Oral .................................................................. 19

2-3. Levels of Significant Exposure to Molybdenum – Dermal ............................................................. 30

2-4. Genotoxicity of Molybdenum Compounds In Vivo ........................................................................ 55

2-5. Genotoxicity of Molybdenum Compounds In Vitro........................................................................ 55

3-1. Transfer Rates (Day-1

) for the Molybdenum Model........................................................................ 69

4-1. Chemical Identity of Molybdenum and Compounds....................................................................... 78

4-2. Physical and Chemical Properties of Molybdenum and Compounds............................................... 80

5-1. Facilities that Produce, Process, or Use Molybdenum Trioxide ...................................................... 86

5-2. Molybdenum U.S. Production, Import, and Export Data from 2010 to 2014 and 2018 in 

Metric Tons...................................................................................................................................88

5-3. Releases to the Environment from Facilities that Produce, Process, or Use Molybdenum 

Trioxide ........................................................................................................................................91

5-4. Lowest Limit of Detection Based on Standards.............................................................................. 97

5-5. Summary of Environmental Levels of Molybdenum...................................................................... 98

5-6. Molybdenum Levels in Water, Soil, and Air of National Priorities List (NPL) Sites....................... 98

5-7. 24-Hour Molybdenum Concentrations (μg/m3

) in Air Samples (2018 Data)................................... 99

5-8. Molybdenum Levels Detected in Foods in the 2006–2011 and 2013–2014 Market Basket 

Surveys....................................................................................................................................... 105

5-9. Urinary Molybdenum Levels in U.S. Adults................................................................................ 106

5-10. Molybdenum Levels in Breast Milk in Mothers of Term and Preterm Infants............................. 109

5-11. Urinary Molybdenum Levels in U.S. Children and Adolescents................................................. 110

7-1. Regulations and Guidelines Applicable to Molybdenum .............................................................. 122
MOLYBDENUM 1

CHAPTER 1. RELEVANCE TO PUBLIC HEALTH

1.1 OVERVIEW AND U.S. EXPOSURES

Molybdenum (Mo) is a naturally occurring trace element that can be found extensively in nature. 

Molybdenum is a metal that exists as a dark-gray or black powder with a metallic luster or as a silverywhite mass (NLM 2020a). It does not occur naturally in the pure metallic form, but principally as oxide 

or sulfide compounds (Barceloux 1999; EPA 1979). Therefore, almost all exposure is to a molybdenum 

compound rather than the actual metal. Important naturally occurring molybdenum compounds are the 

minerals molybdenite, powellite, wulfenite, ferrimolybdite, and ilsemannite. In this toxicological profile, 

“molybdenum” is used to refer to the element (molybdenum metal) and generically for substances or 

compounds containing molybdenum. The most common forms used in commerce and found in the 

environment are molybdenum trioxide and molybdate salts (sodium molybdate or ammonium 

molybdate). Industrial applications of molybdenum nanoparticles have also been identified; however, 

molybdenum nanoparticle exposure is not discussed in this toxicological profile because their physicalchemical properties differ from that of larger molybdenum particles and the toxicological and 

toxicokinetic properties of nanoparticles can vastly differ from those of larger particles.

Biologically, molybdenum plays an important role as a micronutrient in plants and animals, including 

humans. It is used widely in industry for metallurgical applications; some of these applications include 

high temperature furnaces, as a support wire for tungsten filaments in incandescent light bulbs, and as a 

component of steel used in solar panels and wind turbines (EPA 1979; Stiefel 2011).

Molybdenum is more abundant in areas of natural mineral deposits and can be found in all environmental 

media. Higher concentrations in air, water, and soil can be found near industrial operations due to 

contamination. Molybdenum concentrations in ambient air have been reported to range from below 

detection limits to 0.03 mg/m3 (EPA 1979). Concentrations of molybdenum in ambient air of urban 

areas, 0.01–0.03 μg/m3

, are higher than those found in rural areas, 0.001–0.0032 μg/m3. It has been 

reported that concentrations of molybdenum in surface waters are generally <1.0 μg/L (USGS 2006) and 

drinking water (USGS 2011) and groundwaters contain about 1.0 μg/L (USGS 2011). Near mining 

activities, surface water molybdenum concentrations can be orders of magnitude higher (Frasacoli and 

Hudson-Edwards 2018). Concentrations as high as 1,400 μg/L have been detected in drinking waters in 

areas impacted by mining and milling operations (USGS 2011), far exceeding the U.S. Environmental 

Protection Agency (EPA) health-based screening level of 40 μg/L (EPA 2018a). Globally, most soils 
MOLYBDENUM 2

1. RELEVANCE TO PUBLIC HEALTH

contain molybdenum at concentrations between 0.6 and 3.5 ppm, although total concentrations in soils 

can vary widely depending on geological composition or industrial contamination. The average 

concentration of soils is generally 1–2 ppm. In the United States, it has been reported that the median 

concentration of molybdenum in soils is 1.2–1.3 ppm, with a range of 0.1–40 ppm (EPA 1979).

The exposure to molybdenum to the general population is almost entirely through food. Foods derived 

from above-ground plants, such as legumes, leafy vegetables, and cauliflower, generally have a relatively 

higher concentration of molybdenum in comparison to food from tubers or animals. Beans, cereal grains, 

leafy vegetables, legumes, liver, and milk are reported as the richest sources of molybdenum in the 

average diet (Barceloux 1999). Drinking water coming from sources close to areas with high 

molybdenum contamination from industrial effluents may contain a higher concentration of molybdenum. 

1.2 SUMMARY OF HEALTH EFFECTS

Molybdenum is an essential nutrient; the nutritional requirement for adults is 45 μg/day (0.64 μg/kg/day) 

(NAS 2001). Exposure to excess levels has been associated with adverse health outcomes. The most 

sensitive effects appear to be respiratory effects following inhalation exposure to molybdenum trioxide, 

and decreases in body weight, kidney damage, decreases in sperm count, and anemia following oral 

exposure (see Figure 1-1). A systematic review of the available human and laboratory animal health 

effects database resulted in the following hazard identification conclusions: 

• Respiratory effects are a presumed health effect for humans for molybdenum oxides.

• Renal effects are a presumed health effect for humans.

• The data were inadequate to conclude whether hepatic, uric acid level, reproductive, or 

developmental effects will occur in humans.

Respiratory Effects. Decreases in lung function, dyspnea, and cough were reported in a study of workers 

exposed to fine or ultrafine molybdenum trioxide dust (Ott et al. 2004). Another study of workers at a 

molybdenite roasting facility exposed to molybdenum trioxide and other oxides did not have alterations in 

lung function (Walravens et al. 1979). In studies of rats and mice exposed to molybdenum trioxide for 

2 years, hyaline degeneration of the nasal epithelium, squamous metaplasia of the epiglottis, and chronic 

inflammation (rats only) were observed (NTP 1997). However, no effects were observed following a 

13-week exposure to similar concentrations (NTP 1997). 
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1. RELEVANCE TO PUBLIC HEALTH

Figure 1-1. Health Effects Found in Animals Following Oral Exposure to 

Molybdenum

Hepatic Effects. Liver effects, which consisted of decreases in glycogen content, increases in 

aminotransferase activities, and increases in lipid content, have been observed at higher doses 

(≥300 mg/kg/day) that are often associated with body weight losses (Rana and Chauhan 2000; Rana and 

Kumar 1980b, 1980c; Rana et al. 1980, 1985). No hepatic effects have been observed at lower 

(≤60 mg/kg/day) doses (Bersenyi et al. 2008; Murray et al. 2014a).

Renal Effects. Several studies have reported renal effects in rats exposed to ≥60 mg/kg/day (Bompart et 

al. 1990; Murray et al. 2014a; Rana and Kumar 1980c, 1983; Rana et al. 1980). The effects included 

hyperplasia of the renal proximal tubules, degeneration, increases in total lipid levels in the kidney, and 

diuresis and creatinuria. 

Reproductive Effects. Cross-sectional epidemiological studies have reported significant associations 

between blood molybdenum levels and sperm concentration and morphology (Meeker et al. 2008) or 

testosterone levels (Lewis and Meeker 2015; Meeker et al. 2010). No significant alterations in sperm 

parameters or estrous cycling were observed in a 90-day rat study (Murray et al. 2014a) or in a 

2-generation reproductive toxicity study (Murray et al. 2019). Studies providing limited information on 

molybdenum doses and/or the copper content of the diet have reported reproductive effects. Decreases in 

sperm motility and concentration and increases in sperm morphological changes have been observed in 
MOLYBDENUM 4

1. RELEVANCE TO PUBLIC HEALTH

rats exposed to approximately 25 mg molybdenum/kg/day as sodium molybdate (Pandey and Singh 2002; 

Zhai et al. 2013). Degeneration of the seminiferous tubules was also observed at similar molybdenum 

doses (Jeter and Davis 1954). Effects have also been observed in the female reproductive system (oocyte 

morphological alterations, abnormal rate of ovulation, and irregularities in the estrous cycle) at ≥1.5 mg 

molybdenum/kg/day in rats (Fungwe et al. 1990; Jeter and Davis 1954; Zhang et al. 2013).

Developmental Effects. Mixed results have been observed in animal developmental toxicity studies. 

Decreases in the number of live fetuses and fetal growth were observed in rats administered 14 mg 

molybdenum/kg as sodium molybdate (Pandey and Singh 2002). Interpretation of the results of this study 

is limited by the lack of information on the copper content of the diet and the lack of developmental 

effects reported in two high-quality studies in which rats were exposed to doses as high as 40 mg 

molybdenum/kg/day as sodium molybdate (Murray et al. 2014b, 2019). 

Uric Acid Levels. A study of workers at a molybdenite roasting facility exposed to molybdenum trioxide 

and other oxides reported an increase in serum uric acid levels (Walravens et al. 1979). An increased 

occurrence of gout-like symptoms and increased blood uric acid levels were also observed in residents 

living in an area of high molybdenum levels in the soil (Koval’skiy et al. 1961); no alterations in urinary 

uric acid levels were found in a 10-day experimental study in men (Deosthale and Gopalan 1974). 

Cancer Effects. No increases in the risk of lung cancer were reported in workers who self-reported 

exposure to molybdenum (Droste et al. 1999). An increase in alveolar/bronchiolar adenomas or 

carcinomas was observed in mice exposed to molybdenum trioxide for 2 years (NTP 1997); in rats 

chronically exposed to airborne molybdenum trioxide, the incidence of alveolar/bronchiolar adenoma/

carcinoma was within the range of historical controls (NTP 1997). The potential carcinogenicity of 

molybdenum in humans has not been evaluated by the Department of Health and Human Services or the 

EPA. The International Agency for Research on Cancer (IARC 2018) categorized molybdenum trioxide 

as possibly carcinogenic to humans (Group 2B).

1.3 MINIMAL RISK LEVELS (MRLs)

As summarized in Table 1-1, an inhalation MRL has been derived for chronic-duration exposure to 

molybdenum trioxide and an oral MRL has been derived for intermediate-duration exposure to 

molybdenum. As presented in Figure 1-2, available data have identified the kidney as a sensitive target of 
MOLYBDENUM 5

1. RELEVANCE TO PUBLIC HEALTH

molybdenum toxicity following oral exposure. The available data were not considered adequate for 

derivation of acute- or intermediate-duration inhalation MRLs or acute- or chronic-duration oral MRLs.

Table 1-1. Minimal Risk Levels (MRLs) for Molybdenuma

Exposure 

duration MRL Critical effect

Point of 

departure

Uncertainty 

and modifying 

factors Reference

Inhalation exposure (mg molybdenum/m3)

Acute Insufficient data for derivation of an MRL

Intermediate Insufficient data for derivation of an MRL

Chronic 

(molybdenum 

trioxide)

0.002 Squamous metaplasia of 

the epiglottis in female 

rats 

0.071

(BMCLHEC)

UF: 30 NTP 1997

Oral exposure (mg/kg/day)

Acute Insufficient data for derivation of an MRL

Intermediate 0.06 Renal proximal tubule 

hyperplasia

17 (NOAEL) UF: 100

MF: 3

Murray et 

al. 2014a

Chronic Insufficient data for derivation of an MRL

aSee Appendix A for additional information. 

BMCL = benchmark concentration lower confidence limit; HEC = human equivalent concentration; MF = modifying 

factor; NOAEL = no-observed-adverse-effect level; UF = uncertainty factor

Figure 1-2. Summary of Sensitive Targets of Molybdenum – Oral

The kidney is the most sensitive target of molybdenum oral exposure. 

Numbers in circles are the lowest LOAELs for all health effects in animals.

No reliable dose response data were available for humans.
MOLYBDENUM 6

CHAPTER 2. HEALTH EFFECTS

2.1 INTRODUCTION 

The primary purpose of this chapter is to provide public health officials, physicians, toxicologists, and 

other interested individuals and groups with an overall perspective on the toxicology of molybdenum. It 

contains descriptions and evaluations of toxicological studies and epidemiological investigations and 

provides conclusions, where possible, on the relevance of toxicity and toxicokinetic data to public health.

A glossary and list of acronyms, abbreviations, and symbols can be found at the end of this profile.

To help public health professionals and others address the needs of persons living or working near hazardous 

waste sites, the information in this section is organized by health effect. These data are discussed in terms of 

route of exposure (inhalation, oral, and dermal) and three exposure periods: acute (≤14 days), intermediate 

(15–364 days), and chronic (≥365 days).

As discussed in Appendix B, a literature search was conducted to identify relevant studies examining health 

effect endpoints. Figure 2-1 provides an overview of the database of studies in humans or experimental 

animals included in this chapter of the profile. These studies evaluate the potential health effects associated 

with inhalation, oral, or dermal exposure to molybdenum, but may not be inclusive of the entire body of 

literature. A systematic review of the scientific evidence of the health effects associated with exposure to 

molybdenum was also conducted; the results of this review are presented in Appendix C.

Human and animal inhalation studies are presented in Table 2-1 and Figure 2-2; animal oral studies are 

presented in Table 2-2 and Figure 2-3; and animal dermal studies are presented in Table 2-3.

Levels of significant exposure (LSEs) for each route and duration are presented in tables and illustrated in 

figures. The points in the figures showing no-observed-adverse-effect levels (NOAELs) or lowestobserved-adverse-effect levels (LOAELs) reflect the actual doses (levels of exposure) used in the studies. 

LOAELs have been classified into "less serious" or "serious" effects. "Serious" effects are those that 

evoke failure in a biological system and can lead to morbidity or mortality (e.g., acute respiratory distress 

or death). "Less serious" effects are those that are not expected to cause significant dysfunction or death, 

or those whose significance to the organism is not entirely clear. ATSDR acknowledges that a 

considerable amount of judgment may be required in establishing whether an endpoint should be 
MOLYBDENUM 7

2. HEALTH EFFECTS

classified as a NOAEL, "less serious" LOAEL, or "serious" LOAEL, and that in some cases, there will be 

insufficient data to decide whether the effect is indicative of significant dysfunction. However, the 

Agency has established guidelines and policies that are used to classify these endpoints. ATSDR believes 

that there is sufficient merit in this approach to warrant an attempt at distinguishing between "less 

serious" and "serious" effects. The distinction between "less serious" effects and "serious" effects is 

considered to be important because it helps the users of the profiles to identify levels of exposure at which 

major health effects start to appear. LOAELs or NOAELs should also help in determining whether or not 

the effects vary with dose and/or duration, and place into perspective the possible significance of these 

effects to human health. Levels of exposure associated with cancer (Cancer Effect Levels, CELs) of 

molybdenum are indicated in Table 2-1 and Figure 2-2.

A User's Guide has been provided at the end of this profile (see Appendix D]). This guide should aid in 

the interpretation of the tables and figures for LSEs and MRLs.

Molybdenum, as a component of pterin-based cofactor, is an essential element. Historically, three 

molybdenum cofactor-containing enzymes have been identified: sulfite oxidase, xanthine oxidase, and 

aldehyde oxidase (NAS 2001; Sardesai 1993). These enzymes are involved in the degradation of sulfurcontaining amino acids and sulfatides, purine degradation pathway catalyzing the oxidation of 

hypoxanthine to xanthine and of xanthine to uric acid, and oxidation of aromatic and nonaromatic 

heterocycles and aldehydes to carboxylic acids (Wahl et al. 2010). Within the last 10 years, a fourth 

enzyme, mitochondrial amidoxime reducing component (mARC), has been identified in mammals (Wahl 

et al. 2010). Clear signs of molybdenum deficiency have not been found in healthy humans (NAS 2001). 

However, a deficiency in molybdenum cofactor has been observed in individuals with a severe metabolic 

defect. The lack of molybdenum cofactor and subsequent deficiencies in molybdoenzymes is manifested 

in central nervous system effects (Bayram et al. 2013). The effects that typically occur shortly after birth 

include intractable seizures and feeding difficulties; the patients develop severe psychomotor retardation 

due to progressive cerebral atrophy and ventricular dilatation (Bayram et al. 2013). The nutritional 

requirements for molybdenum are based on maintaining molybdenum balance; the Institute of Medicine 

has established the following age-specific Recommended Dietary Allowances (RDAs) (NAS 2001):

• 17 μg/day for 1–3 year olds

• 22 μg/day for 4–8 year olds

• 34 μg/day for 9–13 year olds

• 43 μg/day for 14–18 year olds
MOLYBDENUM 8

2. HEALTH EFFECTS

• 45 μg/day (0.64 μg/kg/day) for adults 

• 50 μg/day in pregnant and lactating women 

As illustrated in Figure 2-1, a number of human and laboratory animal studies have evaluated the toxicity 

of molybdenum following inhalation, oral, or dermal exposure; this toxicological profile on molybdenum 

does not include discussion of the health effects of molybdenum nanoparticles, which could have 

different toxicological and toxicokinetic properties than larger molybdenum particles. Of the 

92 identified toxicity publications, 84% evaluated health outcomes in laboratory animals; most (74%) 

were conducted by the oral route of exposure. Inhalation studies primarily focused on the respiratory 

tract, although intermediate- and chronic-duration studies examined a wide range of endpoints in rats and 

mice exposed to molybdenum trioxide. Although a large number of laboratory oral exposure studies have 

been identified, most had a limited scope (examined one or two potential targets). However, a small 

number of studies evaluated a wide range of endpoints. The most studied endpoints following oral 

exposure were potential hematological, musculoskeletal, and reproductive outcomes. No human dermal 

exposure studies were identified; the animal studies primarily focused on dermal and immunological 

endpoints.

A number of factors can influence the toxicity of molybdenum including the animal species; previous 

dietary history; relative amounts of dietary molybdenum, copper, and sulfur; and the form of 

molybdenum. The oral toxicity of molybdenum has been well-established in ruminants, particularly cows 

and sheep. The toxicity is likely due to an interaction between molybdate and sulfide in the rumen, 

resulting in the formation of thiomolybdates (Gould and Kendall 2011). In the absence of adequate 

copper in the rumen, the thiomolybdate is absorbed through the rumen or small intestine and can bind to 

copper-containing compounds such as ceruloplasmin and cytochrome oxidase, resulting in symptoms 

resembling copper deficiency (a condition often referred to as molybdenosis). The observed effects can 

include decreases in weight gain, alterations in hair/wool texture and pigmentation, delayed puberty, and 

reduced conception rates. Molybdenum also interacts with copper in monogastric animals; however, the 

mode of interaction differs between the species. The available data suggest that the findings in ruminants 

do not appear to be relevant to humans or monogastric animals (NAS 2001). Thus, ruminant data will not 

be further discussed in the toxicological profile.

Studies in rats provide evidence that copper status, particularly the copper content of the diet, can 

influence the toxicokinetics and toxicity of molybdenum; see Section 3.4 for a more detailed discussion of 

the interaction between molybdenum and copper. Administration of 150 or 500 mg/kg molybdenum in 
MOLYBDENUM 9

2. HEALTH EFFECTS

the diet for up to 6 weeks to rats fed a copper-deficient or copper-adequate diet resulted in profound 

differences in the distribution of copper and molybdenum in the plasma, liver, and kidneys (Nederbragt 

1980, 1982). For example, at a molybdenum dietary concentration of 150 mg/kg, molybdenum levels in 

the liver and kidneys were 3.5 and 9 times higher than pre-exposure levels, respectively, in the copperadequate rats as compared to 6 and 4 times higher, respectively, in the copper-deficient rats. 

Additionally, the relative increases in copper levels in the liver and kidneys associated with molybdenum 

exposure were greater in the rats fed the copper-deficient diet, as compared to those fed the copperadequate diet. Exposure to elevated levels of dietary molybdenum in animals maintained on basal diets 

with inadequate copper levels resulted in marked toxicity (for example, Brinkman and Miller 1961; 

Johnson et al. 1969; Sasmal et al. 1968). Similar effects were not observed when animals were fed 

similar molybdenum levels and maintained on a copper-adequate diet (for example, Mills et al. 1958; 

Murray et al. 2014a; Peredo et al. 2013). In the United States, the average copper intake is 1.0–

1.6 mg/day and the copper RDA is 0.9 mg/day (NAS 2001). Thus, studies in which laboratory animals 

were fed a copper-deficient diet may not be relevant to evaluating the risk of molybdenum toxicity to the 

general population with adequate copper intake. Studies in which the laboratory animals were fed a basal 

diet with inadequate copper levels are clearly identified in the text, are discussed separately from studies 

in which there were adequate dietary copper levels, and are not included in the LSE table or figure. The 

current recommended dietary copper concentrations of 5, 6, and 3 ppm have been established for rats, 

mice, and rabbits, respectively (NAS 1977, 1995); for rats and mice, a copper dietary level of 8 ppm has 

been established to support gestation and lactation (NAS 1995). 

Ammonium tetrathiomolybdate is an experimental chelating agent used to decrease excess copper levels 

in individuals with Wilson’s disease, a genetic disease that limits copper excretion resulting in an 

accumulation of toxic levels of copper in the liver, brain, and eyes. Administration of tetrathiomolybdate 

compounds, as compared to other molybdate compounds, results in more dramatic shifts in copper levels 

in rats fed copper-adequate diets (Mills et al. 1981a), and the toxicity may differ from other molybdenum 

compounds. Significant increases in serum and kidney copper levels, decreases in liver copper levels, and 

increases in serum, liver, and kidney molybdenum levels were found in rats exposed to ammonium 

tetrathiomolybdate as compared to rats receiving the same molybdenum dose as sodium molybdate (Mills 

et al. 1981a); these results suggest that the tetrathiomolybdate impaired utilization of dietary copper, 

utilization of stored copper, or both. A study in rats demonstrated that administration of supplemental 

copper could reverse the adverse effects observed following administration via gavage of 12 mg 

molybdenum/kg/day as ammonium tetrathiomolybdate (Lyubimov et al. 2004). This study suggests that 

ammonium tetrathiomolybdate may interfere with copper homeostasis. No studies evaluating whether 
MOLYBDENUM 10

2. HEALTH EFFECTS

copper supplementation would reverse the toxicity of other molybdenum compounds were identified. 

Because tetrathiomolybdate compounds may not be representative of other molybdenum compounds, 

studies involving exposure to tetrathiomolybdate compounds are not included in the LSE table and figure, 

but are discussed in the text. 

The human and animal studies suggest several sensitive targets of molybdenum toxicity: 

• Respiratory Endpoints: Respiratory effects are a presumed health effect for humans based on 

inadequate evidence in molybdenum oxide workers and a high level of evidence in rats and mice 

chronically exposed to airborne molybdenum trioxide.

• Renal Endpoints: Renal effects are a presumed health effect for humans based on no data in 

humans and a high level of evidence in laboratory animals. The observed effects include 

histological alterations in the kidneys and alterations in renal function.

• Other Endpoints: Although there is some evidence that molybdenum exposure may result in 

hepatic, reproductive, or developmental effects, the data are not considered adequate to classify 

whether molybdenum is a hepatic or developmental hazard to humans.

o Hepatic Effects: There is inadequate evidence of increased risk of liver disease in humans. 

There is high evidence that inhalation or oral exposure to molybdenum compounds will result 

in histological alterations in rats, mice, or rabbits. There is moderate evidence in rats that 

exposure may result in alterations in serum clinical chemistry parameters and/or lipid levels 

in laboratory animals. 

o Reproductive Effects: There is low evidence of male reproductive effects in cross-sectional 

studies that do not establish causality. Two high-quality animal studies have not found 

evidence of reproductive effects in rats. Several lower-quality studies have reported male and 

female reproductive effects; other studies have not reported any reproductive alterations.

o Developmental Effects: There is low evidence of developmental effects in epidemiological 

studies that do not establish causality. There are mixed results in laboratory animal studies, 

with most studies not finding evidence of developmental toxicity.
MOLYBDENUM 11

2. HEALTH EFFECTS

Figure 2-1. Overview of the Number of Studies Examining Molybdenum Health Effects

Most studies examined the potential body weight, hematological, musculoskeletal, and reproductive effects of molybdenum

Fewer studies evaluated health effects in humans than animals (counts represent studies examining endpoint)

*Includes studies discussed in Chapter 2. A total of 91 studies (including those finding no effect) have examined toxicity; most studies examined multiple 

endpoints.
MOLYBDENUM 12

2. HEALTH EFFECTS

Table 2-1. Levels of Significant Exposure to Molybdenum – Inhalation

Figure 

keya

Species 

(strain)

No./group

Exposure 

parameters

Doses 

(mg/m3)

Parameters 

monitored Endpoint

NOAEL 

(mg

Mo/m3)

Less serious 

LOAEL (mg

Mo/m3)

Serious 

LOAEL (mg

Mo/m3) Effects

ACUTE EXPOSURE

1 Rat 

(SpragueDawley) 

5 M, 5 F

4 hours 0, 1,200 CS, BW, FI, 

WI, OW, HP

Bd wt 1,200 Weight loss or no body weight gain during 

first 2–3 post-exposure days; thereafter,

weight gain was similar to controls

Resp 1,200

Ammonium dimolybdate

Jackson et al. 1991a

2 Rat 

(SpragueDawley) 

5 M, 5 F

4 hours 0, 3,890 CS, BW, FI, 

WI, OW, HP

Bd wt 3,890 Weight loss during first 2–3 postexposure days; thereafter, weight gain 

was similar to controls

Resp 3,890

Molybdenum trioxide

Jackson et al. 1991b

3 Rat 

(SpragueDawley) 

5 M, 5 F

4 hours 0, 899 CS, BW, FI, 

WI, OW, HP

Bd wt 899 Weight loss during first 2–3 postexposure days; thereafter, weight gain 

was similar to controls

Resp 899

Sodium molybdate

Jackson et al. 1991c

4 Rat 

(SpragueDawley) 

5 M, 5 F

4 hours 0, 2,613 CS, BW, FI, 

WI, OW, HP

Bd wt 2,613 14% decrease in body weight gain on 

post-exposure day 3

Resp 2,613

Molybdenum trioxide

Jackson et al. 1991d

5 Rat (CD) 

3 M, 3 F

4 hours 3,360 CS, GN, HP Resp 3,360

Molybdenum trioxide

Leuschner 2010
MOLYBDENUM 13

2. HEALTH EFFECTS

Table 2-1. Levels of Significant Exposure to Molybdenum – Inhalation

Figure 

keya

Species 

(strain)

No./group

Exposure 

parameters

Doses 

(mg/m3)

Parameters 

monitored Endpoint

NOAEL 

(mg

Mo/m3)

Less serious 

LOAEL (mg

Mo/m3)

Serious 

LOAEL (mg

Mo/m3) Effects

6 Rat 

(Fischer344) 5 M, 

5 F

6 hours/day

5 days/week

14 days

0, 2, 6.7, 

20, 67, 

200

CS, BW, HP Bd wt 67 200 Decreased body weight gain in males at 

67 mg/m3 (10%) and females exposed to 

200 mg/m3 (13%); weight loss in males at 

200 mg/m3 (terminal weight 5% less than 

initial weight)

Resp 200

Molybdenum trioxide

NTP 1997

7 Mouse 

(B6C3F1) 

5 M, 5 F

6 hours/day

5 days/week

14 days

0, 2, 6.7, 

20, 67, 

200

CS, BW, HP Bd wt 200 Body weight loss in males and decrease 

in body weight gain in females

Resp 200

Molybdenum trioxide

NTP 1997

INTERMEDIATE EXPOSURE

8 Rat 

(Fischer344) 10 M, 

10 F

6.5 hours/day

5 days/week

13 weeks

0, 0.67, 

2, 6.7, 

20, 67

CS, BW, 

OW, HP, RX

Bd wt 67

Resp 67

Cardio 67

Gastro 67

Hemato 67

Musc/skel 67

Hepatic 67

Renal 67

Endocr 67

Repro 67 M

Molybdenum trioxide

NTP 1997

9 Mouse 

(B6C3F1) 

10 M, 10 F

6.5 hours/day

5 days/week

13 weeks

0, 0.67, 

2, 6.7, 

20, 67

CS, BW, 

OW, HP, RX

Bd wt 67

Resp 67

Cardio 67

Gastro 67
MOLYBDENUM 14

2. HEALTH EFFECTS

Table 2-1. Levels of Significant Exposure to Molybdenum – Inhalation

Figure 

keya

Species 

(strain)

No./group

Exposure 

parameters

Doses 

(mg/m3)

Parameters 

monitored Endpoint

NOAEL 

(mg

Mo/m3)

Less serious 

LOAEL (mg

Mo/m3)

Serious 

LOAEL (mg

Mo/m3) Effects

Hemato 67

Musc/skel 67

Hepatic 67

Renal 67

Endocr 67

Repro 67 M

Molybdenum trioxide

NTP 1997

CHRONIC EXPOSURE

10 Human 

25 M

Occupational 0, 9.47 BI, OF Resp 9.47 

Other 

noncancer

9.47 Increased serum uric acid levels

Molybdate

Walravens et al. 1979

11 Rat 

(Fischer344) 50 M, 

50 F

6 hours/day

5 days/week

105 weeks

0, 6.7, 

20, 67

CS, BW, HP Bd wt 67

Resp 6.7b Hyaline degeneration of nasal respiratory 

and olfactory epithelium (females only), 

squamous metaplasia of the epiglottis, 

and chronic lung inflammation (only 

significant at 20 and 67 mg/m3

concentrations);

BMCLHEC of 0.071 mg/m3

Cardio 67

Gastro 67

Musc/skel 67

Hepatic 67

Renal 67

Endocr 67

Molybdenum trioxide

NTP 1997
MOLYBDENUM 15

2. HEALTH EFFECTS

Table 2-1. Levels of Significant Exposure to Molybdenum – Inhalation

Figure 

keya

Species 

(strain)

No./group

Exposure 

parameters

Doses 

(mg/m3)

Parameters 

monitored Endpoint

NOAEL 

(mg

Mo/m3)

Less serious 

LOAEL (mg

Mo/m3)

Serious 

LOAEL (mg

Mo/m3) Effects

12 Mouse 

(B6C3F1) 

50 M, 50 F

6 hours/day

5 days/week

105 weeks

0, 6.7, 

20, 67

CS, BW, HP Bd wt 67

Resp 6.7 Squamous metaplasia of the epiglottis, 

histiocytic cellular infiltration in the lungs, 

and alveolar epithelial metaplasia were 

observed at ≥6.7 mg/m3

; nasal 

suppurative inflammation in males at 

20 or 67 mg/m3 and hyaline degeneration 

of nasal respiratory and olfactory 

epithelium (females only) at 67 mg/m3

Cardio 67

Gastro 67

Musc/skel 67

Hepatic 67

Renal 67

Endocr 67

Cancer 6.7 Alveolar/bronchiolar carcinoma in males 

at ≥6.7 mg/m3 and increased incidence of 

alveolar/bronchiolar adenoma in females 

at ≥20 mg/m3; an increase in alveolar/

bronchiolar adenoma or carcinoma in 

male mice exposed to 6.7 or 20 mg/m3

Molybdenum trioxide

NTP 1997

aThe number corresponds to entries in Figure 2-2; differences in levels of health effects and cancer effects between male and females are not indicated in Figure 2-2. 

Where such differences exist, only the levels of effect for the most sensitive gender are presented. bUsed to derive a chronic-duration oral MRL for molybdenum trioxide of 0.002 mg molybdenum/m3 based on a BMCL10 human equivalent concentration (HEC) of 0.071 mg 

molybdenum/m3 and an uncertainty factor of 30.

Bd wt or BW = body weight; BI = biochemical changes; BMCL = 95% lower confidence limit on the benchmark concentration; Cardio = cardiovascular; CS = clinical signs; 

Endocr = endocrine; F = female(s); FI = food intake; Gastro = gastrointestinal; GN = gross necropsy; Hemato = hematological; HP = histopathology; LOAEL = lowestobserved-adverse-effect level; M = male(s); MRL = minimal risk level; Musc/skel = muscular skeletal; NOAEL = no-observed-adverse-effect level; OF = organ function; 

OW = organ weight; Repro = reproductive; Resp = respiratory; RX = reproductive effects; WI = water intake
MOLYBDENUM 16

2. HEALTH EFFECTS

Figure 2-2. Levels of Significant Exposure to Molybdenum – Inhalation

Acute (≤14 days)
MOLYBDENUM 17

2. HEALTH EFFECTS

Figure 2-2. Levels of Significant Exposure to Molybdenum – Inhalation

Intermediate (15-364 days)
MOLYBDENUM 18

2. HEALTH EFFECTS

Figure 2-2. Levels of Significant Exposure to Molybdenum – Inhalation

Chronic (≥365 days)
MOLYBDENUM 19

2. HEALTH EFFECTS

Table 2-2. Levels of Significant Exposure to Molybdenum – Oral

Figure 

keya

Species 

(strain)

No./group

Exposure 

parameters

Doses 

(mg/kg/day)

Parameters 

monitored Endpoint

NOAEL (mg

Mo/kg/day)

Less serious 

LOAEL (mg

Mo/kg/day)

Serious 

LOAEL (mg

Mo/kg/day) Effects

ACUTE EXPOSURE

1 Human 

4 M

10 days

(F)

0.00237, 

0.00771, 

0.022

UR Other 

noncancer

0.022 No alterations in urinary uric acid 

levels

Ammonium molybdate

Deosthale and Gopalan 1974

2 Rat 

(SpragueDawley) 

5 M, 5 F

Once

(GO)

1,900, 2,400, 

3,000

LE, CS, BW, 

GN

Death 2,291 LD50

Gastro 2,400 3,000 Thickening of the glandular 

stomach

Ammonium dimolybdate

Baldrick and Healing 1990e

3 Rat 

(SpragueDawley) 

5 M, 5 F

Once

(GO)

2,000, 2,500 

(males only), 

3,200, 4,000 

(females 

only), 5,000

LE, CS, BW, 

GN

Death 2,566 F, 

1,802 M

LD50

Molybdenum trioxide

Baldrick and Healing 1990f

4 Rat 

(SpragueDawley) 

5 M, 5 F

Once

(GO)

1,500, 2,300, 

3,000

LE, CS, BW, 

GN

Death 2,079 F,

1,912 M

LD50

Sodium molybdate

Baldrick and Healing 1990g

5 Rat 

(SpragueDawley) 

22 M

PNDs 4–17

(G)

0, 50 BW, HP Bd wt 50 

Musc/skel 50 Increased buccal and sulcal 

enamel lesions following preeruptive exposure to molybdenum 

and administration of a caries 

promoting diet

Sodium molybdate

Hunt and Navia 1975
MOLYBDENUM 20

2. HEALTH EFFECTS

Table 2-2. Levels of Significant Exposure to Molybdenum – Oral

Figure 

keya

Species 

(strain)

No./group

Exposure 

parameters

Doses 

(mg/kg/day)

Parameters 

monitored Endpoint

NOAEL (mg

Mo/kg/day)

Less serious 

LOAEL (mg

Mo/kg/day)

Serious 

LOAEL (mg

Mo/kg/day) Effects

6 Mouse 

(ICR) 25 F

14 days

(W)

0, 1.3, 2.6, 

5.3, 11

HP Repro 2.6 5.3 Increase in the rate of abnormal 

MII oocytes and decrease in 

ovarian weights at 11 mg/kg/day; 

ovarian hyperemia at 5.3 and 

11 mg/kg/day (incidence not 

reported)

Sodium molybdate

Zhang et al. 2013

7 Mouse 

(ICR) 10 M

14 days

(W)

0, 3, 6, 12, 

25, 49

RX Repro 12 25 Decreases in relative epididymides

weight, sperm concentration, and 

sperm motility and increase in rate 

of sperm abnormalities

Sodium molybdate

Zhai et al. 2013

8 Rabbit 

(New 

Zealand) 

5 M

14 days

(F)

0, 0.58 BW, HP Bd wt 0.58 

Hepatic 0.58 

Renal 0.58 

Ammonium heptamolybdate

Bersenyi et al. 2008

INTERMEDIATE EXPOSURE

9 Rat 

(SpragueDawley) 

7 M

8 weeks

(GW)

0, 40, 80 BW, OW, UR Bd wt 40 80 Decrease in body weight gain; 

terminal body weight was 26% 

lower than in controls

Renal 40 80 Increases in diuresis and 

creatinuria, decreases in 

creatinine clearance, increases in 

urinary kallikrein (distal tubule 

enzyme) levels, and increases in 

relative and absolute kidney 

weights

Ammonium heptamolybdate

Bompart et al. 1990
MOLYBDENUM 21

2. HEALTH EFFECTS

Table 2-2. Levels of Significant Exposure to Molybdenum – Oral

Figure 

keya

Species 

(strain)

No./group

Exposure 

parameters

Doses 

(mg/kg/day)

Parameters 

monitored Endpoint

NOAEL (mg

Mo/kg/day)

Less serious 

LOAEL (mg

Mo/kg/day)

Serious 

LOAEL (mg

Mo/kg/day) Effects

10 Rat 

(SpragueDawley) 

6 F

8 weeks

(W)

0, 0.76, 1.5, 

7.6, 15

BW, WI, RX Repro 0.76 1.5 Prolonged estrus phase (6–

12 hours) of the estrous cycle at 

≥1.5 mg/kg/day; no effects on 

fertility

Sodium molybdate

Fungwe et al. 1990

11 Rat 

(SpragueDawley) 3–

6 M, 2–3 F

6 weeks

(F)

0, 70 BW, HE Hemato 70

Sodium molybdate

Gray and Daniel 1954

12 Rat (LongEvans) 

4 M, 4 F

At least 

8 weeks

(F)

0, 7 BW, HE Bd wt 7

Hemato 7

Repro 7

Develop 7

Sodium molybdate

Jeter and Davis 1954

13 Rat 

(Wistar) 

4 M

5 weeks

(F)

0, 74 BW, BI Bd wt 74 36% decrease in body weight gain

Sodium molybdate

Mills et al. 1958

14 Rat 

(SpragueDawley) 

10 M, 10 F

90 days

(F)

0, 5, 17, 60 CS, BW, BC, 

HE, FI, GN, 

HP, OW

Bd wt 17 M 60 M 15.2% lower terminal body weight 

in males

Resp 60

Cardio 60

Gastro 60

Hemato 60

Hepatic 60

Renal 17 Fb 60 F Slight diffuse hyperplasia in the 

renal proximal tubules were 

observed in 2/10 female rats
MOLYBDENUM 22

2. HEALTH EFFECTS

Table 2-2. Levels of Significant Exposure to Molybdenum – Oral

Figure 

keya

Species 

(strain)

No./group

Exposure 

parameters

Doses 

(mg/kg/day)

Parameters 

monitored Endpoint

NOAEL (mg

Mo/kg/day)

Less serious 

LOAEL (mg

Mo/kg/day)

Serious 

LOAEL (mg

Mo/kg/day) Effects

Ocular 60

Endocr 60

Repro 60 F

60 M

Other 

noncancer

60

Sodium molybdate

Murray et al. 2014a

15 Rat 

(SpragueDawley) 

25 F

GDs 6–20

(F)

0, 3, 10, 20, 

40

DX Develop 40 

Sodium molybdate

Murray et al. 2014b

16 Rat 

(SpragueDawley) 

24 M, 24 F

2 generations 

10 weeks 

prior to 

mating, 10–

17 days 

mating period, 

and gestation 

and lactation 

periods

(W)

0, 5, 17, 40 CS, BW, 

OW, HP, RX, 

DX

Bd wt 40

Resp 40

Renal 40

Endocr 40

Repro 40

Develop 40

Sodium molybdate

Murray et al. 2019

17 Rat 

(SpragueDawley) 

24 M, 24 F

2 generations 

10 weeks 

prior to 

mating, 10–

17 days 

mating period, 

and gestation 

and lactation 

periods

(F)

0, 40 CS, BW, 

OW, HP, RX, 

DX

Bd wt 40 F Decreased maternal weight gain 

(22%) on GDs 0–7

Resp 40

Renal 40

Endocr 40

Repro 40

Develop 40
MOLYBDENUM 23

2. HEALTH EFFECTS

Table 2-2. Levels of Significant Exposure to Molybdenum – Oral

Figure 

keya

Species 

(strain)

No./group

Exposure 

parameters

Doses 

(mg/kg/day)

Parameters 

monitored Endpoint

NOAEL (mg

Mo/kg/day)

Less serious 

LOAEL (mg

Mo/kg/day)

Serious 

LOAEL (mg

Mo/kg/day) Effects

Sodium molybdate

Murray et al. 2019

18 Rat 

(Druckery) 

10 M

5 days/week 

60 days

(GW)

0, 4.7, 14, 24 BW Bd wt 24 

Repro 4.7 14 Decreases in sperm count and 

sperm motility and increases in 

sperm abnormalities at 

≥14 mg/kg/day; degeneration of 

seminiferous tubules in the testes 

at 24 mg/kg/day; it is unclear 

whether this was also observed at 

14 mg/kg/day

Sodium molybdate

Pandey and Singh 2002

19 Rat 

(Druckery) 

20 M

5 days/week 

60 days

(GW)

0, 14 DX, RX Repro 14 Decrease in fertility (60% versus 

80% in controls) and increased 

pre-implantation losses

Develop 14 Increased post-implantation 

losses, increased resorptions, 

decreased number of live fetuses, 

and decreases in fetal weight and 

crown-rump length

Sodium molybdate

Pandey and Singh 2002

20 Rat 

(Wistar) 

6 M

9 weeks

(W)

0, 100 BW, BI, OW Bd wt 100 

Cardio 100 

Other 

noncancer

100 No alterations in blood triglyceride, 

glucose, or insulin levels

Sodium molybdate

Peredo et al. 2013
MOLYBDENUM 24

2. HEALTH EFFECTS

Table 2-2. Levels of Significant Exposure to Molybdenum – Oral

Figure 

keya

Species 

(strain)

No./group

Exposure 

parameters

Doses 

(mg/kg/day)

Parameters 

monitored Endpoint

NOAEL (mg

Mo/kg/day)

Less serious 

LOAEL (mg

Mo/kg/day)

Serious 

LOAEL (mg

Mo/kg/day) Effects

21 Rat 

(Wistar) 

10 M or 

5 M, 5 F

4–5 weeks

(F)

0, 110 BW, BI Bd wt 110 M 46–48% decrease in body weight 

gain

Sodium molybdate

Van Reem and Williams 1956

22 Rat 

(Wistar) 

8 NR

6 weeks

(F)

0, 85 BW, BI Bd wt 85

Sodium molybdate

Williams and Van Reem 1956

23 Rat 

(Wistar) 

8 NR

6 weeks

(F)

0, 90, 144, 

185

BW, BI Bd wt 90 Decreases in body weight gain of 

22, 44, and 60% in the 90, 144, 

and 185 mg/kg/day groups

Sodium molybdate

Williams and Van Reem 1956

24 Rat 

(SpragueDawley) 

10 F

8 weeks

(W)

0, 0.015, 

0.076, 0.15, 

0.30, 0.76, 

1.5

BW, BI, OW Bd wt 1.5 

Sodium molybdate

Yang and Yang 1989

25 Mouse 

(Kunming) 

20 M

100 days

(W)

0, 100 BW, BC, HP, 

RX

Bd wt 100 

Repro 100 Decreased sperm density and 

motility; testicular atrophy (no 

incidence data reported)

Molybdenum

Wang et al. 2016
MOLYBDENUM 25

2. HEALTH EFFECTS

Table 2-2. Levels of Significant Exposure to Molybdenum – Oral

Figure 

keya

Species 

(strain)

No./group

Exposure 

parameters

Doses 

(mg/kg/day)

Parameters 

monitored Endpoint

NOAEL (mg

Mo/kg/day)

Less serious 

LOAEL (mg

Mo/kg/day)

Serious 

LOAEL (mg

Mo/kg/day) Effects

26 Rabbit 

(Dutch) 2–

5 M, F

30–84 days

(F)

0, 7.1, 25, 

54, 120, 240

CS, LE, BW, 

HE

Death 120 4/5 and 2/2 died at 120 and 

240 mg/kg/day; average survival 

was 44 and 30 days, respectively

Bd wt 25 120 Weight loss at 120 and 

240 mg/kg/day

Hemato 25 54 Anemia in 2/5, 5/5, and 4/5 rabbits 

at 54, 120, and 240 mg/kg/day

Musc/skel 25 54 Front leg abnormality described as 

weakness progressing to inability 

to “maintain weight and legs 

spread outward”

Dermal 25 54 Alopecia

Sodium molybdate

Arrington and Davis 1953

CHRONIC EXPOSURE

27 Human 

262 M, F

NR

(F)

0.21 BC Other 

noncancer

0.21 Increased incidence of symptoms 

of gout and an increased blood 

uric acid levels

Molybdenum

Koval’sky et al. 1961

aThe number corresponds to entries in Figure 2-3; differences in levels of health effects and cancer effects between male and females are not indicated in Figure 2-3. 

Where such differences exist, only the levels of effect for the most sensitive gender are presented. bUsed to derive an intermediate-duration oral MRL of 0.06 mg/kg/day based on a NOAEL of 17 mg molybdenum/kg/day, a total uncertainty factor of 100, and a modifying 

factor of 3.

BC = biochemistry; Bd wt or BW = body weight; BI = biochemical changes; Cardio = cardiovascular; CS = clinical signs; Develop = developmental; DX = developmental 

effects; Endocr = endocrine; (F) = feed; F = female(s); FI = food intake; (GO) = gavage in oil; (GW) = gavage in water; Gastro = gastrointestinal; GD = gestation day; 

GN = gross necropsy; HE = hematology; Hemato = hematological; HP = histopathology; LE = lethality; LOAEL = lowest-observed-adverse-effect level; M = male(s); 

MRL = minimal risk level; Musc/skel = muscular/skeletal; NOAEL = no-observed-adverse-effect level; NR = not reported; OW = organ weight; PND = postnatal day; 

Repro = reproductive; Resp = respiratory; RX = reproductive effects; UR = urinalysis; (W) = water; WI = water intake
MOLYBDENUM 26

2. HEALTH EFFECTS

Figure 2-3. Levels of Significant Exposure to Molybdenum – Oral

Acute (≤14 days)
MOLYBDENUM 27

2. HEALTH EFFECTS

Figure 2-3. Levels of Significant Exposure to Molybdenum – Oral

Intermediate (15-364 days)
MOLYBDENUM 28

2. HEALTH EFFECTS

Figure 2-3. Levels of Significant Exposure to Molybdenum – Oral

Intermediate (15-364 days)
MOLYBDENUM 29

2. HEALTH EFFECTS

Figure 2-3. Levels of Significant Exposure to Molybdenum – Oral

Chronic (≥365 days)
MOLYBDENUM 30

2. HEALTH EFFECTS

Table 2-3. Levels of Significant Exposure to Molybdenum – Dermal

Species (strain)

No./group

Exposure 

parameters

Doses 

(mg/kg/day)

Parameters 

monitored Endpoint

NOAEL (mg

Mo/kg/day)

Less serious 

LOAEL (mg

Mo/kg/day)

Serious LOAEL 

(mg

Mo/kg/day) Effects

ACUTE EXPOSURE

Guinea pig 

(Dunkin/Hartley) 

20 F

Twice 90% CS, BW, IX Immuno 90 

Ammonium dimolybdate

Allan 1996a

Guinea pig 

(Dunkin/Hartley) 

20 F

Twice 70% CS, BW, IX Immuno 70 

Molybdenum trioxide

Allan 1996c

Guinea pig 

(Dunkin/Hartley) 

20 F

Twice 70% CS, BW, IX Immuno 70 

Sodium molybdate

Allan 1996d

Guinea pig 

(Dunkin/Hartley) 

20 F

Twice 70% CS, BW, IX Immuno 70 

Molybdenum trioxide

Allan 1996b

Rat (CD) 

5 M, 5 F

24 hours 0, 

1,200 mg/kg

CS, BW, GN Dermal 1,200

Ammonium dimolybdate

Baldrick and Healing 1990a

Rat (CD) 

5 M, 5 F

24 hours 0, 

1,300 mg/kg

CS, BW, GN Dermal 1,300

Molybdenum trioxide

Baldrick and Healing 1990b

Rat (CD) 

5 M, 5 F

24 hours 0, 930 mg/kg CS, BW, GN Dermal 930

Sodium molybdate

Baldrick and Healing 1990c
MOLYBDENUM 31

2. HEALTH EFFECTS

Table 2-3. Levels of Significant Exposure to Molybdenum – Dermal

Species (strain)

No./group

Exposure 

parameters

Doses 

(mg/kg/day)

Parameters 

monitored Endpoint

NOAEL (mg

Mo/kg/day)

Less serious 

LOAEL (mg

Mo/kg/day)

Serious LOAEL 

(mg

Mo/kg/day) Effects

Rat (CD) 

5 M, 5 F

24 hours 0, 

1,300 mg/kg

CS, BW, GN Dermal 1,300

Molybdenum trioxide

Baldrick and Healing 1990d

Rabbit (New 

Zealand) 

6 M

Once 0, 56 mg CS Ocular 56 Mild conjunctival inflammation

Ammonium dimolybdate

Liggett and McRae 1990a

Rabbit (New 

Zealand) 

6 M

Once 0, 67 mg CS Ocular 67 Mild conjunctival inflammation

Molybdenum trioxide

Liggett and McRae 1990b

Rabbit (New 

Zealand) 

6 M

Once 0, 46 mg CS Ocular 46 Mild conjunctival inflammation

Sodium molybdate

Liggett and McRae 1990c

Rabbit (New 

Zealand) 

6 M

Once 0, 67 CS Ocular 67 Conjunctival inflammation

Molybdenum trioxide

Liggett and McRae 1990d

Rabbit (New 

Zealand) 

6 F

4 hours 280 mg CS Dermal 280 

Ammonium dimolybdate

Liggett and McRae 1990e

Rabbit (New 

Zealand) 

6 F

4 hours 340 mg CS Dermal 340 

Molybdenum trioxide

Liggett and McRae 1990f
MOLYBDENUM 32

2. HEALTH EFFECTS

Table 2-3. Levels of Significant Exposure to Molybdenum – Dermal

Species (strain)

No./group

Exposure 

parameters

Doses 

(mg/kg/day)

Parameters 

monitored Endpoint

NOAEL (mg

Mo/kg/day)

Less serious 

LOAEL (mg

Mo/kg/day)

Serious LOAEL 

(mg

Mo/kg/day) Effects

Rabbit (New 

Zealand) 

6 F

4 hours 230 mg CS Dermal 230 

Sodium molybdate

Liggett and McRae 1990g

BW = body weight; CS = clinical signs; F = female(s); GN = gross necropsy; IX = immune function; LOAEL = lowest-observed-adverse-effect level; M = male(s); 

NOAEL = no-observed-adverse-effect level
MOLYBDENUM 33

2. HEALTH EFFECTS

2.2 DEATH

The lethality of molybdenum compounds has been investigated in several inhalation and oral exposure 

studies in laboratory animals. In inhalation studies, no deaths were reported in rats or mice exposed to 

≤200 mg molybdenum/m3 for 14 days (NTP 1997) or ≤67 mg molybdenum/m3 for 90 days or 2 years 

(NTP 1997).

Oral LD50 values have been estimated in rats exposed to several molybdenum compounds. The estimated 

LD50 values were 2,291 mg molybdenum/kg for ammonium dimolybdate (Baldrick and Healing 1990e), 

1,802 and 2,566 mg molybdenum/kg for pure molybdenum trioxide for males and females, respectively 

(Baldrick and Healing 1990f), and 1,912 and 2,079 mg molybdenum/kg for sodium molybdate for males 

and females, respectively (Baldrick and Healing 1990g). A study of technical-grade molybdenum 

trioxide did not report deaths occurring in rats administered a single dose of 3,400 mg molybdenum/kg 

(Baldrick and Healing 1990h).

Several oral studies have reported deaths in rabbits repeatedly exposed to molybdenum. Mortality rates 

of 42–100% were observed in rabbits exposed to 59–120 mg molybdenum/kg/day for intermediate 

durations (Arrington and Davis 1953; Robinson et al. 1969; Valli et al. 1969; Widjajakusuma et al. 1973). 

Although the causes of death were not reported, anorexia, body weight loss, and anemia were observed in 

most of the studies at the lethal concentrations, suggesting that the deaths may be related to a functional 

copper deficiency. The copper content of the diet was adequate in the Arrington and Davis (1953) study 

and was not reported in the Widjajakusuma et al. (1973), Robinson et al. (1969), and Valli et al. (1969) 

studies. No deaths have been reported in rat studies (e.g., Lyubimov et al. 2004; Murray et al. 2014a, 

2014; Pandey and Singh 2002).

2.3 BODY WEIGHT

There are limited epidemiological data evaluating possible associations between molybdenum and body 

weight. A cross-sectional study of National Health and Nutrition Examination Survey (NHANES) 

participants did not find an association between urinary molybdenum levels and the risk of being 

overweight (Mendy et al. 2012).

Several inhalation exposure studies have reported body weight effects in laboratory animals. Single 

4-hour exposures to 1,200 mg molybdenum/m3 as ammonium dimolybdate (Jackson et al. 1991a), 
MOLYBDENUM 34

2. HEALTH EFFECTS

3,890 mg molybdenum/m3 as molybdenum trioxide (Jackson et al. 1991b), 2,613 mg molybdenum/m3 as 

molybdenum trioxide (Jackson et al. 1991d), or 899 mg molybdenum/m3 as sodium molybdate (Jackson 

et al. 1991c) resulted in decreases in body weight gain or weight loss during the first 2–3 days postexposure; thereafter, the body weight gain was similar to controls. Decreases in body weight gain and

weight loss were observed in rats and mice exposed via inhalation to molybdenum trioxide for 14 days 

(NTP 1997). Terminal body weights were 10% lower in male rats exposed to 67 mg molybdenum/m3

compared to controls, and weight loss was observed in male rats and mice exposed to 200 mg 

molybdenum/m3

. In female rats and mice exposed to 200 mg molybdenum/m3, the terminal body weights 

were 13 and 10%, respectively, lower than the control groups. No significant alterations in body weight 

gain were observed in rats or mice exposed to airborne molybdenum trioxide concentrations as high as 

67 mg molybdenum/m3 for 13 weeks or 2 years (NTP 1997).

A large number of animal studies reported alterations in body weight following acute- or intermediateduration oral exposure to molybdenum. Large differences in terminal body weights between controls and 

molybdenum-exposed groups and weight loss have been reported in many studies in which the basal diet 

did not provide adequate levels of copper (Brinkman and Miller 1961; Fell et al. 1979; Johnson and 

Miller 1961; Ostrom et al. 1961; Sasmal et al. 1968; Van Reen 1959). In one study, exposure to 500 mg 

molybdenum/kg/day as sodium molybdate resulted in weight loss in rats (Sasmal et al. 1968); no 

alterations in weight loss were observed at 50 or 100 mg molybdenum/kg/day. The weight loss began 

early in the study; the animals weighed about 35% less than at the start of the study after 1 week of 

exposure. In another study by this group (Sasmal et al. 1968), exposure to 50 mg molybdenum/kg/day as 

ammonium molybdate resulted in weight loss. Although the study suggests differences between the two 

molybdenum compounds, the very low copper content of the diet (no additional copper was added to the 

purified diet) precludes extrapolating these data to other conditions. In another study comparing 

molybdenum compounds, a 10-day dietary exposure to 0.6 mg molybdenum/kg/day as ammonium 

tetrathiomolybdate resulted in a 10% decrease in body weight in rats; however, no alterations in body 

weight gain were observed in rats exposed to 0.6 mg molybdenum/kg/day as ammonium heptamolybdate 

under the same exposure conditions (Parry et al. 1993). The copper content of the diet was 3 ppm, which 

is lower than the recommendation of 5 ppm in the diet (NAS 1995).

Decreases in body weight gain have been observed in studies in which the basal diet provided a 

nutritionally adequate level of copper (Arrington and Davis 1953; Bompart et al. 1990; Jeter and Davis 

1954; Johnson et al. 1969; Lyubimov et al. 2004; Mills et al. 1958; Murray et al. 2014a; Van Reen and 

Williams 1956). Significant decreases in body weight gain were observed at 60–110 mg 
MOLYBDENUM 35

2. HEALTH EFFECTS

molybdenum/kg/day as sodium molybdate or ammonium heptamolybdate in intermediate-duration studies 

(Bompart et al. 1990; Mills et al. 1958; Murray et al. 2014a; Van Reen and Williams 1956; Williams and 

Van Reen 1956). The magnitude of the decrease in body weight gain appeared to be related to the dose, 

with approximately 15% decreases observed at 60 mg molybdenum/kg/day and 48% decreases observed 

at 110 mg molybdenum/kg/day. Administration of ammonium tetrathiomolybdate resulted in a LOAEL 

of 4.4 mg molybdenum/kg/day for decreases in body weight gain (Lyubimov et al. 2004); the interaction 

between the ammonium tetrathiomolybdate and copper may have resulted in copper insufficiency and 

contributed to the body weight effect. Decreases in food intake have also been reported in dietary 

exposure studies (Murray et al. 2014a; Williams and Van Reen 1956) and a gavage study (Lyubimov et 

al. 2004). Williams and Van Reen (1956) found that when the control group food intake was matched to 

the molybdenum group, body weight was not adversely affected after 5 weeks of exposure to 85 mg 

molybdenum/kg/day as sodium molybdate. However, when the control group had ad libitum access to 

food, exposure to 90 mg molybdenum/kg/day as sodium molybdate resulted in a 22% decrease in body 

weight gain. In contrast, Murray et al. (2014a) found a decrease in food conversion efficiency suggesting 

that factors other than the reduction in feed intake resulted in the decreased body weight gain. Similarly, 

in a study by Johnson and Miller (1961) in which the basal diet contained 3.2 ppm copper, large 

differences (50–60% less) in food intake were observed between the control group and the group exposed 

to 20 ppm molybdenum/kg/day as sodium molybdate. However, when the control intake was matched to 

the molybdenum group’s intake, significant decreases in body weight gain were still observed.

2.4 RESPIRATORY

Limited data are available on the toxicity of molybdenum to the respiratory tract of humans. A cohort 

study of workers exposed to molybdenum trioxide and other oxides at a molybdenite roasting plant 

reported normal lung function test results in 20/25 workers (Walravens et al. 1979). Some alterations in 

lung function (forced expiratory volume in 1 second, FEV1) were observed in the remaining five workers; 

the decrease in FEV1 was characterized as mild in three of the workers and “more marked” in two 

workers, which may be indicative of mild obstructive lung disease. The study did not provide lung 

function data for a reference group. The estimated 8-hour time-weighted average (TWA) molybdenum 

concentration in total dust was 9.46 mg molybdenum/m3

; the molybdenum content of the respirable dust 

ranged from 1.02 to 4.49 mg molybdenum/m3

. Another cohort study of workers exposed to fine and 

ultrafine molybdenum trioxide dust reported dyspnea and cough in symptomatic workers (Ott et al. 2004). 

Radiographic abnormalities were noted in the lungs of most of the symptomatic workers and in half of the 

asymptomatic workers, although none of the radiographs showed evidence of interstitial lung disease. 
MOLYBDENUM 36

2. HEALTH EFFECTS

Significant differences in lung function (increased predicted FEV1 and forced vital capacity) were also 

observed in the workers, as compared to a control group. In symptomatic workers, alterations in 

bronchioalveolar lavage cytology suggestive of subclinical alveolitis were noted. This study (Ott et al. 

2004) has several limitations including the lack of monitoring data, minimal information on the control 

group, which does not appear to be comprised of workers at this facility, and differences in the mean and 

ranges of ages of the different groups (40.0 years [range of 24–58 years], 30.5 years [22–45 years], and 

30.0 years [14–72 years] in the symptomatic workers, asymptomatic workers, and controls, respectively), 

which were not adjusted for in the statistical analyses.

The potential respiratory toxicity of molybdenum has been investigated in laboratory animals exposed to 

airborne molybdenum trioxide for acute, intermediate, and chronic durations and in intermediate-duration 

oral studies in rats. No histological alterations were observed in the lungs of rats exposed for 4 hours to 

1,200 mg molybdenum/m3 as ammonium dimolybdate (Jackson et al. 1991a), 2,613–3,890 mg 

molybdenum/m3 as molybdenum trioxide (Jackson et al. 1991b, 1991d; Leuschner 2010), or 899 mg 

molybdenum/m3 as sodium molybdate (Jackson et al. 1991c). In inhalation studies conducted by the 

National Toxicology Program (NTP 1997), no histological alterations were observed in the nasal cavity of 

rats and mice exposed to 200 mg molybdenum/m3 as molybdenum trioxide for 14 days (NTP 1997); no 

other regions of the respiratory tract were examined. Similarly, no histological alterations were observed 

in the respiratory tract of rats or mice exposed to ≤67 mg molybdenum/m3 as molybdenum trioxide for 

13 weeks (NTP 1997). In contrast, chronic exposure resulted in lesions in the nose, larynx, and lungs in 

rats and mice exposed to molybdenum trioxide for 2 years (NTP 1997). In the nose, hyaline degeneration 

of the respiratory and olfactory epitheliums was observed in rats exposed to ≥6.7 mg molybdenum/m3 and 

in mice exposed to 67 mg molybdenum/m3

; other nasal lesions observed in mice included suppurative 

inflammation at ≥20 mg molybdenum/m3 and olfactory epithelial atrophy at 67 mg molybdenum/m3

. 

Squamous metaplasia of the epiglottis was observed in rats and mice exposed to ≥6.7 mg 

molybdenum/m3

. In the lungs, chronic inflammation was observed in rats exposed to ≥20 mg 

molybdenum/m3.and alveolar epithelial metaplasia and histiocytic cellular infiltration were observed at 

≥6.7 mg molybdenum/m3

.

Two laboratory animal studies examined the respiratory tract following oral exposure to molybdenum. 

No lesions were observed in the lungs of rats exposed to ≤60 mg molybdenum/kg/day as sodium 

molybdate in the diet for 90 days (Murray et al. 2014a) or ≤40 mg molybdenum/kg/day as sodium 

molybdate in the drinking water or diet for 147–158 days (Murray et al. 2019).
MOLYBDENUM 37

2. HEALTH EFFECTS

2.5 CARDIOVASCULAR

Using the dataset from the NHANES cross-sectional study (2009–2012), Shiue and Hristova (2014) 

found an association between urinary molybdenum levels and high blood pressure among adults after 

adjusting for potential confounders (adjusted odds ratio [OR] of 1.45; 95% confidence interval [CI] of 

1.04–2.02). The investigators estimated that molybdenum accounted for 6.3% of the variance in the

population risk and significant associations were also found for other metals including cesium, lead, 

platinum, antimony, arsenic, and tungsten and industrial pollutants including phthalates, bisphenol A, and 

parabens. In a cross-sectional study examining the possible association between municipal water 

constituents and cardiovascular mortality in residents of 94 large cities in the United States, Schroeder 

and Kraemer (1974) found a weak negative correlation between arteriosclerotic heart disease deaths and 

molybdenum levels among white males, but not white females or nonwhite males or females. The mean 

concentration of molybdenum in the municipal water samples was 1.25 μg/L (0.00003 mg 

molybdenum/kg/day, assuming a water intake of 2 L/day and body weight of 70 kg) with a range of 0–

16 μg/L. These studies appear to provide conflicting results, with one study suggesting a beneficial effect 

of increased molybdenum (Schroeder and Kraemer 1974) and the other a detrimental effect (Shiue and 

Hristova 2014). However, a number of etiological factors contribute to the overall risk of both diseases 

and the contribution of molybdenum to the overall risk was low in both studies.

In the only laboratory animal study evaluating blood pressure, Peredo et al. (2013) reported a slight 

decrease (approximately 4%) in systolic blood pressure in rats exposed to 100 mg molybdenum/kg/day as 

sodium molybdate in drinking water for 9 weeks; this slight decrease in blood pressure was not 

considered biologically relevant. No histological alterations were observed in the hearts of rats or mice 

exposed to airborne molybdenum trioxide concentrations as high as 67 mg molybdenum/m3 for 13 weeks 

or 2 years (NTP 1997) or in rats ingesting ≤60 mg molybdenum/kg/day as sodium molybdate in the diet 

for 90 days (Murray et al. 2014a). 

2.6 GASTROINTESTINAL

Intermediate- or chronic-duration inhalation exposure to ≤67 mg molybdenum/m3 as molybdenum 

trioxide did not result in histological alterations in the gastrointestinal tract (NTP 1997).

A single-dose oral lethality study reported thickening of the glandular stomach in rats receiving a gavage 

dose of 3,000 mg molybdenum/kg as ammonium dimolybdate (Baldrick and Healing 1990e). No 
MOLYBDENUM 38

2. HEALTH EFFECTS

histological alterations were observed in the gastrointestinal tract of rats exposed to ≤60 mg 

molybdenum/kg/day as sodium molybdate in the diet for 90 days (Murray et al. 2014a). In contrast, Fell 

et al. (1979) reported soft feces and diarrhea and a number of histological alterations in the 

gastrointestinal tract of rats exposed for up to 21 days to 0.5 mg molybdenum/kg/day as ammonium 

tetrathiomolybdate in the diet (diet provided an inadequate amount of copper). The alterations included 

shortening of the gastric pits with a reduction in the amount of mucin in the stomach, an increase in the 

crypt to villus ratio in the small intestine due to a lengthening of the crypts, edema of the lamina propria 

in the ileum, and submucosal edema of the cecum resulting in a thickening of the cecum but no effect on 

the brush border. However, the investigators did not provide incidence data, which limits the assessment 

of these alterations.

2.7 HEMATOLOGICAL

No significant alterations in hematological parameters were observed in rats or mice following inhalation 

exposure to molybdenum trioxide at concentrations as high as 67 mg molybdenum/m3 for 13 weeks (NTP 

1997).

In general, the hematological system does not appear to be a target of molybdenum oral toxicity when the 

basal diet contains adequate levels of copper. In rats exposed to sodium molybdate or ammonium 

heptamolybdate, the highest NOAEL values for hematological alterations ranged from 3.35 to 150 mg 

molybdenum/kg/day for intermediate-duration exposure (Brinkman and Miller 1961; Franke and Moxon 

1937; Gray and Daniel 1954; Hunt and Navia 1973; Jeter and Davis 1954; Johnson et al. 1969; Murray et 

al. 2014a). One study reported decreases in erythrocyte counts, hemoglobin, and hematocrit in rats 

exposed to 4.4 mg molybdenum/kg/day as ammonium tetrathiomolybdate administered via gavage for 

59–61 days (Lyubimov et al. 2004). Although the basal diet contained the National Research Council’s 

(NRC’s) recommended amount of copper (NAS 1995), hematological effects were not observed in rats 

exposed to the same molybdenum dose receiving a diet containing additional copper (110 ppm), 

suggesting that the hematological effects may have been secondary to a molybdenum-induced copper 

deficiency (anemia is a sign of copper deficiency). In young rabbits, exposure to 54 mg 

molybdenum/kg/day as sodium molybdate in the diet resulted in anemia (Arrington and Davis 1953). 

Even though the reported copper concentration in the diet exceeded the more recently recommended 

standard of 3 ppm (NAS 1977), administration of additional copper resulted in increases in hemoglobin 

levels. In a similar study using mature rabbits, anemia was observed in one of two rabbits exposed to 

30 mg molybdenum/kg/day as sodium molybdate in the diet (Arrington and Davis 1953). Decreases in 
MOLYBDENUM 39

2. HEALTH EFFECTS

hemoglobin levels and packed cell volume were also observed in two other rabbit studies (Valli et al. 

1969; Widjajakusuma et al. 1973) in which rabbits were exposed to 77 or 59 mg molybdenum/kg/day in 

the diet for approximately 4 weeks. Mortality was observed in both studies and neither study reported the 

copper levels of the basal diet; Valli et al. (1969) did note that the rabbits were fed a diet with a low 

copper content. In pigs, no hematological alterations were observed following dietary exposure to 20–

100 ppm molybdenum as sodium molybdate or ammonium heptamolybdate in the diet for at least 

8 weeks (Gipp et al. 1967; Kline et al. 1973); the studies did not provide sufficient information to allow 

for an estimation of the molybdenum dose.

2.8 MUSCULOSKELETAL

No histological alterations were observed in the bones of rats or mice exposed via inhalation to 6.7–67 mg 

molybdenum/m3 as molybdenum trioxide for 13 weeks or 2 years (NTP 1997). Chronic molybdenum 

inhalation exposure also did not affect femoral bone density or curvature in groups of 10 rats exposed to 

concentrations as high as 67 mg molybdenum/m3 (NTP 1997).

A number of oral exposure studies in laboratory animals have examined the effect of molybdenum on 

bone growth and strength and on the promotion of dental caries. Musculoskeletal effects were observed 

in two studies in which the diet contained at least the recommended level of copper. In a study by 

Johnson et al. (1969) in which rats were exposed to 150 mg molybdenum/kg/day as sodium molybdate in 

the diet for 6 weeks (the basal diet contained copper levels that were 3 times higher than the 

recommended amount), decreases in femur breaking strength (22% less than controls) and tail ring 

rupture strength (32% less than controls) were observed. Young rabbits exposed to ≥54 mg 

molybdenum/kg/day as sodium molybdate for 30–84 days exhibited a front limb abnormality 

characterized by weakness progressing to an inability to “maintain weight and legs spread outward” 

(Arrington and Davis 1953). This was not observed in mature rabbits exposed to ≤120 mg 

molybdenum/kg/day as sodium molybdate for at least 54 days (Arrington and Davis 1953). The 

investigators noted that in three of the seven affected animals, one or both feet bent inward at the carpus 

joint, the articular surface of the radius was exposed, and the tendon slipped out of normal position. It 

should also be noted that increases in mortality were also observed in the young rabbits exposed to 54 mg 

molybdenum/kg/day, and in two of the rabbits with limb abnormalities, administration of additionally 

copper did not reverse the skeletal effect, although there was improvement of other effects including 

anemia and body weight gain. 
MOLYBDENUM 40

2. HEALTH EFFECTS

In an acute-duration study, femurs were significantly shorter in rats exposed to 0.6 mg 

molybdenum/kg/day as ammonium heptamolybdate or ammonium tetrathiomolybdate for 13 days (Parry 

et al. 1993). No alterations in the width of the growth plate or the bone composition (dry matter content, 

ash content, or percentage of calcium or phosphorus) were found. Similar findings were found in a 

26-day study conducted by Parry et al. (1993); significant decreases in femur length were noted in rats 

exposed to 0.6 mg molybdenum/kg/day as ammonium heptamolybdate or ammonium tetrathiomolybdate 

in the diet. Although no direct comparisons were made between the two molybdenum groups, the 

magnitude of the decrease in femur length, as compared to the controls, was greater in the 

tetrathiomolybdate group. Increases in growth plate width were also observed in the rats exposed to 

ammonium tetrathiomolybdate, but not in rats exposed to ammonium heptamolybdate. In both 

experiments, the rats were fed a basal diet with inadequate copper levels (60% of the recommended 

concentration); in the ammonium tetrathiomolybdate study, plasma and liver copper levels indicated that 

the animals were extremely copper deficient. Spence et al. (1980) examined the development of 

widening of the epiphyseal growth plate over time in rats exposed to 1 mg molybdenum/kg/day as 

ammonium tetrathiomolybdate in the diet for 2–21 days. The study found cartilaginous dysplasia at the 

epiphyseal growth plate with impaired or arrested endochondral ossification, increases in periosteal 

osteogenesis and production of large amounts of disorganized bone, resorption of most trabecular bone, 

hemorrhaging within and tearing of tendons and ligaments, rotation and slipping of the distal epiphysis in 

the femur without fracture, and impaired fibrogenesis at ligamentous attachments to bone. Thickening 

and widening of the epiphyseal growth plate were observed in the distal femur and proximal and in the 

epiphyses of the humeral head, distal radius, and ulna; these effects were observed within the first 

2 weeks of the study. Other morphological alterations in the bone were observed after 7 days of 

exposure; these included loss of alignment of hypertrophic cells at the periphery of the epiphyseal 

cartilage and localized increases in cell numbers. In rats allowed to recover for 39 days following the 

21-day exposure period, osteogenesis and fibrogenesis returned to normal, and remodeling and growth 

returned (although some abnormal cartilage and bone were present). As with the Parry et al. (1993) 

study, the rats in the Spence et al. (1980) study were fed a basal diet containing an inadequate amount of 

copper (60% of the recommended level). Fejery et al. (1983) found an increase in femur breaking 

strength in rats exposed to 0.17 or 1.7 mg molybdenum/kg/day (copper content of the diet was not 

reported), which was considered a beneficial effect; at 17 mg molybdenum/kg/day, breaking strength was 

similar to controls. However, if the rats were maintained on a protein-deficient diet, decreases in 

breaking strength were observed at 1.7 and 17 mg molybdenum/kg/day. In rabbits exposed to a lethal 

concentration of sodium molybdate (77 mg molybdenum/kg/day) in the diet for 4 weeks, fractures of the 

humeral bone epiphyses were observed in 50% of the animals (Valli et al. 1969). Other effects included 
MOLYBDENUM 41

2. HEALTH EFFECTS

longitudinal widening of the epiphyseal cartilage, marked reduction in trabecular bone, irregularly 

arranged spicules, and irregular metaphyseal calcification. In addition, the investigators noted that there 

was marked muscular degeneration in the pelvic limbs in 25% of the rabbits. The copper content of the 

basal diet was not reported in this study, although the investigators noted that the diet had a low copper 

content. 

Alterations in tooth enamel and caries formation have also been observed in laboratory animals exposed 

to molybdenum. In rat pups administered 50 mg molybdenum/kg/day as sodium molybdate via gavage 

on postnatal days (PNDs) 4–17 (prior to tooth eruption) and fed a caries-promoting diet on PNDs 18–35, 

a 25% increase in buccal enamel lesion and 85 and 12.5% increases in lesions penetrating to the buccal 

and sulcal dentine-enamel junctions, respectively, were observed in the mandibular molars (Hunt and 

Navia 1975). Fejery et al. (1983) reported biphasic alterations in incisor tooth enamel microhardness in 

rats exposed to sodium molybdate in drinking water for 6 weeks (the copper content of the basal diet was 

not reported). At 1.7 mg molybdenum/kg/day, there were increases in microhardness (6–7% increases in 

surface and deep enamel microhardness), which was considered a beneficial effect. However, at 17 mg 

molybdenum/kg/day, tooth surface and deep enamel microhardness was decreased by 14.5 and 7.5%, 

respectively. The study also examined the possible effect of a low protein diet (3% in the low-protein 

groups compared to 18% in the protein-adequate groups) and found that the beneficial effect of 1.7 mg 

molybdenum/kg/day did not occur in the rats in the low-protein diet; a 4–5% reduction in microhardness 

was found at 1.7 mg/kg/day. Van Reen et al. (1962) did not find increases in dental caries in weanling 

NMRI-D rats (a caries susceptible strain) exposed to 8 mg molybdenum/kg/day as sodium molybdate for 

5 weeks (the basal diet provided adequate copper levels).

2.9 HEPATIC

There are limited data on the hepatotoxicity of molybdenum in humans. Using the NHANES 2007–2008 

cross-sectional study data, Mendy et al. (2012) found a significant association between urinary 

molybdenum levels and the risk of having a self-reported liver condition (OR 3.09; 95% CI 1.24–7.73). 

The geometric mean urinary molybdenum level of the population was 43.8 μg molybdenum/g creatinine 

(95% CI 42.61–45.19); the investigators did not report the urinary concentration associated with the 

increased risk of liver conditions. This study does not establish causality between molybdenum exposure 

and liver damage, and significant associations were also found between uranium and cesium levels and 

liver conditions.
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The liver does not appear to be a sensitive target of molybdenum toxicity in laboratory animals, although 

some studies have reported biochemical alterations. No significant alterations in serum clinical chemistry 

parameters or liver weights were observed in rats or mice exposed to airborne molybdenum trioxide 

concentrations as high as 67 mg molybdenum/m3 for 13 weeks (NTP 1997). No significant alterations in 

the incidence of hepatic lesions were observed following 13 weeks or 2 years of exposure (NTP 1997). 

No histological alterations were observed in livers of rabbits exposed to 1.2 mg molybdenum/kg/day as 

ammonium heptamolybdate in the diet for 14 days (Bersenyi et al. 2008), rabbits exposed to 0.58 mg 

molybdenum/kg/day from carrots grown in ammonium heptamolybdate rich soil, or rats exposed to 

60 mg molybdenum/kg/day in the diet for 90 days (Murray et al. 2014a); these are the only studies that 

included histological examination of the liver. The Bersenyi et al. (2008) female rabbit study did not find 

alterations in serum alanine or aspartate aminotransferases levels, γ-glutamyl transferase, alkaline 

phosphatase, or cholesterol levels; however, a 60% increase in serum triglyceride levels was found at 

1.2 mg molybdenum/kg/day. In contrast, the Murray et al. (2014a) study examined similar serum clinical 

chemistry parameters (including triglyceride levels) and did not find any significant alterations. 

A series of studies conducted by Rana and associates have also reported some liver alterations in rats 

exposed to 300–490 mg molybdenum/kg/day as ammonium molybdate. The reported alterations included 

increases in total lipid levels (Rana et al. 1980; Rana and Kumar 1980b, 1980c), decreases in “total 

carbohydrate” levels (Rana and Kumar 1980c), decreases in glycogen content (Rana et al. 1985), and 

increases in serum alanine aminotransferase and aspartate aminotransferase activities (Rana and Chauhan 

2000). The addition of 100 mg/kg body weight/day copper to the basal diet (approximately 5 ppm) 

appeared to reverse the effects of molybdenum on hepatic lipid and carbohydrate levels (Rana and Kumar 

1980c). There was low confidence in these studies due to the poor reporting of the study design 

(including route of oral administration, whether the dose was reported in terms of molybdenum or 

ammonium molybdate, and copper content of the diet), the lack of histological examination of the liver, 

and the reported body weight losses (Rana et al. 1980; Rana and Chauhan 2000).

2.10 RENAL

Intermediate- or chronic-duration inhalation exposure to molybdenum trioxide (highest concentration 

tested was 67 mg molybdenum/m3

) did not result in histological alterations in the kidney of rats or mice 

(NTP 1997).
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The available data from laboratory animal studies suggest that the kidney may be a target of molybdenum 

toxicity following oral exposure. In the only available acute-duration study, no histological alterations 

were observed in the kidneys of female rabbits exposed to 1.2 mg molybdenum/kg/day as ammonium 

heptamolybdate in the diet for 14 days (Bersenyi et al. 2008) or male rabbits exposed to 0.58 mg 

molybdenum/kg/day from carrots grown in ammonium heptamolybdate-rich soil for 14 days (Bersenyi et 

al. 2008). Murray et al. (2014a) reported a slight diffuse hyperplasia in the renal proximal tubules in 

2/10 female rats exposed to 60 mg molybdenum/kg/day as sodium molybdate in the diet for 90 days; no 

renal lesions were observed in females exposed to 60 mg molybdenum/kg/day for 90 days and allowed to 

recover for 60 days. No alterations were observed in the male rats. Although the incidence was low, the 

investigators considered it to be treatment-related because it is an uncommon finding in female rats of this 

age. In a subsequent 2-generation study by this group, no histological alterations were observed in male 

or female rats exposed to 40 mg molybdenum/kg/day as sodium molybdate in drinking water or diet for 

147–158 days (Murray et al. 2019). Degenerative changes in the kidneys were noted in male rats exposed 

to 240 mg molybdenum/kg/day as ammonium molybdate (Bandyopadhyay et al. 1981). It should be 

noted that the food intake in the molybdenum group was paired to another group of rats fed a low-protein 

diet and exposed to molybdenum; the basal diet likely provided adequate copper levels. No other studies 

included histological examination of the kidneys.

Several studies reported alterations in serum and urinary parameters that could be suggestive of altered 

renal function. Diuresis and creatinuria and a decrease in creatinine clearance were observed in rats 

administered via gavage 80 mg molybdenum/kg/day as ammonium heptamolybdate for 8 weeks (Bompart 

et al. 1990). The study did not find significant alterations in urinary protein or glucose levels. Studies by 

Rana and associates have reported increases in total lipid levels in the kidneys (Rana et al. 1980; Rana 

and Kumar 1980c), decreases in “total carbohydrate” levels in the kidney (Rana and Kumar 1980c), 

increases in serum urea and urinary albumin levels (Rana and Kumar 1983), and increases in urine 

specific gravity (Rana and Kumar 1983) in rats exposed to high doses of ammonium molybdate (300–

490 mg molybdenum/kg/day). The addition of copper (approximately 5 ppm) to the basal diet appeared 

to reverse the increased lipid and decreased carbohydrate levels (Rana and Kumar 1980c). As noted in 

the hepatic effects section, there is low confidence in these studies and the results should be interpreted 

cautiously.
MOLYBDENUM 44

2. HEALTH EFFECTS

2.11 DERMAL

Information on the dermal toxicity of molybdenum comes from a small number of oral exposure studies 

reporting skin and hair effects and acute-exposure dermal studies. In an oral exposure study of weanling 

rabbits (Arrington and Davis 1953), alopecia and slight dermatosis were observed in four of five rabbits 

exposed to 54 mg molybdenum/kg/day as sodium molybdate in the diet for 84 days; no dermal effects 

were observed at 25 mg molybdenum/kg/day. In another study by this group, alopecia and slight 

dermatosis were observed in one of two mature rabbits exposed to 30 mg molybdenum/kg/day as sodium 

molybdate. Anemia was also observed at these doses. In the study of weanling rabbits, administration of 

additional copper resulted in a return to a normal hair coat, suggesting that copper insufficiency, possibly 

molybdenum induced, was a contributing factor to the dermal toxicity. Johnson et al. (1969) reported 

decreases (25% lower than controls) in skin rupture strength in rats exposed to 150 mg 

molybdenum/kg/day as sodium molybdate in the diet for 6 weeks.

No dermal effects were observed in rats following a 24-hour dermal application of 280 or 1,200 mg 

molybdenum/kg as ammonium dimolybdate (Baldrick and Healing 1990a; Liggett and McRae 1990e), 

340 or 1,300 mg molybdenum/kg as pure molybdenum trioxide (Baldrick and Healing 1990b; Liggett and 

McRae 1990f), 230 or 930 mg molybdenum/kg as sodium molybdate (Baldrick and Healing 1990c; 

Liggett and McRae 1990g), or 1,333 mg molybdenum/kg as technical-grade molybdenum trioxide 

(Baldrick and Healing 1990d). 

2.12 OCULAR

No ocular lesions were observed in rats exposed to 60 mg molybdenum/kg/day as sodium molybdate in 

the diet for 90 days (Murray et al. 2014a); no other oral or inhalation studies examined ocular endpoints.

Instillation of 56 mg molybdenum/kg as ammonium dimolybdate (Liggett and McRae 1990a), 67 mg 

molybdenum/kg as pure molybdenum trioxide (Liggett and McRae 1990b), 67 mg molybdenum/kg as 

technical grade molybdenum trioxide (Liggett and McRae 1990d), or 46 mg molybdenum/kg as sodium 

molybdate (Liggett and McRae 1990c) resulted in conjunctival inflammation in rabbits.
MOLYBDENUM 45

2. HEALTH EFFECTS

2.13 ENDOCRINE

The possible association between molybdenum and thyroid effects was investigated in adults (subjects did 

not report having thyroid disease, thyroid cancer, or taking thyroid medication on a medical questionnaire 

completed at the blood sampling) using the NHANES 2007–2008 cross-sectional study data set (Yorita 

Christensen 2013). Associations between decreased levels of triiodothyronine (free and total) and 

thyroxine (free) and higher urinary molybdenum levels were found. Although the study found 

associations, these data are inadequate for establishing causality. Another study of NHANES participants 

did not find an association between urinary molybdenum levels and thyroid problems (Mendy et al. 

2012). A cross-sectional study of men at a fertility clinic found a significant inverse relationship between 

blood molybdenum levels and prolactin levels (Meeker et al. 2009). The men were categorized into three 

groups based on blood molybdenum levels (<70th, 70th–85th, and >85th percentile); the association was 

found in the men with blood molybdenum levels >85th percentile, as compared to men with levels 

<70th percentile. The study did not find a significant association with thyroid stimulating hormone and 

blood molybdenum levels.

Inhalation studies did not find histological alterations in the adrenal, pituitary, pancreas, parathyroid, or 

thyroid glands in rats and mice exposed to ≤67 mg molybdenum/m3 as molybdenum trioxide for 13 weeks 

or 2 years (NTP 1997).

In oral exposure laboratory animal studies, increases in serum cortisol, prolactin, and follicle stimulating 

hormone levels were found in male rats administered 240 mg molybdenum/kg/day as ammonium 

molybdate for 4 weeks (Bandyopadhyay et al. 1981); as noted in the renal effects section, food intake was 

matched to a low-protein molybdenum group. No increases in the incidence of histological alterations in 

the adrenal glands, pituitary gland, or thyroid were observed in rats exposed to 60 mg 

molybdenum/kg/day as sodium molybdate in the diet for 90 days (Murray et al. 2014a) or up to 40 mg 

molybdenum/kg/day as sodium molybdate in drinking water or diet for 147–158 days (Murray et al. 

2019). Several thyroid effects were reported in rabbits exposed to 59 mg molybdenum/kg/day as sodium 

molybdate in the diet for 25–31 days (Widjajakusuma et al. 1973). The investigators did not report the 

copper content of the diet; it is likely to be low based on the severe decreases in body weight, 

hematological parameters, and increased mortality. The effects included decreases in thyroxine secretion 

rates; decreases in follicle size (height and diameter); atrophy of the follicular epithelium, colloids, and 

stroma; and degenerative alterations in the follicular epithelium and interfollicular connective tissue. 

With the exception of the degenerative changes, similar, but less prominent, thyroid effects were also 
MOLYBDENUM 46

2. HEALTH EFFECTS

observed in pair-fed controls, suggesting that the decreases in food intake and body weight contributed to 

the thyroid toxicity.

2.14 IMMUNOLOGICAL

There are limited data on the immunotoxicity of molybdenum in humans. Studies of patients with 

stainless steel stents (which contain nickel, chromate, and molybdenum) or in patients prior to hip or knee 

replacements found a low rate of positive results in patch tests with molybdenum (Koster et al. 2000; 

Menezes et al. 2004; Zeng et al. 2014). In patients with stainless steel stents, 3% had a positive delayedtype contact hypersensitivity reaction to molybdenum chloride (Koster et al. 2000). In the other studies, 

exposure to an unspecified molybdenum compound did not result in any positive hypersensitivity results 

(Menezes et al. 2004; Zeng et al. 2014).

No studies have examined immune function following inhalation exposure to molybdenum. 

Intermediate- and chronic-duration studies in rats and mice did not report histological alterations in the 

thymus or spleen at molybdenum trioxide levels as high as 67 mg molybdenum/m3 (NTP 1997). No 

studies were located regarding immune effects in laboratory animals following oral exposure to 

molybdenum.

Guinea pigs showed contact sensitization to a topical challenge with molybdenum pentachloride after 

induction via intradermal injection with 0.03% molybdenum and topical exposure to 5.2% molybdenum 

and an epicutaneous challenge with ≥0.35% molybdenum as molybdenum pentachloride (Boman et al. 

1979). Similarly, guinea pigs were sensitized to 3.2% molybdenum as sodium molybdate following 

intradermal (3.2% molybdenum) or topical (8% molybdenum) induction (Boman et al. 1979). In contrast, 

other studies of skin sensitization in guinea pigs were negative for ammonium dimolybdate (Allan 

1996a), pure and technical-grade molybdenum trioxide (Allan 1996b, 1996c), and sodium molybdate 

(Allan 1996d); these studies tested higher molybdenum concentrations (70–90% molybdenum) than the 

Boman et al. (1979) study.

2.15 NEUROLOGICAL

Information on the potential neurotoxicity of molybdenum comes from inhalation and oral exposure 

studies in laboratory animals evaluating brain histology or monitoring for overt signs of neurotoxicity. 

None of these studies included function testing. No overt signs of neurotoxicity were observed in 
MOLYBDENUM 47

2. HEALTH EFFECTS

laboratory animal studies (e.g., Murray et al. 2014a; NTP 1997). No histological alterations were 

observed in the brain of rats and mice exposed via inhalation to ≤67 mg molybdenum/m3 as molybdenum 

trioxide for 13 weeks or 2 years (NTP 1997) or rats exposed to ≤60 mg molybdenum/kg/day as sodium 

molybdate in the diet for 90 days (Murray et al. 2014a). In contrast, Helaly et al. (2018) reported dense 

inflammation and neurocyte degeneration in the cerebral cortex and hippocampus of rats receiving gavage 

doses of 30 mg molybdenum/kg/day as molybdenum dihydrate for 30 days; however, the study did not 

include incidence data.

2.16 REPRODUCTIVE

There are limited data on reproductive effects of molybdenum in humans. The available studies have 

evaluated correlations between ambient molybdate exposure and reproductive health measures, including 

semen quality (Meeker et al. 2008) and sex hormone levels (Meeker et al. 2010). A cross-sectional study 

by Meeker et al. (2008) reported an inverse association between higher molybdenum blood levels 

(>85th percentile, based on molybdenum levels in blood) and sperm concentration (adjusted OR 3.48; 

95% CI 1.12–10.8) after adjustment for potential confounders and other metal exposures. No associations 

were found for sperm morphology (adjusted OR 2.61; 95% CI 0.97–7.0) or sperm motility (adjusted OR 

2.24; 95% CI 0.77–6.49). In another cross-sectional study, Meeker et al. (2010) reported an inverse 

correlation between higher molybdenum blood levels (≥70th percentile) and testosterone and free 

androgen index (molar ratio of total testosterone sex hormone-binding globulin) levels. The men in these 

studies, who were recruited from Michigan infertility clinics and were not all considered to be infertile 

(i.e., their partners may have been infertile), were only exposed to molybdenum from their surroundings. 

An inverse association between a biomarker of molybdenum exposure (urinary levels) and serum 

testosterone levels was also observed in a cross-sectional study of males participating in NHANES (Lewis 

and Meeker 2015). The study found a 3.82% decrease in serum testosterone levels when urinary 

molybdenum levels doubled (after adjustment for age, body mass index [BMI], income, race, and 

smoking). Although these studies found associations, they do not establish causality and the alterations in 

reproductive parameters may be due to multiple factors rather than only to molybdenum exposure. 

Studies in laboratory animals have evaluated potential alterations in male reproductive tissues, female 

reproductive tissue, and fertility following inhalation (no evaluation of fertility) or oral exposure. No 

studies have evaluated reproductive toxicity following dermal exposure.
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2. HEALTH EFFECTS

Several studies have evaluated the reproductive toxicity in male laboratory animals. No alterations in 

sperm count or motility or histological alterations of male reproductive tissues were observed in rats or 

mice exposed via inhalation to molybdenum trioxide concentrations as high as 67 mg molybdenum/m3

(NTP 1997). Murray et al. (2014a) did not find any alterations in spermatid, sperm counts, sperm 

motility, or sperm morphology in rats exposed to 60 mg molybdenum/kg/day as sodium molybdate in the 

diet for 90 days. Although the study found no alterations in the percentage of motile sperm, a slight, but 

statistically significant, decrease in the percentage of progressively motile sperm was observed at 60 mg 

molybdenum/kg/day (59.0% compared to 69.4% in controls). The investigators noted that the decrease 

was likely attributable to the control group having a value that approached the upper end of the range for 

historical controls (mean of 59.8±16.2%). No alterations in sperm parameters were observed in male rats 

exposed to ≤40 mg molybdenum/kg/day as sodium molybdate in drinking water in a 2-generation study 

(Murray et al. 2019). In parental-generation males exposed to 40 mg molybdenum/kg/day as sodium 

molybdate in the diet, an increase in the number of sperm with no head was found (Murray et al. 2019). 

However, the investigators did not consider this to be treatment-related since it was largely due to one 

male rat, was not observed in the F1 males, and the values were within the range of historical controls.

In contrast to these findings, other studies have reported male reproductive effects. Decreases in sperm 

motility and concentration and increases in sperm morphological changes were observed in rats 

administered via gavage 14 mg molybdenum/kg/day as sodium molybdate for 60 days (Pandey and Singh 

2002), and in mice exposed to 25 mg molybdenum/kg/day as sodium molybdate in the drinking water for 

14 days (Zhai et al. 2013). These studies also found decreases in epididymides, seminal vesicles, and/or 

prostate gland weights (Pandey and Singh 2002; Zhai et al. 2013). The Zhai et al. (2013) study also 

found increases in sperm motility and concentration and decreases in the occurrence of sperm 

morphological alterations in rats exposed to lower molybdenum doses (6 mg molybdenum/kg/day as 

sodium molybdate). A study in rabbits reported reductions in the number of germ cells and mature 

spermatocytes in the testes (Bersenyi et al. 2008); the investigators also noted a large number of syncytial 

giant cells and degenerated cells in the seminiferous tubules. Interpretation of these results are limited 

since incidence data or statistical analyses were not reported. Degeneration of the seminiferous tubules 

was found in rats at 7 mg molybdenum/kg/day as sodium molybdate, which was administered in the diet 

from weaning through sexual maturity (Jeter and Davis 1954); although this study provided an adequate 

amount of copper, there was evidence of copper deficiency (achromotrichia) at ≥7 mg 

molybdenum/kg/day. Degeneration of the seminiferous tubules was also reported by Pandey and Singh 

(2002) for intermediate-duration (60 days) exposures in rats administered molybdenum at doses up to 

24 mg molybdenum/kg/day (sodium molybdate); however, the dose(s) producing the effects are unclear 
MOLYBDENUM 49

2. HEALTH EFFECTS

and incidence data were not reported. The Pandey and Singh (2002) and Zhai et al. (2013) studies did not 

report the copper content of the basal diet, although both studies used commercial diets. Lyubimov et al. 

(2004) reported delayed spermiation, increased sperm and seminal fluid concentration, and increased 

sloughing of epididymal tail epithelial cells at 4.4 mg molybdenum/kg/day as ammonium 

tetrathiomolybdate. Although the basal diet in the Lyubimov et al. (2004) study provided 11 ppm of 

copper, which is above the National Academy of Sciences (NAS 1995) recommended amount for rats 

(5 ppm), dietary copper supplementation (110 ppm) prevented testicular toxicity. It is likely that the 

tetrathiomolybdate interfered with the absorption of dietary copper, resulting in a secondary effect of 

copper insufficiency. 

As with the male reproductive effects, conflicting results have been reported for female reproductive 

effects. Murray et al. (2014a) did not find any alterations in vaginal cytology or estrus cycle in female 

rats exposed to ≤60 mg molybdenum/kg/day as sodium molybdate in the diet for 90 days or in a 

2-generation study in which rats were exposed to ≤40 mg molybdenum/kg/day as sodium molybdate in 

the drinking water or the diet (Murray et al. 2019). No histological alterations were observed in female 

reproductive tissues in rats or mice following inhalation exposure to ≤67 mg molybdenum/m3 for 

13 weeks or 2 years (NTP 1997), in rats exposed to ≤60 mg molybdenum/kg/day as sodium molybdate in 

the diet for 90 days (Murray et al. 2014a), or in rabbits exposed to 1.2 mg molybdenum/kg/day as 

ammonium heptamolybdate in the diet for 14 day (Bersenyi et al. 2008). Zhang et al. (2013) reported an 

increase in the rate of MII oocyte morphological abnormalities and decreases in relative ovarian weights 

were observed in mice exposed to 11 mg molybdenum/kg/day as sodium molybdate in drinking water for 

14 days. The investigators also reported ovarian hyperemia in mice exposed to 5.3 and 11 mg 

molybdenum/kg/day; however, the incidence and statistical significance were not reported. Irregularities 

in the estrous cycle were reported in rats administered 1.5 mg molybdenum/kg/day in the drinking water 

from weaning through sexual maturity (Fungwe et al. 1990). 

Several intermediate-duration oral studies evaluated fertility. No alterations in fertility were observed in 

female rats exposed to ≤15 mg molybdenum/kg/day as sodium molybdate in drinking water (Fungwe et 

al. 1990), in a 2-generation study in rats exposed to ≤40 mg molybdenum/kg/day as sodium molybdate in 

drinking water or diet (Murray et al. 2019), or in male and female rats exposed to 7 mg 

molybdenum/kg/day as sodium molybdate in the diet when a high copper diet was administered (Jeter and 

Davis 1954). In contrast, Pandey and Singh (2002) reported decreases in fertility in males exposed to 

14 mg molybdenum/kg/day as sodium molybdate and mated to unexposed females. Another study 

conducted by Jeter and Davis (1954) in which rats were exposed to 7 mg molybdenum/kg/day from 
MOLYBDENUM 50

2. HEALTH EFFECTS

weaning to maturity also found impaired male fertility; in this study, there is some indication that the diet 

did not provide an adequate level of copper.

2.17 DEVELOPMENTAL

Information on the potential developmental toxicity of molybdenum is limited to two epidemiological 

studies and oral exposure studies in laboratory animals. Vazquez-Salas et al. (2014) found an association 

between third-trimester maternal urinary molybdenum levels (mean level of 54.0 μg/g creatinine) and 

infant psychomotor development indices, including gross and fine motor coordination, during the first 

30 months of life in a cross-sectional study of women in Mexico participating in a prospective study of 

neurodevelopment in children. A doubling of creatinine corrected urinary molybdenum levels resulted in 

significant decreases in psychomotor development index scores. No association was found between 

maternal urinary molybdenum levels during pregnancy (mean levels ranged from 45.6 to 54.6 μg/g 

creatinine during the first, second, and third trimesters) and newborn body weight or infant mental 

development indices (sensory ability, memory, learning, problem solving, and verbal ability). Shirai et al. 

(2010) found no association between maternal urinary molybdenum levels and newborn body weight, 

length, or head circumference in a cross-sectional study of women in Japan with mean urinary 

molybdenum levels of 79.0 μg/g creatinine. As noted elsewhere in this document, these observational 

epidemiology studies do not establish causality between molybdenum and developmental effects, and 

other factors are likely to have contributed to the risk.

No developmental effects were reported in three studies of rats exposed to molybdenum in the presence of 

adequate copper concentrations in the basal diet (Jeter and Davis 1954; Murray et al. 2014b, 2019). In a 

2-generation study, no alterations in pup survival, sex ratios, pup body weight, or developmental 

landmarks were observed in the F1 or F2 offspring of rats exposed to up to 40 mg molybdenum/kg/day as 

sodium molybdate in the drinking water or diet (Murray et al. 2019). In a single-generation study, 

Murray et al. (2014b) reported no effects on litter size, embryofetal survival, sex ratio, fetal body weight, 

or fetal malformations and variations in rats exposed to 40 mg molybdenum/kg/day as sodium molybdate 

in the diet on gestation days (GDs) 6–20. No alterations in birth weights were observed in the offspring 

of male and female rats exposed to 7 mg molybdenum/kg/day as sodium molybdate for at least 14 weeks 

(Jeter and Davis 1954). In contrast to these findings, one study found decreases in the number of live 

fetuses, fetal crown-rump length, and fetal body weight in the offspring of male rats administered 14 mg 

molybdenum/kg as sodium molybdate via gavage for 60 days prior to mating to untreated females 
MOLYBDENUM 51

2. HEALTH EFFECTS

(Pandey and Singh 2002). The copper content of the commercial diet was not reported but was assumed 

to be adequate. 

Developmental effects have also been reported in studies in which the copper content of the diets was 

lower than the NAS-recommended standard of 8 ppm for pregnant rats (NAS 1995). Fungwe et al. 

(1990) reported increases in fetal resorptions and decreases in litter weights in female rats exposed to 

1.3 mg molybdenum/kg/day as sodium molybdate in the drinking water for 8 weeks prior to mating 

through GD 21; the copper content in the basal diet was 6.3 ppm. Decreased maternal body weight gain 

was also observed at doses resulting in developmental toxicity. Decreased weaning weights were 

observed in the offspring of rats exposed to ≥2 mg molybdenum/kg/day as sodium molybdate; the copper 

content of the diet was 5 ppm (Jeter and Davis 1954). Lyubimov et al. (2004) found no effects on litter 

size or fetal survival in rats administered molybdenum daily via gavage at 4.4 mg molybdenum/kg/day as 

ammonium tetrathiomolybdate for 59–61 days (for 29 days prior to mating, during mating, and thereafter 

until sacrifice) in males or for 22–35 days (for 15 days prior to mating, during mating, and during GDs 0–

6) in females. Two studies only available as abstracts provide additional information on the potential 

developmental toxicity of molybdenum. Lyubimov et al. (2002) found no developmental effects in rats 

exposed to 6 mg/kg/day as tetrathiomolybdate on GDs 6–17. Exposure on GDs 7–20 resulted in an 

increase in carpal/tarsal flexure in the offspring of dams exposed to 20 mg/kg/day ammonium 

tetrathiomolybdate (Lyubimov et al. 2003). Although neither study provided information on the copper 

content of the diet, it is assumed to be adequate based on Lyubimov et al. (2004).

2.18 OTHER NONCANCER

Several studies have evaluated the possible associations between molybdenum and uric acid levels. 

Slight, but significant increases in serum uric acid levels were observed in molybdenite roasting facility 

workers exposed to a TWA concentration of 9.47 mg molybdenum/m3 as molybdenum trioxide and other 

oxides (Walravens et al. 1979). The serum uric acid levels were 5.90 mg/dL in the exposed workers and 

5.01 mg/dL in the controls; these levels are within the normal range. No significant associations between 

serum molybdenum levels and serum uric acid levels were found, and none of the workers reported goutlike symptoms.

Koval’skiy et al. (1961) reported a significant increase in blood uric acid levels and symptoms of gout in 

a cross-sectional study of residents living in an area of Armenia with high levels of molybdenum in the 

soil and food, as compared to residents living outside of this area. The mean uric acid levels in a subset 
MOLYBDENUM 52

2. HEALTH EFFECTS

of the examined population (n=52) was 6.2 mg/dL, as compared to levels in five control subjects who had 

a mean level of 3.8 mg/dL; the mean uric acid levels were 8.1 mg/dL among the subjects with gout 

symptoms and 5.3 mg/dL among the exposed subjects without symptoms. The investigators reported that 

copper intakes (5–10 mg/day) were lower in the high molybdenum area as compared to copper intake for 

residents outside of this area (10–15 mg/day). It was also noted that gout-like symptoms have not been 

observed in other high molybdenum areas that have higher copper intakes (Koval’skiy et al. 1961). 

Interpretation of the result of this study is limited by the small control group, as compared to the exposed 

group; lack of information on the selection of controls, particularly if they were matched to the exposed 

group; and lack of information on diet and alcohol exposure, which could influence uric acid levels. 

Additionally, NAS (2001) noted potential analytical problems with the serum and urine copper 

measurements. Based on the levels of molybdenum in the foodstuff, the investigators estimated a daily 

dose of 10–15 mg (0.14–0.21 mg/kg/day assuming a 70-kg body weight). Deosthale and Gopalan (1974) 

did not find significant increases in urinary uric acid levels in four subjects exposed to a low molybdenum 

diet for 10 days followed by a high molybdenum diet with an ammonium molybdate supplement for 

7 days (TWA molybdenum intake was 0.014 mg molybdenum/kg/day), as compared to uric acid levels 

when the subjects were fed a low molybdenum diet. A series of studies in Colorado investigated uric acid 

levels in communities with high molybdenum levels in the drinking water from mine tailings pollution 

(EPA 1979). Comparisons between subjects living in areas with high molybdenum in the drinking water 

(80–200 μg/L; approximately 0.002–0.006 mg/kg/day) to those living in areas with lower levels 

(<40 μg/L; <0.001 mg/kg/day) did not result in any significant differences in serum uric acid levels or 

urinary molybdenum levels. Another study (EPA 1979) noted that serum uric acid levels were within the 

normal range in students with an estimated molybdenum intake of 500 μg/day (0.007 mg/kg/day) (EPA 

1979). A third study found significant increases in uric acid levels in residents with low molybdenum 

(20 μg/L; 0.0006 mg/kg/day) levels in the water and in residents with high molybdenum levels (150–

200 μg/L; 0.004–0.006 mg/kg/day) in the drinking water; as compared to residents with drinking water 

levels of 0–50 μg/L (0–0.001 mg/kg/day). The inconsistencies in the results could be explained by the 

lack of control of several variables including age, sex, alcohol intake, dietary habits, and altitude.

Murray et al. (2014s) found a statistically significant decrease in serum uric acid levels in female rats 

exposed to ≥5 mg molybdenum/kg/day as sodium molybdate in the diet for 90 days; no alterations were 

observed in male rats exposed to up to 60 mg molybdenum/kg/day. Other statistically significant 

alterations in serum clinical chemistry parameters noted in the Murray et al. (2014a) study include 

decreases in total protein and calcium at 60 mg molybdenum/kg/day in males and decreases in serum 

creatinine at ≥5 mg molybdenum/kg/day in females. The investigators noted that the changes in serum 
MOLYBDENUM 53

2. HEALTH EFFECTS

clinical chemistry (including uric acid levels) were not considered treatment-related because the 

alterations were of small magnitude, not dose-related, due to outliers in the controls, and/or consistent 

with normal variability. Quantitative data for the serum clinical chemistry parameters were not provided 

in the published paper.

Possible associations between molybdenum and diabetes and related outcomes have also been 

investigated in a limited number of epidemiological and laboratory animal studies. In a cross-sectional 

study of 9,447 NHANES participants, Menke et al. (2016) found an association between urinary 

molybdenum levels and diabetes. The ORs and 95% CIs for subjects with urinary molybdenum levels in 

the second, third, and fourth quartiles, as compared to the first quartile were 1.46 (1.09–1.97), 1.89 (1.35–

2.66), and 1.76 (1.24–2.50), respectively. Associations were also found for Homeostatic Model 

Assessment (HOMA) insulin resistance levels for all subjects and in subjects without diabetes. 

Two studies in rats did not find significant alterations in serum glucose levels following intermediateduration exposure to 60 or 100 mg molybdenum/kg/day (Murray et al. 2014a; Peredo et al. 2013); 

additionally, serum insulin levels were not altered by exposure to 100 mg molybdenum/kg/day (Peredo et 

al. 2013). Prakash (1989) reported decreases in glycogen levels in the hindlimb muscles of rats 

administered 490 mg molybdenum/kg/day as ammonium molybdate via gavage for 30 days. The 

significance of this effect is difficult to determine since the study did not provide information on body 

weight gain.

2.19 CANCER

The potential carcinogenicity of molybdenum compounds has been evaluated in an occupational exposure 

study and in a rat and mouse inhalation study. In a case-control study examining the potential association 

between lung cancer and exposure to 16 potential carcinogens, Droste et al. (1999) did not find a

significant increase in lung cancer among workers who self-reported exposure to molybdenum. However, 

an increased risk of lung cancer was found in workers who self-reported working in industries that could 

involve exposure to molybdenum (OR 2.1; 95% CI 1.2–3.7); the job most often related to molybdenum 

exposure was processing of stainless steel in the manufacture of metal goods, which could also involve 

exposure to other carcinogens including chromium, nickel, and arsenic. Limitations of this study, 

including self-reported exposure and the potential exposure to other lung carcinogens, preclude its use in 

assessing the potential carcinogenicity of molybdenum.
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2. HEALTH EFFECTS

In the 2-year NTP rat study (NTP 1997), an increase in the combined incidence of alveolar/bronchiolar 

adenoma or carcinoma was observed in male rats exposed to 67 mg molybdenum/m3 as molybdenum 

trioxide; however, the incidence was within the range of historical controls and NTP considered this to be 

equivocal evidence of carcinogenic activity of molybdenum trioxide. No other concentration-related 

increases in neoplastic lesions were observed in the rats. In mice, there were significant increases in the 

incidences of alveolar/bronchiolar carcinoma in males at ≥6.7 mg molybdenum/m3

, alveolar/bronchiolar 

adenoma or carcinoma in males at 6.7 and 20 mg molybdenum/m3

, alveolar/bronchiolar adenoma in 

females at 20 and 67 mg molybdenum/m3

, and alveolar/bronchiolar adenoma or carcinoma in females at 

67 mg molybdenum/m3 (NTP 1997). The incidences of alveolar/bronchiolar adenoma and carcinoma 

were highest in the 6.7 mg molybdenum/m3 groups and lowest in the 67 mg molybdenum/m3 groups. 

NTP (1997) concluded that the male and female mouse data provided some evidence of carcinogenic 

activity of molybdenum trioxide.

The Department of Health and Human Services (NTP 2016) and EPA have not evaluated the carcinogenic 

potential of molybdenum. IARC has categorized molybdenum trioxide as possibly carcinogenic to 

human (Group 2B).

2.20 GENOTOXICITY

No studies were available regarding genotoxic effects of molybdenum compounds in humans following 

environmental or occupational exposure to these compounds. The genotoxicity of molybdenum 

compounds has been studied mostly in in vitro assays utilizing prokaryotic organisms and in mammalian 

cells. Limited information is available regarding the in vivo genotoxicity of molybdenum.

As shown in Table 2-4, sodium molybdate induced a modest, but statistically significant, increase in 

micronucleated bone marrow cells (polychromatic erythrocytes [PCEs]) from male C57BL/6J mice 

following two intraperitoneal injections of 200 or 400 mg/kg sodium molybdate on 2 consecutive days 

(Titenko-Holland et al. 1998). The increase in micronucleated cells per 1,000 PCE or in micronuclei per 

1,000 PCE were about half of those produced by colchicine, the positive control. The same group of 

investigators reported that sodium molybdate induced a positive response in the dominant lethal assay in 

mice. In these experiments, male C57BL/6J mice were treated with 200 or 400 mg/kg sodium molybdate 

and were mated with non-treated female C3H/J mice at various times after dosing. Sodium molybdate 

did not significantly affect pregnancy rate, but induced a significant dose-related increase in postimplantation loss.
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2. HEALTH EFFECTS

Table 2-4. Genotoxicity of Molybdenum Compounds In Vivo

Species Compound Endpoint Results Reference

Mouse (male C57BL/6J) Sodium 

molybdate

Micronuclei in bone 

marrow cells

(+) Titenko-Holland 

et al. 1998

Mouse (male C57BL/6J) Sodium 

molybdate

Dominant lethal assay (+) Titenko-Holland 

et al. 1998

Drosophila melanogaster 

wing spot test

Molybdenum 

chloride

Gene mutation + Ogawa et al. 

1994

+ = positive result; (+) = weakly positive result

Assessment of the activity of molybdenum chloride in the Drosophila melanogaster wing spot test 

showed that the compound induced spots with one or two mutant hairs (small spots) (Ogawa et al. 1994). 

Almost all of the spots detected were mutant clones expressing the mwh phenotype which, according to 

the investigators, suggested a nonlethal genetic change such as gene mutation or mitotic recombination 

occurring at a late developmental stage, or a semi-lethal change such as partial aneuploidy for a 

chromosomal region containing the mwh locus.

Table 2-5 summarizes studies of genotoxic effects of molybdenum compounds in in vitro systems. 

Results of gene mutation and DNA tests performed in prokaryotic organisms, almost all conducted 

without metabolic activation, were mixed, but negative results outnumbered positive results. It is worth 

noting the positive results reported for potassium molybdate and ammonium molybdate in the DNA 

repair assay (Nishioka 1975). 

Table 2-5. Genotoxicity of Molybdenum Compounds In Vitro

Results

Species (test system) Compound Endpoint

With 

activation

Without 

activation Reference

Prokaryotic organisms:

Salmonella typhimurium, 

TA98, TA100, TA1535, 

TA1537, 1538

Ammonium 

molybdate

Gene mutation No data – Arlauskas 

et al. 1985

S. typhimurium, TA97, 

TA98, TA100, TA 1535, 

TA1537

Molybdenum 

trioxide

Gene mutation – – NTP 1997; 

Zeiger et 

al. 1992

S. typhimurium, TA98, 

TA100, TA1535, TA1537

Molybdenum 

trioxide

Gene mutation – – Jones 2004
MOLYBDENUM 56

2. HEALTH EFFECTS

Table 2-5. Genotoxicity of Molybdenum Compounds In Vitro

Results

Species (test system) Compound Endpoint

With 

activation

Without 

activation Reference

S. typhimurium, TA98, 

TA100, TA1535, TA1537, 

TA102

Sodium 

molybdate

Gene mutation – – Beevers 

2009

S. typhimurium, TA98, 

TA100, TA1535, TA1537, 

TA102

Sodium 

molybdate 

Gene mutation – – Burzlaff et 

al. 2017

S. typhimurium, TA98, 

TA100, TA1535, TA1537, 

TA102

Sodium 

molybdate 

Gene mutation – – Burzlaff et 

al. 2017

Saccharomyces 

cerevisiae D3

Sodium 

molybdate

Gene conversion 

and mutation

No data – Singh 1983

Escherichia coli, 

WP2uvrAAmmonium molybdate

Reverse gene 

mutation

No data – Arlauskas 

et al. 1985

E. coli, WP2uvrA Molybdenum 

trioxide

Gene mutation – – Jones 2004

E. coli, 2 WP2 strains Ammonium 

heptamolybdate

Reverse gene 

mutation

No data + Nishioka 

1975

E. coli, CM571 Ammonium 

heptamolybdate

Reverse gene 

mutation

No data – Nishioka 

1975

E. coli PQ37 Molybdenum 

chloride

DNA damage No data – Olivier and 

Marzin 

1987

E. coli WP2s(ƛ) Sodium 

molybdate

DNA damage No data (+) Rossman 

et al. 1984

E. coli WP2s(ƛ) Sodium 

molybdate

DNA damage No data (+) Rossman 

et al. 1991

Bacillus subtilis, H17 and 

M45

Molybdic acid DNA repair assay No data – Kanematsu 

et al. 1980

B. subtilis H17 and M45 Molybdenum 

disulfide

DNA repair assay No data – Kanematsu 

et al. 1980

B. subtilis H17 and M45 Molybdenum 

pentachloride

DNA repair assay No data – Nishioka 

1975

B. subtilis H17 and M45 Potassium 

molybdate

DNA repair assay No data (+) Nishioka 

1975

B. subtilis H17 and M45 Ammonium 

heptamolybdate

DNA repair assay No data + Nishioka 

1975

Photobacterium fischeri Sodium 

molybdate

Direct mutation No data – Ulitzur and 

Barak 1988
MOLYBDENUM 57

2. HEALTH EFFECTS

Table 2-5. Genotoxicity of Molybdenum Compounds In Vitro

Results

Species (test system) Compound Endpoint

With 

activation

Without 

activation Reference

Mammalian cells: 

Mouse lymphoma 

(L5178Y) cells

Sodium 

molybdate

Gene mutation – – Lloyd 2009

Mouse lymphoma L5178Y 

tk (+/-) cells

Sodium 

molybdate 

dihydrate

Gene mutation – – Burzlaff et 

al. 2017

Human peripheral 

lymphocytes

Sodium 

molybdate

Micronucleus 

assay

No data (+) TitenkoHolland et 

al. 1998

Human peripheral 

lymphocytes

Sodium 

molybdate

Micronucleus 

assay

– – Taylor 

2009

Human peripheral blood 

lymphocytes

Sodium 

molybdate 

dihydrate

Micronucleus 

assay

– – Burzlaff et 

al. 2017

Human peripheral 

lymphocytes

Ammonium 

molybdate

Micronucleus 

assay

No data + TitenkoHolland et 

al. 1998

Human peripheral 

lymphocytes

Molybdenum 

Trioxide

Micronucleus 

assay

– – Fox 2005

Syrian hamster embryo 

(SHE) cells

Molybdenum 

trioxide

Micronucleus 

assay

No data + Gibson et 

al. 1997

Chinese hamster ovary 

(CHO) cells

Molybdenum 

trioxide

Chromosomal 

aberrations

– – NTP 1997

CHO cells Molybdenum 

trioxide

Sister chromatid 

exchanges

– – NTP 1997

+ = positive result; (+) = weakly positive result; – = negative result; ± = equivocal result

The few studies that tested molybdenum compounds in mammalian cells provided mixed results 

(Table 2-4). For molybdenum trioxide, NTP (1997) reported negative results for chromosomal 

aberrations; Fox (2005) and Gibson et al. (1997) reported negative and positive results, respectively, for 

micronuclei formation, with both studies evaluating overlapping dose ranges. Titenko-Holland et al. 

(1998) reported positive results for micronuclei formation in human peripheral lymphocytes incubated 

with sodium or ammonium molybdate. However, cell viability was affected by treatment, and blood was 

collected from only two donors; therefore, the results from this study should be interpreted with caution. 

More recent studies with human peripheral lymphocytes did not find increases in micronuclei formation 

for molybdenum trioxide (Fox 2005) or sodium molybdate (Burzlaff et al. 2017; Taylor 2009).
MOLYBDENUM 58

2. HEALTH EFFECTS

In summary, the limited information regarding effects in vivo of molybdenum compounds is insufficient 

to infer possible outcomes of exposure in human populations. In vitro studies in prokaryotic organisms 

mostly found no alterations in gene mutations and mixed results for DNA damage and repair. In vitro

studies in mammalian cells primarily found no alterations in the occurrence of clastogenic effects. 

2.21 MECHANISMS OF ACTION

The mechanism of molybdenum toxicity has not been well-established. There are some indications that 

the mode of action may involve altered copper utilization; however, it is likely that other mechanisms, 

including direct molybdenum alterations, are involved. Support of the mode of action involving impaired 

copper utilization comes from toxicology studies demonstrating more severe effects when animals are 

maintained on a copper-deficient diet; molybdenum induced increases in copper levels in the plasma, 

liver, and kidneys; and apparent reversal of adverse effects following administration of high doses of 

copper. A number of the effects observed in animals orally exposed to molybdenum, particularly 

decreases in body weight and anemia (Arrington and Davis 1953; Brinkman and Miller 1961; Franke and 

Moxon 1937; Gray and Daniel 1954; Johnson et al. 1969), are similar to those observed in copperdeficient animals. Administration of high levels of copper results in a fairly rapid improvement or 

prevents the effect from occurring (Arrington and Davis 1953; Lyubimov et al. 2004). In rats fed a 

copper-adequate diet, exposure to high levels of molybdenum in the diet resulted in significant increases 

in plasma copper levels (Nederbragt 1980, 1982), most of which were in a “tightly bound form” that did 

not appear to be associated with ceruloplasmin (major copper-carrying protein in the blood), as evidenced 

by the lack of an increase in ceruloplasmin levels (Nederbragt 1980). Significant increases in liver and 

kidney copper levels were also observed in rats exposed to molybdenum in the diet and maintained on a 

copper-adequate diet.

In ruminants, which appear to be very sensitive to molybdenum toxicity, it is believed that molybdenum 

reacts with sulfate generated in the rumen to form thiomolybdates; copper can bind to these 

thiomolybdates, which impairs its absorption. There is also some indication that cupric thiomolybdates 

can form in the blood if dietary copper levels are inadequate (Telfer et al. 2004). The copper in these 

cupric thiomolybdates is unavailable to ceruloplasmin and other copper-containing proteins, resulting in a 

functional copper deficiency (Vyskocil and Viau 1999). In monogastric animals exposed to sodium 

molybdate, administration of sulfate decreases the toxicity of molybdenum (Miller et al. 1956; Van Reen 

1959). However, when rats were fed diets containing molybdate and sulfide, there was a substantial 

increase in plasma molybdenum and copper levels and liver molybdenum levels and a decrease in 
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2. HEALTH EFFECTS

ceruloplasmin activity. In the plasma, there was a shift in the fraction of copper associated with albumin 

and ceruloplasmin (Mills et al. 1981a). Similar findings were observed in rats administered 

tetrathiomolybdates, but not in rats exposed to molybdates in the absence of sulfide (Mills et al. 1981a). 

In rats, exposure to tetrathiomolybdates resulted in effects similar to those observed in ruminants 

including signs of copper deficiency, such as loss of pigmentation in hair and a similar distribution of 

copper between the plasma proteins (Mills et al. 1981b). However, these interactions between 

tetrathiomolybdate and copper only occurred when both were present in the gastrointestinal tract (Mills et 

al. 1981b). It is not known if the interactions between copper and molybdenum only occur at higher 

molybdenum doses. As discussed by Brewer et al. (2000), tetrathiomolybdate can form a tripartite 

complex with copper and protein, which can prevent copper absorption through the gastrointestinal tract. 

When tetrathiomolybdate is not administered with food, it can complex with copper and serum albumin, 

which prevents cellular uptake of copper. Due to these mechanisms, tetrathiomolybdate is used to treat 

individuals with Wilson’s disease, which is a metabolic defect that limits the excretion of copper. Other 

molybdenum compounds may also interfere with copper balance in humans. Significant increases in 

serum and urine copper levels were observed in men exposed to 0.022 mg molybdenum/kg/day (the 

source of molybdenum was high molybdenum sorghum supplemented with ammonium molybdate) for 

10 days, as compared to exposure to 0.00771 mg molybdenum/kg/day for 10 days (Deosthale and 

Gopalan 1974). However, there was no difference in fecal excretion of copper, suggesting that copper 

absorption was not affected. In contrast, another study (Turnlund and Keys 2000) did not find any 

significant alterations in serum copper levels in humans exposed to molybdenum levels of 22–

1,490 μg/day (0.0003–0.02 mg/kg/day) for 24 days (subjects were fed diets containing naturally high or 

low levels of molybdenum).

A number of studies have reported that molybdenum induces oxidative stress. An in vitro study in mouse 

fibroblasts and liver cancer cells found that trivalent molybdenum induced oxidative stress as indicated by 

increases in reactive oxygen species generation and increases in malondialdehyde concentration 

(Terpilowska and Siwicki 2019). This possible mechanism of action is supported by several in vivo

studies. A general population study found an association between urinary molybdenum levels and ratio of 

oxidized glutathione to reduced glutathione in the general population suggestive of a relationship between 

molybdenum and oxidative stress (Domingo-Relloso et al. 2019). Zhai et al. (2013) showed that the 

levels of two enzymatic antioxidants (superoxide dismutase and glutathione peroxidase) in the testes of 

mice paralleled the molybdenum-induced sperm effects. Increases in antioxidant levels and 

improvements in sperm parameters were observed at lower molybdenum doses. However, at higher 

molybdenum doses, there were significant decreases in antioxidant levels and significant decreases in 
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2. HEALTH EFFECTS

sperm motility and concentration and an increase in the rate of sperm abnormalities. Zhang et al. (2013) 

reported a similar finding for superoxide dismutase and glutathione peroxidase levels in the ovaries of 

mice and the rate of MII oocyte abnormalities. Molybdenum-induced hepatocyte apoptosis was observed 

in goats orally exposed to ammonium molybdate for 50 days (Zhuang et al. 2016). Molybdenum 

exposure resulted in down-regulation of superoxide dismutase and catalase expression in liver cells and 

an up-regulation of malondialdehyde, nitric oxide, and total nitric oxide synthase expression. The 

investigators suggested that the observed effect may be due to a disruption of the mitochondrial 

antioxidant defense system resulting in apoptosis via activation of the mitochondrial signaling pathways. 
MOLYBDENUM 61

CHAPTER 3. TOXICOKINETICS, SUSCEPTIBLE POPULATIONS, 

BIOMARKERS, CHEMICAL INTERACTIONS

3.1 TOXICOKINETICS 

• A number of factors can influence the oral absorption of molybdenum; absorption can range 

between 40 and 100%. The amount absorbed decreased with increasing doses and was lower 

when the molybdenum was ingested with a meal. There is evidence for absorption of airborne 

molybdenum, but no data on the amount absorbed. Molybdenum is poorly absorbed 

(approximately 0.2%) through the skin.

• Absorbed molybdenum is widely distributed throughout the body, with the highest concentrations 

found in the kidneys and liver.

• Molybdenum is not metabolized; however, it can undergo oxidation and reduction.

• Molybdenum is primarily excreted in the urine, with lesser amounts excreted in feces.

3.1.1 Absorption 

Inhaled molybdenum particles that deposit in the respiratory tract are subject to three general distribution 

processes: (1) bronchial and tracheal mucociliary transport to the gastrointestinal tract; (2) transport to 

thoracic lymph nodes (e.g., lung, tracheobronchial, mediastinal); or (3) absorption into blood and/or 

lymph and transfer to other tissues (e.g., peripheral lymph tissues, liver, kidney). The above processes 

apply to all forms of deposited molybdenum, although the relative contributions of each pathway and 

rates associated with each pathway vary with the physical characteristics (e.g., particle size, solubility). 

Particles having diameters >5 μm are deposited primarily in the upper airways (extrathoracic, 

tracheobronchial regions) and are cleared from the respiratory tract primarily by mucociliary transport to 

the gastrointestinal tract (Bailey et al. 2007; ICRP 1994). Smaller particles (≤5 μm) are deposited 

primarily in the pulmonary region (terminal bronchioles and alveoli). Particles are cleared from the 

pulmonary region primarily by absorption, lymph drainage, macrophage phagocytosis and migration, and 

upward mucociliary flow. Dissolved molybdenum is absorbed into blood. The rate of absorption will 

depend on solubility. ICRP (2012) assigns molybdenum sulfide, oxides, and hydroxides to a “slow” 

classification in their absorption, which equates to an expected terminal absorption half-time of 

approximately 19 years (Bailey et al. 2007; ICRP 1994). More soluble forms of molybdenum, such as 

molybdenum trioxide (MoVIO3), would be expected to undergo more rapid dissolution and absorption.

Quantitative estimates of absorption following inhalation exposure to molybdenum in humans or animals 

were not identified. Evidence for absorption of molybdenum trioxide is available from inhalation studies 
MOLYBDENUM 62

3. TOXICOKINETICS, SUSCEPTIBLE POPULATIONS, BIOMARKERS, CHEMICAL INTERACTIONS

on molybdenum trioxide conducted in rodents (Fairhall et al. 1945; NTP 1997). Fairhall et al. (1945) 

showed distribution to several tissues following inhalation exposure of guinea pigs to molybdenum 

trioxide. In rats and mice exposed to inhaled molybdenum trioxide (6.7–67 mg molybdenum/m3

, 

6 hours/day, 5 days/week for 2 years), exposure-dependent increases in blood molybdenum were 

observed (NTP 1997). The respective blood molybdenum levels in the 0, 6.7, 20, and 67 mg 

molybdenum/m3 groups were 0.221, 0.800, 1.774, and 6.036 μg/g in male rats, 0.059, 0.355, 0.655, and 

2.411 μg/g in female rats, 0.102, 0.208, and 0.770 μg/g in male mice (no data were available for controls), 

and 0.043, 0.066, 0.198, and 0.523 μg/g for female mice.

Absorption of ingested molybdenum has been studied in human adults and infants (Cantone et al. 1993, 

1997; Engel et al. 1967; Giussani et al. 1998, 2006, 2007; Novotny and Turnlund 2006, 2007; Robinson 

et al. 1973; Sievers et al. 2001a, 2001b; Turnlund et al. 1995a, 1995b; Werner et al. 1998; Yoshida et al. 

2006). These studies fall into two general categories: mass balance studies and bioavailability studies. 

Mass balance studies estimate the absorption fraction from measurements of the difference between the 

ingested dose of molybdenum and fecal excretion (the difference being net absorption). Bioavailability 

studies estimate the absorption fraction from measurements of the plasma concentration of molybdenum 

following the oral dose. These methods provide estimates of net absorption in that absorbed molybdenum 

that is excreted into the gastrointestinal tract (e.g., biliary excretion) may not be accurately quantified 

from mass balance or bioavailability estimates. However, both approaches have been facilitated by the 

use of stable isotopes of molybdenum (95Mo, 96Mo), which allow measurement of plasma and excretion 

kinetics following concurrent intravenous and oral dosing. The use of stable isotopes also allows 

measurement of the administered molybdenum in plasma and excreta, distinct from background sources 

of molybdenum derived from other sources and preexisting body stores. By incorporating elimination 

kinetics data into mathematical models that include parameters representing absorption and fecal 

excretion of absorbed molybdenum, the absorption fraction can be estimated. In most reported stable 

isotope studies, the exact form of molybdenum administered was not reported. However, the isotope 

dosing material was typically prepared from an acid dissolution of metallic molybdenum (Mo0

). The 

resulting material dissolved in water most likely was a mixture of soluble molybdate anion (MoVIO4

2-

) and 

other soluble molybdenum oxide hydrates.

Balance and bioavailability studies conducted in humans have shown that the fraction of ingested 

molybdenum that is absorbed depends on numerous factors, including molybdenum dose level, fasting, 

diet, and nutritional status. Absorption was estimated to be 80–100% in replete fasted adults who 

ingested molybdenum dissolved in water or in a beverage (Giussani et al. 2006; Novotny and Turnlund 
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3. TOXICOKINETICS, SUSCEPTIBLE POPULATIONS, BIOMARKERS, CHEMICAL INTERACTIONS

2006, 2007; Turnlund et al. 1995a). Absorption was 80–100% following a single dose of 20–40 μg 

molybdenum/kg dissolved in water and decreased with increasing dose level; absorption was 60% after a 

dose of 60 μg molybdenum /kg (Giussani et al. 2006). Absorption was lower when molybdenum was 

ingested with a meal (40–60%), when dissolved in black tea (20%), or when incorporated into vegetables 

cultivated with 96Mo (30–60%), compared to when ingested without a meal (80–100%) (Giussani et al. 

2006; Werner et al. 1998). Absorption was lower when molybdenum was incorporated into the diet 

(83%) compared to when it was administered in a beverage (90–94%) (Novotny and Turnlund 2007). 

Absorption appears to be affected by dietary molybdenum intake and molybdenum nutritional status. The 

absorption fraction was 90% in adults fed a diet containing 22 μg/day (approximately 0.3 μg 

molybdenum/kg/day), compared to 94% when fed a diet containing 467 μg molybdenum/day 

(approximately 7 μg molybdenum/kg/day) (Novotny and Turnlund 2007). Absorption in infants 

(gestational age 30–39 weeks) was 96–99% when a stable isotope of molybdenum was mixed with breast 

milk or infant formula (Sievers et al. 2001a, 2001b).

Long-term diet mass balance studies, without the aid of stable isotopes, have been conducted in adults and 

children (Engel et al. 1967; Robinson et al. 1973; Tipton et al. 1966). Because these studies cannot 

distinguish between the ingested dose of molybdenum and molybdenum excreted from body stores, these 

studies will underestimate the absorption fraction. A 50-week balance study conducted in two adult 

males (age 23 and 25 years) found absorption to range from 60 to 80% (Tipton et al. 1966). A 3-week 

balance study conducted in women (age 19–21 years) found absorption to range from 40 to 70% 

(Robinson et al. 1973). An 8-day balance study conducted in women (age 18–23 years) found absorption 

to range from 72 to 84% (Yoshida et al. 2006). Balance studies (18–30 days) conducted in female 

children (age 6–10 years) estimated the absorption fraction from diet to range from 67 to 85% (Engel et 

al. 1967).

Measurements of the time course of plasma molybdenum following oral doses of molybdenum indicate 

that absorption is relatively rapid, with peak concentrations in plasma attained within 100 minutes of 

dosing (Giusanni et al. 2006; Novotny and Turnlund 2007).

Studies of absorption and elimination kinetics conducted in swine provide estimates of absorption of 

ingested molybdenum. Based on cumulative urinary and fecal excretion measurements in swine dosed 

with a stable isotope of molybdenum, absorption was estimated to be between 80 and 90% (Bell et al. 

1964). Studies conducted in rats have shown that tetrathiomolybdate (MoVIS4

2-

) is more poorly absorbed 
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when ingested in the diet; approximately 21% was absorbed when the copper content of the diet was 

8 ppm (Mills et al. 1981b).

Roper (2009) evaluated the in vitro percutaneous absorption of sodium molybdate through human skin. 

Following an 8-hour application of 3.97 or 19.83 mg molybdenum/mL, the potentially absorbable doses 

were 0.21 and 0.16%, respectively.

Mechanisms that participate in absorptive transport of molybdenum in the gastrointestinal tract have not 

been characterized. Molybdate (MoO4

2-

) and sulfate (SO4

2-

) show mutually competitive inhibition for 

absorptive transport in rat small intestine, suggesting involvement of a common transporter for both 

anions (Cardin and Mason 1975, 1976). This transporter may be the Na+

/SO4

2-

symporter (NaS1 or 

SLC13A1) expressed in rodent small intestine and renal proximal tubule (Markovich and Aronson 2007; 

Murer et al. 1994). In humans, NaS1 is expressed in kidney but not small intestine, suggesting that 

mechanisms of absorptive transport in humans may be different from that of rodents (Lee at al. 2000).

3.1.2 Distribution 

Very little information on the distribution on molybdenum following inhalation exposure is available. 

Following exposure of guinea pigs to inhaled molybdenum trioxide (150–300 mg/m3

, 1 hour/day, 

5 days/week for 5 weeks), molybdenum was distributed to the lungs, liver, kidneys, and bone (Fairhall et 

al. 1945). Tissue levels decreased by approximately 20% in the 2-week postexposure period. 

Absorbed molybdenum distributes to various tissues. Human autopsy studies have found that the kidney 

and liver have the highest amounts of molybdenum (Iyengar et al. 1978; Schroeder et al. 1970; Sorensen 

and Archambault 1963; Sumino et al. 1975; Tipton and Cook 1963; Tipton et al. 1965; Yoo et al. 2002; 

Zeisler et al. 1988). Based on a review of these data, Giussani (2008) estimated liver and kidney 

molybdenum concentrations to be approximately 0.5–1.5 μg molybdenum/g wet in liver (700–2,700 μg) 

and 0.2–0.4 μg molybdenum/g wet in kidney (55–120 μg). Coughtrey and Thorne (1983) reported 

relatively high concentrations (56 μg molybdenum/g) in bone, based on their recalculation of 

measurements of molybdenum in bone ash reported in Nusbaum et al. (1965) and Iyengar et al. (1978). 

However, these results are not supported by other studies (previously cited) and have been attributed to 

overestimation of tissue concentrations measured by arc emission spectrometry in the Nusbaum et al. 

(1965) and Iyengar et al. (1978) studies (Giussani 2008).
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Results of studies in rats and guinea pigs exposed to oral molybdenum show that molybdenum is widely 

distributed (Bibr et al. 1977; Howell et al. 1993; Murray et al. 2014b; Pandey et al. 2002). Generally, the 

highest molybdenum tissue concentration is observed in the kidney. Molybdenum also is distributed to 

liver, spleen, brain, lungs, heart, bone, muscles, testes, epididymides, seminal vesicles, prostate, blood 

cells, and plasma. 

Maternal-Fetal Transfer. Results of studies in humans and animals show that molybdenum is distributed 

to the fetus. In humans, maternal and fetal cord blood levels obtained from 33 maternal-fetal pairs at 

parturition show similar molybdenum levels (maternal: 1.44±0.75 μg/L, mean±standard deviation [SD]; 

fetal: 1.44±0.89 μg/L) (Bougle et al. 1989). Molybdenum concentrations in venous cord blood (flowing 

from the placenta to the fetus; 0.7±0.2 μg/L, mean±SD) were slightly higher than in arterial cord blood 

(flowing from the fetus to the placenta; 0.6±0.2 μg/L), indicating fetal retention of molybdenum (Krachler 

et al. 1999); the study did not evaluate whether there was a statistical difference between the molybdenum 

concentrations in venous and arterial blood.

Gestational exposure of rats to ammonium molybdate and thiomolybdate shows distribution of 

molybdenum to fetal liver, kidney, bone, and brain (Howell et al. 1993). Levels in liver, kidney, and bone 

were similar, with lower levels in brain. In rats, dose-dependent increases in placental and maternal liver 

content of molybdenum were observed following gestational exposure to molybdenum (sodium 

molybdate) in drinking water (5–100 mg molybdenum/L; equivalent to approximately 0.76–

15 mg/kg/day, based on intermediate exposure to nonpregnant female rats) over the full dose range 

(Fungwe et al. 1989). However, neonatal whole-body levels of molybdenum reached a plateau at 

drinking water concentrations ≥50 mg/L (Fungwe et al. 1989). Results suggest that molybdenum levels 

in the fetus reach steady state more rapidly than in dams.

Maternal-Infant Transfer. Several studies have measured molybdenum in breast milk (Anderson 1992; 

Aquilio et al. 1996; Biego et al. 1998; Bougle et al. 1988; Casey and Neville 1987; Dang et al. 1984; Friel 

et al. 1999a; Krachler et al. 1998; Wappelhorst et al. 2002); the mean concentrations ranged from 0.02 to 

24 μg/L. Breast milk concentrations are highest at the start of breast feeding and then decline (EFSA 

2013). In the only study comparing maternal intake to breast milk levels, Wappelhorst et al. (2002) did 

not find a correlation between breast milk concentrations of molybdenum and maternal molybdenum 

intake. The mean concentration in breast milk was 72 μg/L and the mean maternal intake was 

132 μg/day.
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Bacteria and eukaryotes express cell membrane molybdate transporters, one of which (MoT2) also 

appears to be expressed in humans (Tejada-Jimenez et al. 2007, 2011). In eukaryotes, this transporter 

participates in the delivery of molybdate into cells for incorporation into molybdopterin-cofactor (Moco), 

the biologically active prosthetic group in molybdenum-dependent enzymes (Schwarz et al. 2009). MoT2 

transport of molybdate is inhibited by sulfate, suggesting a common carrier for molybdate and sulfate. A 

sulfate-insensitive oxalate-sensitive molybdate transporter has been described in mammalian MEK-293T 

cells grown in culture (Nakanishi et al. 2013). Uptake of molybdate into human red blood cells involves 

participation of the Cl-1

/HCO3

-1 anion exchanger (Gimenez et al. 1993).

3.1.3 Metabolism 

Molybdenum exists in several valence states and may undergo oxidation and reduction. The primary 

form of molybdenum that interacts with enzyme systems is MoVI, as the molybdate anion (MoVIO2

2-

) 

(Nakanishi et al. 2013). After molybdate is taken into a cell, it is incorporated into a molybdopterin to 

form molybdenum cofactor (Moco). Moco is a sulfur-molybdate complex that forms the prosthetic group 

in molybdenum-dependent enzymes (Mendel and Kruse 2012; Schwarz et al. 2009). Given that Moco is 

extremely sensitive to oxidation, it is believed that it is bound to a Moco-binding protein in the cell 

(Mendel and Kruse 2012). This stored Moco would be readily available to meet the cell’s demand for 

molybdenum enzymes. Molybdate forms complexes with copper and binds to plasma proteins as a 

copper-molybdenum-sulfur (Cu-Mo-S) complex (Nederbragt 1980, 1982).

3.1.4 Excretion 

Studies investigating the elimination and excretion of molybdenum following inhalation exposure were 

not identified.

Absorbed molybdenum is excreted in urine and feces in humans. Urine is the dominant excretion route, 

accounting for the excretion of approximately 75–90% of the absorbed dose (Giussani 2008; Novotny and 

Turnlund 2007). The fraction excreted in urine increases with increasing dietary intake (Novotny and 

Turnlund 2007). Urine also is the dominant excretory route for absorbed molybdenum in swine. 

Following an oral dose, approximately 90% of the dose was excreted in urine (Bell et al. 1964). To 

measure urinary and fecal excretion of molybdenum, Turnlund et al. (1995a, 1995b) exposed four healthy 

adult males to various doses of a radioactive isotope of molybdenum (24–1,378 μg 100Mo/day) and 

administered intravenous doses of stable isotope of molybdenum (33 μg 97Mo). Six days after exposure 
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to 100Mo in the diet, 17.8% of the 100Mo label was excreted in the urine at the lowest dose tested (total 

molybdenum dose of 24 μg/day). As the molybdenum dose increased, the amount excreted in the urine 

also increased; at the highest dose (1488 μg/day), 82.1% of the 100Mo was excreted in the urine. A similar 

pattern of urinary excretion was found when 97Mo was measured: 32.7% of the label at 24 μg/day and 

86.7% at 1,488 μg/day. The percentage of the molybdenum dose excreted in the feces decreased with 

increasing doses. At the lowest dose tested, 9.4% of the 100Mo dose was excreted in the feces; at the 

highest dose, 7.5% of the 100Mo dose was excreted in the feces. In contrast, no consistent pattern of fecal 

97Mo excretion was found. When total molybdenum excretion was measured, the study found that 41% 

was excreted in feces and 59% was excreted in urine at the lowest dose tested and 6% was excreted in 

feces and 94% was excreted in urine at the highest dose tested. Fecal excretion of absorbed molybdenum 

is thought to result from biliary secretion. Studies conducted in bile-duct cannulated rats have shown that, 

following an intravenous dose of Mo[V] or Mo[VI], approximately 1% of the molybdenum dose was 

secreted into bile in a period of 4 hours (Lener and Bibr 1979).

The rate of elimination of molybdenum from plasma has been studied in human clinical studies (Cantone 

et al. 1997; Rosoff and Spencer 1964; Thompson et al. 1996; Werner et al. 2000). Elimination is 

approximately biphasic, with mean half-times of 30 minutes and 6.6 hours (Giussani 2008).

The whole-body elimination rate in rats is dose-dependent (Bibr and Lener 1973). Following oral 

administration of Mo[VI] at doses <3 μg molybdenum/kg, elimination was mono-phasic with a half-time of 

approximately 47 hours. Following doses >3 μg molybdenum/kg, the rate of elimination increased, with 

an increasing proportion of elimination contributed by a fast phase having a half-time of 6 hours.

Mechanisms that participate in the renal excretion of molybdenum have not been characterized. In sheep, 

reabsorption of filtered molybdate (MoO4

2-

) is saturable, which results in an increase in the fraction of 

filtered molybdate excreted as the plasma molybdate concentration increases and approaches or exceeds 

the tubular maximum (Ryan et al. 1987). In sheep and rat kidney, sodium-dependent reabsorptive 

transport of molybdate (MoO4

2-

) and sulfate (SO4

-2

) exhibit mutual inhibition (Ryan et al. 1987). This is 

consistent with participation of the Na+

/SO4

2- symporter (NaS1 or SLC13A1) in the reabsorption of 

molybdate. This symporter is also expressed in the human renal proximal tubule (Markovich and 

Aronson 2007; Murer et al. 1994).
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3.1.5 Physiologically Based Pharmacokinetic (PBPK)/Pharmacodynamic (PD) Models 

PBPK models use mathematical descriptions of the uptake and disposition of chemical substances to 

quantitatively describe the relationships among critical biological processes (Krishnan et al. 1994). PBPK 

models are also called biologically based tissue dosimetry models. PBPK models are increasingly used in 

risk assessments, primarily to predict the concentration of potentially toxic moieties of a chemical that 

will be delivered to any given target tissue following various combinations of route, dose level, and test 

species (Clewell and Andersen 1985). Physiologically based pharmacodynamic (PBPD) models use 

mathematical descriptions of the dose-response function to quantitatively describe the relationship 

between target tissue dose and toxic endpoints. 

Several multi-compartmental models of the kinetics of molybdenum in humans have been developed 

(Giussani 2008; Giussani et al. 1998, 2000; Novotny and Turnlund 2007; Thompson et al. 1996). The 

latest of these are the Giussani (2008) and Novotny and Turnlund (2007) models. Both models yield 

similar predictions when applied to the same dosing scenarios (Giusanni 2008). The Giussani (2008) 

model has been adopted for use by the International Commission on Radiological Protection (ICRP) and 

is described in this section.

Giussani (2008) Model

Giussani (2008) developed a model of molybdenum kinetics in humans. The structure of the model is 

shown in Figure 3-1 and parameter values are presented in Table 3-1. Data used to derive and evaluate 

the model are described in Giusanni (2008) and included human clinical studies in which subjects were 

administered intravenous or oral doses of stable isotopes of molybdenum (Giusanni et al. 2006, 2007; 

Novotny and Turnlund 2006, 2007; Turnlund et al. 1995a; Werner et al. 1998, 2000). The Giussani 

(2008) model has been adopted for use by the ICRP and is described in this section.

The model consists of two central compartments representing extracellular fluids (ECF) and 

compartments representing liver, kidney (two subcompartments), and a lumped compartment representing 

all other tissues. All transfers of molybdenum between compartments are first order and governed by 

first-order rate coefficients (day-1

). The two ECF compartments represent fast and slow transfer pathways 

out of the ECF and were based on studies conducted in humans, which provide evidence for multi-phasic 

clearance of molybdenum from plasma (Giussani et al. 2007; Werner et al. 2000). The half-times for the 

two ECF compartments are approximately 30 minutes for ECF1 and 280 minutes for ECF2. Transfers 
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3. TOXICOKINETICS, SUSCEPTIBLE POPULATIONS, BIOMARKERS, CHEMICAL INTERACTIONS

from the fast compartment (ECF1) are to liver, kidney, and urine. Transfers from the slow compartment 

(ECF2) are to urine, kidney, and other tissues; the slow compartment also receives molybdenum from the 

liver. Retention half-times in tissues are 41 days for liver, 14.5 days for kidney, and 21.5 days for the 

other tissue compartment. Excretion of absorbed molybdenum occurs in urine (88%) and transfer from 

liver to the gastrointestinal tract (12%).

Figure 3-1. The Proposed Systemic Model for Molybdenum Radionuclides

ECF = extracellular fluid

Source: Reprinted from Giussani (2008) with permission from Elsevier.

Table 3-1. Transfer Rates (Day-1) for the Molybdenum Model 

Transfer rate Value (day-1)

ECF1 to ECF2 12.5

ECF1 to liver 14.2

ECF1 to urinary bladder contents 6.5

ECF2 to urinary path 1.7

ECF2 to other kidney tissues 0.115

ECF2 to other tissues 1.73

Liver to alimentary tract 0.0048

ECF1

ECF2

Liver

Right colon

(fecal 

excretion)

Other 

tissues

Urinary

path

Other 

kidney

tissues

Urinary

bladder 

contents

From alimentary tract,

respiratory tract, wounds
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Table 3-1. Transfer Rates (Day-1) for the Molybdenum Model 

Transfer rate Value (day-1)

Liver to ECF2 0.0122

Other kidney tissues to ECF2 0.0474

Other tissues to ECF2 0.0323

Urinary path to urinary bladder contents 1.40

Urinary bladder contents to urine 12

Modified parameters of the alimentary tract

Stomach to small intestine (liquid form) 100

Stomach to small intestine (diet) 40

Small intestine to right colon (liquid form) 10

Small intestine to right colon (diet) 16

ƒA (liquid form)a 0.9

ƒA (diet)a 0.6

Source: Reprinted from Giussani (2008) with permission from Elsevier.

The model can simulate absorption from the gastrointestinal tract and respiratory tract. The absorption 

fraction for the gastrointestinal pathway uses an absorption fraction of 0.9 for molybdenum ingested in 

liquids and 0.6 for molybdenum ingested in the diet. The model predicts a steady state for constant 

dietary intake of molybdenum in adults, in which approximately 52% of the molybdenum body burden is 

in liver, 3% is in kidney, 45% is in other tissues, 53% of the daily dose is excreted in urine, and 47% of

the daily dose is excreted in feces (Giussani 2008). The model is constructed to be able to interface with 

output from the ICRP Human Respiratory Tract Model (HRTM) (Bailey et al. 2007; ICRP 1994). The 

inputs to the Giussani (2008) model from the HRTM would be simulated transfers of molybdenum to the 

gastrointestinal tract (mucociliary transfer) and to blood (absorption from the respiratory tract), depending 

on the particle size and solubility of the inhaled molybdenum and other physiological factors (e.g., age, 

activity).

Novotny and Turnlund (2007) Model 

The major difference between the structures of the Giussani (2008) and Novotny and Turnlund (2007) 

models is that the Novotny and Turnlund (2007) model has a single lumped compartment representing all 

tissues outside of the vasculature. The Novotny and Turnlund (2007) model illustrated in Figure 3-2 has 

two configurations: an intravenous configuration, which has two plasma compartments, representing fast 
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and slower clearance, and an oral configuration, which has a single plasma compartment. Molybdenum 

exchanges between plasma and a lumped tissue compartment. Urinary excretion is represented as a direct 

transfer from plasma. Absorbed molybdenum is also transferred to the gastrointestinal tract.

Figure 3-2. Diagram of the Compartment Molybdenum Model*

*Circles show distinct kinetic compartments and arrows show pathways of flow between 

compartments. Dashed lines show structures that are specific to the intravenous (IV) dosing. Bold 

arrows show paths that appear to be involved in molybdenum regulation as suggested by kinetic 

modeling.

Source: Novotny and Turnlund (2007), by permission of the American Society for Nutrition (via 

Oxford University Press)

Novotny and Turnlund (2006, 2007) conducted mass balance studies with subjects who ingested stable 

isotopes of molybdenum in the context of varying dietary intakes of molybdenum (22–1,490 μg 

molybdenum/day) and found that certain model parameters were dependent on dietary intake. These 

included, in association with increasing dietary intake, increases in the first-order rate coefficients for 

gastrointestinal absorption and urinary excretion and a decrease in the rate coefficients for transfer from 
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plasma to tissues. The largest adjustments were needed to simulate molybdenum kinetics in subjects who 

consumed >121 μg molybdenum/day and included a 70% decrease in the coefficient for transfer of 

molybdenum from plasma to tissues and a 660% increase in the rate coefficient for transfer from plasma 

to urine. These results suggest that high molybdenum intakes (>121 μg molybdenum/day) result in 

physiological adaptations that increase excretion of absorbed molybdenum (Novotny and Turnlund 2007).

3.1.6 Animal-to-Human Extrapolations 

There are limited data to evaluate potential differences in the toxicity of molybdenum between laboratory 

animals and humans. Most of the available oral exposure studies were conducted in rats, and human data 

are mostly limited to a small number of cross-sectional studies. Within laboratory animal species, some 

differences have been observed between rats and rabbits, with rabbits appearing to be more sensitive than 

rats. However, the studies are not directly comparable due to differences in the copper content and other 

dietary constituents. In the absence of data to the contrary, it is assumed that the toxicity of molybdenum 

will be similar across species (excluding ruminants).

3.2 CHILDREN AND OTHER POPULATIONS THAT ARE UNUSUALLY SUSCEPTIBLE

This section discusses potential health effects from exposures during the period from conception to 

maturity at 18 years of age in humans. Potential effects on offspring resulting from exposures of parental 

germ cells are considered, as well as any indirect effects on the fetus and neonate resulting from maternal 

exposure during gestation and lactation. Children may be more or less susceptible than adults to health 

effects from exposure to hazardous substances and the relationship may change with developmental age. 

This section also discusses unusually susceptible populations. A susceptible population may exhibit 

different or enhanced responses to certain chemicals than most persons exposed to the same level of these 

chemicals in the environment. Factors involved with increased susceptibility may include genetic 

makeup, age, health and nutritional status, and exposure to other toxic substances (e.g., cigarette smoke). 

These parameters can reduce detoxification or excretion or compromise organ function. 

Populations at greater exposure risk to unusually high exposure levels to molybdenum are discussed in 

Section 5.7, Populations with Potentially High Exposures.
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There are limited data on the toxicity of molybdenum in children. In studies in rat pups maintained on a 

caries-promoting diet, administration of 50 mg molybdenum/kg/day as sodium molybdate resulted in an 

increase in buccal enamel lesions (Hunt and Navia 1975), but exposure to 8 mg molybdenum/kg/day did 

not result in increases in dental caries (Van Reen et al. 1962). Arrington and Davis (1953) exposed young 

(6 weeks of age at the start of the study) and mature rabbits to sodium molybdate in the diet for 30–

84 days. Marked muscular/skeletal effects were observed in the young rabbits, but not in the mature 

rabbits. Since the investigators did not provide information on dietary intake, it is difficult to make direct 

comparisons across the studies.

An observational study did not find an association between maternal urinary molybdenum levels and 

newborn body weight or infant mental development (Shirai et al. 2010). However, another study did find 

an association between third-trimester maternal urinary molybdenum levels and infant psychomotor 

development indices (Vazquez-Salas et al. 2014). Two rat studies in which the copper content of the diet 

was adequate did not find significant alterations in fetal growth, survival, or malformations at maternal 

doses of 40 mg molybdenum/kg/day (Murray et al. 2014b, 2019). However, a third study reported 

decreases in growth and number of live fetuses in the offspring of male rats administered 14 mg 

molybdenum/kg as sodium molybdate 5 days/week for 60 days prior to mating with unexposed females 

(Pandey and Singh 2002).

Studies in laboratory animals have found that maintenance on a copper-deficient diet enhances the 

toxicity of molybdenum (Brinkman and Miller 1961; Franke and Moxon 1937; Johnson and Miller 1961; 

Sasmal et al. 1968; Valli et al. 1969; Van Reen 1959; Widjajakuma et al. 1973). Administration of 

additional copper results in a reversal of the adverse effect (Arrington and Davis 1953). Thus, individuals 

with low copper intakes may be unusually susceptible to the toxicity of molybdenum. Additionally, 

individuals with high dietary molybdenum intake, including individuals taking supplements containing 

high levels of molybdenum, may be at an increased risk from exposure to high levels of molybdenum in 

the environment.

Studies in rats suggest that the toxicity of molybdenum may be increased in animals maintained on a low 

protein diet. The magnitudes of the decrease in body weight gain (Bandyopadhyay et al. 1981; Cox et al. 

1960) and decreases in femur breaking strength (Fejery et al. 1983) were greater in rats exposed to a low 

protein diet, as compared to those maintained on a diet with sufficient protein.
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3.3 BIOMARKERS OF EXPOSURE AND EFFECT 

Biomarkers are broadly defined as indicators signaling events in biologic systems or samples. They have 

been classified as biomarkers of exposure, biomarkers of effect, and biomarkers of susceptibility 

(NAS/NRC 1989).

A biomarker of exposure is a xenobiotic substance or its metabolite(s) or the product of an interaction 

between a xenobiotic agent and some target molecule(s) or cell(s) that is measured within a compartment 

of an organism (NAS/NRC 1989). The preferred biomarkers of exposure are generally the substance 

itself, substance-specific metabolites in readily obtainable body fluid(s), or excreta. Biomarkers of 

exposure to molybdenum are discussed in Section 3.3.1. The National Report on Human Exposure to 

Environmental Chemicals provides an ongoing assessment of the exposure of a generalizable sample of 

the U.S. population to environmental chemicals using biomonitoring (see http://www.cdc.gov/

exposurereport/). If available, biomonitoring data for molybdenum from this report are discussed in 

Section 5.6, General Population Exposure. 

Biomarkers of effect are defined as any measurable biochemical, physiologic, or other alteration within an 

organism that (depending on magnitude) can be recognized as an established or potential health 

impairment or disease (NAS/NRC 1989). This definition encompasses biochemical or cellular signals of 

tissue dysfunction (e.g., increased liver enzyme activity or pathologic changes in female genital epithelial 

cells), as well as physiologic signs of dysfunction such as increased blood pressure or decreased lung 

capacity. Note that these markers are not often substance specific. They also may not be directly 

adverse, but can indicate potential health impairment (e.g., DNA adducts). Biomarkers of effect caused 

by molybdenum are discussed in Section 3.3.2.

A biomarker of susceptibility is an indicator of an inherent or acquired limitation of an organism's ability 

to respond to the challenge of exposure to a specific xenobiotic substance. It can be an intrinsic genetic or 

other characteristic or a preexisting disease that results in an increase in absorbed dose, a decrease in the 

biologically effective dose, or a target tissue response. If biomarkers of susceptibility exist, they are 

discussed in Section 3.2, Children and Other Populations that are Unusually Susceptible.
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3.3.1 Biomarkers of Exposure

Molybdenum levels can readily be measured in tissues, body fluids, and excreta. Dose-related increases 

in serum molybdenum levels were observed in rats and mice exposed via inhalation to molybdenum 

trioxide for 2 years (NTP 1997). In a study examining the relationship between plasma molybdenum 

levels and dietary intake, Turnland and Keyes (2004) reported a baseline plasma molybdenum level of 

8.2±0.5 nmol/L; 25 days after the subjects were maintained on a low molybdenum diet (22 μg/day), the 

plasma molybdenum level was 5.1±0.5 nmol/L. Although a significant correlation between plasma 

molybdenum and dietary molybdenum was observed, comparison between plasma molybdenum levels at 

different dietary intakes showed that a significant increase in plasma molybdenum was not observed until 

the dietary intake exceeded 460 μg/day (6.6 mg/kg/day) and that tripling the intake resulted in a doubling 

of the plasma molybdenum levels. Urinary molybdenum levels were also significantly correlated to 

dietary intakes (Turnland and Keyes 2004) and appeared to be more responsive to changes in dietary 

intake. At all dietary concentrations, the urinary molybdenum levels were slightly lower than the dietary 

intakes (Turnland and Keyes 2004). The investigators concluded that plasma molybdenum levels are an 

indicator of dietary intake, but urinary levels had a more direct relationship with dietary intake. 

Molybdenum levels were measured in urine samples collected during the NHANES study. The geometric 

mean urinary molybdenum levels in the United States in 2011–2012 was 37.1 μg/L and the creatininecorrected value was 42.0 μg/g creatinine (CDC 2015); see Section 5.6 for additional information.

Although several studies have reported molybdenum levels in hair samples (DiPietro et al. 1989; Nagra et 

al. 1992; Paschal et al. 1989), no relationship between molybdenum exposure and hair levels has been 

established. Furthermore, Miekeley et al. (1998) demonstrated large interlaboratory differences in the 

molybdenum levels measured in hair.

3.3.2 Biomarkers of Effect

No biomarkers to characterize effects caused by molybdenum have been identified.

3.4 INTERACTIONS WITH OTHER CHEMICALS

The interaction between copper and molybdenum has been well-established in animals. The levels of 

copper in the diet have been shown to influence the toxicity of molybdenum. Marked toxicity has been 
MOLYBDENUM 76

3. TOXICOKINETICS, SUSCEPTIBLE POPULATIONS, BIOMARKERS, CHEMICAL INTERACTIONS

reported in studies in which the copper content of the diet was inadequate. Observed effects included 

mortality in rabbits (Valli et al. 1969; Widjajakuma et al. 1973), marked decreases in body weight gain 

and weight loss in rats and rabbits (Brinkman and Miller 1961; Johnson and Miller 1961; Sasmal et al. 

1968; Valli et al. 1969; Van Reen 1959), and anemia in rats and rabbits (Brinkman and Miller 1961; 

Franke and Moxon 1937; Gray and Daniel 1954; Johnson et al. 1969; Valli et al. 1969). In general, these 

effects (or the severity of the effects) have not been observed when the diet contains an adequate level of 

copper (Mills et al. 1958; Murray et al. 2014a; Pandey and Singh 2002; Peredo et al. 2013). Exposure to 

high levels of copper has been shown to reduce the toxicity of molybdenum. Administration of high 

doses of copper to moribund rabbits resulted in a return to normal body weight gain and increases in 

hemoglobin levels within 2–3 weeks (Arrington and Davis 1953). Lyubimov et al. (2004) showed that 

administration of a high dose of copper prevented the molybdenum-induced testicular toxicity observed in 

rats fed a copper-adequate diet. Similarly, in an environmental exposure study of men at infertility 

clinics, Meeker et al. (2008) found a greater decline in sperm concentration in men with high 

molybdenum blood levels and copper blood levels below the median, as compared to when the men were 

not stratified by blood copper levels. 

Kinetic studies have demonstrated differences in plasma, liver, and kidney copper and molybdenum 

concentrations in rats fed copper-deficient, copper-adequate, and copper-excessive diets (Nederbragt 

1980). Administration of molybdenum results in increases in plasma, liver, and kidney copper levels in 

rats fed a copper-adequate diet (Nederbragt 1980); the increases in copper appear to be molybdenumdose-related. Most of the rise in plasma copper levels was in the tightly-bound fraction, which is likely to 

be poorly available for copper metabolism. Excess copper in the diet resulted in a smaller increase in 

copper concentrations in plasma, liver, and kidneys and molybdenum concentrations in the liver and 

kidney, as compared to levels in rats fed a copper-adequate diet. Similarly, lower rises in liver copper and 

molybdenum and kidney molybdenum levels were observed in rats fed a copper-deficient and highmolybdenum diet, as compared to the copper-adequate diet. At the lowest molybdenum dose, kidney 

molybdenum levels were higher in the copper-deficient groups. In another study (Nederbragt 1982), 

kidney levels of copper and molybdenum were 5 and 3 times higher, respectively, in the copper-adequate 

groups as compared to the copper-deficient group. Two human studies have also evaluated the effect of 

molybdenum on copper levels. In one study, increases in serum and urine copper levels were found 

following a 10-day exposure to 0.022 mg molybdenum/kg/day (Deosthale and Gopalan 1974). Another 

study found no significant alterations in serum copper levels in humans exposed to 0.0003–0.02 mg 

molybdenum/kg/day for 24 days (Turnlund and Keys 2000). 
MOLYBDENUM 77

CHAPTER 4. CHEMICAL AND PHYSICAL INFORMATION

4.1 CHEMICAL IDENTITY

Molybdenum (Mo) is a naturally occurring metallic trace element found in natural minerals, but not as the 

free metal. Biologically, it is an important micronutrient in plants and animals, including humans. It is 

used widely in industry for metallurgical applications (EPA 1979).

Molybdenum metal is a dark-gray or black powder with a metallic luster (NLM 2020a). It is a transition 

element in Group 6 of the Periodic Table. It has oxidation states from -2 to +6. Commonly encountered 

compounds are those of molybdenum in oxidation state +6 (Mo(VI), MoO3, molybdates) and +4 (Mo(IV), 

MoS2). It does not occur naturally in the pure metallic form; it more commonly occurs in the mineral,

molybdenite (Sebenik et al. 2012). Other naturally occurring molybdenum-containing minerals are

powellite, wulfenite, ferrimolybdite, and ilsemannite; however, molybdenite is the primary commercial 

source of molybdenum. Molybdenum (VI) anions include molybdate (MoO4

-2

) with molybdenum at the 

center of a tetrahedron of four oxygen atoms, and polyberic anions (‘isopolymolybdates’) of which the 

most common are heptamolybdate (Mo7O24

6-

) and octamolybdate (Mo8O26

4-

) (EPA 1979). These anions 

occur in sodium and ammonium salts, often hydrated, which are the common sources of molybdenum in 

commerce and industrial applications.

There are 33 known isotopes of molybdenum. Seven isotopes occur naturally: mass numbers 92, 94, 95, 

96, 97, 98, and 100. 98Mo is the most abundant isomer, comprising approximately 24.3% (Rumble et al. 

2018). Radioisotopes of masses 83–91, 93, 99, and 101–115 have been reported. The only one of major 

worldwide importance is Mo-99 (99Mo), a 100% beta-emitting isotope with a 65.976-hour radioactive 

half-life that is used to produce technetium-99m (Tc-99m or 99mTc) for medical scans (Doll et al. 2014; 

Parma 2009; Richards 1989).

Under physiological conditions (pH >6.5), the molybdate anion, [MoO4]

2-

, is the sole molybdenum 

species in aqueous media (Cruywagen 2000; Cruywagen et al. 2002). Molybdenum compounds (e.g., 

molybdenum trioxide and polymolybdates) transform rapidly to the [MoO4]

2- ion under environmentally

relevant test conditions (Deltombe et al. 1974; Greenwood and Earnshaw 1997). Protonated forms, such 

as [HMoO4]

- and H2MoO4, are found at pH <5 (Smedley and Kinniburgh 2017). Molybdenum tends to be 

more mobile under alkaline conditions, but adsorption increases with decreasing pH (Goldberg et al. 

2002). 
MOLYBDENUM 78

4. CHEMICAL AND PHYSICAL INFORMATION

Information regarding the chemical identity of molybdenum and molybdenum compounds is provided in 

Table 4-1.

Table 4-1. Chemical Identity of Molybdenum and Compounds

Characteristic Information

Chemical name Molybdenuma Molybdenum disulfideb Molybdenum trioxidec

Synonym(s) and 

registered trade 

namesd

MChVL; TsM1; 

Amperit 105.054; Amperit 

106.2; Metco 63

Molybdenite (natural 

mineral); molybdenum(IV) 

sulfide; DAG 325; Molykote

Molybdenum(VI) oxide; 

molybdic acid anhydride; 

molybdic anhydride; 

molybdic oxide

Chemical formula Mo MoS2 MoO3

CAS Registry 

Number

7439-98-7 1309-56-4/1317-33-5 

(natural mineral form)e; 

12612-50-9 (synthetic form)

1313-27-5

Chemical name Sodium molybdatef Ammonium dimolybdate

Ammonium 

heptamolybdate 

tetrahydrateg

Synonym(s) and 

registered trade 

namesd

Disodium molybdate; 

molybdic acid, disodium 

salte

Ammonium molybdenum 

oxidee

Ammonium paramolybdate 

tetrahydrate; 

hexammonium molybdate

Chemical formula Na2MoO4 (NH4)2Mo2O7 (NH4)6Mo7O24.4H2O

CAS Registry 

Number

7631-95-0 27546-07-2 12054-85-2/12027-67-7 

(anhydrous)h

Chemical name Diammonium molybdate Ammonium tetrathiomolybdate

Synonym(s) and 

registered trade 

namesd

Ammonium molybdate; 

molybdic acid, 

diammonium salti

Tiomolibdate diammonium; ammonium molybdenum 

sulfide; ammonium tetrathiomolybdate; thiomolybdic 

acid, diammonium salt; Coprexa; TM; ATTMj

Chemical formula (NH4)2MoO4 (NH4)2MoS4

CAS Registry 

Number

13106-76-8 15060-55-6

aAll information in this column obtained from NLM (2020a), unless otherwise noted. bAll information in this column obtained from NLM (2020b), unless otherwise noted. cAll information in this column obtained from NLM (2020c), unless otherwise noted. dAdditional synonyms and trade names may be queried using the Common Chemistry service from Chemical 

Abstracts Service (http://www.commonchemistry.org/).

eEPA 2019a. f

All information in this column obtained from NLM (2020d), unless otherwise noted. gAll information in this column obtained from NLM (2020e), unless otherwise noted. hOECD 2013. i

EPA 2019b. j

NLM 2019.

CAS = Chemical Abstracts Service
MOLYBDENUM 79

4. CHEMICAL AND PHYSICAL INFORMATION

4.2 PHYSICAL AND CHEMICAL PROPERTIES

Metallic molybdenum, in the form of fine molybdenum powder, is considered nonflammable.

Information regarding the physical and chemical properties of molybdenum and molybdenum compounds 

is provided in Table 4-2. Much of the information presented was obtained from the chapter, Molybdenum 

and Molybdenum Compounds, in the Ullmann's Encyclopedia of Industrial Chemistry (Sebenik et al. 

2012), or handbooks such as the CRC Handbook of Chemistry and Physics or the European Chemicals 

Agency (ECHA) registration dossiers. 
MOLYBDENUM 80

4. CHEMICAL AND PHYSICAL INFORMATION

Table 4-2. Physical and Chemical Properties of Molybdenum and Compoundsa

Property Information

Chemical name Molybdenum Molybdenite (natural 

mineral)/molybdenum 

disulfide

Molybdenum trioxide

Molecular weight 95.94 160.07 143.95

Color Dull gray Black White, turns slightly 

blue in light

Physical state Powder Crystalline solid Crystalline solid

Melting point 2,617°C >1,600°C 

(rhombohedral crystal);

did not melt at 

1,800°Cb

801°C

Boiling point 4,612°C No data 1,155°C 

Density/specific gravity 10.22 g/cm3 5.05 g/cm3 4.692 g/cm3 (21°C)

Odor No data Odorless Odorlessc

Odor threshold:

Water No data No data No data

Air No data No data No data

Solubility:

Water at 25°C Insoluble; 5.5–12 mg/L 

at 20°C and pH 3.5–

4.3d

Insoluble 490 mg/L (28°C)

Organic solvents No data Insoluble Insoluble

Inorganic solvents Dissolved by a mixture 

of concentrated nitric 

and concentrated 

hydrofluoric acids

Dissolves only in

strongly oxidizing acids 

(e.g., aqua regia)

Soluble in aqueous 

alkali and ammonia; 

14,000 mg/L in nitric 

acid (4 mol/L, 20°C)

Partition coefficients:

Log Kow NA NA NA

Log Koc NA NA NA

Vapor pressuree:

at 20°C

at 2,469°C

at 2,721°C

at 3,039°C

at 3,434°C

at 3,939°C

at 4,606°C

No data

7.5x10-3 mm Hg

7.5x10-2 mm Hg

0.75 mm Hg

7.5 mm Hg

75 mm Hg

750 mm Hg

No data No data

Henry's law constant at 25°C NA NA NA

Autoignition temperature NA NA NA

Flashpoint NA NA NA

Flammability limits Not flammable Not flammable Not flammablec

Explosive limits NA NA NA

Conversion factors NA NA NA
MOLYBDENUM 81

4. CHEMICAL AND PHYSICAL INFORMATION

Table 4-2. Physical and Chemical Properties of Molybdenum and Compoundsa

Property Information

Chemical name Sodium molybdate Ammonium 

dimolybdate

Ammonium 

heptamolybdate 

tetrahydrate

Molecular weight 205.92 339.95 1,235.8

Color White White

Physical state Crystalline powderf Solid, powderg Crystalline solidf

Melting point 687°Cf Decomposes from ca. 

150°Cg

Decomposition at

90°Ch

Boiling point Not applicable Decomposes from ca. 

150°Cf

Decomposition at 

90°Ch

Density/specific gravity 3.5 g/cmh 2.97 at 20°Cg 2.86 (20°C)f

Odor Odorlessf Odorlessf Odorlessf

Odor threshold:

Water No data No data No data

Air No data No data No data

Solubility:

Water 40 wt% (anhydrous salt 

in 100 g saturated 

solution, 25°C)

228.4 g/L (20°C, 

pH 6.8)g

206.5 g/L (20°C, 

tetrahydrate)f

Organic solvents No data No data No data

Inorganic solvents No data No data No data

Partition coefficients:

Log Kow NA NA NA

Log Koc NA NA NA

Vapor pressure at 20°C No data No data No data

Henry's law constant at 25°C NA NA NA

Autoignition temperature NA No data NA

Flashpoint No data No data No data

Flammability limits No data Non flammableg No data

Explosive limits NA NA NA

Conversion factors NA NA NA
MOLYBDENUM 82

4. CHEMICAL AND PHYSICAL INFORMATION

Table 4-2. Physical and Chemical Properties of Molybdenum and Compoundsa

Property Information

Chemical name Diammonium molybdate Ammonium tetrathiomolybdate

Molecular weight 196.01 260.28

Color Colorless, white, or slightly greenishyellowishiDeep redh

Physical state Crystalline solidh Crystalline solidh

Melting point No data >300°Cj

Boiling point No data No data

Density 1.4i No data

Odor Odorlessi No data

Odor threshold:

Water No data No data

Air No data No data

Solubility:

Water >10,000 mg/Lk; 39 wt% (in 100 g 

saturated solution, 25°C)

Insoluble (hygroscopic)l

Organic solvents No data No data

Inorganic solvents No data No data

Partition coefficients:

Log Kow NA No data

Log Koc No data No data

Vapor pressure at 25°C No data NA

Henry's law constant at 25°C NA No data

Autoignition temperature NA NA

Flashpoint NA NA

Flammability limits Not flammablei No data

Explosive limits NA NA

Conversion factors NA NA

aAll information was obtained from Sebenik et al. (2012) unless otherwise noted. bCannon 1959.

c

NOAA 2015.

dECHA 2019a. eLide 2005. f

OECD 2013. gECHA 2019b. hHaynes et al. 2014. i

NJDOH 2009. j

Sigma-Aldrich 2015. k

ECHA 2019c. l

Alfa Aesar 2015.

NA = not applicable
MOLYBDENUM 83

CHAPTER 5. POTENTIAL FOR HUMAN EXPOSURE

5.1 OVERVIEW

Molybdenum has been identified in at least 86 of the 1,854 hazardous waste sites that have been proposed 

for inclusion on the EPA National Priorities List (NPL) (ATSDR 2017). However, the number of sites in 

which molybdenum has been evaluated is not known. The number of sites in each state is shown in 

Figure 5-1. Of these sites, all are located within the United States.

Figure 5-1. Number of NPL Sites with Molybdenum Contamination

• The general population is primarily exposed to molybdenum through dietary intake. 

• Inhalation exposure and ingestion of molybdenum from drinking water is typically low for the 

general population; however, water levels near mining operations may be higher and exposure 

may be greater for populations near these activities. 

• Molybdenum compounds (e.g., molybdenum trioxide and polymolybdates) transform to the 

[MoO4]

2- ion under neutral or alkaline conditions; however, protonated forms, such as [HMoO4]-

and H2MoO4, are found at pH <5. 
MOLYBDENUM 84

5. POTENTIAL FOR HUMAN EXPOSURE

Molybdenum is a naturally occurring trace element found extensively in nature. Biologically, 

molybdenum plays an essential role as a micronutrient in plants and animals, including humans. It is also 

used widely in industry for metallurgical applications (EPA 1979). A radioactive isotope of molybdenum 

(

99Mo) is used as a source for producing metastable technetium-99 (99mTc), which is an important 

radiopharmaceutical that is used in the vast majority of high resolution medical imaging tests (Parma 

2009). Important, naturally occurring molybdenum compounds are the minerals molybdenite, powellite, 

wulfenite, ferrimolybdite, and ilsemannite. When in the form of molybdate, a tetrahedral polyatomic 

anion, or other isopolyanions, it can be processed into salts used in industrial applications. The 

molybdate ion is the most common form of molybdenum found in the aqueous environment (EPA 1979).

If released to the atmosphere, molybdenum will be returned to earth by wet and dry deposition. In water, 

pH levels and oxidation/reduction conditions of the sediment govern the speciation of molybdenum and 

adsorption potential in natural aquifers. In the pH range of 3–5, molybdenum tends to exist as hydrogen 

molybdate and is adsorbed to sediment composed of clay and other oxic minerals (Fitzgerald et al. 2008). 

The adsorption and mobility of molybdenum in soils is also inversely correlated with pH. Adsorption of 

molybdenum to 36 surface and subsurface soils was maximized under acidic conditions (pH 2–5) and 

decreased rapidly at pH 5–8 (Goldberg et al. 2002). The availability of molybdenum to plants and 

vegetation is also affected by pH and soil properties. Since adsorption to soil decreases with increasing 

pH, it becomes more bioavailable for uptake to vegetation under nonacidic conditions.

Molybdenum is infrequently detected in ambient air, but is a natural constituent of water and soils. The 

earth’s crust contains an average of 0.0001% (1 ppm) of molybdenum, and surface waters usually have 

molybdenum concentrations of <5 μg/L (EPA 1979). A decade-long study conducted by the U.S. 

Geological Survey (USGS) of >5,000 monitoring and drinking water wells from over 40 major aquifers in 

the United States reported a median molybdenum concentration of 1 μg/L (USGS 2011).

Anthropogenic activities such as mining operations may result in localized areas where molybdenum 

levels greatly exceed background levels.

The primary route of exposure for the general population to molybdenum is through the ingestion of food. 

NAS has estimated that the average dietary intakes (AVDIs) of molybdenum by adult men and women 

are 109 and 76 μg/day, respectively (NAS 2001). Other routes of exposure, such as inhalation of ambient 

air, ingestion of drinking water, and dermal exposure, are negligible for the general population; however, 

they may be important routes of exposure in certain occupational settings such as mining activities and 
MOLYBDENUM 85

5. POTENTIAL FOR HUMAN EXPOSURE

metallurgical applications where molybdenum is used. For example, molybdenum levels in air samples 

of two plants that produce molybdenum salts were 0.5–200 and 0.2–30 mg/m3

, depending upon the 

location of the sample and operation being performed (EPA 1979). Respirable dust samples contained 

molybdenum at levels of 0.471, 1.318, 0.142, and 0.318 mg/m3 during mining, crushing, milling, and 

open pit operations, respectively, at a Colorado mine (EPA 1979).

The extensive nationwide use of radioactive 99Mo in generators that produce 99mTc for nuclear medicine 

imaging scans can expose medical staff and the public in medical facilities to low levels of ionizing 

radiation. The extent of those exposures is limited by U.S. Nuclear Regulatory Commission (USNRC) 

and agreement state regulations (USNRC 2016a, 2016b).

5.2 PRODUCTION, IMPORT/EXPORT, USE, AND DISPOSAL

5.2.1 Production

Molybdenum is a naturally occurring trace element that can be found extensively in nature. Biologically, 

it plays an important role as a micronutrient in plants and animals, including humans. It is also used 

widely in industry for metallurgical applications (USGS 2015a).

Molybdenum does not occur naturally in the pure metallic form, but is in minerals, principally as oxide or 

sulfide compounds (Barceloux 1999; EPA 1979). Important naturally occurring molybdenum compounds 

are the minerals molybdenite (MoS2, the predominant source), powellite, wulfenite, ferrimolybdite, and 

ilsemannite. Molybdenum may also form molybdate, a tetrahedral poly atomic anion, or other 

isopolyanions, which can form salts used in industrial applications. The earth’s crust contains an average 

of 0.0004% (4 ppm) of molybdenum (Sebenik et al. 2012). Deposits that are economically feasible for 

mining contain ≥200 ppm of molybdenum, with lower concentrations obtained as a byproduct of copper 

mining (EPA 1979).

Molybdenite (MoS2) is the principal mineral from which molybdenum is obtained. Mining and milling of 

crude ore produce molybdenite concentrate containing ≥90% of MoS2, almost all of which is converted to 

technical-grade molybdenum trioxide. Molybdenum trioxide is the base material for the production of a 

variety of chemical compounds, ferromolybdenum, and purified molybdenum (EPA 1979).
MOLYBDENUM 86

5. POTENTIAL FOR HUMAN EXPOSURE

Roasting molybdenite concentrate in a multiple hearth furnace at temperatures up to 600°C produces 

technical-grade molybdenum trioxide. This can be further purified by sublimation or selective 

recrystallization at about 1,000–1,100°C (Sebenik et al. 2012).

Worldwide mine production of molybdenum was estimated to be 258,000 metric tons in 2013, with 

approximately 92% produced, in descending order, by China, the United States, Chile, Peru, Mexico, and 

Canada. The United States accounted for 24% of world production with 60,700 metric tons in 2013, 

down slightly from 61,500 metric tons in 2012. Primary molybdenum operations accounted for 53% of 

total U.S. molybdenum production, while byproduct production made up 47% of the total in 2013. All 

U.S. molybdenum concentrates and products are from the mining of ore (USGS 2015a). U.S. production 

of molybdenum increased roughly 8% in 2014 to 65,500 metric tons (USGS 2015b). U.S. production of 

molybdenum for 2018 was 41,900 metric tons (USGS 2019). The USGS Mineral Industry Survey for 

molybdenum reported that domestic production for the first 3 months of 2019 was 3,620 metric tons

(January), 3,420 metric tons (February) and 3,650 metric tons (March) (USGS 2019).

Table 5-1 contains a list the number of facilities per state that produced, processed, or used molybdenum 

trioxide in 2017, as well as information on the amount of molybdenum trioxide on site and related

activities and uses (TRI17 2018). 

Table 5-1. Facilities that Produce, Process, or Use Molybdenum Trioxide

Statea

Number of 

facilities

Minimum 

amount on site 

in poundsb

Maximum 

amount on site 

in poundsb Activities and usesc

AL 5 100 99,999 7, 10, 12

AR 2 100,000 999,999 7

AZ 3 100,000 9,999,999 1, 4, 7, 9

CA 16 1,000 9,999,999 1, 2, 3, 7, 9, 10, 12, 13

CO 1 100,000 999,999 1, 6, 12, 13

DE 1 0 99 12

GA 1 0 0 0

IA 3 1,000,000 9,999,999 1, 3, 4, 6, 9

ID 1 10,000 99,999 12

IL 8 10,000 999,999 1, 5, 6, 7, 10

IN 7 1,000 999,999 1, 5, 6, 7, 8, 9, 10

KS 4 0 999,999 2, 3, 8, 10

KY 5 10,000 999,999 1, 2, 3, 4, 6, 7, 10

LA 27 0 49,999,999 1, 2, 3, 5, 6, 7, 8, 9, 10, 11, 12, 13
MOLYBDENUM 87

5. POTENTIAL FOR HUMAN EXPOSURE

Table 5-1. Facilities that Produce, Process, or Use Molybdenum Trioxide

Statea

Number of 

facilities

Minimum 

amount on site 

in poundsb

Maximum 

amount on site 

in poundsb Activities and usesc

MD 1 100,000 999,999 7

ME 1 10,000 99,999 1, 5, 6

MI 4 0 99,999 1, 5, 6, 7, 10

MN 3 10,000 9,999,999 1, 7, 9, 10, 11, 13

MS 2 10,000 999,999 1, 5, 7, 10

MT 3 10,000 999,999 1, 2, 3, 5, 10, 12, 13

ND 2 10,000 99,999 10

NE 1 10,000 99,999 10

NJ 2 100,000 999,999 10

NM 2 10,000 999,999 10

NV 1 10,000 99,999 12

OH 8 1,000 999,999 1, 6, 7, 8, 9, 11, 13

OK 4 10,000 999,999 1, 4, 5, 7, 10, 13

OR 1 10,000 99,999 7

PA 17 100 9,999,999 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 12

TX 40 0 99,999,999 1, 2, 3, 4, 6, 7, 8, 9, 10, 11, 12

UT 3 100 999,999 10, 11

WA 3 1,000 999,999 7, 10, 11, 12

WI 1 10,000 99,999 10, 11

WV 2 10,000 9,999,999 2, 3, 7, 10

WY 3 10,000 999,999 10

aPost office state abbreviations used.

bAmounts on site reported by facilities in each state. c

Activities/Uses:

1. Produce

2. Import

3. Used Processing

4. Sale/Distribution

5. Byproduct

6. Reactant

7. Formulation Component

8. Article Component

9. Repackaging

10. Chemical Processing Aid

11. Manufacture Aid

12. Ancillary

13. Manufacture Impurity

14. Process Impurity

Source: TRI17 2018 (Data are from 2017)

Molybdenum-99 (99Mo) is a radioactive form of molybdenum and the only molybdenum radioisotope of 

commercial importance. It is produced in nuclear reactors, and then processed, packaged, and shipped to 

medical facilities throughout the world, where the 99Tc progeny into which it transforms is eluted and 

injected into patients for imaging purposes (e.g., cardiac stress tests). 
MOLYBDENUM 88

5. POTENTIAL FOR HUMAN EXPOSURE

99Mo was produced in one of eight nuclear reactors (mainly at the Chalk River complex in Canada) using 

highly enriched uranium, and then commercialized at five processing facilities and six generator 

manufacturing facilities. The availability of those reactors was reduced by the closure of the Chalk River 

facility, and this impacted the supply stream. The United States has established a high national priority 

on assuring an adequate supply of 99Mo and urged manufacturers to switch from using highly enriched 

uranium (HEU) to low enriched uranium (LEU) to reduce the use of HEU for civilian applications 

(Ballinger 2010; The White House 2012; USNRC 2015; Van Noorden 2013). At a NAS symposium in 

2017, several companies discussed their plans to produce 99Mo in the United States (NAS 2018).

Currently, 99Mo can be produced by placing HEU or LEU targets in an operating nuclear reactor and 

allowing the neutron flux to produce 99Mo and its radioactive precursors. The quantity of 99Mo peaks 

after approximately 6 days, at which time, the target is removed, processed, and prepared for shipment. 

New facilities for producing 99Mo from LEU in the United States are being planned (Welsh et al. 2015).

5.2.2 Import/Export

Molybdenum-containing exports rose from 49,900 metric tons in 2010 to 55,300 metric tons in 2014, 

while imports for consumption rose from 19,700 metric tons in 2010 to 23,600 metric tons in 2014 

(USGS 2015b). Imports of molybdenum (excluding ore) were 22,190 metric tons in 2018 and exports 

totaled 44,700 metric tons (USGS 2019). These data, along with U.S. production volumes, are 

summarized in Table 5-2.

Table 5-2. Molybdenum U.S. Production, Import, and Export Data from 2010 to 

2014 and 2018 in Metric Tons

2010a 2011a 2012a 2013a 2014a 2018b

Total U.S. production 59,400 63,700 61,500 60,700 65,500 41,900

U.S. imports for consumption 19,700 21,100 19,800 20,200 23,600 22,219c

U.S. exports for consumption 49,900 56,700 48,900 53,100 55,300 44,700d

aUSGS 2015b. bUSGS 2019. c

Excludes imports of ore and concentrate. dIncludes ores and concentrates.
MOLYBDENUM 89

5. POTENTIAL FOR HUMAN EXPOSURE

5.2.3 Use

Molybdenum is used primarily in metallurgical applications, including as an alloying agent in cast iron, 

steel, and superalloys to enhance properties such as hardenability, strength, toughness, and wear- and 

corrosion-resistance. Molybdenum is commonly used in combination with other alloy metals like 

chromium, cobalt, manganese, nickel, niobium, and tungsten. The leading form of molybdenum used by 

industry, particularly in stainless steel production, is molybdenum trioxide (USGS 2015a).

Molybdenum is used significantly as a refractory metal and molybdenum compounds in a variety of nonmetallurgical chemical applications, such as catalysts, lubricants, and pigments. For example, MoS2 is 

used along with cobalt during the desulfurization process of petroleum (Sebenik et al. 2012). Most 

molybdenum nitride catalysts are nitrogen deficient due to thermodynamically unfavorable conditions at 

atmospheric pressure; however, molybdenum nitride was recently produced in a high temperature and 

pressure environment by solid state ion exchange. Testing found its catalytic activity to be 3 times that of 

MoS2 and its selectivity to hydrogenation to be 3 times that of MoS2 for hydrodesulfurizing dibenzothiophene (Wang et al. 2015). As green technology is becoming more popular, molybdenum has become 

increasingly important in areas like biofuels, catalysts, ethanol, solar panels, and wind power (USGS 

2015a). 

A radioactive isotope of molybdenum, 99Mo, is used as a source to produce the metastable radioisotope 

technetium-99m (99mTc), which is used in the vast majority of medical imaging tests performed today 

(Doll et al. 2014; Parma 2009; Richards 1989). It was estimated that 85% of all medical radioisotope 

procedures use 99mTc and that about 50,000 99mTc-based diagnostic procedures are performed in the 

United States each day, resulting in about 13 million procedures annually (Parma 2009).

Molybdenum concentrate produced by U.S. mines is roasted, exported for conversion, or purified to 

lubricant-grade molybdenum disulfide. Purified MoS2 is used directly as a solid or in coatings that are 

bonded onto the metal surface by burnishing, vapor deposition, or bonding processes that use binders, 

solvents, and mechanochemical procedures (Stiefel 2011). 

Metallurgical applications accounted for about 87% of total molybdenum use in 2013. The principle nonmetallurgical use was in catalysts, primarily catalysts used in petroleum refining. Molybdenum 

compounds are also used to produce pigments (USGS 2015a).
MOLYBDENUM 90

5. POTENTIAL FOR HUMAN EXPOSURE

5.2.4 Disposal

Recycling is the most environmentally acceptable means of disposal for stable molybdenum (USGS 

2015b). Because molybdenum is difficult to remove from waste water, it often is adsorbed to biosolids in 

municipal waste water treatment facilities. Biosolids are beneficial and are often used as fertilizer or 

compost for agricultural applications. In the United States, the land application ceiling limit for 

molybdenum in biosolids is 75 mg/kg (EPA 2018b). 

A 99mTc generator containing a depleted uranium shield or sufficient residual 99Mo radioactivity to be 

considered radioactive can be disposed of by shipping to an authorized licensee following USNRC

agreement state requirements along with those of the Department of Transportation (USNRC 2015). If 

the 99Mo is allowed to decay sufficiently (typically ≥10 half-lives) and the internal shield is lead or 

tungsten, then disposal should follow state and local requirements.

5.3 RELEASES TO THE ENVIRONMENT

The Toxics Release Inventory (TRI) data should be used with caution because only certain types of 

facilities are required to report (EPA 2005). This is not an exhaustive list. Manufacturing and processing 

facilities are required to report information to the TRI only if they employ ≥10 full-time employees; if 

their facility is included in Standard Industrial Classification (SIC) Codes 10 (except 1011, 1081, and 

1094), 12 (except 1241), 20–39, 4911 (limited to facilities that combust coal and/or oil for the purpose of 

generating electricity for distribution in commerce), 4931 (limited to facilities that combust coal and/or 

oil for the purpose of generating electricity for distribution in commerce), 4939 (limited to facilities that 

combust coal and/or oil for the purpose of generating electricity for distribution in commerce), 4953 

(limited to facilities regulated under RCRA Subtitle C, 42 U.S.C. section 6921 et seq.), 5169, 5171, and 

7389 (limited S.C. section 6921 et seq.), 5169, 5171, and 7389 (limited to facilities primarily engaged in 

solvents recovery services on a contract or fee basis); and if their facility produces, imports, or processes 

≥25,000 pounds of any TRI chemical or otherwise uses >10,000 pounds of a TRI chemical in a calendar 

year (EPA 2005).

Molybdenum mining, milling, and smelting, along with its association with uranium mining and milling, 

copper mining and milling, shale oil production, oil refining, and coal-fired power plants, have resulted in 

major releases to the environment (EPA 1979). 
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5. POTENTIAL FOR HUMAN EXPOSURE

5.3.1 Air

Estimated releases of 83,484 pounds (~37.87 metric tons) of molybdenum trioxide to the atmosphere 

from 188 domestic manufacturing and processing facilities in 2017, accounted for about 4.23% of the 

estimated total environmental releases from facilities required to report to the TRI (TRI17 2018). These 

releases are summarized in Table 5-3.

Table 5-3. Releases to the Environment from Facilities that Produce, Process, or 

Use Molybdenum Trioxidea

Reported amounts released in pounds per yearb

Statec RFd Aire Waterf UIg Landh Otheri

Total release

On-sitej Off-sitek

On- and 

off-site

AL 5 59 0 0 8,295 0 4,539 3,815 8,354

AR 2 105 0 0 0 4,398 105 4,398 4,503

AZ 3 7,400 0 0 14,011 5 7,400 14,016 21,416

CA 16 638 43 0 11,305 70,649 10,152 72,484 82,635

CO 1 26 0 0 0 0 26 0 26

DE 1 14 0 0 2 0 14 2 15

GA 1 0 0 0 0 0 0 0 0

IA 3 8,650 6,100 0 5,705 0 15,500 4,955 20,455

ID 1 3 0 0 22,265 0 22,268 0 22,268

IL 8 18,361 2,685 0 16,464 2,449 21,046 18,913 39,959

IN 7 394 12,002 0 420,655 3,091 73,394 362,748 436,142

KS 4 250 0 0 5 0 255 0 255

KY 5 297 0 0 1,071 21 307 1,082 1,390

LA 27 6,761 1,733 81,533 279,468 9,012 258,237 120,270 378,507

MD 1 500 6,500 0 250 0 7,250 0 7,250

ME 1 147 0 0 0 0 147 0 147

MI 4 5 44 0 0 0 49 0 49

MN 3 124 5 0 227 0 129 227 356

MS 2 85 740 0 3,100 0 825 3,100 3,925

MT 3 129 0 0 24 0 129 24 153

ND 2 3 0 0 46 0 3 46 49

NE 1 0 0 0 0 0 0 0 0

NJ 2 0 0 0 13,620 0 0 13,620 13,620

NM 2 3 0 0 0 0 3 0 3

NV 1 1 0 0 42,143 0 42,144 0 42,144

OH 8 281 2,029 50,320 547 83 50,606 2,654 53,259

OK 4 3,501 20 0 0 46,900 3,521 46,900 50,421
MOLYBDENUM 92

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Table 5-3. Releases to the Environment from Facilities that Produce, Process, or 

Use Molybdenum Trioxidea

Reported amounts released in pounds per yearb

Statec RFd Aire Waterf UIg Landh Otheri

Total release

On-sitej Off-sitek

On- and 

off-site

OR 1 17 0 0 1,266 0 1,153 130 1,283

PA 17 25,837 743 0 25,890 340 46,536 6,274 52,810

TX 40 9,616 6,224 54,360 659,999 0 593,414 136,784 730,199

UT 3 12 0 0 60 0 12 60 72

WA 3 260 0 0 7 0 260 7 267

WI 1 0 0 0 0 0 0 0 0

WV 2 0 0 0 0 0 0 0 0

WY 3 5 0 0 0 0 5 0 5

Total 188 83,484 38,868 186,213 1,526,424 136,948 1,159,428 812,509 1,971,937

aThe TRI data should be used with caution since only certain types of facilities are required to report. This is not an 

exhaustive list. Data are rounded to nearest whole number.

bData in TRI are maximum amounts released by each facility.

cPost office state abbreviations are used. dNumber of reporting facilities.

eThe sum of fugitive and point source releases are included in releases to air by a given facility.

f

Surface water discharges, waste water treatment-(metals only), and publicly owned treatment works (POTWs) (metal 

and metal compounds).

gClass I wells, Class II-V wells, and underground injection. hResource Conservation and Recovery Act (RCRA) subtitle C landfills; other onsite landfills, land treatment, surface 

impoundments, other land disposal, other landfills. i

Storage only, solidification/stabilization (metals only), other off-site management, transfers to waste broker for 

disposal, unknown

j

The sum of all releases of the chemical to air, land, water, and underground injection wells.

k

Total amount of chemical transferred off-site, including to POTWs.

RF = reporting facilities; UI = underground injection

Source: TRI17 2018 (Data are from 2017)

The primary source of molybdenum emissions to the atmosphere is coal combustion. In 1970, it was 

estimated that 550 metric tons of molybdenum were released via coal combustion in the United States, in 

comparison to 900 metric tons estimated from all air pollution sources (EPA 1979). A total of 909 metric 

tons of molybdenum can be emitted from a single 1,000 megawatt power plant per year (EPA 1979). 

Historical concentrations of molybdenum in fly ash from coal combustion were reported to range from 

7 to 160 mg/kg (Barceloux 1999). Advances in sorbent and air pollution control technology such as 

fabric filters and electrostatic precipitators in power plants have resulted in a reduction of atmospheric 

emissions of molybdenum and other metals as compared to emissions from decades ago (Cho and Wu 

2004; EPA 2009a). A report from the EPA, which compiled data on 73 coal combustion residues (CCR),
MOLYBDENUM 93

5. POTENTIAL FOR HUMAN EXPOSURE

typically found molybdenum levels of 8–30 μg/g (mg/kg) in fly ash and scrubber sludges and about 1–

10 μg/g (mg/kg) in gypsum (EPA 2009a). The study reported that no correlation was observed in 

molybdenum content and coal type or air pollution control system employed.

5.3.2 Water

Estimated releases of 38,868 pounds (~17.63 metric tons) of molybdenum trioxide to surface water from 

188 domestic manufacturing and processing facilities in 2017, accounted for about 1.97% of the estimated 

total environmental releases from facilities required to report to the TRI (TRI17 2018). This estimate 

includes releases to waste water treatment and publicly owned treatment works (POTWs) (TRI17 2018). 

These releases are summarized in Table 5-3.

Per year, it has been estimated that natural processes result in the release of 3.6x1010 g of molybdenum 

into surface waters (EPA 1979).

Aqueous effluents from industries with a high presence of molybdenum, including molybdenum mining, 

milling, and smelting; uranium mining and milling; copper mining and milling; shale oil production; oil 

refining; and coal-fired power plants, contain molybdenum at concentrations ranging from 100 to 

800,000 μg/L (EPA 1979). Molybdenum levels in leachate samples obtained from a landfill located in 

Caledonia, Wisconsin ranged from 1.28 to 16 μg/L (WDNR 2013).

Effluent concentrations of molybdenum from three molybdenum mining and milling operations (two in 

Colorado, one in New Mexico) ranged on the order of 1,000–10,000 μg/L. In 1972, a mine in Colorado 

released approximately 100,000 kg of molybdenum into a receiving stream. Releases of molybdenum 

from coal power plants to surface waters in the United States average about 1,800 metric tons/year. A 

uranium mill in Colorado reported leaking of the tailings ponds containing 860,000 μg/L molybdenum in 

1965. Some uranium operations in New Mexico reported as much as 1,000 μg/L molybdenum in aqueous 

effluents. Copper milling operations have reported molybdenum effluent concentrations as high as 

30,000 μg/L (EPA 1979). 

Frasacoli and Hudson-Edwards (2018) compiled monitoring data on molybdenum levels in miningaffected areas in different parts of the world, which included groundwaters, nearby rivers, and tailing pore 

water. The largest levels of molybdenum were observed in mine waste from a coal mine located in 

Poland (2,332,000 μg/L). Groundwater from an area near 13 nonactive mines in Mexico ranged from 
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5. POTENTIAL FOR HUMAN EXPOSURE

<5 to 150 μg/L. Tailing pore water from an inactive mining operation in Manitoba, Canada had 

molybdenum levels of <5–1,100 μg/L. Molybdenum concentrations from tailings channel water from an 

active copper mining facility in Chile ranged from 2,670 to 3,900 μg/L. Mine drainage samples obtained 

from an operational mine in Peru had molybdenum levels of 0.001–13.9 ppm (1–13,900 μg/L) 

(Skierszkan et al. 2016). 

5.3.3 Soil

Estimated releases of 1,526,424 pounds (~692.37 metric tons) of molybdenum trioxide to soil from 

188 domestic manufacturing and processing facilities in 2017, accounted for about 77.41% of the 

estimated total environmental releases from facilities required to report to the TRI (TRI17 2018). An 

additional 186,213 pounds (~84.46 metric tons), accounted for about 9.44% of the total environmental 

emissions, were released via underground injection (TRI17 2018). These releases are summarized in 

Table 5-3.

Metals, such as molybdenum, may leach into soil via municipal solid waste incineration bottom ash 

(IMOA 2015).

5.4 ENVIRONMENTAL FATE

5.4.1 Transport and Partitioning

Air. Molybdenum released to the air by industrial processes will be subject to atmospheric deposition 

(IMOA 2015). Deposition from the atmosphere is only a minor source to terrestrial and aquatic 

environments (Fitzgerald et al. 2008).

Water. Molybdenum can be leached into the aquatic environment near industrial use areas via direct 

release or atmospheric wet deposition by rain (IMOA 2015). The pH of water, along with the 

composition and redox conditions of the sediment, greatly affect the speciation and adsorption behavior 

of molybdenum in natural waterbodies. Molybdenum accumulation in the sediment phase is favored 

under conditions of low pH and in sediments with low redox potential and high iron and organic matter 

content (Fitzgerald et al. 2008). In more favorable reducing geochemical conditions, solid-phase iron and 

manganese oxyhydroxides tend to undergo dissolution, and sorbed molybdenum may be released back 

into the water phase.
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5. POTENTIAL FOR HUMAN EXPOSURE

Sediment and Soil. In a seasonally anoxic basin, the distribution of molybdenum in the pore water of 

sediments was relatively uniform. In a perennially oxic basin, however, there was a redistribution of 

molybdenum in the sediment-water interface subsequent to deposition. This was determined to be a 

consequence of adsorption of molybdenum to iron oxyhydroxides at a rate of 36 cm3

/molecule-second in 

the first 1–2 cm depth (IMOA 2015).

Geological uplift and atmospheric deposition result in the molybdenum enrichment of surface soils 

(IMOA 2015). Molybdenum concentrations are found to be the highest in the topsoil layer, due to strong 

binding to natural organic matter. Goldberg et al. (2002) studied the adsorption potential of molybdenum 

as a function of pH on 36 surface and subsurface soil samples from 27 soil series belonging to six 

different soil orders, which provided a wide range of soil physical-chemical characteristics such as 

organic carbon content, cation exchange capacity, and iron content. In general, maximum adsorption 

occurred under acidic pH conditions (pH 2–5) in which molybdenum adsorbed to oxyhydroxide mineral 

surfaces and sorption decreased rapidly from pH 5 to 8 and was minimal in all soils at pH >9. Skierszkan 

et al. (2016) studied the stable isotopic composition of molybdenum and zinc in mine wastes and noted a 

large variation in δ98Mo (a measure of how the isotopic composition in the liquid or solid waste differed 

from a National Institute of Standards and Technology [NIST] standard solution) as a function of 

adsorption. At lower pH, adsorption of molybdenum is greatest, and the molybdenum isotope profile

shifts toward heavier isotopic composition, as the adsorption process preferentially removes lighter 

isotopes. In contrast, zinc has the opposite behavior as it is more mobile under acidic conditions and 

adsorption is enhanced under alkaline conditions with lighter zinc isotopes more prominent. These results 

suggest the possibility of using isotopic composition as a method to understand attenuation mechanisms 

such as adsorption and molybdate precipitation during the weathering process. 

Other Media. As reviewed by Regoli et al. (2012), the bioaccumulation factor (BAF) ranged from 

30.1 to 71.6 (average of 49) in fish exposed to molybdenum levels <65 ug/L. At higher molybdenum 

levels (up to 766 μg/L), the BAF ranged from 0.4 to 9.9 (average 1.4). A laboratory study in rainbow 

trout found a similar inverse relationship between molybdenum concentration in the water and 

bioconcentration factor (BCF) (Regoli et al. 2012). A 60-day exposure to 880 μg/L resulted in tissue 

levels below the limit of detection. Exposure to 11,100 μg/L for 28 days resulted in whole-body 

molybdenum levels of 0.53 mg/kg fish; the calculated average BCF was 0.05. In another study, fish in a 

creek near a molybdenum tailings pile had measured BCFs of <100 after a 2-week exposure (CCME 

1999). The accumulation data show that the BAF decreases with increasing molybdenum levels. At low 
MOLYBDENUM 96

5. POTENTIAL FOR HUMAN EXPOSURE

molybdenum concentrations, there is an active accumulation of essential metals in organisms (and often 

non-essential metals via the same uptake mechanisms) to ensure that metabolic requirements are met. 

This active uptake process decreases when organisms are exposed to higher metal concentrations. At 

higher concentrations, organisms with active regulation mechanisms are even limiting their uptake by 

excretion of excess metals. EPA published a framework for metals risk assessment that discusses the 

difference in interpreting BCF and BAF values for organic versus inorganic compounds (EPA 2017a). It 

was generally concluded that the most recent scientific data on bioaccumulation do not currently support 

the use of BCF and BAF values when applied as generic threshold criteria when assessing the hazardous 

potential of metals. Moreover, single-value BCF/BAF data are most applicable to site-specific 

assessments; for more general regional or national assessments, the media chemistry and metal 

concentrations for a particular species should be considered for BCF/BAF studies. 

5.4.2 Transformation and Degradation

As a naturally occurring trace element, molybdenum can be found extensively in nature. The 

predominant form of molybdenum in natural waters is as the molybdate anion, [MoO4]

2- (Barceloux 

1999), while naturally occurring molybdenum salts are the dominant form in dry environments (EPA 

1979).

Air. No information regarding the chemical forms of molybdenum in the atmosphere and their 

transformations could be located. It is generally assumed that metals, especially those from combustion 

sources, exist in the atmosphere as oxides since metallic species are readily attacked by atmospheric 

oxidants.

Water. The speciation of molybdenum in aqueous media as a function of pH and molybdenum 

concentration, has been thoroughly investigated and reported upon in open literature. As discussed in 

Chapter 4, at pH >6.5, the sole molybdenum species is the molybdate anion, [MoO4]

2- (Cruywagen 2000; 

Cruywagen et al. 2002). Molybdenum compounds transform rapidly into the [MoO4]

2- ion under 

environmentally relevant conditions (Greenwood and Earnshaw 1997). In low redox environments, the 

molybdate anion can be reduced to molybdenum disulfide or molybdenite (Fitzgerald et al. 2008).

Sediment and Soil. Molybdenum is found naturally in soil as the minerals molybdenite, powellite, 

wulfenite, ferrimolybdite, and ilsemannite (EPA 1979; Fitzgerald et al. 2008).
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5. POTENTIAL FOR HUMAN EXPOSURE

The predominant form of molybdenum in wet soil is as the molybdate anion, [MoO4]

2- (Barceloux 1999).

Other Media. No data for the degradation of molybdenum in other media were located.

5.5 LEVELS IN THE ENVIRONMENT

Reliable evaluation of the potential for human exposure to molybdenum depends, in part, on the 

reliability of supporting analytical data from environmental samples and biological specimens. 

Concentrations of molybdenum in unpolluted atmospheres and in pristine surface waters are often so low 

as to be near the limits of current analytical methods. In reviewing data on molybdenum levels monitored 

or estimated in the environment, it should also be noted that the amount of chemical identified 

analytically is not necessarily equivalent to the amount that is bioavailable.

Table 5-4 shows the typical limits of detection that are achieved by commonly employed analytical 

methods in environmental media. Smedley and Kinniburgh (2017) compiled a list of ranges for 

molybdenum in environmental matrices from primary references, including analytical detection limits. 

The American Public Health Association publishes analytical methods for molybdenum and other metals 

in aqueous samples and the EPA publishes laboratory analytical methods and procedures to test for 

analytes in air, water, solids, and hazardous waste. An overview summary of the range of concentrations 

detected in environmental media is also presented in Table 5-5. 

Table 5-4. Lowest Limit of Detection Based on Standardsa

Media Detection limit Reference

Air 0.48 ng/m3 EPA 1999 (Method IO-3.3)

Drinking water 0.3 μg/L EPA 1994 (Method 200.8)

Surface water and groundwater 0.3 μg/L; 8 μg/L APHA 1989 (Method 3120B); EPA 1994 

(Method 200.8)

Soil 0.000090–0.0023 mg/kg Campillo et al 2002 

Sediment 0.000090–0.0023 mg/kg Campillo et al. 2002

Whole blood ~0.1 ng/mL (μg/L) Keyes and Turnland (2002)

aDetection limits based on using appropriate preparation and analytics. These limits may not be possible in all 

situations.
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5. POTENTIAL FOR HUMAN EXPOSURE

Table 5-5. Summary of Environmental Levels of Molybdenum

Media Low High For more information

Outdoor air (μg/m3) 0.2 8.05 Section 5.5.1

Surface water (μg/L) <1 157 Section 5.5.2

Groundwater (μg/L) 1 4,700 Section 5.5.2

Drinking water (μg/L) <1 >40 Section 5.5.2

Food (ppb) <1 1,800 Section 5.6

Soil (mg/kg) <0.05 94.7 Section 5.5.3

Detections of molybdenum in air, water, and soil at NPL sites are summarized in Table 5-6.

Table 5-6. Molybdenum Levels in Water, Soil, and Air of National Priorities List 

(NPL) Sites

Medium Mediana

Geometric 

meana

Geometric 

standard 

deviationa

Number of 

quantitative 

measurements NPL sites

Water (μg/L) 340 229 14.4 16 10

Soil (mg/kg) 57 56.2 0.00794 7 6

Air (μg/m3) 0.0655 0.0515 2.30 4 2

aConcentrations found in ATSDR site documents from 1981 to 2017 for 1,854 NPL sites (ATSDR 2017). Maximum 

concentrations were abstracted for types of environmental media for which exposure is likely. Pathways do not 

necessarily involve exposure or levels of concern.

5.5.1 Air

Molybdenum concentrations in ambient air have been reported to range from below detection limits to 

0.03 mg/m3 (EPA 1979). Concentrations of molybdenum in ambient air of urban areas, 0.01–0.03 μg/m3

, 

are higher than those found in rural areas, 0.001–0.0032 μg/m3 (Barceloux 1999). Data from the EPA Air 

Quality System Database reported 24-hour concentrations of molybdenum at several locations in the 

United States for 2018 (EPA 2018c). These data are summarized in Table 5-7.
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5. POTENTIAL FOR HUMAN EXPOSURE

Table 5-7. 24-Hour Molybdenum Concentrations (μg/m3) in Air Samples 

(2018 Data)

State (sample 

type)

Arithmetic 

mean 99th percentile 75th percentile 50th Percentile 10th Percentile

California (TSP) 0.0007 0.0015 0.0007 0.0007 0.0007

California (TSP) 0.0008 0.0019 0.0007 0.0007 0.0007

California (TSP) 0.0009 0.0016 0.0015 0.0007 0.0007

California (TSP) 0.0015 0.0053 0.0019 0.0007 0.0007

California (TSP) 0.0010 0.0023 0.0015 0.0007 0.0007

California (TSP) 0.0030 0.0063 0.0042 0.0028 0.0007

California (TSP) 0.0037 0.0068 0.0047 0.0038 0.0007

California (TSP) 0.0008 0.0016 0.0007 0.0007 0.0007

California (TSP) 0.0009 0.0023 0.0007 0.0007 0.0007

California (TSP) 0.0008 0.0019 0.0007 0.0007 0.0007

California (TSP) 0.0008 0.0021 0.0007 0.0007 0.0007

California (TSP) 0.0013 0.0042 0.0015 0.0007 0.0007

California (TSP) 0.0010 0.0026 0.0015 0.0007 0.0007

California (TSP) 0.0010 0.0025 0.0015 0.0007 0.0007

California (TSP) 0.0015 0.0046 0.0019 0.0007 0.0007

California (TSP) 0.0007 0.0014 0.0007 0.0007 0.0007

California (PM10) 2.0000 2.0000 2.0000 2.0000 2.0000

California (PM10) 2.0000 2.0000 2.0000 2.0000 2.0000

California (PM10) 0.0002 0.0020 0.0000 0.0000 0.0000

California (PM2.5) 0.0003 0.0030 0.0000 0.0000 0.0000

California (PM2.5) 0.0001 0.0030 0.0000 0.0000 0.0000

California (PM2.5) 0.0003 0.0030 0.0000 0.0000 0.0000

California (PM2.5) 0.0003 0.0030 0.0000 0.0000 0.0000

California (PM2.5) 0.0003 0.0020 0.0000 0.0000 0.0000

California (PM2.5) 0.0013 0.0043 0.0017 0.0010 0.0004

Michigan (TSP) 0.0011 0.0026 0.0016 0.0010 0.0002

Michigan (TSP) 3.0787 49.5000 1.4000 0.9000 0.5000

Michigan (PM10) 0.9935 2.2000 1.5000 0.9000 0.4000

Michigan (PM10) 1.0057 2.6000 1.4000 0.8000 0.4000

Michigan (PM10) 1.1778 2.4000 1.5000 1.1000 0.4000

Texas (PM2.5) 0.0005 0.0040 0.0010 0.0000 0.0000
MOLYBDENUM 100

5. POTENTIAL FOR HUMAN EXPOSURE

Table 5-7. 24-Hour Molybdenum Concentrations (μg/m3) in Air Samples 

(2018 Data)

State (sample 

type)

Arithmetic 

mean 99th percentile 75th percentile 50th Percentile 10th Percentile

Texas (PM2.5) 0.0006 0.0030 0.0010 0.0000 0.0000

Texas (PM2.5) 0.0006 0.0020 0.0010 0.0000 0.0000

Vermont (PM2.5) 0.0726 0.4700 0.1100 0.0400 0.0100

Vermont (PM10) 0.0655 0.2700 0.1100 0.0400 0.0000

Vermont (PM10) 0.1955 0.3800 0.2700 0.1500 0.0900

Vermont (PM10) 0.1787 0.5900 0.3000 0.1000 0.0400

PM10 = particulate matter ≤10 μm in diameter; PM2.5 = particulate matter ≤2.5 μm in diameter; TSP = total 

suspended particulate

Source: EPA 2018c

5.5.2 Water

It has been reported that concentrations of molybdenum are generally <1.0 μg/L in surface waters (USGS 

2006) and 1.0 μg/L in drinking water (USGS 2011). Groundwaters contain about 1.0 μg/L (USGS 2011). 

Smedley and Kinniburgh (2017) compiled ranges of molybdenum levels in rain water, stream water, 

rivers, lakes, estuaries, and oceans. Most surface water levels were <1 μg/L; however, there was wide 

variability, with levels tending to be higher with increasing salinity of the body of water (for example, 

molybdenum levels in the Salton Sea, California were reported as high as 37 μg/L). 

A USGS study of surface water from 51 of the nation’s major river basins was conducted from 1991 to 

2002 (USGS 2006). The median concentration of molybdenum in 2,773 surface water samples was 

<1.0 μg/L, with a maximum concentration of 157 μg/L. There were eight samples (approximately 0.29% 

of the total) that exceeded the health-based screening level of 40 μg/L for molybdenum.

In a study of surface waters collected from 197 sampling stations in Colorado, molybdenum was found at 

concentrations <10 μg/L in 87% of the 299 samples. Samples that contained concentrations >5 μg/L 

were concluded to be the result of proximity to mineralization or mining and milling operations (EPA 

1979). However, another study comparing surface waters draining highly mineralized areas to those with 

baseline molybdenum areas found that molybdenum mineralization did not contribute significantly to 
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concentrations in surface waters. The waters from streams draining the highly mineralized areas rarely 

had molybdenum concentrations above 1–2 μg/L (EPA 1979).

Huang et al. (2010) discussed surface water concentrations of metals including molybdenum in the 

Gyama Valley, an area impacted by four metal mining operations. Molybdenum concentrations ranged 

from <0.6 (detection limit) to 10.4 μg/L in the Gyamaxung-chu stream/river. 

DOI (1967) collected river and lake water samples from 100 sampling stations around the United States 

from 1962 to 1967. The samples were taken from areas susceptible to contamination, including highly 

populated areas, industrial areas, recreational use areas, and state and national boundaries. Molybdenum 

was detected in the water samples at maximum concentrations >100 μg/L at 38 of the sample sites, while 

26 sites had mean molybdenum concentrations >50 μg/L.

Molybdenum levels of 9.3–10.4 μg/L for open oxic seawater and 0.67–3.74 μg/L in euxinic waters of the 

Black Sea were reported (Smedley and Kinniburgh 2017). Kulathilake and Chatt (1980) reported the 

molybdenum concentration in the Atlantic Ocean as 7.2–7.9 μg/L. Another study reported that the 

molybdenum concentration in the North Atlantic ranged from 0.5 to 1.0 μg/L (Chan and Riley 1966). In 

the Pacific Ocean, measured molybdenum concentrations included 8.8 μg/L in the Eastern Pacific 

(Kiriyama and Kuroda 1984) and 1.5 μg/L in the Western Pacific (Nakata et al. 1983). Kawabuchi and 

Kuroda (1969) reported a mean molybdenum concentration of 7.7 μg/L in Tokyo Bay. Molybdenum 

concentrations measured in the English Channel ranged from 12 to 16 μg/L (Chan and Riley 1966), while 

the Irish Sea was reported to have a mean molybdenum concentration of 8.4 μg/L (Riley and Taylor 

1968). 

A comprehensive groundwater monitoring study conducted from 1992 to 2003 by the USGS of 

5,183 monitoring and drinking-water wells representative of over 40 principal aquifers in humid and dry 

regions and in various land-use settings reported that the median concentration of molybdenum in 

3,063 samples was 1.0 μg/L, with a maximum value of 4,700 μg/L (USGS 2011). Approximately 1.5% 

of the groundwater samples had molybdenum levels exceeding the health-based screening level of 

40 μg/L (USGS 2011). Levels of molybdenum tended to be greatest in glacial unconsolidated sand and 

gravel aquifers as compared to other major aquifer groups in the study.

A report issued by the Wisconsin Department of Natural Resources found elevated levels of molybdenum 

in private supply wells and groundwater monitoring wells near the We Energies Oak Creek power plant 
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located in Caledonia, Wisconsin (WDNR 2013). Molybdenum levels in 21 private well samples 

exceeded the state of Wisconsin groundwater enforcement standard of 40 μg/L. It was not determined 

whether the elevated levels of molybdenum were naturally occurring or were a consequence of the 

activities of the power plant and the coal ash fill areas located nearby the plant.

In January of 2017, the EPA published the final results of the third Unregulated Contaminant Monitoring 

Rule (UCMR 3) program. Molybdenum levels >1 μg/L were measured in 25,377 out of 62,981 analyzed 

drinking water samples, and 151 samples had levels greater than the health-based screening level of 

40 μg/L. In 40 of the 4,922 public water systems tested, at least one measurable level above 40 μg/L was 

found (EPA 2017b). Concentrations as high as 1,400 μg/L have been detected in drinking waters in areas 

impacted by mining and milling operations (USGS 2011).

In a study of finished drinking water supplies from the 100 largest cities in the United States in 1964, 

median and maximum molybdenum concentrations of 1.4 and 68 μg/L, respectively, were reported 

(USGS 1964). Another study reported a mean molybdenum concentration of 8 μg/L in samples collected 

from 161 drinking water sources from 44 states in the United States (Hadjimarkos 1967). Molybdenum 

levels measured onsite at 12 public water facilities across England and Wales ranged from below the 

detection limit (0.03 μg/L) to 1.51 μg/L over an 18-month collection period (Smedley et al. 2014). 

Corresponding molybdenum levels in tap water from 24 residences in three towns (North Wales, the 

English Midlands, and South East England) served by these public water facilities ranged from <0.03 to 

1.00 μg/L. The study indicated that there was little variability in molybdenum concentrations when

comparing levels in tap water versus respective water supply facilities, construction ages of the residences 

(i.e., new homes versus older homes), and pre-flush versus post-flush tap samples, suggesting that water 

distribution pipework has a negligible effect on supplied tap water levels of molybdenum.

Drinking water may also be affected by industrial contamination, as water treatment facilities are 

ineffective at removing molybdenum from source waters. In tap waters samples collected in 1971 from 

Golden, Colorado, a community that derives its water supply from a stream draining a molybdenum mine 

and mill, the mean molybdenum concentration was reported to be 440 μg/L. However, after the mine 

closed in 1974, the mean concentration in drinking water samples decreased to 150 μg/L by January 

1975, 60 μg/L by June 1975, and 30 μg/L by 1977 (EPA 1979).
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5.5.3 Sediment and Soil

Globally, most soils contain molybdenum at concentrations between 0.6 and 3.5 mg/kg, although total 

concentrations in soils can vary widely depending on geological composition, soil horizon, or industrial 

contamination. Statistical analysis of 4,841 samples of soil collected from a depth of 0–5 cm in the 

conterminous United States showed molybdenum levels ranging from <0.05 to 75.7 mg/kg (USGS 2014). 

The 5th, 25th, 50th, 75th, and 95th percentile concentrations were 0.24, 0.51, 0.78, 1.14, and 2.27 mg/kg, 

respectively (USGS 2014). From 4,780 samples of C horizon (substratum) soils in the United States, the 

molybdenum levels were reported as ranging from <0.05 to 94.7 mg/kg and the 5th, 25th, 50th, 75th, and 

95th percentile concentrations were 0.20, 0.51, 0.83, 1.27, and 2.88 mg/kg, respectively (USGS 2014). A 

review of 25,673 deep soil samples from the British Geological Survey reported molybdenum 

concentrations of <0.6–885 mg/kg, with a median value of 1.4 mg/kg (Smedley and Kinniburgh 2017). 

The Forum of European Geological Surveys (FOREGS), under the International Union of Geological 

Sciences/International Association of Geochemistry (IUGS/IAGC) Global Geochemical Baselines 

Programme, collected 840 topsoil samples from 26 European countries and reported molybdenum 

concentrations ranging from <0.1 to 21.3 mg/kg (mean 0.943 mg/kg) (FOREGS 2005).

Above average molybdenum soil concentrations may occur in areas containing molybdenum-rich rock 

formations or in areas of industrial contamination. Natural sources sampled, including soils covering a 

mineralized area, soil derived from a marine black shale, alluvial soils on the eastern footslopes of Sierra 

Nevada, and soils formed from volcanic ash in Kauai, Hawaii, contained mean molybdenum 

concentrations of 76, 12, 17.4, and 14.9 mg/kg, respectively. Soils sampled near industrial 

contamination, such as soils downstream from a molybdenum mine and mill in Colorado, soil irrigated 

with water contaminated by a uranium mill, and soils 2 miles from a molybdenum smelter in 

Pennsylvania, had mean molybdenum concentrations of 59, 61, and 29 mg/kg, respectively (EPA 1979).

Typical molybdenum concentrations found in stream sediments were reported to range from 1 to 5 mg/kg 

(EPA 1979). Sediments in streams that drain water from natural deposits of molybdenum in the United 

States have been reported to have molybdenum concentrations ranging from 10 to 200 ppm (10–

200 mg/kg). Another study reported molybdenum levels of up to 300 mg/kg in sediments derived from 

black marine shales in England. Stream sediment collected from water below a molybdenum mine and 

mill in Colorado had molybdenum concentrations ranging from 50 to 1,800 mg/kg (mean of 530 mg/kg). 

Molybdenum content in stream sediments have been shown to reflect mineralization, as the concentration 

increases with decreasing sediment grain size (EPA 1979). FOREGS collected 848 freshwater sediment 
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samples from 26 European countries and reported molybdenum concentrations ranging from 0.12 to 

117 mg/kg (mean 1.34 mg/kg) (FOREGS 2005). An analysis of 65,477 stream sediments in the British 

Geological Survey G-Base reported a range of molybdenum concentrations of <0.1–309 mg/kg, with a 

median of 0.4 mg/kg (Smedley and Kinniburgh 2017). Sediment samples collected from river/streams in 

Tibet close in proximity to mining operations had molybdenum levels of 9.1–20.8 mg/kg (Huang et al. 

2010). 

5.5.4 Other Media

In a study detecting and comparing trace elements in the milk of guinea pigs (n=87), dairy cattle (n=48), 

horses (n=35), and humans (n=84), the average molybdenum concentrations measured were 26, 22, 16, 

and 17 μg/L, respectively (Anderson 1992). Average concentrations of molybdenum detected in six 

kinds of milk, including cow’s milk-based formula, breast milk, soya milk, bottled milk, dried milk, and 

evaporated milk, were 18, 4, 160, 34, 35, and 29 μg/L, respectively (Biego et al. 1998). Most of the 

molybdenum is in the cream fraction (Archibald 1951).

Food derived from aboveground plants, such as legumes, leafy vegetables, and cauliflower generally has 

a relatively higher concentration of molybdenum in comparison to food from tubers or animals. Beans, 

cereal grains, leafy vegetables, legumes, liver, and milk are reported as the richest sources of 

molybdenum in the average diet (Barceloux 1999).

Typical concentrations of molybdenum in plants are 1–2 mg/kg; however, a range of tenths to hundreds 

of mg/kg have been reported (EPA 1979). Tobacco contains molybdenum concentrations of 0.3–

1.76 mg/kg (Barceloux 1999).

5.6 GENERAL POPULATION EXPOSURE

Molybdenum exposure to the general population via ambient air and drinking water is expected to be 

negligible compared with exposure through food (Barceloux 1999). Molybdenum does not occur 

naturally in the pure metallic form. It is principally found as oxide or sulfide compounds (Barceloux 

1999; EPA 1979). Therefore, almost all exposure is to a molybdenum compound rather than the metal 

alone. The average dietary intake of molybdenum in the United States by adult men and women are 

109 and 76 μg/day, respectively (NAS 2001). A study of the dietary intake of adult residents in Denver, 

Colorado reported a mean molybdenum ingestion rate of 180 μg/day (range 120–240 μg/day) (Barceloux 
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1999). Daily intakes ranged from 74 to 126 μg molybdenum in a study of older children and adults in the 

northeastern United States (Barceloux 1999).

The European Food Safety Authority (EFSA) used dietary intake studies to derive estimates of which 

foods were most responsible for molybdenum intake in European populations (EFSA 2013). Cereals and 

cereal-based products (including bread) are the largest contributors to molybdenum intake in a Western 

diet; these products contribute one-third to one-half of the total molybdenum intake. Other contributors to 

total molybdenum intake include dairy products and vegetables.

A summary of molybdenum concentrations positively identified in foods analyzed during the FDA Total 

Diet Study (TDS) of 2006–2011 and 2013–2014 is summarized in Table 5-8 (FDA 2017). The data for 

molybdenum arose from Market Basket Surveys conducted in 2010 and 2011 and 2013–2014, in which 

382 store-bought foods purchased in four geographic regions of the United States (northeast, southeast, 

central, and west) were analyzed. Only those food items in which the molybdenum content of at least one 

sample was above the detection limit of the analytical method are reported. Another survey of levels of 

molybdenum in food found the highest molybdenum concentrations in legumes; grains and grain 

products; nuts; meat, fish, and poultry (including liver); eggs; and milk, yogurt, and cheese (76.7, 30.0, 

29.5, 8.9, 6.3, and 4.6 μg/100 g, respectively) (Pennington and Jones 1987). 

Table 5-8. Molybdenum Levels Detected in Foods in the 2006–2011 and 2013–

2014 Market Basket Surveysa

Food

Number of 

samples

Positive 

detections

Mean 

(mg/kg)

Median 

(mg/kg)

Maximum 

(mg/kg)

LOD 

(mg/kg)

LOQ 

(mg/kg)

Liver (beef/calf), pan-cooked 

with oil

8 8 1.500 1.400 1.700 0.700 3.000

Pinto beans, dry, boiled 8 8 1.300 1.270 1.600 0.700 3.000

Pork and beans, canned 8 1 0.088 0 0.700 0.700 3.000

Peanut butter, 

smooth/creamy

8 3 0.508 0 1.900 0.900 3.000

Shredded wheat cereal 8 5 0.554 0.883 0.984 0.700 3.000

Raisin bran cereal 8 1 0.088 0 0.701 0.700 3.000

Crisped rice cereal 8 8 0.898 0.837 11.300 0.700 3.000

Granola with raisins 8 6 0.589 0.772 0.815 0.700 3.000

Oat ring cereal 8 8 1.300 1.300 1.400 0.700 3.000

Collards, fresh/frozen, boiled 8 2 0.262 0 1.600 0.500 2.000

Chili con carne with beans, 

canned 

8 2 0.179 0 0.730 0.700 3.000

Refried beans, canned 8 2 0.254 0 1.100 0.800 3.000
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Table 5-8. Molybdenum Levels Detected in Foods in the 2006–2011 and 2013–

2014 Market Basket Surveysa

Food

Number of 

samples

Positive 

detections

Mean 

(mg/kg)

Median 

(mg/kg)

Maximum 

(mg/kg)

LOD 

(mg/kg)

LOQ 

(mg/kg)

White beans, dry, boiled 8 8 1.100 1.116 1.800 0.700 3.000

Granola bar, with raisins 8 1 0.164 0 1.300 0.800 3.000

Candy bar, chocolate, 

nougat, and nuts 

8 1 0.115 0 0.922 0.800 3.000

aTrace values were defined as results ≥LOD and <LOQ. Results ≥LOD and <LOQ (trace values) were used as 

reported when calculating the means.

LOD = limit of detection; LOQ = limit of quantification

Source: FDA 2017 (Data were initially released in 2014 and revised April 2017.)

Molybdenum is an essential dietary element and is often included in nutritional supplements. Based on 

data from NHANES, the median molybdenum intake from dietary supplements was about 23 and 

24 μg/day for men and women who reported supplement use, respectively. Dietary supplements 

generally contain molybdenum in the form of sodium molybdate or ammonium molybdate (Momcilovic 

1999; NAS 2001), although the molybdenum can also be in the form of molybdenum chloride, 

molybdenum glycinate, and molybdenum amino acid chelate (NIH 2019).

It was reported in 1979 that in the United States, the average human intake of molybdenum via drinking 

water was <5 μg/day (EPA 1979). Drinking water coming from sources close to areas with high 

molybdenum contamination from industrial effluents may contain a higher concentration of molybdenum 

(>50 μg/L) (EPA 1979).

Urinary levels of molybdenum were measured for the U.S. population from NHANES studies from 1999 

to 2016 (CDC 2019) and are summarized in Table 5-9.

Table 5-9. Urinary Molybdenum Levels in U.S. Adults 

Survey years Geometric mean 50th percentile 95th percentile Sample size

Urinary molybdenum (μg/L)a

1999–2000 41.7 (36.7–47.4) 46.6 (40.5–52.5) 168 (143–206) 1,299

2001–2002 41.1 (38.3–44.1) 47.6 (43.7–51.2) 150 (130–166) 1,560

2003–2004 35.9 (34.0–38.0) 40.3 (37.6–42.1) 133 (119–144) 1,543

2005–2006 41.3 (38.7–44.0) 46.0 (41.7–49.6) 153 (135–171) 1,520

2007–2008 40.8 (38.7–43.0) 44.5 (42.2–47.8) 152 (145–164) 1,857
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Table 5-9. Urinary Molybdenum Levels in U.S. Adults 

Survey years Geometric mean 50th percentile 95th percentile Sample size

2009–2010 39.6 (37.5–41.8) 42.0 (39.8–43.9) 144 (130–163) 2,019

2011–2012 34.1 (31.8–36.5) 37.3 (33.6–39.8) 136 (120–146) 1,715

2013–2014 30.8 (28.58–33.3) 32.7 (28.3–36.0) 129 (116–137) 1,811

2015–2016 32.0 (29.9–34.1) 35.9 (33.1–37.8) 124 (112–136) 1,793

Creatinine corrected urinary molybdenum (μg/g creatinine)

1999–2000 39.6 (36.9–42.6) 38.5 (36.1–41.0) 122 (116–147) 1,299

2001–2002 39.3 (36.8–42.0) 39.6 (36.4–42.1) 123 (109–139) 1,559

2003–2004 36.9 (35.0–38.9) 37.0 (35.7–38.4) 118 (101–134) 1,543

2005–2006 41.2 (39.3–43.1) 40.5 (38.8–42.7) 119 (103–132) 1,520

2007–2008 43.5 (42.1–44.9) 42.9 (41.3–44.7) 122 (110–132) 1,857

2009–2010 41.9 (40.0–43.9) 41.2 (39.4–43.0) 127 (115–141) 2,019

2011–2012 38.6 (37.5–42.2) 40.0 (36.0–43.6) 118 (108–131) 1,261

2013–2014 35.9 (33.7–38.2) 36.9 (35.1–38.4) 97.8 (88.5–111) 1,810

2015–2016 34.9 (33.3–36.6) 36.3 (34.4–38.1) 97.4 (85.5–102) 1,791

aLimit of detection for survey years 1999–2001, 2001–2002, 2003–2004, 2005–2006, 2007–2008, 2009–2010,

2011–2012, 2013–2014, 2015–2016 were 0.8, 0.8, 1.5, 0.92, 0.92, 0.99, 0.8, and 0.8 μg/L, respectively.

Source: CDC 2019

Paschal et al. (1998) analyzed the levels of molybdenum and 12 other metals in the urine of 496 residents 

of the United States obtained from the NHANES III survey conducted from 1988 to 1994. The 

specimens randomly selected were from a broad spectrum of the population (e.g., both urban and rural 

communities, both males and females, and persons aged 6–88 years from all major ethnicities). The 

geometric mean molybdenum concentration of the samples was 46.8 μg/L and the 25th, 50th, 75th, and 

95th percentiles were 27.9, 56.5, 93.9, and 168.0, μg/L, respectively. The creatinine-adjusted 25th, 50th, 

75th, and 95th percentiles were 30.9, 45.7, 64.3, and 133.8 μg/g, respectively, with a geometric mean of 

39.6 μg/g. 

Molybdenum levels in whole blood are typically <5 ng/mL in the general population; however, blood 

samples from persons from areas with natural molybdenum deposits or from molybdenum mining areas 

may have concentrations of up to 150 μg/mL (Barceloux 1999).

Blood samples collected from 18 miners at a molybdenum mine in New Mexico had plasma molybdenum 

levels <5 μg/L in 12 of the 18 samples and 6–18 μg/L in the remaining 6 samples. The concentration of 

molybdenum in urine collected from 11 of the miners ranged from 20 to 74 μg/L. It was noted that 
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5. POTENTIAL FOR HUMAN EXPOSURE

molybdenum levels in urine and blood of miners mainly exposed to molybdenite may not be above 

average, since molybdenite is a relatively insoluble compound (EPA 1979).

In a survey of a molybdenite mining, crushing, and milling operation in Colorado, mean molybdenum 

levels in respirable dust samples were 0.471, 1.318, 0.142, and 0.318 mg/m3 during mining, crushing, 

milling, and open pit operations, respectively (EPA 1979). In settled dust and air samples collected from 

a molybdenum smelting operation, concentrations of molybdenum, in the form of molybdenum trioxide, 

were 57–61% and 3–33 mg/m3

, respectively (EPA 1979). Forty air samples collected above a crucible in 

a molybdenum trioxide smelting plant contained a mean molybdenum concentration of 0.22 mg/m3

, while 

air samples collected in the breathing zone of workers had molybdenum concentrations ranging from 

1.4 to 5.4 mg/m3 (EPA 1979). The air concentrations of molybdenum in two plants that produce 

molybdenum salts were 0.5–200 and 0.2–30 mg/m3 (EPA 1979). More recent monitoring data for mining 

and milling operations were not located; current levels may be lower due to possible changes in 

occupational standards, engineering and administrative controls, and personal protective equipment 

requirements.

Workers involved in metal refining and metal working may be exposed to airborne particulates containing 

molybdenum. In a study assessing the exposure of a group of 20 workers performing welding, polishing, 

and assembly of stainless steel constructions, molybdenum was detected in personal air samplers at 

concentrations of 0.27–9.7, 0.03–4.2, and 0.14–0.60 μg/m3

, respectively. Stationary air samplers in the 

facility detected course (equivalent aerodynamic diameter [EAD] 2–10 μm) and fine (EAD <2 μm) 

molybdenum particles at concentrations of 0.015–0.087 and 0.093–0.54 μg/m3

, respectively (Kucera et al. 

2000).

The National Occupational Exposure Survey (NOES) conducted by NIOSH in 1983 estimated that 

245,024 workers employed at 15,996 facilities were potentially exposed to molybdenum (pure, powder, 

and unknown forms) in the United States (RTECS 2009). The NOES database does not contain 

information on the frequency, concentration, or duration of exposure; the survey provides only estimates 

of workers potentially exposed to chemicals in the workplace.

The extensive nationwide use of radioactive 99Mo in generators that produce 99mTc for nuclear medicine 

imaging scans can expose medical staff and the public in medical facilities to low levels of ionizing 

radiation. The extent of those exposures is limited by the USNRC and agreement state regulations 

(USNRC 2016a, 2016b).
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Breast milk and infant formula are the primary sources of molybdenum in infants aged 0–6 months (NAS 

2001). The primary source of dietary molybdenum intake among children in the United States is milk 

(EPA 1979). Several studies have measured molybdenum levels in human breast milk; average 

molybdenum levels ranged from 1.5 to 17 μg/L (Anderson 1992; Aquilio et al. 1996; Biego et al. 1998; 

Bougle et al. 1988). As shown in Table 5-10, the highest molybdenum concentrations occur within the 

first week after birth and tend to be higher in the mothers of term infants, as compared to preterm infants 

(Aquilio et al. 1996; Bougle et al. 1988). 

Table 5-10. Molybdenum Levels in Breast Milk in Mothers of Term and Preterm 

Infants

Molybdenum levels in breast milk (μg/L)

Lactation day Term infants Preterm infants Reference

2–6 6.8 3.9a Aquilio et al. 1996

12–16b 5.7 2.4a

21c 3.6 1.9a

3–5 10.2 4.0a Bougle et al. 1988

7–10d 4.8 3.7

14d 1.5 1.4

30d 2.6 1.9

60e No data 1.2

aSignificantly different from term infant levels (p<0.05).

bSignificantly different from molybdenum concentration at 2-6 days (p<0.01). cSignificantly different from molybdenum concentration at 2-6 days (p<0.05). dSignificantly different from molybdenum concentration for whole group at 3-5 days (p<0.01). eSignificantly different from molybdenum concentration at for whole group at 3-5 days (p<0.05).

Krachler and colleagues studied the trace element concentrations in human milk during the course of 

lactation (Krachler et al. 1998; Rossipal and Krachler 1998). In total, 79 samples of human milk from 

46 healthy mothers were sampled in Austria in 1995 and 1996 at 1–293 days after the mothers gave birth 

(Rossipal and Krachler 1998). In colostrum milk (1–3 days postpartum), the molybdenum concentration 

was 8.88±3.74 μg/L. In samples collected 42–60 days postpartum, the concentration was 

1.43±1.77 μg/L, and at 97–293 days, the milk contained 1.78±1.62 μg/L. In a later study, the same group 

analyzed a further set of samples of colostrum milk only (Krachler et al. 1999). Previous results were 

confirmed, with the mean concentration being reported as 7.0±3.8 μg/L (median 5.7 μg/L, range 3.4–

18.8 μg/L). Another study from Europe reported molybdenum concentrations in human milk 

(Wappelhorst et al. 2002). In samples taken daily in 2002 from 19 mothers from Germany, Poland, and 
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the Czech Republic, for sample periods between 2 and 8 weeks for each mother and covering weeks 3–68 

of lactation, the median molybdenum concentration was 0.53 μg/L (mean 0.72 μg/L, range 0.27–

1.61 μg/L). Data on molybdenum concentrations in human milk are also available from Japan (Hattori et 

al. 2004). In 17 samples provided by three mothers during days 96 and 327 after delivery, the 

molybdenum concentrations ranged from 1.97 to 8.93 μg/L, with an estimated average of ~4.3 μg/L 

(estimated from three median values given for the individual mothers). In comparison, the concentration 

of molybdenum in formula milk after preparation is reported as 2.38±0.75 μg/L (n=6). In Canada, 

20 samples of mother’s milk were analyzed for molybdenum in the context of a balance study on lowbirth-weight infants on parenteral and enteral nutrition (Friel et al. 1999b). The median molybdenum 

concentration is reported as 5 μg/L, with a range of 2.1–23 μg/L.

Urinary levels of molybdenum in children 6–11 and 12–19 years old were measured during the NHANES 

study assessing exposure from 1999 to 2016 (CDC 2019) and in children 3–5 years of age during 

NHANES 2015–2016; these data are summarized in Table 5-11. 

Table 5-11. Urinary Molybdenum Levels in U.S. Children and Adolescents 

Survey years Geometric mean 50th percentile 95th percentile Sample size

Urinary molybdenum (μg/L)a

1999–2000

Age 6–11 years 78.2 (61.0–100) 84.8 (67.7–105) 267 (159–840) 310

Age 12–19 years 54.3 (47.6–62.0) 60.6 (52.2–70.3) 188 (146–216) 648

2001–2002

Age 6–11 years 63.3 (53.4–75.0) 69.2 (63.0–77.6) 197 (161–291) 368

Age 12–19 years 60.6 (55.5–66.2) 65.7 (58.7–73.1) 179 (155–227) 762

2003–2004

Age 6–11 years 62.2 (56.7–68.3) 71.3 (55.7–84.1) 181 (138–235) 290

Age 12–19 years 52.5 (49.0–56.3) 59.6 (55.5–65.1) 143 (130–156) 725

2005–2006

Age 6–11 years 65.6 (56.6–76.0) 73.5 (62.8–85.5) 181(154–205) 355

Age 12–19 years 59.1 (53.7–65.1) 63.8 (57.9–69.4) 173 (148–202) 701

2007–2008

Age 6–11 years 69.3 (60.8–79.0) 72.8 (62.1–83.9) 235 (169–282) 394

Age 12–19 years 64.1 (58.6–70.2) 68.6 (63.7–80.2) 174 (151–196) 376

2009–2010

Age 6–11 years 65.0 (57.8–73.2) 69.7 (61.1–84.2) 218 (180–263) 378

Age 12–19 years 52.4 (47.5–57.7) 58.5 (51.4–65.6) 178 (151–201) 451
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Table 5-11. Urinary Molybdenum Levels in U.S. Children and Adolescents 

Survey years Geometric mean 50th percentile 95th percentile Sample size

2011–2012

Age 6–11 years 58.4 (51.5–66.2) 65.1 (52.8–74.5) 211 (187–283) 399

Age 12–19 years 46.4 (40.2–53.7) 51.0 (44.2–64.8) 163 (145–173) 390

2013–2014

Age 6–11 years 51.7 (47.1–56.6) 54.7 (49.3–61.9) 182 (159–210) 402

Age 12–19 years 48.2 (41.5–55.8) 55.9 (49.0–64.8) 156 (136–180) 451

2015–2016

Age 3–5 years 47.3 (43.9–50.8) 51.6 (45.8–61.1) 191 (146–218) 486

Age 6–11 years 56.2 (51.3–61.5) 57.5 (50.2–69.1) 173(165–224) 379

Age 12–19 years 47.7 (43.3–52.6) 53.0 (45.9–57.0) 149 (135–166) 402

Creatinine corrected urinary molybdenum (μg/g creatinine)

1999–2000

Age 6–11 years 85.9 (73.7–100) 79.3 (71.6–88.4) 214 (154–1,040) 310

Age 12–19 years 41.9 (39.3–44.6) 40.5 (37.7–44.4) 112 (78.4–185) 648

2001–2002

Age 6–11 years 77.2 (73.1–81.5) 77.6 (71.8–84.5) 185 (165–219) 368

Age 12–19 years 43.4 (40.8–46.1) 44.1 (40.8–47.2) 106 (94.8–118) 762

2003–2004

Age 6–11 years 72.5 (65.2–80.7) 73.5 (65.1–79.9) 160 (129–257) 290

Age 12–19 years 37.5 (35.4–39.8) 38.9 (36.9–41.8) 81.0 (74.3–102) 725

2005–2006

Age 6–11 years 81.0 (71.9–91.3) 78.6 (72.1–89.0) 201(160–261) 355

Age 12–19 years 45.5 (42.5–48.7) 45.7 (41.3–49.2) 109 (95.0–131) 701

2007–2008

Age 6–11 years 90.4 (81.8–99.8) 88.2 (79.2–101) 274 (224–354) 394

Age 12–19 years 50.1 (47.2–53.2) 50.1 (44.2–53.4) 129 (99.5–138) 376

2009–2010

Age 6–11 years 88.6 (81.9–95.4) 89.0 (79.2–95.4) 195 (178–216) 378

Age 12–19 years 49.0 (45.3–53.0) 50.7 (44.6–56.2) 126 (96.4–134) 451

2011–2012

Age 6–11 years 83.5 (76.1–91.6) 81.7 (74.3–91.2) 259 (185–300) 398

Age 12–19 years 44.4 (40.8–48.4) 43.7 (39.1–48.0) 109 (92.4–131) 390

2013–2014

Age 6–11 years 77.0 (73.5–80.8) 73.5 (70.0–81.4) 184 (164–225) 402

Age 12–19 years 43.6 (40.3–47.2) 44.0 (39.1–47.3) 113 (95.9–140) 451
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5. POTENTIAL FOR HUMAN EXPOSURE

Table 5-11. Urinary Molybdenum Levels in U.S. Children and Adolescents 

Survey years Geometric mean 50th percentile 95th percentile Sample size

2015–2016

Age 3–5 years 109 (102–116) 107 (98.4–117) 275 (239–329) 485

Age 6–11 years 79.7 (75.5–84.1) 79.6 (69.9–86.7) 200(171–229) 379

Age 12–19 years 44.6 (41.9–47.5) 45.0 (41.8–48.7) 107 (89.3–133) 402

aLimit of detection for survey years 1999–2001, 2001–2002, 2003–2004, 2005–2006, 2007–2008, 2009–2010, 

2011–2012, 2013–2014, 2015–2016 were 0.8, 0.8, 1.5, 0.92, 0.92, 0.99, 0.8, and 0.8 μg/L, respectively.

NHANES = National Health and Nutrition Examination Survey

Source: CDC 2019; NHANES data are periodically updated, and the most recent information can be found at 

https://cdc.gov/exposurereport/index.html.

5.7 POPULATIONS WITH POTENTIALLY HIGH EXPOSURES

Workers in an industrial setting such as mining, metal refining, and metal working can be exposed to 

significant levels of molybdenum (Kucera et al. 2000). Populations living close to areas with high 

molybdenum contamination from industrial effluents and high mineral deposits are at risk for higher 

exposures (EPA 1979).

99Mo generators are the major source of ionizing radiation exposure to nuclear medicine staff in medical 

facilities that perform 99mTc diagnostic imaging scans (Ahasan 2004).
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CHAPTER 6. ADEQUACY OF THE DATABASE

Section 104(i)(5) of CERCLA, as amended, directs the Administrator of ATSDR (in consultation with the 

Administrator of EPA and agencies and programs of the Public Health Service) to assess whether 

adequate information on the health effects of molybdenum is available. Where adequate information is 

not available, ATSDR, in conjunction with NTP, is required to assure the initiation of a program of 

research designed to determine the adverse health effects (and techniques for developing methods to 

determine such health effects) of molybdenum.

Data needs are defined as substance-specific informational needs that, if met, would reduce the 

uncertainties of human health risk assessment. This definition should not be interpreted to mean that all 

data needs discussed in this section must be filled. In the future, the identified data needs will be 

evaluated and prioritized, and a substance-specific research agenda will be proposed. 

6.1 INFORMATION ON HEALTH EFFECTS

Studies evaluating the health effects of inhalation, oral, and dermal exposure of humans and animals to 

molybdenum that are discussed in Chapter 2 are summarized in Figure 6-1. The purpose of this figure is 

to illustrate the information concerning the health effects of molybdenum. The number of human and 

animal studies examining each endpoint is indicated regardless of whether an effect was found and the 

quality of the study or studies. 

6.2 IDENTIFICATION OF DATA NEEDS

Missing information in Figure 6-1 should not be interpreted as a “data need.” A data need, as defined in 

ATSDR’s Decision Guide for Identifying Substance-Specific Data Needs Related to Toxicological 

Profiles (ATSDR 1989), is substance-specific information necessary to conduct comprehensive public 

health assessments. Generally, ATSDR defines a data gap more broadly as any substance-specific 

information missing from the scientific literature.
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6. ADEQUACY OF THE DATABASE

Figure 6-1. Summary of Existing Health Effects Studies on Molybdenum By 

Route and Endpoint* 

 

Potential body weight, hematological, musculoskeletal, and reproductive effects were the most 

studied endpoints 

The majority of the studies examined oral exposure in animals (versus humans) 

*Includes studies discussed in Chapter 2; the number of studies include those finding no effect

and studies may have examined more than one endpoint. 
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6. ADEQUACY OF THE DATABASE

Acute-Duration MRLs. No data were located regarding health effects after acute inhalation exposure 

to molybdenum in humans. In laboratory animals, the inhalation exposure data are limited to studies of 

molybdenum trioxide conducted in rats and mice (NTP 1997); however, the studies only examined the 

nasal cavity and body weight. Although increased mortality and decreases in body weight gain were 

observed, the studies are not adequate for identifying the primary target of toxicity. Thus, they were not 

considered adequate for derivation of an acute-duration inhalation MRL. Additional studies examining a 

wide-range of endpoints and several different molybdenum compounds would be useful for characterizing 

the hazard of molybdenum following acute inhalation exposure.

In an acute oral exposure experiment, no alterations in uric acid levels were observed in volunteers 

(Deosthale and Gopalan 1974); the study did not examine other potential endpoints. A small number of 

studies have examined the acute oral toxicity in laboratory animals, and none of them examined a widerange of endpoints. One study found an increase in serum triglyceride levels in rabbits but did not find 

any histological alterations in the liver or kidneys (Bersenyi et al. 2008). Three acute laboratory animal 

studies have reported reproductive effects (Bersenyi et al. 2008; Zhai et al. 2013; Zhang et al. 2013). 

However, interpretation of the results is limited by the lack of statistical analyses (Bersenyi et al. 2008) or 

limited information on molybdenum and copper intake (Zhai et al. 2013; Zhang et al. 2013). 

Additionally, reproductive effects have not been reported in high-quality intermediate-duration studies 

(Murray et al. 2014a, 2019). Given these limitations, the database was not considered suitable for 

derivation of an acute-duration oral MRL. Additional studies that report molybdenum doses and copper 

content of the diet, and evaluate a wide range of endpoints, including the reproductive system, are needed.

Intermediate-Duration MRLs. The available data on the toxicity of molybdenum following 

intermediate-duration inhalation exposure are limited to 90-day studies of molybdenum trioxide 

examining a wide range of potential targets of toxicity in rats and mice (NTP 1997). No adverse effects 

were observed in these studies, and the studies were not considered suitable for derivation of an 

intermediate-duration inhalation MRL for molybdenum. Additional studies testing higher concentrations 

and several molybdenum compounds may identify sensitive targets.

A number of studies have examined the intermediate-duration toxicity of ingested molybdenum. Among 

studies in which the laboratory animals were provided a diet with adequate levels of copper, a number of 

targets of toxicity were identified including the kidney, hematological system, reproductive system, and 

the developing organism (Bompart et al. 1990; Fungwe et al. 1990; Jeter and Davis 1954; Lyubimov et al. 

2004; Murray et al. 2014a, 2019; Pandey and Singh 2002). The lowest LOAEL values were identified for 
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6. ADEQUACY OF THE DATABASE

reproductive and developmental effects. However, these values were identified in lower quality studies 

and were not confirmed in higher quality studies; thus, they were not considered suitable as a point of 

departure (POD) for an MRL. An intermediate-duration oral MRL was derived based on kidney effects 

in a high-quality study (Murray et al. 2014a). Additional studies are needed to confirm that the kidney is 

the most sensitive target of oral molybdenum toxicity. 

Chronic-Duration MRLs. Two occupational exposure studies have reported mixed results on the 

effect of molybdenum on the respiratory tract (Ott et al. 2004; Walravens et al. 1979). There is 

insufficient information on the specific molybdenum compounds involved and limited data on exposure 

levels. Chronic exposure studies in rats and mice have identified the respiratory tract as a sensitive target 

of molybdenum trioxide toxicity (NTP 1997), and an inhalation MRL was derived based on the findings 

in the animal studies. Additional studies are needed to evaluate the inhalation toxicity of other 

molybdenum compounds.

A number of studies have evaluated the chronic toxicity of ingested molybdenum in humans. Studies of 

populations potentially exposed to high concentrations of molybdenum have evaluated potential 

alterations in uric acid levels (EPA 1979; Koval’skiy et al. 1961); there are a number of limitations with 

both of these studies restricting their usefulness in evaluating the chronic toxicity of molybdenum in 

humans. Epidemiological studies that examined the potential of molybdenum to induce adverse health 

effects presumably involved background environmental exposure (Meeker et al. 2008, 2010; Mendy et al. 

2012; Schroeder and Kraemer 1974; Shiue and Hristova 2014; Vazquez-Salas et al. 2014; Yorita 

Christensen 2013). Although some of these studies reported statistically significant associations between 

biomarkers of molybdenum exposure (plasma or urine levels) and adverse effects, the studies do not 

establish causality and there may have been factors other than molybdenum exposure. No laboratory 

animal studies evaluated the chronic oral toxicity of molybdenum. Additional studies examining a wide 

range of potential endpoints are needed to identify the hazards associated with chronic ingestion of high 

levels of molybdenum and establish dose-response relationships; these data could be used to derive a 

chronic-duration oral MRL.

Health Effects. 

Reproductive. A study of men at an infertility clinic found associations between blood 

molybdenum levels and altered sperm parameters and reproductive hormone levels (Meeker et al. 

2008, 2010). These studies do not establish causality. Oral exposure studies in laboratory

animals have provided mixed results on whether the reproductive system is a target of 
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6. ADEQUACY OF THE DATABASE

molybdenum toxicity (Bersenyi et al. 2008; Fungwe et al. 1990; Lyubimov et al. 2004; Murray et 

al. 2014a, 2019; Pandey and Singh 2002; Zhai et al. 2013; Zhang et al. 2013). High-quality 

studies did not find any significant alterations in sperm parameters, estrous cycling, or male or 

female reproductive tissue (Murray et al. 2014a, 2019), and no effects on fertility were found in a 

2-generation study (Murray et al. 2019). In contrast, other studies have found alterations in 

estrous cycling (Fungwe et al. 1990), sperm parameters (Pandey and Singh 2002; Zhai et al. 

2013), oocytes (Zhang et al. 2013), and male fertility (Pandey and Singh 2002). Interpretation of 

the results of these studies was limited by inadequate information on molybdenum doses (the 

investigators did not provide adequate information on body weight or water consumption, which 

could be used to estimate doses) or did not report the copper content of the commercial diet used. 

Additional studies are needed to provide insight into the apparent conflicting results for 

reproductive toxicity.

Immunotoxicity. The immunotoxicity of molybdenum has not been adequately addressed. No 

inhalation or oral exposure studies addressed immune function; intermediate- and chronicduration inhalation studies did not find histological alterations in the thymus or spleen (NTP 

1997). Very low levels of positive results of patch tests were observed in patients undergoing hip 

or knee replacements (Koster et al. 2000; Menezes et al. 2004; Zeng et al. 2014). In animals, 

contact sensitization was observed in a guinea pigs in a sensitization assay with molybdenum 

pentachloride (Boman et al. 1979); other studies with other molybdenum compounds—

ammonium dimolybdate, molybdenum trioxide, and sodium molybdate—have not found 

evidence of skin sensitization (Allan et al. 1996, 1996b, 1996c, 1996d). Studies examining 

immune function and systemic immunological endpoints (e.g., changes in white cell populations, 

cytokine levels, macrophage infiltration) would be useful in evaluating whether this is a target of 

molybdenum toxicity; it would be useful if the studies evaluated different molybdenum 

compounds.

Mechanisms of Action. The mechanisms of molybdenum toxicity are poorly understood. 

Although there are data suggesting that molybdenum toxicity may be related to alterations in 

copper utilization, it is also likely that other mechanisms, such as oxidative damage, are also 

involved. Studies examining the mode of action are needed to support the identification of 

critical endpoints and derivation of MRLs.
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6. ADEQUACY OF THE DATABASE

Epidemiology and Human Dosimetry Studies. A small number of epidemiology studies were 

identified for molybdenum; however, most of these studies presumably involved background 

environmental exposure to molybdenum. Two occupational exposure studies found conflicting results 

regarding the respiratory toxicity of molybdenum (Walravens et al. 1979; Ott et al. 2004). Additional 

studies of worker populations examining a wide range of potential endpoints, including the respiratory 

tract, would provide valuable information on the toxicity of inhaled molybdenum. General population 

studies have identified a number of potential targets of toxicity of ingested molybdenum including blood 

pressure (Shiue and Hrisova 2014), liver (Mendy et al. 2012), the reproductive system (Meeker et al. 

2008, 2010), and the developing organism (Shirai et al. 2010); however, none of the studies established 

causality. Studies of populations exposed to high levels of molybdenum in drinking water or from foods 

grown in molybdenum-rich soil would provide support for establishing sensitive targets of molybdenum 

toxicity. 

Biomarkers of Exposure and Effect. 

Exposure. Molybdenum levels can be measured in blood, tissues, and excreta, and background urinary 

levels of molybdenum have been established in healthy individuals (CDC 2019). Blood and urinary 

levels have been shown to increase in response to increased molybdenum ingestion (Turnland and Keyes 

2004), although plasma molybdenum levels are likely to be reflective of recent dietary intake. Studies 

that quantified the relationship between blood and/or urinary levels and intake would provide valuable 

information on screening and comparison with adverse effect levels. Studies evaluating biochemical 

and/or genomic biomarkers of exposure would also be useful for evaluating potential inhalation and/or 

oral exposure.

Effect. No biomarkers of effect were identified. The available data have identified the following 

sensitive targets: respiratory tract (inhalation only), kidney, and possibly the reproductive system. 

Studies examining the possible relationship between blood or urinary levels of molybdenum with these 

adverse health effects could facilitate medical surveillance leading to early detection and possible 

treatment.

Absorption, Distribution, Metabolism, and Excretion. For humans, detailed quantitative 

information is available regarding the absorption, distribution, and excretion of ingested molybdate 

(Mo[VI]O4

2-

) and molybdenum incorporated into food. Although molybdate is most likely the dominant 

chemical species of molybdenum in the body, there are no data for humans on toxicokinetics following 
MOLYBDENUM 119

6. ADEQUACY OF THE DATABASE

exposures to other forms of molybdenum that could occur in the environment, such as MoIV compounds. 

No quantitative information is available on the toxicokinetics of molybdenum in humans following 

chronic oral exposure. There is no information on inhalation, and dermal toxicokinetic data are limited to 

an in vitro percutaneous absorption study (Roper 2009). A study conducted in mice showed that 

molybdenum is absorbed following inhalation exposure to molybdenum trioxide (NTP 1997).

Limited information was identified on the relative bioavailability of different molybdenum compounds 

following inhalation or oral exposure. It is likely that the solubility of the molybdenum compound would 

greatly influence the amount that is absorbed through the lungs or gastrointestinal tract. Studies 

examining relative bioavailability would provide valuable information on extrapolating data across 

molybdenum compounds and species.

Studies conducted in humans have provided data for the development of PBPK models of molybdenum 

kinetics in humans (Giussani 2008; Novotny and Turnlund 2007). Models have not been developed for 

rodents or other animal species that could be used in dosimetry extrapolation of animal bioassay results.

Comparative Toxicokinetics. The available data on the toxicity of molybdenum in humans and 

laboratory animals suggest that they have similar targets of toxicity; however, there are limited 

epidemiology data. The available data suggest similarities in the absorption, distribution, and elimination 

of ingested molybdenum in humans and rats. Additional studies are needed to compare the toxicokinetics 

of inhaled molybdenum and to assess whether there are species differences.

Children’s Susceptibility. Two epidemiological studies have examined possible developmental 

effects associated with maternal urinary molybdenum levels (Shirai et al. 2010; Vazquez-Salas et al. 

2014); interpretation of the results of these studies is limited. Studies in laboratory animals have not 

reported alterations in pup survival, body weight, occurrence of malformations, or developmental 

landmarks in rats orally exposed to molybdenum (Jeter and Davis 1954; Murray et al. 2014, 2019). There 

are limited data on the toxicity of molybdenum in children; studies are needed to evaluate whether the 

susceptibility of children differs from adults.

Physical and Chemical Properties. The physical-chemical properties of molybdenum are 

provided in Chapter 4. No data needs are identified.
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6. ADEQUACY OF THE DATABASE

Production, Import/Export, Use, Release, and Disposal. According to the Emergency 

Planning and Community Right-to-Know Act of 1986, 42 U.S.C. Section 11023, industries are required 

to submit substance release and off-site transfer information to the EPA. The TRI is updated yearly and 

should provide a list of industrial production facilities and emissions.

Environmental Fate. Molybdenum is a naturally occurring trace element that can be found 

extensively in nature (EPA 1979). Its transport and partitioning are well understood. No data needs are 

identified.

Bioavailability from Environmental Media. Biologically, molybdenum plays an important role as 

a micronutrient in plants and animals, including humans (EPA 1979). Its bioavailability is well 

documented. No data needs are identified.

Food Chain Bioaccumulation. Measured BCFs of molybdenum in fish suggest that 

bioaccumulation in aquatic organisms is not high. No data needs are identified.

Exposure Levels in Environmental Media. Reliable monitoring data for the levels of 

molybdenum in contaminated media at hazardous waste sites are needed so that the information obtained 

on levels of molybdenum in the environment can be used in combination with the known body burden of 

molybdenum to assess the potential risk of adverse health effects in populations living in the vicinity of 

hazardous waste sites.

Exposure Levels in Humans. Exposure to molybdenum to the general population is almost entirely 

through food. Food derived from aboveground plants, such as legumes, leafy vegetables, and cauliflower 

generally has a relatively higher concentration of molybdenum in comparison to food from tubers or 

animals. Beans, cereal grains, leafy vegetables, legumes, liver, and milk are reported as the richest 

sources of molybdenum in the average diet. Nutritional supplements are also a source of dietary 

exposure. Drinking water coming from sources close to areas with high molybdenum contamination from 

industrial effluents may contain a higher concentration of molybdenum. Exposure to molybdenum in an 

industrial setting such as mining can be significant (Barceloux 1999; EPA 1979; Momcilovic 1999; NAS 

2001).

This information is necessary for assessing the need to conduct health studies on these populations.
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6. ADEQUACY OF THE DATABASE

Exposures of Children. There are limited data on estimates of molybdenum exposure in children. 

Milk is reported to be the primary source of dietary molybdenum intake among children in the United 

States (Biego et al. 1998; EPA 1979); however, this is based on older data. More recent monitoring data 

would be valuable in assessing whether molybdenum exposure sources vary between children and adults.

6.3 ONGOING STUDIES

No ongoing studies on the toxicity of molybdenum or its toxicokinetic properties were identified in the 

National Institute of Health (NIH) RePORTER (2019) database. 
MOLYBDENUM 122

CHAPTER 7. REGULATIONS AND GUIDELINES

Pertinent international and national regulations, advisories, and guidelines regarding molybdenum in air, 

water, and other media are summarized in Table 7-1. This table is not an exhaustive list, and current 

regulations should be verified by the appropriate regulatory agency.

ATSDR develops MRLs, which are substance-specific guidelines intended to serve as screening levels by 

ATSDR health assessors and other responders to identify contaminants and potential health effects that 

may be of concern at hazardous waste sites. See Section 1.3 and Appendix A for detailed information on 

the MRLs for molybdenum.

Table 7-1. Regulations and Guidelines Applicable to Molybdenum

Agency Description Information Reference

Air

EPA RfC No data IRIS 2003

WHO Air quality guidelines Not listed WHO 2010

Water & Food

EPA Drinking water standards and health advisories for 

molybdenum

EPA 2018a

1-Day health advisory (10-kg child) 0.08 mg/L

10-Day health advisory (10-kg child) 0.08 mg/L

DWEL 0.2 mg/L

Lifetime health advisory 0.04 mg/L

National primary drinking water regulations Not listed EPA 2009b

RfD (molybdenum) 5x10-3 mg/kg/daya IRIS 2003

WHO Drinking water quality guidelines Not establishedb WHO 2017

FDA Substances added to food Not listedc FDA 2018

USNRC Annual limit on intake, oral ingestion NRC 2018

99Molybdenum compounds except oxides, 

hydroxides, and molybdenum disulfide

2x103 μCi

Cancer

HHS Carcinogenicity classification No data NTP 2016

EPA Carcinogenicity classification No data IRIS 2003

IARC Carcinogenicity classification IARC 2018

Molybdenum trioxide Group 2Bd

Occupational

OSHA PEL (8-hour TWA) for general industry, shipyards 

and construction (molybdenum, as molybdenum)

OSHA 2018a, 

2018b, 2018c

Soluble compounds 5 mg/m3

Insoluble compounds, total dust 15 mg/m3
MOLYBDENUM 123

7. REGULATIONS AND GUIDELINES

Table 7-1. Regulations and Guidelines Applicable to Molybdenum

Agency Description Information Reference

NIOSH REL (up to 10-hour TWA) Not establishede NIOSH 2016a, 

2016b

IDLH (molybdenum, as molybdenum)

Soluble compounds 1,000 mg Mo/m3 NIOSH 1994a

Insoluble compounds 5,000 mg Mo/m3 NIOSH 1994b

USNRC Annual limit on intake, inhalation NRC 2018

99Molybdenum compounds except oxides, 

hydroxides, and molybdenum disulfide

3x103 μCi

Derived air concentration

99Molybdenum compounds except oxides, 

hydroxides, and molybdenum disulfide

1x10-6 μCi/mL

Emergency Criteria

EPA AEGLs-air No data EPA 2016

DOE PACs-air DOE 2018b

PAC-1f

Molybdenum 30 mg/m3

Ammonium heptamolybdate 2.6 mg/m3

Ammonium molibdate 3.5 mg/m3

Ammonium molybdate(VI) tetrahydrate 2.8 mg/m3

Diammonium dimolybdate 2.6 mg/m3

Diammonium molybdate 3.1 mg/m3

Disodium molybdate 3.2 mg/m3

Molybdenum carbide 34 mg/m3

Molybdenum dioxide 40 mg/m3

Molybdenum hexacarbonyl 83 mg/m3

Molybdenum pentachloride 4.3 mg/m3

Molybdenum trioxide 2.3 mg/m3

Molybdenum(IV) sulfide 50 mg/m3

Sodium molybdate dihydrate 3.8 mg/m3

PAC-2f

Molybdenum 330 mg/m3

Ammonium heptamolybdate 230 mg/m3

Ammonium molibdate 290 mg/m3

Ammonium molybdate(VI) tetrahydrate 30 mg/m3

Diammonium dimolybdate 29 mg/m3

Diammonium molybdate 22 mg/m3

Disodium molybdate 17 mg/m3

Molybdenum carbide 360 mg/m3

Molybdenum dioxide 430 mg/m3

Molybdenum hexacarbonyl 920 mg/m3

Molybdenum pentachloride 410 mg/m3

Molybdenum trioxide 43 mg/m3
MOLYBDENUM 124

7. REGULATIONS AND GUIDELINES

Table 7-1. Regulations and Guidelines Applicable to Molybdenum

Agency Description Information Reference

Molybdenum(IV) sulfide 260 mg/m3

Sodium molybdate dihydrate 34 mg/m3

PAC-3f

Molybdenum 2,000 mg/m3

Ammonium heptamolybdate 1,400 mg/m3

Ammonium molibdate 1,700 mg/m3

Ammonium molybdate(VI) tetrahydrate 180 mg/m3

Diammonium dimolybdate 170 mg/m3

Diammonium molybdate 130 mg/m3

Disodium molybdate 100 mg/m3

Molybdenum carbide 2,200 mg/m3

Molybdenum dioxide 2,600 mg/m3

Molybdenum hexacarbonyl 5,500 mg/m3

Molybdenum pentachloride 2,400 mg/m3

Molybdenum trioxide 260 mg/m3

Molybdenum(IV) sulfide 1,600 mg/m3

Sodium molybdate dihydrate 210 mg/m3

aThe RfD is based on a LOAEL of 0.14 mg/kg/day for increased uric acid levels in humans (Koval’skiy et al. 1961).

bReason for not establishing guideline value: occurs in drinking water at concentrations well below those of health 

concern.

cThe Substances Added to Food inventory replaces EAFUS and contains the following types of ingredients: food 

and color additives listed in FDA regulations, flavoring substances evaluated by FEMA or JECFA, GRAS substances 

listed in FDA regulations, substances approved for specific uses in food prior to September 6, 1958, substances that 

are listed in FDA regulations as prohibited in food, delisted color additives, and some substances "no longer FEMA 

GRAS."

dGroup 2B: possibly carcinogenic to humans.

eIn 1988, NIOSH provided comments to OSHA in which NIOSH questioned whether proposed PELs for particular 

substances, including the TWA 5 mg/m3 PEL for molybdenum (soluble compounds as molybdenum), were adequate 

to protect workers from recognized health hazards. At that time, NIOSH also concluded that the documentation 

cited by OSHA was inadequate to support a proposed PEL of 10 mg/m3 for particular substances including

molybdenum (insoluble compounds as molybdenum) (NIOSH 2018). f

Definitions of PAC terminology are available from U.S. Department of Energy (DOE 2018a).

AEGL = acute exposure guideline level; DOE = Department of Energy; DWEL = drinking water equivalent level; 

EAFUS = Everything Added to Food in the United States; EPA = Environmental Protection Agency; FDA = Food and 

Drug Administration; FEMA = Flavor and Extract Manufacturers Association; GRAS = generally recognized as safe; 

HHS = Department of Health and Human Services; IARC = International Agency for Research on Cancer; 

IDLH = immediately dangerous to life or health; IRIS = Integrated Risk Information System; JECFA = Joint 

FAO/WHO Expert Committee on Food Additives; LOAEL = lowest-observed-adverse-effect level; NIOSH = National 

Institute for Occupational Safety and Health; NTP = National Toxicology Program; OSHA = Occupational Safety and 

Health Administration; PAC = protective action criteria; PEL = permissible exposure limit; REL = recommended 

exposure limit; RfD = oral reference dose; TWA = time-weighted average; USNRC = U.S. Nuclear Regulatory 

Commission; WHO = World Health Organization
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MOLYBDENUM A-1

APPENDIX A. ATSDR MINIMAL RISK LEVEL WORKSHEETS

MRLs are derived when reliable and sufficient data exist to identify the target organ(s) of effect or the 

most sensitive health effect(s) for a specific duration for a given route of exposure. An MRL is an 

estimate of the daily human exposure to a hazardous substance that is likely to be without appreciable risk 

of adverse noncancer health effects over a specified route and duration of exposure. MRLs are based on 

noncancer health effects only; cancer effects are not considered. These substance-specific estimates, 

which are intended to serve as screening levels, are used by ATSDR health assessors to identify 

contaminants and potential health effects that may be of concern at hazardous waste sites. It is important 

to note that MRLs are not intended to define clean-up or action levels.

MRLs are derived for hazardous substances using the NOAEL/uncertainty factor approach. They are 

below levels that might cause adverse health effects in the people most sensitive to such chemicalinduced effects. MRLs are derived for acute (1–14 days), intermediate (15–364 days), and chronic 

(≥365 days) durations and for the oral and inhalation routes of exposure. Currently, MRLs for the dermal 

route of exposure are not derived because ATSDR has not yet identified a method suitable for this route 

of exposure. MRLs are generally based on the most sensitive substance-induced endpoint considered to 

be of relevance to humans. Serious health effects (such as irreparable damage to the liver or kidneys, or 

birth defects) are not used as a basis for establishing MRLs. Exposure to a level above the MRL does not 

mean that adverse health effects will occur.

MRLs are intended only to serve as a screening tool to help public health professionals decide where to 

look more closely. They may also be viewed as a mechanism to identify those hazardous waste sites that 

are not expected to cause adverse health effects. Most MRLs contain a degree of uncertainty because of 

the lack of precise toxicological information on the people who might be most sensitive (e.g., infants, 

elderly, nutritionally or immunologically compromised) to the effects of hazardous substances. ATSDR 

uses a conservative (i.e., protective) approach to address this uncertainty consistent with the public health 

principle of prevention. Although human data are preferred, MRLs often must be based on animal studies 

because relevant human studies are lacking. In the absence of evidence to the contrary, ATSDR assumes 

that humans are more sensitive to the effects of hazardous substance than animals and that certain persons 

may be particularly sensitive. Thus, the resulting MRL may be as much as 100-fold below levels that 

have been shown to be nontoxic in laboratory animals.
MOLYBDENUM A-2

APPENDIX A

Proposed MRLs undergo a rigorous review process: Health Effects/MRL Workgroup reviews within the 

Division of Toxicology and Human Health Sciences, expert panel peer reviews, and agency-wide MRL 

Workgroup reviews, with participation from other federal agencies and comments from the public. They 

are subject to change as new information becomes available concomitant with updating the toxicological 

profiles. Thus, MRLs in the most recent toxicological profiles supersede previously published MRLs. 

For additional information regarding MRLs, please contact the Division of Toxicology and Human 

Health Sciences, Agency for Toxic Substances and Disease Registry, 1600 Clifton Road NE, Mailstop 

S102-1, Atlanta, Georgia 30329-4027.
MOLYBDENUM A-3

APPENDIX A

MINIMAL RISK LEVEL (MRL) WORKSHEET

Chemical Name: Molybdenum

CAS Numbers: 7439-98-7

Date: May 2020

Profile Status: Final

Route: Inhalation

Duration: Acute

MRL Summary: There are insufficient data for derivation of an acute-duration inhalation MRL for 

molybdenum due to the limited number of endpoints examined in the only available animal studies.

Rationale for Not Deriving an MRL: The database on the acute inhalation toxicity of molybdenum is 

limited to several 4-hour studies in rats exposed to ammonium dimolybdate (Jackson et al. 1991a), 

molybdenum trioxide (Jackson et al. 1991b, 1991d; Leuschner 2010), or sodium molybdate (Jackson et 

al. 1991c) and a 14-day study in rats and mice exposed to molybdenum trioxide (NTP 1997). No effects 

on lethality or the respiratory tract (most only examined the lungs) were observed at concentrations of 

1,200 mg molybdenum/m3 and higher (Jackson et al. 1991a, 1991b, 1991c, 1991d; Leuschner 2010); 

several of the studies reported decreases in body weight on days 2–3 post-exposure (Jackson et al. 1991b, 

1991c, 1991d). The NTP (1997) study evaluated the effect of molybdenum trioxide on the nasal cavity 

and on body weight in rats and mice exposed 6 hours/day, 5 days/week for 14 days. No adverse effects 

were observed in the nasal cavity. However, weight loss was observed at the highest concentration tested 

(200 mg molybdenum/m3

); decreases in body weight gain were observed in male rats exposed to 67 mg 

molybdenum/m3 and in female rats and mice exposed to 200 mg/m3

. Given the limited number of 

endpoints examined, the decrease in body weight gain was not considered a suitable basis for an acuteduration inhalation MRL because the database is inadequate for identifying the critical target of 

molybdenum toxicity following acute-duration inhalation exposure.

Agency Contacts (Chemical Managers): G. Daniel Todd
MOLYBDENUM A-4

APPENDIX A

MINIMAL RISK LEVEL (MRL) WORKSHEET

Chemical Name: Molybdenum

CAS Numbers: 7439-98-7

Date: May 2020

Profile Status: Final

Route: Inhalation

Duration: Intermediate

MRL Summary: There are insufficient data for derivation of an intermediate-duration inhalation MRL 

for molybdenum due to the lack of studies identifying a critical target of toxicity.

Rationale for Not Deriving an MRL: Information on the intermediate-duration toxicity of molybdenum 

is limited to 90-day studies of molybdenum trioxide in rats and mice conducted by NTP (1997) that 

examined a wide range of potential targets, including reproductive endpoints. No toxicologically 

significant alterations were observed at concentrations of molybdenum trioxide as high as 67 mg/m3

. 

Consistent with ATSDR’s practice of not using free-standing NOAELs as a POD, an intermediateduration inhalation MRL was not derived.

Agency Contacts (Chemical Managers): G. Daniel Todd
MOLYBDENUM A-5

APPENDIX A

MINIMAL RISK LEVEL (MRL) WORKSHEET

Chemical Name: Molybdenum trioxide

CAS Numbers: 1313-27-5

Date: May 2020

Profile Status: Final

Route: Inhalation

Duration: Chronic

MRL 0.002 mg molybdenum/m3

Critical Effect: Respiratory effect, squamous metaplasia of the epiglottis in female rats

Reference: NTP 1997

Point of Departure: BMCL10 of 1.60 mg molybdenum/m3 (BMCLHEC of 0.071 mg Mo/m3

)

Uncertainty Factor: 30

LSE Graph Key: 11

Species: Rat

MRL Summary: A chronic-duration inhalation MRL of 0.002 mg molybdenum/m3 was derived for 

molybdenum trioxide based on an increased incidence of squamous metaplasia of the epiglottis in female 

rats exposed to 6.7 mg molybdenum/m3 as molybdenum trioxide 6 hours/day, 5 days/week for 2 years 

(NTP 1997). The MRL is based on a BMCL10 of 1.60 mg molybdenum/m3 (human equivalent 

concentration [HEC] of 0.071 mg molybdenum/m3

) and a total uncertainty factor of 30 (3 for 

extrapolation from animals to humans with dosimetric adjustments and 10 for human variability).

Selection of the Critical Effect: There are limited data on the toxicity of inhaled molybdenum in 

humans. A study of workers at a molybdenite roasting facility exposed to molybdenum trioxide and other 

oxides found no alterations in lung function but did find increases in serum uric acid levels (Walravens et 

al. 1979); the TWA molybdenum concentration was 9.46 mg molybdenum/m3

. Another study of workers 

exposed to ultrafine molybdenum trioxide dust reported respiratory symptoms (dyspnea and cough), 

radiographic abnormalities, and impaired lung function (Ott et al. 2004); the study did not provide 

monitoring data. Confidence in these cohort studies was considered very low (see Appendix C for 

additional information). 

Data on the chronic toxicity of molybdenum in laboratory animals is limited to 2-year studies in rats and 

mice exposed to molybdenum trioxide (NTP 1997). In these studies, NTP (1997) examined a wide range 

of potential targets of toxicity. Adverse effects were limited to the respiratory tract, specifically the nasal 

respiratory and olfactory epithelium, epiglottis, and lungs. The respiratory tract was considered the 

critical target of molybdenum trioxide toxicity. 

Selection of the Principal Study: The NTP (1997) study was selected as the principal study.

Summary of the Principal Study:

NTP. 1997. Toxicology and carcinogenicity studies of molybdenum trioxide (CAS No. 1313-27-5) in 

F344/N rats and B6C3F1 mice (inhalation studies). National Toxicology Program, Research Triangle 

Park, NC. NT PTR 462.

Groups of male and female F344/N rats and B6C3F1 mice (50/sex/species/group) were exposed to target 

concentrations of 0, 10, 30, or 100 mg/m3 molybdenum trioxide (0, 6.7, 20, and 67 mg molybdenum/m3

) 

6 hours/day, 5 days/week for 106 (rats) or 105 (mice) weeks; actual concentrations were within 15% of 

the target level. The average mass median aerodynamic diameter particle sizes (and geometric standard 

deviation, σg) were 1.5 (1.8), 1.6 (1.8), and 1.7 (1.8) μm for the 6.7, 20, and 67 mg/m3 concentrations, 
MOLYBDENUM A-6

APPENDIX A

respectively. The following parameters were used to assess toxicity: twice daily cage-side observations, 

body weights (weekly for 12 weeks, at 15 weeks, monthly thereafter, and at termination), and 

histopathological examination of major tissues and organs. In addition, bone density and femoral 

curvature studies were conducted in 10 animals/sex/species/group.

No significant alterations in survival rates or body weight gain and no toxicologically significant 

alterations in bone density or curvature were found. Non-neoplastic lesions were only observed in the 

nose, larynx, and lungs; a summary of the type of lesions and incidences is presented in Table A-1. The 

severity of the respiratory lesions was concentration related. Significant increases in the incidence of 

alveolar/bronchiolar carcinoma and/or adenoma were observed in mice: carcinoma in male mice at 

≥6.7 mg/m3

, adenoma or carcinoma (combined) in male mice at 6.7 and 20 mg/m3, adenoma in female 

mice at ≥20 and 67 mg/m3

, and adenoma or carcinoma (combined) in female mice at 67 mg/m3. In rats, 

the incidence of alveolar/bronchiolar adenoma or carcinoma (combined) was increased in males; 

however, the incidences (0/50, 1/49, 1/49, 4/60) were within the range of historical controls and NTP 

considered this to be equivocal evidence of carcinogenic activity.

Table A-1. Incidence of Non-Neoplastic Respiratory Tract Lesions in Rats and 

Mice Exposed to Molybdenum Trioxide for 2 Years 

Concentration (mg molybdenum/m3)

0 6.7 20 67

Male rats

Hyaline degeneration of nasal respiratory 

epithelium

2/50 7/49 48/49a 49/50a

Squamous metaplasia of epiglottis 0/49 11/48a 16/49a 39/49a

Chronic lung inflammation in alveolus 2/50 3/50 25/50a 47/50a

Female rats

Hyaline degeneration of nasal respiratory 

epithelium

1/48 13/49a 50/50a 50/50a

Hyaline degeneration of nasal olfactory 

epithelium

39/48 47/49b 50/50a 50/50a

Squamous metaplasia of epiglottis 0/49 18/49a 29/49a 49/50a

Chronic lung inflammation 14/50 13/50 43/50a 49/50a

Male mice

Nasal suppurative inflammation 2/50 6/50 10/49b 8/50b

Nasal olfactory epithelium atrophy 3/50 5/50 3/49 10/50b

Hyaline degeneration of nasal respiratory 

epithelium

11/50 13/50 11/49 41/50a

Squamous metaplasia of epiglottis 0/50 26/49a 37/48a 49/50a

Laryngeal hyperplasia 1/50 3/49 6/48 41/50

Histiocyte infiltration in the lungs 2/50 16/50a 9/49b 9/50b

Alveolar epithelial metaplasia 0/50 32/50a 36/49a 49/50a

Female mice

Hyaline degeneration of nasal respiratory 

epithelium

26/49 23/50 28/49 48/49a

Hyaline degeneration of nasal olfactory 

epithelium

22/49 14/50 14/49 36/49a
MOLYBDENUM A-7

APPENDIX A

Table A-1. Incidence of Non-Neoplastic Respiratory Tract Lesions in Rats and 

Mice Exposed to Molybdenum Trioxide for 2 Years 

Concentration (mg molybdenum/m3)

0 6.7 20 67

Squamous metaplasia of epiglottis 1/49 36/50a 43/49a 49/50a

Laryngeal hyperplasia 1/49 1/50 7/49 35/50

Alveolar epithelial metaplasia 2/50 26/50a 39/49a 46/49b

aSignificantly different from controls; p≤0.01.

bSignificantly different from controls; p≤0.05.

Source: NTP 1997

Selection of the Point of Departure for the MRL: The MRL was based on a BMCL10 of 1.60 mg 

molybdenum/m3 for squamous metaplasia of the epiglottis in female rats.

Benchmark dose (BMD) modeling was conducted for the respiratory tract lesions with statistically 

significant increases in incidence at ≥6.7 mg/m3 (squamous metaplasia of the epiglottis in male and 

female rats and mice, hyaline degeneration of the nasal respiratory and olfactory epithelium in female 

rats, histiocyte infiltration in the lungs in male mice, and alveolar epithelial metaplasia in male and female 

mice). The incidence data (Table A-1) provided adequate fit for four endpoints (squamous metaplasia in 

male rats, female rats, and female mice and hyaline degeneration of the nasal respiratory epithelium in 

female rats). The results of the BMD modeling are presented in the Benchmark Dose Modeling 

subsection and are summarized in Table A-2.

Table A-2. Summary of Benchmark Dose Modeling 

Endpoint Selected model

BMC10

(mg Mo/m3)

BMCL10

(mg Mo/m3)

Squamous metaplasia of the 

epiglottis in male rats

Multistage, 2-degree

(Table A-4 and Figure A-1)

4.36 3.53

Hyaline degeneration of the 

respiratory epithelium in female rats

Log-logistic

(Table A-5 and Figure A-2)

5.87 4.82

Squamous metaplasia of the 

epiglottis in female rats

Weibull

(Table A-6 and Figure A-3)

1.97 1.60

Squamous metaplasia of the 

epiglottis in male mice

Gamma

(Table A-7 and Figure A-4)

1.30 1.06

BMC = benchmark concentration; BMCL = 95% lower confidence limit on the benchmark concentration 

A summary of the potential POD values is presented in Table A-3. Because there are dosimetric 

differences in regional respiratory tract deposition of aerosols between animal species, a comparison was 

made between the human equivalent concentration PODs (PODHEC). The lowest PODHEC, BMCLHEC of 

0.071 mg molybdenum/m3 for squamous metaplasia of the epiglottis in female rats, was selected as the 

POD for the MRL.
MOLYBDENUM A-8

APPENDIX A

Table A-3. Summary of PODs and HECs

Endpoint

PODs

(mg Mo/m3)

RDDR 

valuesa

HECsb

(mg Mo/m3)

Squamous metaplasia of the epiglottis in male rats 3.53 (BMCL) 0.459 0.28

Hyaline degeneration of the respiratory epithelium in female rats 4.82 (BMCL) 0.248 0.21

Hyaline degeneration of the olfactory epithelium in female rats 6.7 (LOAEL) 0.248 0.30

Squamous metaplasia of the epiglottis in female rats 1.60 (BMCL) 0.248 0.071

Squamous metaplasia of the epiglottis in male mice 1.06 (BMCL) 0.441 0.08

Histiocyte infiltration in the lungs of male mice 6.7 (LOAEL) 1.046 1.3

Alveolar epithelial metaplasia in male mice 6.7 (LOAEL) 1.046 1.3

Squamous metaplasia of the epiglottis in female mice 6.7 (LOAEL) 0.367 0.44

Alveolar epithelial metaplasia in female mice 6.7 (LOAEL) 3.067 3.7

aRDDR values specific for each region of the respiratory tract (extrathoracic, tracheobronchial, and pulmonary) were 

calculated using EPA’s RDDR calculator with reference body weights of 0.40, 0.25, 0.040, and 0.035 kg for male 

rats, female rats, male mice, and female mice, respectively, and reported particle sizes and particle size 

distributions. bHEC calculated by multiplying the duration-adjusted POD (POD x 6 hours/24 hours x 5 days/7days) by the RDDR 

value.

BMCL = 95% lower confidence limit on the benchmark concentration; HEC = human equivalent concentration; 

LOAEL = lowest observed adverse effect level; POD = point of departure; RDDR = regional deposited dose ratio for 

the specific region of the respiratory tract

Adjustment for Intermittent Exposure: The PODs were adjusted for intermittent exposure (6 hours/day, 

5 days/week).

Calculation of Human Equivalent Concentration: HECs were calculated for each potential POD by 

multiplying the duration-adjusted POD by the regional deposited dose ratio (RDDR) for the specific 

region of the respiratory tract. The RDDR is a factor used to adjust particulate exposure concentration in 

animals to a predicted concentration in humans that would be associated with the same dose delivered to a 

specific region of the respiratory tract or to the blood (EPA 1994). The RDDRs were calculated using 

EPA’s RDDR calculator with reference body weights of 0.40, 0.25, 0.040, and 0.035 kg for the male rats, 

female rats, male mice, and female mice, respectively, the reported particle sizes, and particle size 

distributions. The particles were assumed to be monodispersed given that the σg was 1.8. 

Uncertainty Factor: The BMCLHEC is divided by a total uncertainty factor (UF) of 30.

• 3 for extrapolation from animals to humans with dosimetric adjustments

• 10 for human variability

BMCLHEC ÷ UFs = MRL

0.071 mg molybdenum/m3 ÷ 30 = 0.002 mg molybdenum/m3

Other Additional Studies or Pertinent Information that Lend Support to this MRL: This MRL is 

specific to molybdenum trioxide; there are insufficient data to evaluate the health effects associated with 

inhalation exposure to other molybdenum compounds. 

Benchmark Dose Modeling: The incidence data (Table A-1) for respiratory tract lesions, which had 

significant increases in incidence at ≥6.7 mg/m3 (squamous metaplasia of the epiglottis in male and 
MOLYBDENUM A-9

APPENDIX A

female rats and mice, hyaline degeneration of the nasal respiratory and olfactory epithelium in female 

rats, histiocyte infiltration in the lungs in male mice, and alveolar epithelial metaplasia in male and female 

mice), were fit to all available dichotomous models in EPA’s Benchmark Dose Software (BMDS, 

version 3.1) using the extra risk option. Adequate model fit was judged by three criteria: goodness-of-fit 

statistics (p-value >0.1), visual inspection of the dose-response curve, and scaled residual at the data point 

(except the control) closest to the predefined benchmark response (BMR). Among all of the models 

providing adequate fit to the data, the lowest BMCL was selected as the POD when the difference 

between the BMCLs estimated from these models was >3-fold; otherwise, the BMCL from the model 

with the lowest Akaike’s Information Criterion (AIC) was chosen. For all lesion types, a BMR of 10% 

was used. Since the incidence of hyaline degeneration in the olfactory epithelium of female rats was 

essentially the same response level across groups, the data were not modeled since they provide limited 

information on the dose-response relationship. The incidence data for histiocyte infiltration in the lungs 

in male mice, alveolar epithelial metaplasia in male mice, squamous metaplasia in female mice, and 

alveolar epithelial metaplasia in female mice did not fit any of the available dichotomous models. The 

model predictions for the other endpoints are presented in Tables A-4, A-5, A-6, and A-7 and the fits of 

the selected models are presented in Figures A-1, A-2, A-3, and A-4. 
MOLYBDENUM A-10

APPENDIX A

Table A-4. Model Predictions for Squamous Metaplasia of the Epiglottis in Male 

Rats Exposed to Molybdenum Trioxide (NTP 1997)

Model DF χ2

χ2

Goodnessof-fit 

p-valuea

Scaled residualsb

AIC

BMC10

(mg/m3)

BMCL10

(mg/m3)

Dose 

below 

BMC

Dose 

above 

BMC

Overall 

largest

Gammac 2 3.07 0.22 0.00 1.55 1.55 169.98 4.36 3.53

Logistic 2 9.45 0.01 1.50 0.93 -2.47 181.70 ND ND

LogLogisticd 2 3.56 0.17 0.00 0.98 -1.42 170.75 3.80 2.23

LogProbitd 2 3.74 0.15 -0.00 0.93 -1.51 170.95 ND ND

Multistage (1-degree)e 3 3.07 0.38 0.00 1.55 1.55 167.98 4.36 3.53

Multistage (2-degree)e,f 3 3.07 0.38 0.00 1.55 1.55 167.98 4.36 3.53

Multistage (3-degree)e 3 3.07 0.38 0.00 1.55 1.55 167.98 4.36 3.53

Probit 2 9.17 0.01 1.60 0.90 -2.37 181.01 ND ND

Weibullc 2 3.07 0.22 0.00 1.55 1.55 169.98 4.36 3.53

aValues <0.1 fail to meet conventional goodness-of-fit criteria. bScaled residuals at doses immediately below and above the BMC; also the largest residual at any dose.

c

Power restricted to ≥1.

dSlope restricted to ≥1.

eBetas restricted to ≥0.

f

Selected model. BMCLs for models providing adequate fit were sufficiently close (differed by <3-fold). Therefore, 

the model with the lowest AIC was selected.

AIC = Akaike Information Criterion; BMC = maximum likelihood estimate of the exposure concentration associated 

with the selected benchmark response; BMCL = 95% lower confidence limit on the BMC (subscripts denote 

benchmark response: i.e., 10 = exposure concentration associated with 10% extra risk); DF = degrees of freedom; 

ND = not determined, goodness-of-fit criteria, p<0.10

Figure A-1. Fit of 2-Degree Multistage Model to Data on Incidence of Squamous 

Metaplasia of the Epiglottis in Male Rats Exposed to Molybdenum Trioxide 
MOLYBDENUM A-11

APPENDIX A

Table A-5. Model Predictions for Hyaline Degeneration of the Nasal Respiratory 

Epithelium in Female Rats Exposed to Molybdenum Trioxide (NTP 1997)

Model DF χ2

χ2

Goodnessof-fit 

p-valuea

Scaled residualsb

AIC

BMC10

(mg/m3)

BMCL10

(mg/m3)

Dose 

below 

BMC

Dose 

above 

BMC

Overall 

largest

Gammac 2 4.41 0.11 0.14 -1.03 1.82 77.98 3.69 2.85

Logistic 3 5.04 0.17 -1.20 -0.37 1.86 77.15 3.78 2.95

LogLogisticd,e 2 0.02 0.99 0.00 -0.00 0.13 70.45 5.87 4.82

LogProbitd 1 0.00 0.99 -0.00 -0.00 -0.00 72.42 5.92 4.73

Multistage (1-degree)f 2 18.41 0.00 0.28 -3.28 -3.28 95.80 ND ND

Multistage (2-degree)f 2 2.81 0.24 0.20 -1.21 -1.21 74.57 3.40 2.54

Multistage (3-degree)f 2 0.02 0.99 0.01 -0.05 0.15 70.46 4.77 2.39

Probit 2 0.48 0.79 0.49 -0.28 0.49 71.03 4.09 3.12

aValues <0.1 fail to meet conventional goodness-of-fit criteria. bScaled residuals at doses immediately below and above the BMC; also the largest residual at any dose.

cPower restricted to ≥1.

dSlope restricted to ≥1.

eSelected model. BMCLs for models providing adequate fit were sufficiently close (differed by <3-fold). Therefore, 

the model with the lowest AIC was selected.

f

Betas restricted to ≥0.

AIC = Akaike Information Criterion; BMC = maximum likelihood estimate of the exposure concentration associated 

with the selected benchmark response; BMCL = 95% lower confidence limit on the BMC (subscripts denote 

benchmark response: i.e., 10 = exposure concentration associated with 10% extra risk); DF = degrees of freedom; 

ND = not determined, goodness-of-fit criteria, p<0.10

Figure A-2. Fit of Log-logistic Model to Data on Incidence of Hyaline 

Degeneration of the Nasal Respiratory Epithelium in Female Rats Exposed to 

Molybdenum Trioxide 
MOLYBDENUM A-12

APPENDIX A

Table A-6. Model Predictions for Squamous Metaplasia of the Epiglottis in 

Female Rats Exposed to Molybdenum Trioxide (NTP 1997)

Model DF χ2

χ2

Goodnessof-fit 

p-valuea

Scaled residualsb

AIC

BMC10

(mg/m3)

BMCL10

(mg/m3)

Dose 

below 

BMC

Dose 

above 

BMC

Overall 

largest

Gammac 2 2.05 0.36 0.00 1.00 1.00 146.51 1.97 1.60

Logistic 2 15.55 0.00 -2.67 2.17 -2.67 163.85 ND ND

LogLogisticd 1 5.02 0.03 -0.00 0.82 -1.58 152.04 ND ND

LogProbite 2 4.16 0.12 -0.00 0.79 -1.51 148.92 2.76 1.41

Multistage (1-degree)e 3 2.05 0.56 -0.00 1.00 1.00 144.51 1.97 1.60

Multistage (2-degree)e 1 2.05 0.15 -0.00 1.04 1.04 148.50 1.99 1.60

Multistage (3-degree)e 1 1.98 0.16 -0.00 1.11 1.11 148.42 2.02 1.61

Probit 2 17.51 0.00 -2.85 2.00 -2.13 166.05 ND ND

Weibullf 3 2.05 0.56 -0.00 1.00 1.00 144.51 1.97 1.60

aValues <0.1 fail to meet conventional goodness-of-fit criteria. bScaled residuals at doses immediately below and above the BMC; also the largest residual at any dose.

c

Power restricted to ≥1.

dSlope restricted to ≥1.

eBetas restricted to ≥0.

f

Selected model. BMCLs for models providing adequate fit were sufficiently close (differed by <3-fold). Therefore, 

the model with the lowest AIC was selected.

AIC = Akaike Information Criterion; BMC = maximum likelihood estimate of the exposure concentration associated 

with the selected benchmark response; BMCL = 95% lower confidence limit on the BMC (subscripts denote 

benchmark response: i.e., 10 = exposure concentration associated with 10% extra risk); DF = degrees of freedom; 

ND = not determined, goodness-of-fit criteria, p<0.10

Figure A-3. Fit of Weibull Model to Data on Incidence of Squamous Metaplasia of 

the Epiglottis in Female Rats Exposed to Molybdenum Trioxide 
MOLYBDENUM A-13

APPENDIX A

Table A-7. Model Predictions for Squamous Metaplasia of the Epiglottis in Male 

Mice Exposed to Molybdenum Trioxide (NTP 1997)

Model DF χ2

χ2

Goodness 

of fit 

p-valuea

Scaled residualsb

AIC

BMC10

(mg/m3)

BMCL10

(mg/m3)

Dose 

below 

BMC

Dose 

above 

BMC

Overall 

largest

Gammac,d 3 5.55 0.14 -0.00 1.60 -1.65 135.46 1.30 1.06

Logistic 2 61.77 0.00 -3.19 2.80 -6.62 164.85 ND-1 ND-1

LogLogistice 1 1.42 0.23 -0.00 0.34 -0.85 134.73 ND-2 ND-2

LogProbitd 1 0.88 0.35 -0.00 0.31 -0.70 136.12 ND-2 ND-2

Multistage (1-degree)f 2 5.55 0.06 -0.00 1.60 -1.65 137.46 ND-1 ND-1

Multistage (2-degree)f 3 5.55 0.14 -0.00 1.60 -1.65 135.46 1.30 1.06

Multistage (3-degree)f 3 5.55 0.14 -0.00 1.60 -1.65 135.46 1.30 1.06

Probit 2 90.03 0.00 -3.63 2.65 -8.24 171.89 ND-1 ND-1

Weibullc 3 5.55 0.14 -0.00 1.60 -1.65 135.46 1.30 1.06

aValues <0.1 fail to meet conventional goodness-of-fit criteria. bScaled residuals at doses immediately below and above the BMC; also the largest residual at any dose.

c

Power restricted to ≥1.

dSelected model. BMCLs for models providing adequate fit were sufficiently close (differed by <3-fold). Therefore, 

the model with the lowest AIC was selected.

eSlope restricted to ≥1.

f

Betas restricted to ≥0.

AIC = Akaike Information Criterion; BMC = maximum likelihood estimate of the exposure concentration associated 

with the selected benchmark response; BMCL = 95% lower confidence limit on the BMC (subscripts denote 

benchmark response: i.e., 10 = exposure concentration associated with 10% extra risk); DF = degrees of freedom; 

ND-1 = not determined, goodness-of-fit criteria, p<0.10; ND-2 = not determined, BMCL was 10 times lower than 

lowest non-zero dose

Figure A-4. Fit of Gamma Model to Data on Incidence of Squamous Metaplasia of 

the Epiglottis in Male Mice Exposed to Molybdenum Trioxide

Agency Contacts (Chemical Managers): G. Daniel Todd
MOLYBDENUM A-14

APPENDIX A

MINIMAL RISK LEVEL (MRL) WORKSHEET

Chemical Name: Molybdenum

CAS Numbers: 7439-98-7

Date: May 2020

Profile Status: Final

Route: Oral

Duration: Acute

MRL Summary: There are insufficient data for derivation of an acute-duration oral MRL for 

molybdenum due inadequate information on the molybdenum and copper intake in the acute-duration 

studies reporting adverse reproductive effects and the conflicting results between the acute-duration 

studies and high-quality, intermediate-duration studies. 

Rationale for Not Deriving an MRL: A small number of studies have evaluated the acute toxicity of 

molybdenum. One human study (Deosthale and Gopalan 1974) examining a limited number of potential 

endpoints did not find alterations in urinary uric acid levels in subjects exposed to doses as high as 

0.022 mg molybdenum/kg/day for 10 days. In rabbits, exposure to 1.2 mg molybdenum/kg/day as 

ammonium heptamolybdate in the diet for 14 days resulted in a 60% increase in serum triglyceride levels 

(Bersenyi et al. 2008); no histological alterations were observed in the liver or kidneys. The toxicological 

significance of this finding is not known and has not been reported in a study of male rabbits exposed to 

0.58 mg molybdenum/kg/day as ammonium heptamolybdate (Bersenyi et al. 2008) or rats exposed to 

60 mg molybdenum/kg/day as sodium molybdate for 90 days (Murray et al. 2014a). 

Reproductive effects have been observed in male and female mice and rabbits. In female mice, an 

increased rate of abnormal MII oocytes was observed at 11 mg molybdenum/kg/day (Zhang et al. 2013). 

A second acute-exposure study in rabbits exposed to 1.2 mg molybdenum/kg/day as ammonium 

heptamolybdate for 14 days (Bersenyi et al. 2008) and an intermediate-duration oral study in rats exposed 

to 60 mg molybdenum/kg/day as sodium molybdate for 90 days (Murray et al. 2014a) did not find 

histological alterations in the ovaries. In males, a significant decrease in sperm concentration and motility 

and an increase in sperm abnormalities were observed at 25 mg molybdenum/kg/day in mice (Zhai et al. 

2013). A rabbit study reported a reduction in mature spermatocytes in rabbits exposed to 0.58 mg 

molybdenum/kg/day, but did not report the incidence or statistical significance (Bersenyi et al. 2008). 

Intermediate-duration studies in rats did not find significant alterations in sperm parameters in rats 

exposed to 60 mg molybdenum/kg/day as sodium molybdate for 90 days (Murray et al. 2014a) or in rats 

exposed to 40 mg molybdenum/kg/day as sodium molybdate in a 2-generation study (Murray et al. 2019). 

Interpretation of the Zhang et al. (2013) and Zhai et al. (2013) studies is limited by the lack of information 

on the copper content of the “commercial standard pellet” diet used in these studies and the lack of 

information on molybdenum doses. ATSDR estimated doses using the reported molybdenum 

concentration in the drinking water and reference values for water consumption and body weight (Zhang 

et al. 2013) or the midpoint of the reported body weights and an estimated water consumption based on 

this body weight (Zhai et al. 2013). 

The acute-duration oral database was not considered suitable for derivation of an MRL due to the limited 

information on the molybdenum and copper intake and the conflicting results between the findings of the 

Zhang et al. (2013) and Zhai et al. (2013) studies with the intermediate-duration Murray et al. (2014a, 

2019) studies.

Agency Contacts (Chemical Managers): G. Daniel Todd
MOLYBDENUM A-15

APPENDIX A

MINIMAL RISK LEVEL (MRL) WORKSHEET

Chemical Name: Molybdenum

CAS Numbers: 7439-98-7

Date: May 2020

Profile Status: Final

Route: Oral

Duration: Intermediate

MRL 0.06 mg molybdenum/kg/day

Critical Effect: Renal effect, proximal tubule hyperplasia

Reference: Murray et al. 2014a

Point of Departure: NOAEL of 17 mg molybdenum/kg/day

Uncertainty Factor: 100

Modifying Factor: 3

LSE Graph Key: 14

Species: Rat

MRL Summary: An intermediate-duration oral MRL of 0.06 mg molybdenum/kg/day was derived for 

molybdenum based on an increased incidence of renal proximal tubule hyperplasia in rats exposed to 

sodium molybdate in the diet for 90 days (Murray et al. 2014a). The MRL is based on a NOAEL of 

17 mg molybdenum/kg/day, a total uncertainty factor of 100 (10 for extrapolation from animals to 

humans, and 10 for human variability), and a modifying factor of 3 (to address concern that 

reproductive/developmental alterations may be sensitive outcomes in populations with marginal copper 

intakes). The MRL is calculated based on the assumption of healthy dietary levels of molybdenum and 

copper and represents the level of exposure above and beyond the normal diet. 

Selection of the Critical Effect: Several adverse effects have been reported in intermediate-duration oral 

studies in laboratory animals. Observed effects include kidney damage (Bompart et al. 1990; Murray et 

al. 2014a, 2019), decreased body weight gain (Bompart et al. 1990; Lyubimov et al. 2004; Mills et al. 

1958; Murray et al. 2014a; Van Reen and Williams 1956; Williams and Van Reen 1956), hematological 

effects (Arrington and Davis 1953; Lyubimov et al. 2004), reproductive effects (Fungwe et al. 1990; Jeter 

and Davis 1954; Lyubimov et al. 2004; Murray et al. 2014a; Pandey and Singh 2002; Wang et al. 2016), 

and developmental effects (Pandey and Singh 2002). 

The toxicity of molybdenum can be influenced by several factors including animal species; previous 

dietary history; relative amounts of dietary molybdenum, copper, and sulfur; and the form of 

molybdenum. Copper nutritional status is particularly important in evaluating the relevance of animal 

toxicity studies for establishing an MRL. Marked differences in the distribution of molybdenum and 

copper and the toxicity of molybdenum have been observed in rats exposed to high doses of molybdenum 

and maintained on a copper-deficient diet compared to those maintained on a copper-adequate diet 

(Brinkman and Miller 1961; Johnson et al. 1969; Nederbragt 1980, 1982; Sasmal et al. 1968). Since the 

average copper intake of the U.S. population exceeds the dietary requirements (NAS 2001), studies in 

which animals were fed inadequate levels of copper were not considered relevant for MRL derivation and 

were excluded from further consideration. Similarly, studies in which the molybdenum was administered 

as ammonium tetrathiomolybdate were also excluded since administration of tetrathiomolybdate 

compounds can result in shifts in the copper levels in rats fed copper-adequate diets (increases in serum 

and kidney copper levels and decreases in liver copper levels) (Mills et al. 1981a), and copper 

supplementation of rats exposed to ammonium tetrathiomolybdate resulted in a reversal of adverse effects 

(Lyubimov et al. 2004). A summary of the NOAEL and LOAEL values for studies with adequate copper 

in the diet is presented in Table A-8. 
MOLYBDENUM A-16

APPENDIX A

Table A-8. Summary of Health Effects Following Intermediate-Duration Oral 

Exposure to Molybdenum

Species, duration 

(route) NOAEL LOAEL Effect Reference (compound)

Body weight

Rat

147–158 days (diet)

40 22% decrease in maternal body 

weight gain on GDs 0–7; <10% 

decrease over entire study

Murray et al. 2019 

(sodium molybdate)

Rat 

90 days (diet)

17 60 Decrease in body weight gain in 

males; terminal weights 15.2% less 

than controls

Murray et al. 2014a 

(sodium molybdate)

Rat

5 weeks (diet)

74 36% decrease in body weight gain Mills et al. 1958 (sodium 

molybdate)

Rat

8 weeks (gavage)

40 80 Decrease in body weight gain; 

terminal body weight was 26% 

lower than in controls

Bompart et al. 1990 

(ammonium 

heptamolybdate)

Rat

6 weeks (diet)

85 Williams and Van Reen 

1956 (sodium molybdate)

Rat

6 weeks (diet)

90 22% decrease in body weight gain Williams and Van Reen 

1956 (sodium molybdate)

Rat 

4–5 weeks (diet)

110 46–48% decrease in body weight 

gain

Van Reen and Williams 

1956 (sodium molybdate)

Rat

147–158 days 

(drinking water)

40 Murray et al. 2019

(sodium molybdate)

Hematological effects

Rabbit

30–84 days (diet)

25 54 Anemia Arrington and Davis 1953 

(sodium molybdate)

Rabbit

≥8 weeks (diet)

7 Jeter and Davis 1954 

(sodium molybdate)

Rat

90 days (diet)

60 Murray et al. 2014a 

(sodium molybdate)

Rat

6 weeks (diet)

70 Gray and Daniel 1954 

(sodium molybdate)

Kidney effects

Rat

90 days (diet)

17 60 Slight diffuse hyperplasia in 

proximal tubules

Murray et al. 2014a 

(sodium molybdate)

Rat

8 weeks (gavage)

40 80 Diuresis and creatinuria and 

decreases in creatinine clearance

Bompart et al. 1990 

(ammonium 

heptamolybdate)

Rat

147–158 days (diet)

40 Murray et al. 2019 (sodium 

molybdate)

Rats

147–158 days 

(drinking water)

40 Murray et al. 2019 (sodium 

molybdate)
MOLYBDENUM A-17

APPENDIX A

Table A-8. Summary of Health Effects Following Intermediate-Duration Oral 

Exposure to Molybdenum

Species, duration 

(route) NOAEL LOAEL Effect Reference (compound)

Reproductive effects

Rat

8 weeks (drinking 

water)

0.76 1.5 Prolonged estrus phase; no effect 

on female fertility

Fungwe et al. 1990

(sodium molybdate)

Rat

60 days (gavage)

3.4a 10a Decreases in sperm count and 

motility; increases in sperm 

abnormalities

Pandey and Singh 2002 

(sodium molybdate)

Rat

60 days (gavage)

10a Decreases in male fertility Pandey and Singh 2002 

(sodium molybdate)

Mouse

100 days (drinking 

water)

100 Decreased sperm density and 

motility

Wang et al. 2016

(unspecified molybdenum 

compound)

Rat

90 days (diet)

60 No treatment-related alterations in 

sperm parameters; no alterations in 

vaginal cytology, estrus cycle, or 

histology of male or female 

reproductive tissues 

Murray et al. 2014a

(sodium molybdate)

Rat

≥8 weeks (diet)

7 No effect on fertility Jeter and Davis 1954 

(sodium molybdate)

Rat

2 generations (diet)

40 No effects on sperm parameters, 

estrous cycling, or fertility

Murray et al. 2019

(sodium molybdate)

Rat

2 generations 

(drinking water)

40 No effects on sperm parameters, 

estrous cycling, or fertility

Murray et al. 2019

(sodium molybdate)

Developmental effectsb

Rat (males only) 

60 days (gavage)

10a Increased post-implantation losses, 

increased resorptions, decreased 

number of live fetuses, and 

decreases in fetal weight and 

crown-rump length

Pandey and Singh 2002 

(sodium molybdate)

Rat 

≥8 weeks (diet)

7 Jeter and Davis 1954 

(sodium molybdate)

Rat 

GDs 6–20 (diet)

37.5 Murray et al. 2014b

(sodium molybdate)

Rat

2 generations (diet)

40 Murray et al. 2019

(sodium molybdate)

Rat

2 generations 

(drinking water)

40 Murray et al. 2019

(sodium molybdate)

aAdjusted for intermittent exposure (5 days/week).

bThe copper content of the basal diet (6 g/kg diet) in the Fungwe et al. (1990) study is below the recommended level 

of 8 g/kg required for pregnancy and lactation. Thus, the observed developmental effects are not included in this 

table.

GD = gestation day; LOAEL = lowest-observed-adverse-effect level; NOAEL = no-observed-adverse-effect level
MOLYBDENUM A-18

APPENDIX A

The Fungwe et al. (1990) study identified the lowest LOAEL value: prolonged estrus phase without an 

effect on fertility in rats exposed to 1.5 mg molybdenum/kg/day as sodium molybdate in drinking water 

for 8 weeks (Fungwe et al. 1990). However, this finding was not selected as the critical effect because 

other high-quality studies have not reported estrus cycle alterations in a 90-day (Murray et al. 2014a) 

study or 2-generation study (Murray et al. 2019). Additionally, confidence in this study is decreased by 

the limited information on doses. The study reported molybdenum drinking water concentrations but did 

not calculate doses. ATSDR estimated doses using reference values for body weight and drinking water 

consumption. As presented in Table A-9, a comparison with the molybdenum liver concentrations in this 

study with levels reported in the Murray et al. (2014a, 2019) studies suggested that these estimated doses 

may have underestimated the actual doses. In the Fungwe et al. (1989) study, the average liver 

molybdenum level was 10.76 μg/g in the 15 mg/kg/day group; in the Murray et al. (2014a, 2014b) 

studies, the liver molybdenum level was 4.10–4.92 μg/g in the 17 mg/kg/day groups. 

Table A-9. Comparison of Molybdenum Liver Concentrations in Female Rats

Study

Dose

Liver molybdenum concentration

Murray et al. 

2019a (water 

exposure, 

unless noted)

0 mg/kg/day 5 mg/kg/day 17 mg/kg/day 40 mg/kg/day 40 mg/kg/day 

(dietary exposure)

2.96 μg/g 3.18 μg/g 4.10 μg/g 6.48 μg/g 7.23 μg/g

Murray et al. 

2014ab (dietary 

exposure)

0 mg/kg/day 5 mg/kg/day 17 mg/kg/day 60 mg/kg/day 

2.46 μg/g 3.51 μg/g 4.92 μg/g 13.0 μg/g

Fungwe et al. 

1989c (water 

exposure)

0 mg/kg/day 0.76 mg/kg/day 1.5 mg/kg/day 7.6 mg/kg/day 15 mg/kg/day

2.63 μg/g 5.01 μg/g 5.03 μg/g 7.77 μg/g 10.76 μg/g

aParental generation. bLiver concentrations reported in Murray et al. (2019).

c

Liver concentrations from a study by Fungwe et al. (1990) utilizing the same water concentrations as Fungwe et al. 

(1989).

The next highest LOAEL is 14 mg molybdenum/kg for decreases in sperm count and motility, increased 

sperm abnormalities, decreased male fertility, increased post-implantation losses, decreased number of 

live fetuses, and decreased fetal weight in a study of male rats receiving gavage doses of sodium 

molybdate 5 days/week during a 60-day period (Pandey and Singh 2002). The reliability of this LOAEL 

is uncertain due to the lack of information on the copper content of the diet and because decreases in 

fertility and alterations in sperm parameters have not been observed in other high-quality studies 

involving exposure to 40 mg molybdenum/kg/day via the diet or drinking water in a 2-generation study 

(Murray et al. 2019) or 60 mg molybdenum/kg/day via the diet in a 90-day study (Murray et al. 2014a). 

Additionally, no developmental effects were observed in single-generation (Murray et al. 2014b) or 

2-generation (Murray et al. 2019) studies in rats exposed to 37.5–40 mg/kg/day.

As with the reproductive and developmental effects, only one study reported hematological effects. 

Anemia was reported in rabbits exposed to 54 mg molybdenum/kg/day as sodium molybdate in the diet 

for 30–84 days (Arrington and Davis 1953). This is considered a low-quality study because the 

molybdenum was sprayed on the food pellets but there was no measurement of actual dietary 
MOLYBDENUM A-19

APPENDIX A

concentrations, only 2–5 animals per group were tested, and no information was provided on which 

hematological parameters were altered. Additionally, the diet may not have provided adequate copper 

levels since copper supplementation was administered to the 54 mg/kg/day group to prevent deaths in the 

3/5 animals that exhibited “severe toxic symptoms” characteristic of copper deficiency.

If the reproductive, developmental, and hematological effects are excluded because they were reported in 

lesser-quality studies and were not confirmed in higher-quality studies, then the lowest LOAEL is 

60 mg/kg/day for body weight and renal effects (Murray et al. 2014a). A 15% decrease in body weight 

gain was observed in male rats; no significant alterations were observed in females. Although a decrease 

in food consumption was also observed at this dose level, decreases in food efficiency observed at this 

dose suggest that the decrease in body weight was not solely related to the decreased food intake. The 

renal effects consisted of slight diffuse hyperplasia in the renal proximal tubules of 2/10 female rats. The 

investigators (Murray et al. 2014a) noted that this effect is an uncommon background finding in rats of 

this age and considered it to be treatment related; they also suggested that the effect may be due to the 

high levels of copper in the kidneys. Kidney effects (degeneration followed by regeneration) have been 

observed in rats exposed to high levels of copper in the diet (Haywood 1985). A second molybdenum 

study (Bompart et al. 1990) reported diuresis, creatinuria, decreases in creatinine clearance, and increases 

in daily excretion of immunoreactive kallikrein in rats administered 80 mg molybdenum/kg/day via 

gavage for 8 weeks. These alterations are suggestive of decreased glomerular function and distal tubule 

damage; the absence of changes in the brush border enzymes alanine aminopeptidase and γ-glutamyl 

transpeptidase suggests no damage to the proximal tubule functional capacity. The study did not include 

histopathological examination of the kidneys. Although the incidence of proximal tubular hyperplasia 

was not statistically significant in the high-dose females in the Murray et al. (2014a) study, support for 

identifying this as the critical effect comes from the Bompart et al. (1990) study, which found evidence of 

impaired renal function in rats exposed to a slightly higher dose.

Several studies have reported decreases in body weight gain; the lowest LOAEL for this effect was 40 mg 

molybdenum/kg/day as sodium molybdate in the diet in a 2-generation study (Murray et al. 2019). This 

study reported a 22% decrease in body weight gain on GDs 0–7 in the parental-generation females. The 

difference in body weight gain over the length of the study was <10% lower than the controls. This was 

not observed in the F1 generation and was not observed in P or F1 generation rats similarly exposed to 

40 mg molybdenum/kg/day as sodium molybdate in the drinking water (Murray et al. 2019). Decreases 

in body weight have also been observed at higher molybdenum doses (Bompart et al. 1990; Mills et al. 

1958; Murray et al. 2014a; Van Reem and Williams 1956). The decrease in body weight gain observed in 

the Murray et al. (2019) study was not selected as the basis of the MRL because it was not replicated in 

the F1 generation or in rats exposed via drinking water (Murray et al. 2019).

Selection of the Principal Study: The Murray et al. (2014a) study was selected as the principal study 

because it identified the lowest LOAEL for renal effects. 

Summary of the Principal Study:

Murray FJ, Sullivan FM, Tiwary AK, et al. 2014a. 90-Day subchronic toxicity study of sodium 

molybdate dihydrate in rats. Regul Toxicol Pharmacol 79:579-588.

Groups of 10 male and 10 female Sprague-Dawley rats were exposed to 0, 5, 17, or 60 mg 

molybdenum/kg/day (actual concentrations were 0, 4.5, 15.1, and 54.8 mg/kg/day, respectively, in males 

and 0, 5.4, 19.0, and 65.2 mg/kg/day, respectively, in females and the average overall intakes were 0, 5.0, 

17.1, and 60.0 mg/kg/day, respectively) as sodium molybdate dihydrate in the diet for 91 and 92 days; 

additional groups of rats (10/sex/group) were similarly exposed to 0 or 60 mg/kg/day for 91–92 days and 

then continued on the basal diet for 60 days. The basal diet contained 906.5 μg/kg molybdenum and 
MOLYBDENUM A-20

APPENDIX A

14.23 mg/kg copper; the investigators estimated that the control group received 0.08 mg 

molybdenum/kg/day. The following parameters were used to assess toxicity: cage-side observations, 

weekly clinical examinations, ophthalmic examination, weekly body weight measurements, measurement 

of hematological (hemoglobin, hematocrit, erythrocyte, platelet, mean corpuscular hemoglobin 

concentration, mean corpuscular volume, red cell distribution width, total and differential leukocyte, 

reticulocyte, and prothrombin time) and serum chemistry (aspartate aminotransferase, alanine 

aminotransferase, alkaline phosphatase, blood urea nitrogen, creatinine, glucose, cholesterol, 

triglycerides, total protein, albumin, uric acid, total bilirubin, sodium, potassium, chloride, calcium, and 

inorganic phosphorus) parameters, organ weights (adrenal glands, brain, epididymides, heart, kidneys, 

liver, ovaries, pituitary gland, prostate gland and seminal vesicles, spleen, testes, thymus, thyroid/

parathyroid glands, and uterus with cervix), and histopathology examination of major tissues and organs 

in control and 60 mg/kg/day groups (primary and recovery groups) and the adrenal glands from males and 

kidneys from females in the 5 and 17 mg/kg/day groups. Additionally, sperm counts and sperm mobility 

and vaginal cytology and estrus cycles were evaluated.

Significant decreases in body weight gain were observed at 60 mg/kg/day in males starting at week 1 and 

in females starting at week 6. Terminal body weights were 15.2 and 5.6% less than controls, with only 

the males being significantly different from controls. At the end of the recovery period, the 60 mg/kg/day 

males weighed significantly less (9.5%) than controls. Decreases in food consumption were observed on 

numerous occasions in the males exposed to 60 mg/kg/day; a decrease in food conversion efficiency was 

also observed in this group. No significant or treatment-related alterations in hematological or serum 

chemistry parameters were observed. Significant decreases in absolute brain, liver, heart, spleen, and 

pituitary weights were observed in males exposed to 60 mg/kg/day; however, there were no significant 

alterations in relative organ weights. Treatment-related histopathological alterations were limited to a 

slight diffuse hyperplasia in the renal proximal tubules in 2/10 females in the 60 mg/kg/day group; the 

investigators considered it to be treatment-related because it is an uncommon finding at this age. No 

significant alterations in vaginal cytology or estrus cycles were observed. Similarly, no significant 

alterations in spermatid or sperm counts or sperm morphology were observed in males. A slight decrease 

in sperm motility was observed at 60 mg/kg/day; however, this was likely attributable to the control group 

having a value that approached the upper limit among historical controls and was not considered 

treatment related. No alterations in reproductive organ weights or histological alterations were observed.

Selection of the Point of Departure for the MRL: The NOAEL of 17 mg molybdenum/kg/day was 

selected as the POD for the MRL. BMD modeling was not considered because a response was only 

observed at the highest dose tested. A dataset exhibiting a response only at the highest dose level would 

likely provide limited information regarding the shape of a dose-response curve. 

Calculations: The investigators estimated doses using body weight and food consumption data.

Intermittent Exposure: Not applicable.

Uncertainty Factor and Modifying Factor: The NOAEL is divided by a total uncertainty factor (UF) of 

100 and a modifying factor (MF) of 3

• 10 UF for extrapolation from animals to humans

• 10 UF for human variability

• 3 MF for concern that reproductive and/or developmental effects may be a more sensitive 

endpoint than kidney effects in populations with marginal copper intakes. The copper content of 

the Murray et al. (2014b, 2019) reproductive/developmental studies used a commercial diet with 

a fairly high copper content. In contrast, the Fungwe et al. (1990) study, which reported 
MOLYBDENUM A-21

APPENDIX A

reproductive effects, utilized a diet that was slightly higher than the dietary requirement. The 

differences in the copper contents of the diet may explain differences between the study results. 

MRL = NOAEL ÷ (UFs x MF)

0.06 mg molybdenum/kg/day = 17 mg molybdenum/kg/day ÷ ((10 x 10) x 3)

Other Additional Studies or Pertinent Information that Lend Support to this MRL: Selection of the 

POD is supported by the Bompart et al. (1990) study, which found decreases in kidney function in rats 

administered sodium molybdate. 

The MRL is calculated based on the assumption of healthy dietary levels of molybdenum and copper and 

represents the level of exposure above and beyond the normal diet.

Agency Contacts (Chemical Managers): G. Daniel Todd
MOLYBDENUM A-22

APPENDIX A

MINIMAL RISK LEVEL (MRL) WORKSHEET

Chemical Name: Molybdenum

CAS Numbers: 7439-98-7

Date: May 2020

Profile Status: Final

Route: Oral

Duration: Chronic

MRL Summary: There are insufficient data for derivation of a chronic-duration oral MRL for 

molybdenum. The only available experimental study was considered a low-quality study that was not 

considered a suitable basis for an MRL. 

Rationale for Not Deriving an MRL: Data on the chronic toxicity of molybdenum come from several 

population-based studies; most of these studies looked for associations between background exposure to 

molybdenum and adverse health outcomes. No laboratory animal studies were identified. 

Koval’skiy et al. (1961) found increases in blood uric acid and symptoms of gout in residents living in 

Armenia with high levels of molybdenum in the soil and food; the investigators estimated that the 

residents were exposed to 10–15 mg/day (0.14–0.21 mg/kg/day). A series of small studies of residents 

living in areas of Colorado with high levels of molybdenum in the drinking water did not find significant 

increases in uric acid levels; one study estimated that molybdenum intake was 500 μg/day 

(0.007 mg/kg/day) (EPA 1979). Other studies have found significant associations between serum or 

urinary molybdenum levels and the severity of complications from diabetes (Rodriguez Flores et al. 

2011), high blood pressure (Yorita Christensen 2013), semen quality (Meeker et al. 2008), testosterone 

levels (Meeker et al. 2010), and psychomotor index in infants (molybdenum levels were measured in the 

mothers) (Vazques-Salas et al. 2014). However, none of these studies established causality, and the 

molybdenum levels accounted for only a small percentage of the variance. 

Although the Koval’sky et al. (1961) study provided an estimated dose, the study was not considered 

suitable for derivation of a chronic-duration oral MRL for molybdenum. The study has a number of 

deficiencies that limit the interpretation of the results: (1) the control group consisted of 5 individuals 

compared to 52 subjects in the exposed group; (2) no information was provided on the controls to assess 

whether they were matched to the exposed group; (3) it does not appear that the study controlled for 

potential confounders, such as diet and alcohol, which can increase uric acid levels; and (4) NAS (2001) 

noted that there were potential analytical problems with the measurement of serum and urine copper 

levels.

Agency Contacts (Chemical Managers): G. Daniel Todd
MOLYBDENUM B-1

APPENDIX B. LITERATURE SEARCH FRAMEWORK FOR MOLYBDENUM

The objective of the toxicological profile is to evaluate the potential for human exposure and the potential 

health hazards associated with inhalation, oral, or dermal/ocular exposure to molybdenum. 

B.1 LITERATURE SEARCH AND SCREEN 

A literature search and screen was conducted to identify studies examining health effects, toxicokinetics, 

mechanisms of action, susceptible populations, biomarkers, chemical interactions, physical and chemical 

properties, production, use, environmental fate, environmental releases, and environmental and biological 

monitoring data for molybdenum. ATSDR primarily focused on peer-reviewed articles without 

publication date or language restrictions. Non-peer-reviewed studies that were considered relevant to the 

assessment of the health effects of molybdenum have undergone peer review by at least three ATSDRselected experts who have been screened for conflict of interest. The inclusion criteria used to identify 

relevant studies examining the health effects of molybdenum are presented in Table B-1.

Table B-1. Inclusion Criteria for the Literature Search and Screen

Health Effects

Species

Human

Laboratory mammals

Route of exposure

Inhalation

Oral

Dermal (or ocular)

Parenteral (these studies will be considered supporting data)

Health outcome

Death

Systemic effects

Body weight effects 

Respiratory effects

Cardiovascular effects

Gastrointestinal effects

Hematological effects

Musculoskeletal effects

Hepatic effects

Renal effects

Dermal effects

Ocular effects

Endocrine effects

Immunological effects

Neurological effects

Reproductive effects

Developmental effects

Other noncancer effects
MOLYBDENUM B-2

APPENDIX B

Table B-1. Inclusion Criteria for the Literature Search and Screen

Cancer

Toxicokinetics

Absorption

Distribution

Metabolism

Excretion

PBPK models

Biomarkers

Biomarkers of exposure

Biomarkers of effect

Interactions with other chemicals

Potential for human exposure

Releases to the environment

Air

Water

Soil

Environmental fate

Transport and partitioning

Transformation and degradation

Environmental monitoring

Air

Water

Sediment and soil

Other media

Biomonitoring

General populations

Occupation populations

B.1.1 Literature Search

The current literature search was intended to update the draft toxicological profile for molybdenum

released for public comment in 2017. The following main databases were searched in January 2018:

• PubMed 

• National Library of Medicine’s TOXLINE

• Scientific and Technical Information Network’s TOXCENTER

The search strategy used the chemical names, Chemical Abstracts Service (CAS) numbers, 

synonyms, Medical Subject Headings (MeSH) headings, and keywords for molybdenum. The query 

strings used for the literature search are presented in Table B-2. 

The search was augmented by searching the Toxic Substances Control Act Test Submissions (TSCATS), 

NTP website, and National Institute of Health Research Portfolio Online Reporting Tools Expenditures 

and Results (NIH RePORTER) databases using the queries presented in Table B-3. Additional databases 
MOLYBDENUM B-3

APPENDIX B

were searched in the creation of various tables and figures, such as the TRI Explorer, the Substance 

Priority List (SPL) resource page, and other items as needed. Regulations applicable to molybdenum 

were identified by searching international and U.S. agency websites and documents.

Review articles were identified and used for the purpose of providing background information and 

identifying additional references. ATSDR also identified reports from the grey literature, which included 

unpublished research reports, technical reports from government agencies, conference proceedings and 

abstracts, and theses and dissertations. 

Table B-2. Database Query Strings 

Database

search date Query string

PubMed

01/2018 ((("Molybdenum/toxicity"[mh] OR "Molybdenum/adverse effects"[mh] OR 

"Molybdenum/poisoning"[mh] OR "Molybdenum/pharmacokinetics"[mh]) OR 

("Molybdenum"[mh] AND ("environmental exposure"[mh] OR ci[sh])) OR 

("Molybdenum"[mh] AND toxicokinetics[mh:noexp]) OR ("Molybdenum/blood"[mh] OR 

"Molybdenum/cerebrospinal fluid"[mh] OR "Molybdenum/urine"[mh]) OR 

("Molybdenum"[mh] AND ("endocrine system"[mh] OR "hormones, hormone substitutes, 

and hormone antagonists"[mh] OR "endocrine disruptors"[mh])) OR ("Molybdenum"[mh] 

AND ("computational biology"[mh] OR "medical informatics"[mh] OR genomics[mh] OR 

genome[mh] OR proteomics[mh] OR proteome[mh] OR metabolomics[mh] OR 

metabolome[mh] OR genes[mh] OR "gene expression"[mh] OR phenotype[mh] OR 

genetics[mh] OR genotype[mh] OR transcriptome[mh] OR ("systems biology"[mh] AND 

("environmental exposure"[mh] OR "epidemiological monitoring"[mh] OR analysis[sh])) OR 

"transcription, genetic "[mh] OR "reverse transcription"[mh] OR "transcriptional 

activation"[mh] OR "transcription factors"[mh] OR ("biosynthesis"[sh] AND (RNA[mh] OR 

DNA[mh])) OR "RNA, messenger"[mh] OR "RNA, transfer"[mh] OR "peptide 

biosynthesis"[mh] OR "protein biosynthesis"[mh] OR "reverse transcriptase polymerase 

chain reaction"[mh] OR "base sequence"[mh] OR "trans-activators"[mh] OR "gene 

expression profiling"[mh])) OR ("Molybdenum/antagonists and inhibitors"[mh]) OR 

("Molybdenum/metabolism"[mh] AND ("humans"[mh] OR "animals"[mh])) OR 

("Molybdenum"[mh] AND cancer[sb]) OR ("Molybdenum/pharmacology"[majr])) AND 

(2013/12/01 : 3000[dp] OR 2014/12/01 : 3000[mhda])) OR (("1317-33-5"[rn] OR "12033-

29-3"[rn] OR "12033-33-9"[rn] OR "11098-99-0"[rn] OR "18868-43-4"[rn] OR "1313-27-

5"[rn] OR "1313-29-7"[rn] OR "11098-84-3"[rn] OR "27546-07-2"[rn] OR "12054-85-2"[rn] 

OR "15060-55-6"[rn] OR "7631-95-0"[rn] OR "10102-40-6"[rn] OR "7789-82-4"[rn] OR 

"12011-97-1"[rn] OR "11119-46-3"[rn] OR "11062-51-4"[rn] OR "10241-05-1"[rn] OR "1309-

56-4"[rn] OR "7783-77-9"[rn] OR "13939-06-5"[rn] OR "14221-06-8"[rn] OR "13814-74-

9"[rn] OR "12027-67-7"[rn] OR "13106-76-8"[rn]) AND (("Disulfides/toxicity"[mh] OR 

"Disulfides/adverse effects"[mh] OR "Disulfides/poisoning"[mh] OR 

"Disulfides/pharmacokinetics"[mh]) OR ("Disulfides/blood"[mh] OR 

"Disulfides/cerebrospinal fluid"[mh] OR "Disulfides/urine"[mh]) OR ("Disulfides/antagonists 

and inhibitors"[mh]) OR ("Disulfides/metabolism"[mh] AND ("humans"[mh] OR 

"animals"[mh])) OR ("Disulfides/pharmacology"[majr]) OR ("Chlorides/toxicity"[mh] OR 

"Chlorides/adverse effects"[mh] OR "Chlorides/poisoning"[mh] OR 

"Chlorides/pharmacokinetics"[mh]) OR ("Chlorides/blood"[mh] OR "Chlorides/cerebrospinal 

fluid"[mh] OR "Chlorides/urine"[mh]) OR ("Chlorides/antagonists and inhibitors"[mh]) OR 

("Chlorides/metabolism"[mh] AND ("humans"[mh] OR "animals"[mh])) OR 

("Chlorides/pharmacology"[majr]) OR ("Oxides/toxicity"[mh] OR "Oxides/adverse 

effects"[mh] OR "Oxides/poisoning"[mh] OR "Oxides/pharmacokinetics"[mh]) OR 

("Oxides/blood"[mh] OR "Oxides/cerebrospinal fluid"[mh] OR "Oxides/urine"[mh]) OR 
MOLYBDENUM B-4

APPENDIX B

Table B-2. Database Query Strings 

Database

search date Query string

("Oxides/antagonists and inhibitors"[mh]) OR ("Oxides/metabolism"[mh] AND 

("humans"[mh] OR "animals"[mh])) OR ("Oxides/pharmacology"[majr]) OR 

(("disulfides"[mh] OR "chlorides"[mh] OR "oxides"[mh]) AND ("environmental 

exposure"[mh] OR ci[sh])) OR (("disulfides"[mh] OR "chlorides"[mh] OR "oxides"[mh]) AND 

toxicokinetics[mh:noexp]) OR (("disulfides"[mh] OR "chlorides"[mh] OR "oxides"[mh]) AND 

("endocrine system"[mh] OR "hormones, hormone substitutes, and hormone 

antagonists"[mh] OR "endocrine disruptors"[mh])) OR (("disulfides"[mh] OR "chlorides"[mh] 

OR "oxides"[mh]) AND ("computational biology"[mh] OR "medical informatics"[mh] OR 

genomics[mh] OR genome[mh] OR proteomics[mh] OR proteome[mh] OR 

metabolomics[mh] OR metabolome[mh] OR genes[mh] OR "gene expression"[mh] OR 

phenotype[mh] OR genetics[mh] OR genotype[mh] OR transcriptome[mh] OR ("systems 

biology"[mh] AND ("environmental exposure"[mh] OR "epidemiological monitoring"[mh] OR 

analysis[sh])) OR "transcription, genetic "[mh] OR "reverse transcription"[mh] OR 

"transcriptional activation"[mh] OR "transcription factors"[mh] OR ("biosynthesis"[sh] AND 

(RNA[mh] OR DNA[mh])) OR "RNA, messenger"[mh] OR "RNA, transfer"[mh] OR "peptide 

biosynthesis"[mh] OR "protein biosynthesis"[mh] OR "reverse transcriptase polymerase 

chain reaction"[mh] OR "base sequence"[mh] OR "trans-activators"[mh] OR "gene 

expression profiling"[mh])) OR (("disulfides"[mh] OR "chlorides"[mh] OR "oxides"[mh]) AND 

cancer[sb])) AND (2013/12/01 : 3000[dp] OR 2014/12/01 : 3000[mhda])) OR "12027-67-

7"[rn] OR ((("7439-98-7"[rn] OR "1317-33-5"[rn] OR "12033-29-3"[rn] OR "12033-33-9"[rn] 

OR "11098-99-0"[rn] OR "18868-43-4"[rn] OR "1313-27-5"[rn] OR "1313-29-7"[rn] OR 

"11098-84-3"[rn] OR "27546-07-2"[rn] OR "12054-85-2"[rn] OR "15060-55-6"[rn] OR "7631-

95-0"[rn] OR "10102-40-6"[rn] OR "7789-82-4"[rn] OR "12011-97-1"[rn] OR "11119-46-

3"[rn] OR "11062-51-4"[rn] OR "10241-05-1"[rn] OR "1309-56-4"[rn] OR "7783-77-9"[rn] 

OR "13939-06-5"[rn] OR "14221-06-8"[rn] OR "13814-74-9"[rn] OR "12027-67-7"[rn] OR 

"13106-76-8"[rn]) NOT ("molybdenum"[mh] OR "disulfides"[mh] OR "chlorides"[mh] OR 

"oxides"[mh])) AND (2013/12/01 : 3000[dp] OR 2014/12/01 : 3000[mhda])) OR 

(("Ammonium molybdenum sulfide"[tw] OR "Ammonium tetrasulfidomolybdate(2-)"[tw] OR 

"Ammonium tetrathiomolybdate"[tw] OR "Ammonium thiomolybdate(VI)"[tw] OR 

"ATTM"[tw] OR "Bis(ammonium)tetrathiomolybdate(2-)"[tw] OR "Calcium molybdate"[tw] 

OR "Calcium molybdate(VI)"[tw] OR "Calcium molybdenate"[tw] OR "Calcium molybdenum 

oxide"[tw] OR "Coprexa"[tw] OR "Diammonium tetrakis(sulfido)molybdate(2-)"[tw] OR 

"Diammonium tetrakis(thioxo)molybdate"[tw] OR "Diammonium tetrasulfidomolybdate"[tw] 

OR "Diammonium tetrathiomolybdate"[tw] OR "Diammonium tetrathiomolybdate(2-)"[tw] 

OR "Diammonium tetrathiooxomolybdate(2-)"[tw] OR "Diammonium 

tetrathioxomolybdate(2-)"[tw] OR "Diammonium thiomolybdate"[tw] OR "Dimolybdenum 

tetraacetate"[tw] OR "Dimolybdenum trioxide"[tw] OR "Dodecachlorohexamolybdenum"[tw] 

OR "Hexafluoromolybdenum"[tw] OR "Hexamolybdenum dodecachloride"[tw] OR "MC 

400WR"[tw] OR "Molybdate, calcium"[tw] OR "Molybdenite"[tw] OR "Molybdenum 

anhydride"[tw] OR "Molybdenum carbide"[tw] OR "Molybdenum chloride"[tw] OR 

"Molybdenum chloride oxide"[tw] OR "Molybdenum dioxide"[tw] OR "Molybdenum 

fluoride"[tw] OR "Molybdenum hexafluoride"[tw] OR "Molybdenum monocarbide"[tw] OR 

"Molybdenum oxide"[tw] OR "Molybdenum oxychloride"[tw] OR "Molybdenum 

oxytrichloride"[tw] OR "Molybdenum sesquioxide"[tw] OR "Molybdenum sulfide"[tw] OR 

"Molybdenum trichloride monoxide"[tw] OR "Molybdenum trichloride oxide"[tw] OR 

"Molybdenum trisulfide"[tw] OR "Molybdenum(6+) fluoride"[tw] OR "Molybdenum(II) 

acetate"[tw] OR "Molybdenum(IV) oxide"[tw] OR "Molybdic acid, calcium salt"[tw] OR 

"Octachlorohexamolybdenum(4+) tetrachloride"[tw] OR "Tetraacetatodimolybdenum"[tw] 

OR "tetrakis(acetato)di-Molybdenum"[tw] OR "Tetrakis(acetato)dimolybdenum"[tw] OR 

"Tetrakis(acetato)molybdenum"[tw] OR "Tetrakis(mu-(acetato-O:O'))dimolybdenum"[tw] 
MOLYBDENUM B-5

APPENDIX B

Table B-2. Database Query Strings 

Database

search date Query string

OR "tetrakis(mu-acetato)di-Molybdenum"[tw] OR "Tetrakis(mu-acetato)dimolybdenum"[tw] 

OR "tetrakis[mu-(acetato-O:O')]di-Molybdenum"[tw] OR "Thiomolybdic acid (H2MoS4), 

diammonium salt"[tw] OR "Thiomolybdic acid, diammonium salt"[tw] OR "Tiomolibdate 

diammonium"[tw] OR "Trichlorooxomolybdenum"[tw] OR 

"Trichlorooxomolybdenum(V)"[tw]) AND (2013/12/01 : 3000[dp] OR 2014/12/01 : 

3000[crdat] OR 2014/12/01 : 3000[edat])) OR ((("3N5"[tw] OR "A Powder"[tw] OR 

"Ammonium dimolybdate"[tw] OR "Ammonium heptamolybdate"[tw] OR "Ammonium 

heptamolybdate tetrahydrate"[tw] OR "Ammonium molibdate"[tw] OR "Ammonium 

molibdenum oxide"[tw] OR "Ammonium molybdate"[tw] OR "Ammonium molybdate 

hydrate"[tw] OR "Ammonium molybdate tetrahydrate"[tw] OR "Ammonium 

molybdate(VI)"[tw] OR "Ammonium molybdenum oxide"[tw] OR "Ammonium molybdenum 

sulfide"[tw] OR "ammonium paramolybdate"[tw] OR "Ammonium paramolybdate 

tetrahydrate"[tw] OR "Ammonium tetrasulfidomolybdate(2-)"[tw] OR "Ammonium 

tetrathiomolybdate"[tw] OR "Ammonium thiomolybdate(VI)"[tw] OR "Amperit 105.054"[tw] 

OR "Amperit 106.2"[tw] OR "ATTM"[tw] OR "Bis(ammonium)tetrathiomolybdate(2-)"[tw] OR 

"Bouen SKN 301"[tw] OR "C-Powder"[tw] OR "Calcium molybdate"[tw] OR "Calcium 

molybdate(VI)"[tw] OR "Calcium molybdenate"[tw] OR "Calcium molybdenum oxide"[tw] 

OR "Coprexa"[tw] OR "DAG 206"[tw] OR "DAG 325"[tw] OR "DAG-V 657"[tw] OR "Defric 

coat HMB 2"[tw] OR "Diammonium dimolybdate"[tw] OR "Diammonium 

tetrakis(sulfido)molybdate(2-)"[tw] OR "Diammonium tetrakis(thioxo)molybdate"[tw] OR 

"Diammonium tetrasulfidomolybdate"[tw] OR "Diammonium tetrathiomolybdate"[tw] OR 

"Diammonium tetrathiomolybdate(2-)"[tw] OR "Diammonium tetrathiooxomolybdate(2-)"[tw] 

OR "Diammonium tetrathioxomolybdate(2-)"[tw] OR "Diammonium thiomolybdate"[tw] OR 

"Dimolybdenum tetraacetate"[tw] OR "Dimolybdenum trioxide"[tw] OR "dimolybdenum 

trisulfide "[tw] OR "Disodium molybdate"[tw] OR "Disodium molybdate dihydrate"[tw] OR 

"Disodium tetraoxomolybdate"[tw] OR "DM 1 (sulfide)"[tw] OR "DMI 7"[tw] OR 

"Dodecachlorohexamolybdenum"[tw] OR "Hexaammonium heptamolybdate 

tetrahydrate"[tw] OR "Hexaammonium molybdate tetrahydrate"[tw] OR 

"Hexacarbonylmolybdenum"[tw] OR "Hexafluoromolybdenum"[tw] OR "Hexamolybdenum 

dodecachloride"[tw] OR "JCPDS 35-0609"[tw] OR "Liqui-Moly LM 11"[tw] OR "Liqui-Moly 

LM 2"[tw] OR "Liqui-Moly Z Powder"[tw] OR "LM 13"[tw] OR "MC 400WR"[tw] OR 

"MChVL"[tw] OR "MD 40"[tw] OR "Metco 63"[tw] OR "MF 000"[tw] OR "MIPO-M 15"[tw] OR 

"Mo 1202T"[tw] OR "Mo-1202T"[tw] OR "Moly Fine Powder Y"[tw] OR "Moly Powder B"[tw] 

OR "Moly Powder C"[tw] OR "Moly Powder PA"[tw] OR "Moly Powder PB"[tw] OR "Moly 

Powder PS"[tw] OR "Molybdate (Mo2O72-), diammonium"[tw] OR "Molybdate (MoO42-), 

disodium, dihydrate, (T-4)-"[tw] OR "Molybdate, calcium"[tw] OR "Molybdena"[tw] OR 

"Molybdenite"[tw] OR "Molybdenum"[tw] OR "Molybdenum anhydride"[tw] OR 

"Molybdenum bisulfide"[tw] OR "Molybdenum carbide"[tw] OR "Molybdenum carbonyl"[tw] 

OR "Molybdenum chloride"[tw] OR "Molybdenum chloride oxide"[tw] OR "Molybdenum 

dioxide"[tw] OR "Molybdenum disulfide"[tw] OR "Molybdenum disulphide"[tw] OR 

"Molybdenum fluoride"[tw] OR "Molybdenum hexacarbonyl"[tw] OR "Molybdenum 

hexafluoride"[tw] OR "Molybdenum metallicum"[tw] OR "Molybdenum monocarbide"[tw] 

OR "Molybdenum oxide"[tw] OR "Molybdenum oxychloride"[tw] OR "Molybdenum 

oxytrichloride"[tw] OR "Molybdenum pentachloride"[tw] OR "Molybdenum peroxide"[tw] OR 

"Molybdenum sesquioxide"[tw] OR "Molybdenum sesquisulfide"[tw] OR "Molybdenum 

sodium oxide"[tw] OR "Molybdenum sulfide"[tw] OR "Molybdenum sulphide"[tw] OR 

"Molybdenum trichloride monoxide"[tw] OR "Molybdenum trichloride oxide"[tw] OR 

"Molybdenum trioxide"[tw] OR "Molybdenum trioxide pentamer"[tw] OR "Molybdenum 

trioxide tetramer"[tw] OR "Molybdenum trisulfide"[tw] OR "Molybdenum(6+) fluoride"[tw] 

OR "Molybdenum(II) acetate"[tw] OR "Molybdenum(II) chloride"[tw] OR "molybdenum(III) 
MOLYBDENUM B-6

APPENDIX B

Table B-2. Database Query Strings 

Database

search date Query string

sulfide"[tw] OR "molybdenum(IV) oxide"[tw] OR "Molybdenum(IV) sulfide"[tw] OR 

"Molybdenum(V) chloride"[tw] OR "Molybdenum(VI) oxide"[tw] OR "Molybdenum(VI) 

trioxide"[tw] OR "Molybdenumperoxide"[tw] OR "Molybdic acid (H2Mo2O7), diammonium 

salt"[tw] OR "Molybdic acid (H2MoO4), calcium salt (1:1)"[tw] OR "Molybdic acid 

anhydride"[tw] OR "Molybdic acid, ammonium salt"[tw] OR "Molybdic acid, calcium salt"[tw] 

OR "Molybdic acid, disodium salt"[tw] OR "Molybdic acid, disodium salt, dihydrate"[tw] OR 

"Molybdic anhydride"[tw] OR "Molybdic oxide"[tw] OR "Molybdic trioxide"[tw] OR 

"Molycolloid CF 626"[tw] OR "Molyform 15"[tw] OR "Molyhibit 100"[tw] OR "Molyka R"[tw] 

OR "Molyka R-L 3"[tw] OR "Molyke R"[tw] OR "Molykote"[tw] OR "Molykote Microsize 

Powder"[tw] OR "Molykote Z"[tw] OR "Molykote Z Powder"[tw] OR "Molysulfide"[tw] OR 

"MOP-P 100"[tw] OR "Mopol M"[tw] OR "Mopol S"[tw] OR "Motimol"[tw] OR "MVCh 1"[tw] 

OR "Natural molybdenite"[tw] OR "Natural molybdite"[tw] OR "NeoZ"[tw] OR "Nichimoly 

C"[tw] OR "Octachlorohexamolybdenum(4+) tetrachloride"[tw] OR "OKS 110"[tw] OR "PA 

Powder"[tw] OR "Pentachloromolybdenum Molybdenite"[tw] OR "Pigment Black 34"[tw] OR 

"Pol-U"[tw] OR "Powder PA"[tw] OR "RAC 01"[tw] OR "SGC 15"[tw] OR "Sodium 

molybdate"[tw] OR "Sodium molybdate dihydrate"[tw] OR "Sodium molybdate(VI)"[tw] OR 

"Sodium molybdate(VI) dihydrate"[tw] OR "Sodium molybdenate"[tw] OR "Sodium 

molybdenum oxide"[tw] OR "Sodium tetraoxomolybdate(2-)"[tw] OR "Solvest 390A"[tw] OR 

"Sumipowder PA"[tw] OR "T-Powder"[tw] OR "Tetraacetatodimolybdenum"[tw] OR 

"tetrakis(acetato)di-Molybdenum"[tw] OR "Tetrakis(acetato)dimolybdenum"[tw] OR 

"Tetrakis(acetato)molybdenum"[tw] OR "Tetrakis(mu-(acetato-O:O'))dimolybdenum"[tw] 

OR "tetrakis(mu-acetato)di-Molybdenum"[tw] OR "Tetrakis(mu-acetato)dimolybdenum"[tw] 

OR "tetrakis[mu-(acetato-O:O')]di-Molybdenum"[tw] OR "Thiomolybdic acid, diammonium 

salt"[tw] OR "Tiomolibdate diammonium"[tw] OR "TMOIO"[tw] OR 

"Trichlorooxomolybdenum"[tw] OR "Trichlorooxomolybdenum(V)"[tw] OR "TsM1"[tw] OR 

"UP 10"[tw] OR "UP 50"[tw] OR ("Hexaammonium heptamolybdate"[tw] OR 

"Hexammonium heptamolybdat"[tw] OR "Hexammonium tetracosaoxoheptamolybdate"[tw] 

OR "Molybdate (Mo7O24), hexammonium"[tw] OR "Molybdate (Mo7O246-), ammonium 

(1:6)"[tw] OR "Molybdate (Mo7O246-), hexaammonium"[tw] OR "Molybdate, 

hexaammonium"[tw] OR "Molybdic acid (H6Mo7O24), hexaammonium salt"[tw] OR 

"Molybdic acid, hexaammonium salt"[tw] OR "Diammonium molybdate"[tw] OR 

"Diammonium tetraoxomolybdate(2-)"[tw] OR "Molybdate (MoO42-), ammonium (1:2), (T4)-"[tw] OR "Molybdate (MoO42-), diammonium, (beta-4)-"[tw] OR "Molybdate (MoO42-), 

diammonium, (T-4)-"[tw] OR "Molybdic acid (H2MoO4), diammonium salt"[tw] OR 

"Molybdic acid, diammonium salt"[tw])) NOT medline[sb]) AND (2013/12/01 : 3000[dp] OR 

2014/12/01 : 3000[crdat] OR 2014/12/01 : 3000[edat]))

Toxline

01/2018 Date limit 2013 to present:

7439-98-7[rn] OR 1317-33-5[rn] OR 12033-29-3[rn] OR 12033-33-9[rn] OR 11098-99-0[rn] 

OR 18868-43-4[rn] OR 1313-27-5[rn] OR 1313-29-7[rn] OR 11098-84-3[rn] OR 27546-07-

2[rn] OR 12054-85-2[rn] OR 15060-55-6[rn] OR 7631-95-0[rn] OR 10102-40-6[rn] OR 

7789-82-4[rn] OR 12011-97-1[rn] OR 11119-46-3[rn] OR 11062-51-4[rn] OR 10241-05-

1[rn] OR 1309-56-4[rn] OR 7783-77-9[rn] OR 13939-06-5[rn] OR 14221-06-8[rn] OR 

13814-74-9[rn]

"3N5" OR "Ammonium dimolybdate" OR "Ammonium heptamolybdate" OR "Ammonium 

heptamolybdate tetrahydrate" OR "Ammonium molibdate" OR "Ammonium molibdenum 

oxide" OR "Ammonium molybdate" OR "Ammonium molybdate hydrate" OR "Ammonium 

molybdate tetrahydrate" OR "Ammonium molybdate(VI)" OR "Ammonium molybdenum 

oxide"
MOLYBDENUM B-7

APPENDIX B

Table B-2. Database Query Strings 

Database

search date Query string

"Ammonium molybdenum sulfide" OR "ammonium paramolybdate" OR "Ammonium 

paramolybdate tetrahydrate" OR "Ammonium tetrasulfidomolybdate(2-)" OR "Ammonium 

tetrathiomolybdate" OR "Ammonium thiomolybdate(VI)" OR "Amperit 105.054" OR 

"Amperit 106.2" OR "ATTM" OR "Bis(ammonium)tetrathiomolybdate(2-)" OR "Bouen SKN 

301" OR "C-Powder" OR "Calcium molybdate" 

"Calcium molybdate(VI)" OR "Calcium molybdenate" OR "Calcium molybdenum oxide" OR 

"Coprexa" OR "DAG 206" OR "DAG 325" OR "DAG-V 657" OR "Defric coat HMB 2" OR 

"Diammonium dimolybdate" OR "Diammonium tetrakis(sulfido)molybdate(2-)" OR 

"Diammonium tetrakis(thioxo)molybdate" OR "Diammonium tetrasulfidomolybdate" OR 

"Diammonium tetrathiomolybdate"

"Diammonium tetrathiomolybdate(2-)" OR "Diammonium tetrathiooxomolybdate(2-)" OR 

"Diammonium tetrathioxomolybdate(2-)" OR "Diammonium thiomolybdate" OR 

"Dimolybdenum tetraacetate" OR "Dimolybdenum trioxide" OR "dimolybdenum trisulfide " 

OR "Disodium molybdate" OR "Disodium molybdate dihydrate" OR "Disodium 

tetraoxomolybdate" OR "DM 1 (sulfide)" OR "DMI 7" 

"Dodecachlorohexamolybdenum" OR "Hexaammonium heptamolybdate tetrahydrate" OR 

"Hexaammonium molybdate tetrahydrate" OR "Hexacarbonylmolybdenum" OR 

"Hexafluoromolybdenum" OR "Hexamolybdenum dodecachloride" OR "JCPDS 35-0609" 

OR "Liqui-Moly LM 11" OR "Liqui-Moly LM 2" OR "Liqui-Moly Z Powder" OR "LM 13" OR 

"M 5" OR "MC 400WR"

"MChVL" OR "MD 40" OR "Metco 63" OR "MF 000" OR "MFR" OR "MIPO-M 15" OR "Mo

1202T" OR "Mo-1202T" OR "Moly Fine Powder Y" OR "Moly Powder B" OR "Moly Powder 

C" OR "Moly Powder PA" OR "Moly Powder PB" OR "Moly Powder PS" OR "Molybdate 

(Mo2O72-), diammonium" OR "Molybdate (MoO42-), disodium, dihydrate, (T-4)-" OR 

"Molybdate, calcium" OR "Molybdena" OR "Molybdenite"

"Molybdenum" OR "Molybdenum anhydride" OR "Molybdenum bisulfide" OR "Molybdenum 

carbide" OR "Molybdenum carbonyl" OR "Molybdenum chloride" OR "Molybdenum 

chloride oxide" OR "Molybdenum dioxide" OR "Molybdenum disulfide" OR "Molybdenum 

disulphide" OR "Molybdenum fluoride" OR "Molybdenum hexacarbonyl" OR "Molybdenum 

hexafluoride"

"Molybdenum metallicum" OR "Molybdenum monocarbide" OR "Molybdenum oxide" OR 

"Molybdenum oxychloride" OR "Molybdenum oxytrichloride" OR "Molybdenum 

pentachloride" OR "Molybdenum peroxide" OR "Molybdenum sesquioxide" OR 

"Molybdenum sesquisulfide" OR "Molybdenum sodium oxide" OR "Molybdenum sulfide" 

OR "Molybdenum sulphide" OR "Molybdenum trichloride monoxide" 

"Molybdenum trichloride oxide" OR "Molybdenum trioxide" OR "Molybdenum trioxide 

pentamer" OR "Molybdenum trioxide tetramer" OR "Molybdenum trisulfide" OR 

"Molybdenum(6+) fluoride" OR "Molybdenum(II) acetate" OR "Molybdenum(II) chloride" 

OR "molybdenum(III) sulfide" OR "molybdenum(IV) oxide" OR "Molybdenum(IV) sulfide" 

OR "Molybdenum(V) chloride" 

"Molybdenum(VI) oxide" OR "Molybdenum(VI) trioxide" OR "Molybdenumperoxide" OR 

"Molybdic acid (H2Mo2O7), diammonium salt" OR "Molybdic acid (H2MoO4), calcium salt 

(1:1)" OR "Molybdic acid anhydride" OR "Molybdic acid, ammonium salt" OR "Molybdic 

acid, calcium salt" OR "Molybdic acid, disodium salt" OR "Molybdic acid, disodium salt, 

dihydrate" OR "Molybdic anhydride"

"Molybdic oxide" OR "Molybdic trioxide" OR "Molycolloid CF 626" OR "Molyform 15" OR 

"Molyhibit 100" OR "Molyka R" OR "Molyka R-L 3" OR "Molyke R" OR "Molykote" OR 

"Molykote Microsize Powder" OR "Molykote Z" OR "Molykote Z Powder" OR "Molysulfide" 
MOLYBDENUM B-8

APPENDIX B

Table B-2. Database Query Strings 

Database

search date Query string

OR "MOP-P 100" OR "Mopol M" OR "Mopol S" OR "Motimol" OR "MVCh 1" OR "Natural 

molybdenite" OR "Natural molybdite" OR "NeoZ" 

"Nichimoly C" OR "Octachlorohexamolybdenum(4+) tetrachloride" OR "OKS 110" OR "PA 

Powder" OR "Pentachloromolybdenum Molybdenite" OR "Pigment Black 34" OR "Pol-U" 

OR "Powder PA" OR "RAC 01" OR "SGC 15" OR "Sodium molybdate" OR "Sodium 

molybdate dihydrate" OR "Sodium molybdate(VI)" OR "Sodium molybdate(VI) dihydrate" 

OR "Sodium molybdenate"

"Sodium molybdenum oxide" OR "Sodium tetraoxomolybdate(2-)" OR "Solvest 390A" OR 

"Sumipowder PA" OR "T-Powder" OR "Tetraacetatodimolybdenum" OR 

"tetrakis(acetato)di-Molybdenum" OR "Tetrakis(acetato)dimolybdenum" OR 

"Tetrakis(acetato)molybdenum" OR "Tetrakis(mu-(acetato-O:O'))dimolybdenum" OR 

"tetrakis(μ-acetato)di-Molybdenum" OR "Tetrakis(μ-acetato)dimolybdenum" 

"tetrakis[μ-(acetato-O:O')]di-Molybdenum" OR "Thiomolybdic acid, diammonium salt" OR 

"Tiomolibdate diammonium" OR "TMOIO" OR "Trichlorooxomolybdenum" OR 

"Trichlorooxomolybdenum(V)" OR "TsM1" 

No date limit:

"Hexaammonium heptamolybdate" OR "Hexammonium heptamolybdat" OR 

"Hexammonium tetracosaoxoheptamolybdate" OR "Molybdate (Mo7O24), hexammonium" 

OR "Molybdate (Mo7O246-), ammonium (1:6)" OR "Molybdate (Mo7O246-), 

hexaammonium" OR "Molybdate, hexaammonium" OR "Molybdic acid (H6Mo7O24), 

hexaammonium salt" OR "Molybdic acid, hexaammonium salt" OR 12027-67-7[rn]

"Diammonium molybdate" OR "Diammonium tetraoxomolybdate(2-)" OR "Molybdate 

(MoO42-), ammonium (1:2), (T-4)-" OR "Molybdate (MoO42-), diammonium, (beta-4)-" OR 

"Molybdate (MoO42-), diammonium, (T-4)-" OR "Molybdic acid (H2MoO4), diammonium 

salt" OR "Molybdic acid, diammonium salt" OR 13106-76-8[rn]

Toxcenter

01/2018 (FILE 'HOME' ENTERED AT 14:16:56 ON 12 JAN 2018)

 FILE 'TOXCENTER' ENTERED AT 14:17:07 ON 12 JAN 2018

CHARGED TO COST=EH011.05.LB.02.05

L1 34132 SEA FILE=TOXCENTER 7439-98-7 OR 1317-33-5 OR 12033-29-3 OR 

 12033-33-9 OR 11098-99-0 OR 18868-43-4 OR 1313-27-5 OR 

 1313-29-7 OR 11098-84-3 OR 27546-07-2 OR 12054-85-2 OR 

 15060-55-6 OR 7631-95-0 OR 10102-40-6 

L2 523 SEA FILE=TOXCENTER 7789-82-4 OR 12011-97-1 OR 11119-46-3 OR 

 11062-51-4 OR 10241-05-1 OR 1309-56-4 OR 7783-77-9 OR 13939-06-

 5 OR 14221-06-8 OR 13814-74-9 

L3 1148 SEA FILE=TOXCENTER 12027-67-7 OR 13106-76-8 

L4 34486 SEA FILE=TOXCENTER L1 OR L2 

L5 636 SEA FILE=TOXCENTER L3 NOT L4 

L6 35122 SEA FILE=TOXCENTER L1 OR L2 OR L3 

L7 22953 SEA FILE=TOXCENTER L6 NOT PATENT/DT 

L8 22927 SEA FILE=TOXCENTER L7 NOT TSCATS/FS 

 ACT TOXQUERY/Q

 ---------

L9 QUE (CHRONIC OR IMMUNOTOX? OR NEUROTOX? OR TOXICOKIN? OR 

 BIOMARKER? OR NEUROLOG?) 

L10 QUE (PHARMACOKIN? OR SUBCHRONIC OR PBPK OR 

EPIDEMIOLOGY/ST,CT,
MOLYBDENUM B-9

APPENDIX B

Table B-2. Database Query Strings 

Database

search date Query string

 IT) 

L11 QUE (ACUTE OR SUBACUTE OR LD50# OR LD(W)50 OR LC50# OR 

 LC(W)50) 

L12 QUE (TOXICITY OR ADVERSE OR POISONING)/ST,CT,IT 

L13 QUE (INHAL? OR PULMON? OR NASAL? OR LUNG? OR RESPIR?) 

L14 QUE ((OCCUPATION? OR WORKPLACE? OR WORKER?) AND EXPOS?) 

L15 QUE (ORAL OR ORALLY OR INGEST? OR GAVAGE? OR DIET OR DIETS 

OR 

 DIETARY OR DRINKING(W)WATER?) 

L16 QUE (MAXIMUM AND CONCENTRATION? AND (ALLOWABLE OR 

PERMISSIBLE))

L17 QUE (ABORT? OR ABNORMALIT? OR EMBRYO? OR CLEFT? OR FETUS?) 

L18 QUE (FOETUS? OR FETAL? OR FOETAL? OR FERTIL? OR MALFORM? 

OR 

 OVUM?) 

L19 QUE (OVA OR OVARY OR PLACENTA? OR PREGNAN? OR PRENATAL?) 

L20 QUE (PERINATAL? OR POSTNATAL? OR REPRODUC? OR STERIL? OR 

 TERATOGEN?) 

L21 QUE (SPERM OR SPERMAC? OR SPERMAG? OR SPERMATI? OR 

SPERMAS? OR 

 SPERMATOB? OR SPERMATOC? OR SPERMATOG?) 

L22 QUE (SPERMATOI? OR SPERMATOL? OR SPERMATOR? OR 

SPERMATOX? OR 

 SPERMATOZ? OR SPERMATU? OR SPERMI? OR SPERMO?) 

L23 QUE (NEONAT? OR NEWBORN? OR DEVELOPMENT OR 

DEVELOPMENTAL?) 

L24 QUE (ENDOCRIN? AND DISRUPT?) 

L25 QUE (ZYGOTE? OR CHILD OR CHILDREN OR ADOLESCEN? OR 

INFANT?) 

L26 QUE (WEAN? OR OFFSPRING OR AGE(W)FACTOR?) 

L27 QUE (DERMAL? OR DERMIS OR SKIN OR EPIDERM? OR CUTANEOUS?) 

L28 QUE (CARCINOG? OR COCARCINOG? OR CANCER? OR PRECANCER? 

OR 

 NEOPLAS?) 

L29 QUE (TUMOR? OR TUMOUR? OR ONCOGEN? OR LYMPHOMA? OR 

CARCINOM?) 

L30 QUE (GENETOX? OR GENOTOX? OR MUTAGEN? OR 

GENETIC(W)TOXIC?) 

L31 QUE (NEPHROTOX? OR HEPATOTOX?) 

L32 QUE (ENDOCRIN? OR ESTROGEN? OR ANDROGEN? OR HORMON?) 

L33 QUE (OCCUPATION? OR WORKER? OR WORKPLACE? OR EPIDEM?) 

L34 QUE L9 OR L10 OR L11 OR L12 OR L13 OR L14 OR L15 OR L16 OR L17 

 OR L18 OR L19 OR L20 OR L21 OR L22 OR L23 OR L24 OR L25 OR L26 

 OR L27 OR L28 OR L29 OR L30 OR L31 OR L32 OR L33 

L35 QUE (RAT OR RATS OR MOUSE OR MICE OR GUINEA(W)PIG? OR 

MURIDAE 

 OR DOG OR DOGS OR RABBIT? OR HAMSTER? OR PIG OR PIGS OR 

SWINE 

 OR PORCINE OR MONKEY? OR MACAQUE?) 
MOLYBDENUM B-10

APPENDIX B

Table B-2. Database Query Strings 

Database

search date Query string

L36 QUE (MARMOSET? OR FERRET? OR GERBIL? OR RODENT? OR 

LAGOMORPHA 

 OR BABOON? OR CANINE OR CAT OR CATS OR FELINE OR MURINE) 

L37 QUE L34 OR L35 OR L36 

L38 QUE (HUMAN OR HUMANS OR HOMINIDAE OR MAMMALS OR MAMMAL? 

OR 

 PRIMATES OR PRIMATE?) 

L39 QUE L37 OR L38 

 ---------

L40 8795 SEA FILE=TOXCENTER L8 AND L39 

L41 1436 SEA FILE=TOXCENTER L40 AND ED>=20141201 

L42 1422 SEA FILE=TOXCENTER L41 AND PY>2012 

L43 184 SEA FILE=TOXCENTER L40 AND L3 

L44 1597 SEA FILE=TOXCENTER L41 OR L43 

L45 0 SEA FILE=TOXCENTER L44 AND MEDLINE/SB 

L46 269 SEA FILE=TOXCENTER L44 AND MEDLINE/FS 

L47 263 SEA FILE=TOXCENTER L44 AND BIOSIS/FS 

L48 1050 SEA FILE=TOXCENTER L44 AND CAPLUS/FS 

L49 15 SEA FILE=TOXCENTER L44 NOT (L46 OR L47 OR L48) 

L50 1425 DUP REM L46 L47 L49 L48 (172 DUPLICATES REMOVED)

 ANSWERS '1-1425' FROM FILE TOXCENTER

L*** DEL 269 S L44 AND MEDLINE/FS

L*** DEL 269 S L44 AND MEDLINE/FS

L51 269 SEA FILE=TOXCENTER L50 

L*** DEL 263 S L44 AND BIOSIS/FS

L*** DEL 263 S L44 AND BIOSIS/FS

L52 234 SEA FILE=TOXCENTER L50 

L*** DEL 1050 S L44 AND CAPLUS/FS

L*** DEL 1050 S L44 AND CAPLUS/FS

L53 908 SEA FILE=TOXCENTER L50 

L*** DEL 15 S L44 NOT (L46 OR L47 OR L48)

L*** DEL 15 S L44 NOT (L46 OR L47 OR L48)

L54 14 SEA FILE=TOXCENTER L50 

L55 211 SEA FILE=TOXCENTER (L51 OR L52 OR L53 OR L54) AND BIOSIS/FS 

 AND ED>=20141201 

L*** DEL 269 S L44 AND MEDLINE/FS

L*** DEL 269 S L44 AND MEDLINE/FS

L56 269 SEA FILE=TOXCENTER L50 

L*** DEL 263 S L44 AND BIOSIS/FS

L*** DEL 263 S L44 AND BIOSIS/FS

L57 234 SEA FILE=TOXCENTER L50 

L*** DEL 1050 S L44 AND CAPLUS/FS

L*** DEL 1050 S L44 AND CAPLUS/FS

L58 908 SEA FILE=TOXCENTER L50 

L*** DEL 15 S L44 NOT (L46 OR L47 OR L48)

L*** DEL 15 S L44 NOT (L46 OR L47 OR L48)

L59 14 SEA FILE=TOXCENTER L50 

L60 826 SEA FILE=TOXCENTER (L56 OR L57 OR L58 OR L59) AND CAPLUS/FS 

 AND ED>=20141201 

L*** DEL 269 S L44 AND MEDLINE/FS
MOLYBDENUM B-11

APPENDIX B

Table B-2. Database Query Strings 

Database

search date Query string

L*** DEL 269 S L44 AND MEDLINE/FS

L61 269 SEA FILE=TOXCENTER L50 

L*** DEL 263 S L44 AND BIOSIS/FS

L*** DEL 263 S L44 AND BIOSIS/FS

L62 234 SEA FILE=TOXCENTER L50 

L*** DEL 1050 S L44 AND CAPLUS/FS

L*** DEL 1050 S L44 AND CAPLUS/FS

L63 908 SEA FILE=TOXCENTER L50 

L*** DEL 15 S L44 NOT (L46 OR L47 OR L48)

L*** DEL 15 S L44 NOT (L46 OR L47 OR L48)

L64 14 SEA FILE=TOXCENTER L50 

L65 0 SEA FILE=TOXCENTER (L61 OR L62 OR L63 OR L64) NOT (CAPLUS/FS 

 OR MEDLINE/FS OR BIOSIS/FS) AND ED>=20141201 

L*** DEL 269 S L44 AND MEDLINE/FS

L*** DEL 269 S L44 AND MEDLINE/FS

L66 269 SEA FILE=TOXCENTER L50 

L*** DEL 263 S L44 AND BIOSIS/FS

L*** DEL 263 S L44 AND BIOSIS/FS

L67 234 SEA FILE=TOXCENTER L50 

L*** DEL 1050 S L44 AND CAPLUS/FS

L*** DEL 1050 S L44 AND CAPLUS/FS

L68 908 SEA FILE=TOXCENTER L50 

L*** DEL 15 S L44 NOT (L46 OR L47 OR L48)

L*** DEL 15 S L44 NOT (L46 OR L47 OR L48)

L69 14 SEA FILE=TOXCENTER L50 

L70 150 SEA FILE=TOXCENTER (L66 OR L67 OR L68 OR L69) NOT 

ED>=20141201 

L71 23 SEA FILE=TOXCENTER L70 AND BIOSIS/FS 

L72 14 SEA FILE=TOXCENTER L70 NOT (MEDLINE/FS OR BIOSIS/FS OR 

 CAPLUS/FS)

L73 82 SEA FILE=TOXCENTER L70 AND CAPLUS/FS 

L74 52 SEA FILE=TOXCENTER L60 AND ?MOLYB?/TI 

L75 29 SEA FILE=TOXCENTER L73 AND ?MOLYB?/TI 

 D SCAN L55

L76 37 SEA FILE=TOXCENTER L71 OR L72 

 D SCAN L76

 D SCAN L74

 D SCAN L75

L*** DEL 269 S L44 AND MEDLINE/FS

L*** DEL 269 S L44 AND MEDLINE/FS

L77 269 SEA FILE=TOXCENTER L50 

L*** DEL 263 S L44 AND BIOSIS/FS

L*** DEL 263 S L44 AND BIOSIS/FS

L78 234 SEA FILE=TOXCENTER L50 

L*** DEL 1050 S L44 AND CAPLUS/FS

L*** DEL 1050 S L44 AND CAPLUS/FS

L79 908 SEA FILE=TOXCENTER L50 

L*** DEL 15 S L44 NOT (L46 OR L47 OR L48)

L*** DEL 15 S L44 NOT (L46 OR L47 OR L48)

L80 14 SEA FILE=TOXCENTER L50 
MOLYBDENUM B-12

APPENDIX B

Table B-2. Database Query Strings 

Database

search date Query string

L81 1275 SEA FILE=TOXCENTER (L77 OR L78 OR L79 OR L80) AND 

ED>=20141201 

L82 1265 SEA FILE=TOXCENTER L81 AND (L1 OR L2) 

L83 1012 SEA FILE=TOXCENTER L81 NOT (L55 OR L74) 

L84 84 SEA FILE=TOXCENTER L70 NOT (L76 OR L75) 

 D SCAN L84

 D SCAN L83

 (FILE 'HOME' ENTERED AT 20:36:59 ON 14 JAN 2018)

 FILE 'TOXCENTER' ENTERED AT 20:37:09 ON 14 JAN 2018

CHARGED TO COST=EH011.05.LB.02.05

 ACT MOLY1/A

 ---------

L1 ( 34132)SEA FILE=TOXCENTER 7439-98-7 OR 1317-33-5 OR 12033-29-3 OR 

 12033-33-9 OR 11098-99-0 OR 18868-43-4 OR 1313-27-5 OR 

 1313-29-7 OR 11098-84-3 OR 27546-07-2 OR 12054-85-2 OR 

 15060-55-6 OR 7631-95-0 OR 10102-40-6 

L2 ( 523)SEA FILE=TOXCENTER 7789-82-4 OR 12011-97-1 OR 11119-46-3 OR 

 11062-51-4 OR 10241-05-1 OR 1309-56-4 OR 7783-77-9 OR 13939-06-

 5 OR 14221-06-8 OR 13814-74-9 

L3 ( 1148)SEA FILE=TOXCENTER 12027-67-7 OR 13106-76-8 

L4 ( 35122)SEA FILE=TOXCENTER L1 OR L2 OR L3 

L5 ( 22953)SEA FILE=TOXCENTER L4 NOT PATENT/DT 

L6 ( 22927)SEA FILE=TOXCENTER L5 NOT TSCATS/FS 

L7 QUE (CHRONIC OR IMMUNOTOX? OR NEUROTOX? OR TOXICOKIN? OR 

 BIOMARKER? OR NEUROLOG?) 

L8 QUE (PHARMACOKIN? OR SUBCHRONIC OR PBPK OR 

EPIDEMIOLOGY/ST,CT,

 IT) 

L9 QUE (ACUTE OR SUBACUTE OR LD50# OR LD(W)50 OR LC50# OR 

 LC(W)50) 

L10 QUE (TOXICITY OR ADVERSE OR POISONING)/ST,CT,IT 

L11 QUE (INHAL? OR PULMON? OR NASAL? OR LUNG? OR RESPIR?) 

L12 QUE ((OCCUPATION? OR WORKPLACE? OR WORKER?) AND EXPOS?) 

L13 QUE (ORAL OR ORALLY OR INGEST? OR GAVAGE? OR DIET OR DIETS 

OR 

 DIETARY OR DRINKING(W)WATER?) 

L14 QUE (MAXIMUM AND CONCENTRATION? AND (ALLOWABLE OR 

PERMISSIBLE))

L15 QUE (ABORT? OR ABNORMALIT? OR EMBRYO? OR CLEFT? OR FETUS?) 

L16 QUE (FOETUS? OR FETAL? OR FOETAL? OR FERTIL? OR MALFORM? 

OR 

 OVUM?) 

L17 QUE (OVA OR OVARY OR PLACENTA? OR PREGNAN? OR PRENATAL?) 

L18 QUE (PERINATAL? OR POSTNATAL? OR REPRODUC? OR STERIL? OR 

 TERATOGEN?) 

L19 QUE (SPERM OR SPERMAC? OR SPERMAG? OR SPERMATI? OR 

SPERMAS? OR 

 SPERMATOB? OR SPERMATOC? OR SPERMATOG?) 

L20 QUE (SPERMATOI? OR SPERMATOL? OR SPERMATOR? OR 

SPERMATOX? OR 
MOLYBDENUM B-13

APPENDIX B

Table B-2. Database Query Strings 

Database

search date Query string

 SPERMATOZ? OR SPERMATU? OR SPERMI? OR SPERMO?) 

L21 QUE (NEONAT? OR NEWBORN? OR DEVELOPMENT OR 

DEVELOPMENTAL?) 

L22 QUE (ENDOCRIN? AND DISRUPT?) 

L23 QUE (ZYGOTE? OR CHILD OR CHILDREN OR ADOLESCEN? OR 

INFANT?) 

L24 QUE (WEAN? OR OFFSPRING OR AGE(W)FACTOR?) 

L25 QUE (DERMAL? OR DERMIS OR SKIN OR EPIDERM? OR CUTANEOUS?) 

L26 QUE (CARCINOG? OR COCARCINOG? OR CANCER? OR PRECANCER? 

OR 

 NEOPLAS?) 

L27 QUE (TUMOR? OR TUMOUR? OR ONCOGEN? OR LYMPHOMA? OR 

CARCINOM?) 

L28 QUE (GENETOX? OR GENOTOX? OR MUTAGEN? OR 

GENETIC(W)TOXIC?) 

L29 QUE (NEPHROTOX? OR HEPATOTOX?) 

L30 QUE (ENDOCRIN? OR ESTROGEN? OR ANDROGEN? OR HORMON?) 

L31 QUE (OCCUPATION? OR WORKER? OR WORKPLACE? OR EPIDEM?) 

L32 QUE L7 OR L8 OR L9 OR L10 OR L11 OR L12 OR L13 OR L14 OR L15 

 OR L16 OR L17 OR L18 OR L19 OR L20 OR L21 OR L22 OR L23 OR L24 

 OR L25 OR L26 OR L27 OR L28 OR L29 OR L30 OR L31

L33 QUE (RAT OR RATS OR MOUSE OR MICE OR GUINEA(W)PIG? OR 

MURIDAE 

 OR DOG OR DOGS OR RABBIT? OR HAMSTER? OR PIG OR PIGS OR 

SWINE 

 OR PORCINE OR MONKEY? OR MACAQUE?) 

L34 QUE (MARMOSET? OR FERRET? OR GERBIL? OR RODENT? OR 

LAGOMORPHA 

 OR BABOON? OR CANINE OR CAT OR CATS OR FELINE OR MURINE) 

L35 QUE L32 OR L33 OR L34 

L36 QUE (HUMAN OR HUMANS OR HOMINIDAE OR MAMMALS OR MAMMAL? 

OR 

 PRIMATES OR PRIMATE?) 

L37 QUE L35 OR L36 

L38 ( 8795)SEA FILE=TOXCENTER L6 AND L37 

L39 ( 1436)SEA FILE=TOXCENTER L38 AND ED>=20141201 

L40 ( 184)SEA FILE=TOXCENTER L38 AND L3 

L41 ( 1597)SEA FILE=TOXCENTER L39 OR L40 

L42 ( 269)SEA FILE=TOXCENTER L41 AND MEDLINE/FS 

L43 ( 263)SEA FILE=TOXCENTER L41 AND BIOSIS/FS 

L44 ( 1050)SEA FILE=TOXCENTER L41 AND CAPLUS/FS 

L45 ( 15)SEA FILE=TOXCENTER L41 NOT (L42 OR L43 OR L44) 

L46 ( 1425)DUP REM L42 L43 L45 L44 (172 DUPLICATES REMOVED)

L47 ( 269)SEA FILE=TOXCENTER L46 

L48 ( 234)SEA FILE=TOXCENTER L46 

L49 ( 908)SEA FILE=TOXCENTER L46 

L50 ( 14)SEA FILE=TOXCENTER L46 

L51 ( 211)SEA FILE=TOXCENTER (L47 OR L48 OR L49 OR L50) AND BIOSIS/FS 

 AND ED>=20141201

L52 ( 269)SEA FILE=TOXCENTER L46 
MOLYBDENUM B-14

APPENDIX B

Table B-2. Database Query Strings 

Database

search date Query string

L53 ( 234)SEA FILE=TOXCENTER L46 

L54 ( 908)SEA FILE=TOXCENTER L46 

L55 ( 14)SEA FILE=TOXCENTER L46 

L56 ( 826)SEA FILE=TOXCENTER (L52 OR L53 OR L54 OR L55) AND CAPLUS/FS 

 AND ED>=20141201

L57 ( 52)SEA FILE=TOXCENTER L56 AND ?MOLYB?/TI

L58 ( 269)SEA FILE=TOXCENTER L46 

L59 ( 234)SEA FILE=TOXCENTER L46 

L60 ( 908)SEA FILE=TOXCENTER L46 

L61 ( 14)SEA FILE=TOXCENTER L46 

L62 ( 1275)SEA FILE=TOXCENTER (L58 OR L59 OR L60 OR L61) AND 

ED>=20141201

L63 1012 SEA FILE=TOXCENTER L62 NOT (L51 OR L57)

 ---------

L64 97 SEA FILE=TOXCENTER L63 AND (MOLYB?/TI OR DIMOLYB?/TI OR 

 DODECACHLOROHEXAMOLYB?/TI OR HEPTAMOLYB?/TI OR 

HEXACARBONYLMOLY

 B?/TI OR HEXAFLUOROMOLYB?/TI OR HEXAMOLYB?/TI OR 

OCTACHLOROHEXA

 MOLYB?/TI OR PARAMOLYB?/TI) 

L65 5 SEA FILE=TOXCENTER L63 AND (PENTACHLOROMOLYB?/TI OR 

TETRAACETAT

 ODIMOLYB?/TI OR TETRAOXOMOLYB?/TI OR TETRASULFIDOMOLYB?/TI 

OR 

 TETRATHIOMOLYB?/TI OR TETRATHIOOXOMOLYB?/TI OR THIOMOLYB?/TI 

 OR TRICHLOROOXOMOLYB?/TI) 

L66 1 SEA FILE=TOXCENTER L63 AND ("3N5"/TI OR "AMMONIUM 

MOLIBDATE"/TI

 OR "AMMONIUM MOLIBDENUM OXIDE"/TI OR "AMPERIT 105.054"/TI OR 

 "AMPERIT 106.2"/TI OR "ATTM"/TI OR "BIS(AMMONIUM)TETRATHIOMOLYB

 DATE(2-)"/TI OR "BOUEN SKN 301"/TI) 

L67 0 SEA FILE=TOXCENTER L63 AND ("C-POWDER"/TI OR "COPREXA"/TI OR 

 "DAG 206"/TI OR "DAG 325"/TI OR "DAG-V 657"/TI OR "DEFRIC COAT 

 HMB 2"/TI OR "DM 1 (SULFIDE)"/TI OR "DMI 7"/TI OR "JCPDS 

 35-0609"/TI OR "LIQUI-MOLY LM 11"/TI OR "LIQUI-MOLY LM 2"/TI) 

L68 0 SEA FILE=TOXCENTER L63 AND ("LIQUI-MOLY Z POWDER"/TI OR "LM 

 13"/TI OR "MC 400WR"/TI OR "MCHVL"/TI OR "MD 40"/TI OR "METCO 

 63"/TI OR "MF 000"/TI OR "MIPO-M 15"/TI OR "MO 1202T"/TI OR 

 "MO-1202T"/TI OR "MOLY FINE POWDER Y"/TI) 

L69 0 SEA FILE=TOXCENTER L63 AND ("MOLY POWDER B"/TI OR "MOLY 

POWDER 

 C"/TI OR "MOLY POWDER PA"/TI OR "MOLY POWDER PB"/TI OR "MOLY 

 POWDER PS"/TI OR "MOLYCOLLOID CF 626"/TI OR "MOLYFORM 15"/TI 

 OR "MOLYHIBIT 100"/TI OR "MOLYKA R"/TI OR "MOLYKA R-L 3"/TI) 

L70 0 SEA FILE=TOXCENTER L63 AND ("MOLYKE R"/TI OR "MOLYKOTE"/TI OR 

 "MOLYKOTE MICROSIZE POWDER"/TI OR "MOLYKOTE Z"/TI OR 

"MOLYKOTE 

 Z POWDER"/TI OR "MOLYSULFIDE"/TI OR "MOP-P 100"/TI OR "MOPOL 

 M"/TI OR "MOPOL S"/TI OR "MOTIMOL"/TI OR "MVCH 1"/TI) 

L71 0 SEA FILE=TOXCENTER L63 AND ("NEOZ"/TI OR "NICHIMOLY C"/TI OR 
MOLYBDENUM B-15

APPENDIX B

Table B-2. Database Query Strings 

Database

search date Query string

 "OKS 110"/TI OR "PA POWDER"/TI OR "PIGMENT BLACK 34"/TI OR 

 "POL-U"/TI OR "POWDER PA"/TI OR "RAC 01"/TI OR "SGC 15"/TI OR 

 "SOLVEST 390A"/TI OR "SUMIPOWDER PA"/TI OR "T-POWDER"/TI OR 

 "TIOMOLIBDATE DIAMMONIUM"/TI OR "TMOIO"/TI OR "TSM1"/TI) 

L72 103 SEA FILE=TOXCENTER L64 OR L65 OR L66 

 D SCAN L72

 ACT MOLY2/A

 ---------

L76 ( 34132)SEA FILE=TOXCENTER 7439-98-7 OR 1317-33-5 OR 12033-29-3 OR 

 12033-33-9 OR 11098-99-0 OR 18868-43-4 OR 1313-27-5 OR 

 1313-29-7 OR 11098-84-3 OR 27546-07-2 OR 12054-85-2 OR 

 15060-55-6 OR 7631-95-0 OR 10102-40-6 

L77 ( 523)SEA FILE=TOXCENTER 7789-82-4 OR 12011-97-1 OR 11119-46-3 OR 

 11062-51-4 OR 10241-05-1 OR 1309-56-4 OR 7783-77-9 OR 13939-06-

 5 OR 14221-06-8 OR 13814-74-9 

L78 ( 1148)SEA FILE=TOXCENTER 12027-67-7 OR 13106-76-8 

L79 ( 35122)SEA FILE=TOXCENTER L76 OR L77 OR L78 

L80 ( 22953)SEA FILE=TOXCENTER L79 NOT PATENT/DT 

L81 ( 22927)SEA FILE=TOXCENTER L80 NOT TSCATS/FS 

L82 QUE (CHRONIC OR IMMUNOTOX? OR NEUROTOX? OR TOXICOKIN? OR 

 BIOMARKER? OR NEUROLOG?) 

L83 QUE (PHARMACOKIN? OR SUBCHRONIC OR PBPK OR 

EPIDEMIOLOGY/ST,CT,

 IT) 

L84 QUE (ACUTE OR SUBACUTE OR LD50# OR LD(W)50 OR LC50# OR 

 LC(W)50) 

L85 QUE (TOXICITY OR ADVERSE OR POISONING)/ST,CT,IT 

L86 QUE (INHAL? OR PULMON? OR NASAL? OR LUNG? OR RESPIR?) 

L87 QUE ((OCCUPATION? OR WORKPLACE? OR WORKER?) AND EXPOS?) 

L88 QUE (ORAL OR ORALLY OR INGEST? OR GAVAGE? OR DIET OR DIETS 

OR 

 DIETARY OR DRINKING(W)WATER?) 

L89 QUE (MAXIMUM AND CONCENTRATION? AND (ALLOWABLE OR 

PERMISSIBLE))

L90 QUE (ABORT? OR ABNORMALIT? OR EMBRYO? OR CLEFT? OR FETUS?) 

L91 QUE (FOETUS? OR FETAL? OR FOETAL? OR FERTIL? OR MALFORM? 

OR 

 OVUM?) 

L92 QUE (OVA OR OVARY OR PLACENTA? OR PREGNAN? OR PRENATAL?) 

L93 QUE (PERINATAL? OR POSTNATAL? OR REPRODUC? OR STERIL? OR 

 TERATOGEN?)

L94 QUE (SPERM OR SPERMAC? OR SPERMAG? OR SPERMATI? OR 

SPERMAS? OR 

 SPERMATOB? OR SPERMATOC? OR SPERMATOG?) 

L95 QUE (SPERMATOI? OR SPERMATOL? OR SPERMATOR? OR 

SPERMATOX? OR 

 SPERMATOZ? OR SPERMATU? OR SPERMI? OR SPERMO?) 

L96 QUE (NEONAT? OR NEWBORN? OR DEVELOPMENT OR 

DEVELOPMENTAL?) 

L97 QUE (ENDOCRIN? AND DISRUPT?) 
MOLYBDENUM B-16

APPENDIX B

Table B-2. Database Query Strings 

Database

search date Query string

L98 QUE (ZYGOTE? OR CHILD OR CHILDREN OR ADOLESCEN? OR 

INFANT?) 

L99 QUE (WEAN? OR OFFSPRING OR AGE(W)FACTOR?) 

L100 QUE (DERMAL? OR DERMIS OR SKIN OR EPIDERM? OR CUTANEOUS?) 

L101 QUE (CARCINOG? OR COCARCINOG? OR CANCER? OR PRECANCER? 

OR 

 NEOPLAS?) 

L102 QUE (TUMOR? OR TUMOUR? OR ONCOGEN? OR LYMPHOMA? OR 

CARCINOM?) 

L103 QUE (GENETOX? OR GENOTOX? OR MUTAGEN? OR 

GENETIC(W)TOXIC?) 

L104 QUE (NEPHROTOX? OR HEPATOTOX?) 

L105 QUE (ENDOCRIN? OR ESTROGEN? OR ANDROGEN? OR HORMON?) 

L106 QUE (OCCUPATION? OR WORKER? OR WORKPLACE? OR EPIDEM?) 

L107 QUE L82 OR L83 OR L84 OR L85 OR L86 OR L87 OR L88 OR L89 OR 

 L90 OR L91 OR L92 OR L93 OR L94 OR L95 OR L96 OR L97 OR L98 OR 

 L99 OR L100 OR L101 OR L102 OR L103 OR L104 OR L105 OR L106 

L108 QUE (RAT OR RATS OR MOUSE OR MICE OR GUINEA(W)PIG? OR 

MURIDAE 

 OR DOG OR DOGS OR RABBIT? OR HAMSTER? OR PIG OR PIGS OR 

SWINE 

 OR PORCINE OR MONKEY? OR MACAQUE?) 

L109 QUE (MARMOSET? OR FERRET? OR GERBIL? OR RODENT? OR 

LAGOMORPHA 

 OR BABOON? OR CANINE OR CAT OR CATS OR FELINE OR MURINE) 

L110 QUE L107 OR L108 OR L109 

L111 QUE (HUMAN OR HUMANS OR HOMINIDAE OR MAMMALS OR MAMMAL? 

OR 

 PRIMATES OR PRIMATE?) 

L112 QUE L110 OR L111 

L113( 8795)SEA FILE=TOXCENTER L81 AND L112 

L114( 1436)SEA FILE=TOXCENTER L113 AND ED>=20141201 

L115( 184)SEA FILE=TOXCENTER L113 AND L78 

L116( 1597)SEA FILE=TOXCENTER L114 OR L115 

L117( 269)SEA FILE=TOXCENTER L116 AND MEDLINE/FS 

L118( 263)SEA FILE=TOXCENTER L116 AND BIOSIS/FS 

L119( 1050)SEA FILE=TOXCENTER L116 AND CAPLUS/FS 

L120( 15)SEA FILE=TOXCENTER L116 NOT (L117 OR L118 OR L119) 

L121( 1425)DUP REM L117 L118 L120 L119 (172 DUPLICATES REMOVED)

L122( 269)SEA FILE=TOXCENTER L121 

L123( 234)SEA FILE=TOXCENTER L121 

L124( 908)SEA FILE=TOXCENTER L121 

L125( 14)SEA FILE=TOXCENTER L121 

L126( 150)SEA FILE=TOXCENTER (L122 OR L123 OR L124 OR L125) NOT 

ED>=20141

L127( 23)SEA FILE=TOXCENTER L126 AND BIOSIS/FS

L128( 14)SEA FILE=TOXCENTER L126 NOT (MEDLINE/FS OR BIOSIS/FS OR 

CAPLUS/

L129( 82)SEA FILE=TOXCENTER L126 AND CAPLUS/FS

L130( 29)SEA FILE=TOXCENTER L129 AND ?MOLYB?/TI
MOLYBDENUM B-17

APPENDIX B

Table B-2. Database Query Strings 

Database

search date Query string

L131( 37)SEA FILE=TOXCENTER L127 OR L128

L132 84 SEA FILE=TOXCENTER L126 NOT (L131 OR L130)

 ---------

L133 15 SEA FILE=TOXCENTER L132 AND (MOLYB?/TI OR DIMOLYB?/TI OR 

 DODECACHLOROHEXAMOLYB?/TI OR HEPTAMOLYB?/TI OR 

HEXACARBONYLMOLY

 B?/TI OR HEXAFLUOROMOLYB?/TI OR HEXAMOLYB?/TI OR 

OCTACHLOROHEXA

 MOLYB?/TI OR PARAMOLYB?/TI) 

L134 0 SEA FILE=TOXCENTER L1L32 AND (PENTACHLOROMOLYB?/TI OR 

TETRAACET

 ATODIMOLYB?/TI OR TETRAOXOMOLYB?/TI OR 

TETRASULFIDOMOLYB?/TI 

 OR TETRATHIOMOLYB?/TI OR TETRATHIOOXOMOLYB?/TI OR 

THIOMOLYB?/TI

 OR TRICHLOROOXOMOLYB?/TI) 

L135 0 SEA FILE=TOXCENTER L132 AND ("3N5"/TI OR "AMMONIUM 

MOLIBDATE"/T

 I OR "AMMONIUM MOLIBDENUM OXIDE"/TI OR "AMPERIT 105.054"/TI OR 

 "AMPERIT 106.2"/TI OR "ATTM"/TI OR "BIS(AMMONIUM)TETRATHIOMOLYB

 DATE(2-)"/TI OR "BOUEN SKN 301"/TI) 

L136 0 SEA FILE=TOXCENTER L132 AND ("C-POWDER"/TI OR "COPREXA"/TI 

OR 

 "DAG 206"/TI OR "DAG 325"/TI OR "DAG-V 657"/TI OR "DEFRIC COAT 

 HMB 2"/TI OR "DM 1 (SULFIDE)"/TI OR "DMI 7"/TI OR "JCPDS 

 35-0609"/TI OR "LIQUI-MOLY LM 11"/TI OR "LIQUI-MOLY LM 2"/TI) 

L137 0 SEA FILE=TOXCENTER L132 AND ("LIQUI-MOLY Z POWDER"/TI OR "LM 

 13"/TI OR "MC 400WR"/TI OR "MCHVL"/TI OR "MD 40"/TI OR "METCO 

 63"/TI OR "MF 000"/TI OR "MIPO-M 15"/TI OR "MO 1202T"/TI OR 

 "MO-1202T"/TI OR "MOLY FINE POWDER Y"/TI) 

L138 0 SEA FILE=TOXCENTER L132 AND ("MOLY POWDER B"/TI OR "MOLY 

 POWDER C"/TI OR "MOLY POWDER PA"/TI OR "MOLY POWDER PB"/TI OR 

 "MOLY POWDER PS"/TI OR "MOLYCOLLOID CF 626"/TI OR "MOLYFORM 

 15"/TI OR "MOLYHIBIT 100"/TI OR "MOLYKA R"/TI OR "MOLYKA R-L 

 3"/TI) 

L139 0 SEA FILE=TOXCENTER L132 AND ("MOLYKE R"/TI OR "MOLYKOTE"/TI 

OR 

 "MOLYKOTE MICROSIZE POWDER"/TI OR "MOLYKOTE Z"/TI OR 

"MOLYKOTE 

 Z POWDER"/TI OR "MOLYSULFIDE"/TI OR "MOP-P 100"/TI OR "MOPOL 

 M"/TI OR "MOPOL S"/TI OR "MOTIMOL"/TI OR "MVCH 1"/TI) 

L140 0 SEA FILE=TOXCENTER L132 AND ("NEOZ"/TI OR "NICHIMOLY C"/TI OR 

 "OKS 110"/TI OR "PA POWDER"/TI OR "PIGMENT BLACK 34"/TI OR 

 "POL-U"/TI OR "POWDER PA"/TI OR "RAC 01"/TI OR "SGC 15"/TI OR 

 "SOLVEST 390A"/TI OR "SUMIPOWDER PA"/TI OR "T-POWDER"/TI OR 

 "TIOMOLIBDATE DIAMMONIUM"/TI OR "TMOIO"/TI OR "TSM1"/TI) 

 D SCAN L133
MOLYBDENUM B-18

APPENDIX B

Table B-3. Strategies to Augment the Literature Search

Source Query and number screened when available

TSCATSa

01/2018 Compounds searched: 7439-98-7, 1317-33-5, 12033-29-3, 12033-33-9, 11098-99-0, 

18868-43-4, 1313-27-5, 1313-29-7, 11098-84-3, 27546-07-2, 12054-85-2, 15060-55-6, 

7631-95-0, 10102-40-6, 7789-82-4, 12011-97-1, 11119-46-3, 11062-51-4, 10241-05-1, 

1309-56-4, 7783-77-9, 13939-06-5, 14221-06-8, 13814-74-9, 12027-67-7, 13106-76-8

NTP

01/2018 14th ROC (https://ntp.niehs.nih.gov/pubhealth/roc/index-1.html):

7439-98-7 OR 1317-33-5 OR 12033-29-3 OR 12033-33-9 OR 11098-99-0 OR 18868-

43-4 OR 1313-27-5 OR 1313-29-7 OR 11098-84-3 OR 27546-07-2 OR 12054-85-2 

OR 15060-55-6 OR 7631-95-0 OR 10102-40-6 OR 7789-82-4 OR 12011-97-1 OR 

11119-46-3 OR 11062-51-4 OR 10241-05-1 OR 1309-56-4 OR 7783-77-9 OR 13939-

06-5 OR 14221-06-8 OR 13814-74-9 OR 12027-67-7 OR 13106-76-8

molybdenum OR molybdate OR molybdic OR dimolybdate OR dimolybdenum OR 

dodecachlorohexamolybdenum OR heptamolybdate OR hexacarbonylmolybdenum 

OR hexafluoromolybdenum OR hexamolybdenum OR molibdate OR molibdenum OR 

octachlorohexamolybdenum OR paramolybdate OR pentachloromolybdenum OR 

tetraacetatodimolybdenum OR tetraoxomolybdate OR tetrasulfidomolybdate OR 

tetrathiomolybdate OR tetrathiooxomolybdate OR thiomolybdate OR thiomolybdic OR 

tiomolibdate OR trichlorooxomolybdenum OR pigment black 34

NTP Site Search (http://ntpsearch.niehs.nih.gov/home) with Content Types 

"Reports & Publications", "Systematic Review" or "Testing Status":

7439-98-7 OR 1317-33-5 OR 12033-29-3 OR 12033-33-9 OR 11098-99-0 OR 18868-

43-4 OR 1313-27-5 OR 1313-29-7 OR 11098-84-3 OR 27546-07-2 OR 12054-85-2 

OR 15060-55-6 OR 7631-95-0 OR 10102-40-6 OR 7789-82-4 OR 12011-97-1 OR 

11119-46-3 OR 11062-51-4 OR 10241-05-1 OR 1309-56-4 OR 7783-77-9 OR 13939-

06-5 OR 14221-06-8 OR 13814-74-9 OR 12027-67-7 OR 13106-76-8

molybdenum OR molybdate OR molybdic OR dimolybdate OR dimolybdenum OR 

dodecachlorohexamolybdenum OR heptamolybdate OR hexacarbonylmolybdenum 

OR hexafluoromolybdenum OR hexamolybdenum OR molibdate OR molibdenum OR 

octachlorohexamolybdenum OR paramolybdate OR pentachloromolybdenum OR 

tetraacetatodimolybdenum OR tetraoxomolybdate OR tetrasulfidomolybdate OR 

tetrathiomolybdate OR tetrathiooxomolybdate OR thiomolybdate OR thiomolybdic OR 

tiomolibdate OR trichlorooxomolybdenum OR pigment black 34

Regulations.gov

01/2018 Notices or rules:

7439-98-7, 1317-33-5, 12033-29-3, 12033-33-9, 11098-99-0, 18868-43-4, 1313-27-5, 

1313-29-7, 11098-84-3, 27546-07-2, 12054-85-2, 15060-55-6, 7631-95-0, 10102-40-6, 

7789-82-4, 12011-97-1, 11119-46-3, 11062-51-4, 10241-05-1, 1309-56-4, 7783-77-9, 

13939-06-5, 14221-06-8, 13814-74-9, 12027-67-7, 13106-76-8

NIH RePORTER

04/2019 Text Search: "3N5" OR "Ammonium dimolybdate" OR "Ammonium heptamolybdate" 

OR "Ammonium heptamolybdate tetrahydrate" OR "Ammonium molibdate" OR 

"Ammonium molibdenum oxide" OR "Ammonium molybdate" OR "Ammonium 

molybdate hydrate" OR "Ammonium molybdate tetrahydrate" OR "Ammonium 

molybdate(VI)" OR "Ammonium molybdenum oxide" OR "Ammonium molybdenum 

sulfide" OR "ammonium paramolybdate" OR "Ammonium paramolybdate tetrahydrate" 

OR "Ammonium tetrasulfidomolybdate(2-)" OR "Ammonium tetrathiomolybdate" OR 

"Ammonium thiomolybdate(VI)" OR "Amperit 105.054" OR "Amperit 106.2" OR 
MOLYBDENUM B-19

APPENDIX B

Table B-3. Strategies to Augment the Literature Search

Source Query and number screened when available

"ATTM" OR "Bis(ammonium)tetrathiomolybdate(2-)" OR "Bouen SKN 301" OR "CPowder" OR "Calcium molybdate" OR "Calcium molybdate(VI)" OR "Calcium 

molybdenate" OR "Calcium molybdenum oxide" OR "Coprexa" OR "DAG 206" OR 

"DAG 325" OR "DAG-V 657" OR "Defric coat HMB 2" OR "Diammonium dimolybdate" 

OR "Diammonium tetrakis(sulfido)molybdate(2-)" OR "Diammonium 

tetrakis(thioxo)molybdate" OR "Diammonium tetrasulfidomolybdate" OR "Diammonium 

tetrathiomolybdate" OR "Diammonium tetrathiomolybdate(2-)" OR "Diammonium 

tetrathiooxomolybdate(2-)" OR "Diammonium tetrathioxomolybdate(2-)" OR 

"Diammonium thiomolybdate" OR "Dimolybdenum tetraacetate" OR "Dimolybdenum 

trioxide" OR "dimolybdenum trisulfide " OR "Disodium molybdate" OR "Disodium 

molybdate dihydrate" OR "Disodium tetraoxomolybdate" OR "DM 1 (sulfide)" OR "DMI 

7" OR "Dodecachlorohexamolybdenum" OR "Hexaammonium heptamolybdate 

tetrahydrate" OR "Hexaammonium molybdate tetrahydrate" OR 

"Hexacarbonylmolybdenum" OR "Hexafluoromolybdenum" OR "Hexamolybdenum 

dodecachloride" OR "JCPDS 35-0609" OR "Liqui-Moly LM 11" OR "Liqui-Moly LM 2" 

OR "Liqui-Moly Z Powder" OR "LM 13" OR "MC 400WR" OR "MChVL" OR "MD 40" 

OR "Metco 63" OR "MF 000" OR "MIPO-M 15" OR "Mo 1202T" OR "Mo-1202T" OR 

"Moly Fine Powder Y" OR "Moly Powder B" OR "Moly Powder C" OR "Moly Powder 

PA" OR "Moly Powder PB" OR "Moly Powder PS" OR "Molybdate (Mo2O72-), 

diammonium" OR "Molybdate (MoO42-), disodium, dihydrate, (T-4)-" OR "Molybdate, 

calcium" OR "Molybdena" OR "Molybdenite" OR "Molybdenum" OR "Molybdenum 

anhydride" OR "Molybdenum bisulfide" OR "Molybdenum carbide" OR "Molybdenum 

carbonyl" OR "Molybdenum chloride" OR "Molybdenum chloride oxide" OR 

"Molybdenum dioxide" OR "Molybdenum disulfide" OR "Molybdenum disulphide" OR 

"Molybdenum fluoride" OR "Molybdenum hexacarbonyl" OR "Molybdenum 

hexafluoride" OR "Molybdenum metallicum" OR "Molybdenum monocarbide" OR 

"Molybdenum oxide" OR "Molybdenum oxychloride" (Advanced), Search in: 

Projects AdminIC: All, Fiscal Year: Active Projects 

Text Search: "Molybdenum oxytrichloride" OR "Molybdenum pentachloride" OR 

"Molybdenum peroxide" OR "Molybdenum sesquioxide" OR "Molybdenum 

sesquisulfide" OR "Molybdenum sodium oxide" OR "Molybdenum sulfide" OR 

"Molybdenum sulphide" OR "Molybdenum trichloride monoxide" OR "Molybdenum 

trichloride oxide" OR "Molybdenum trioxide" OR "Molybdenum trioxide pentamer" OR 

"Molybdenum trioxide tetramer" OR "Molybdenum trisulfide" OR "Molybdenum(6) 

fluoride" OR "Molybdenum(II) acetate" OR "Molybdenum(II) chloride" OR 

"molybdenum(III) sulfide" OR "molybdenum(IV) oxide" OR "Molybdenum(IV) sulfide" 

OR "Molybdenum(V) chloride" OR "Molybdenum(VI) oxide" OR "Molybdenum(VI) 

trioxide" OR "Molybdenumperoxide" OR "Molybdic acid (H2Mo2O7), diammonium salt" 

OR "Molybdic acid (H2MoO4), calcium salt (1:1)" OR "Molybdic acid anhydride" OR 

"Molybdic acid, ammonium salt" OR "Molybdic acid, calcium salt" OR "Molybdic acid, 

disodium salt" OR "Molybdic acid, disodium salt, dihydrate" OR "Molybdic anhydride" 

OR "Molybdic oxide" OR "Molybdic trioxide" OR "Molycolloid CF 626" OR "Molyform 

15" OR "Molyhibit 100" OR "Molyka R" OR "Molyka R-L 3" OR "Molyke R" OR 

"Molykote" OR "Molykote Microsize Powder" OR "Molykote Z" OR "Molykote Z 

Powder" OR "Molysulfide" OR "MOP-P 100" OR "Mopol M" OR "Mopol S" OR 

"Motimol" OR "MVCh 1" OR "Natural molybdenite" OR "Natural molybdite" OR "NeoZ" 

OR "Nichimoly C" OR "Octachlorohexamolybdenum(4) tetrachloride" OR "OKS 110" 

OR "PA Powder" OR "Pentachloromolybdenum Molybdenite" OR "Pigment Black 34" 

OR "Pol-U" OR "Powder PA" OR "RAC 01" OR "SGC 15" OR "Sodium molybdate" OR 

"Sodium molybdate dihydrate" OR "Sodium molybdate(VI)" OR "Sodium molybdate(VI) 

dihydrate" OR "Sodium molybdenate" OR "Sodium molybdenum oxide" OR "Sodium 

tetraoxomolybdate(2-)" OR "Solvest 390A" OR "Sumipowder PA" OR 
MOLYBDENUM B-20

APPENDIX B

Table B-3. Strategies to Augment the Literature Search

Source Query and number screened when available

"Tetraacetatodimolybdenum" OR "tetrakis(acetato)di-Molybdenum" OR 

"Tetrakis(acetato)dimolybdenum" OR "Tetrakis(acetato)molybdenum" OR 

"Tetrakis(mu-(acetato-O:O'))dimolybdenum" OR "tetrakis(mu-acetato)di-Molybdenum" 

OR "Tetrakis(mu-acetato)dimolybdenum" OR "tetrakis[mu-(acetato-O:O')]diMolybdenum" OR "Thiomolybdic acid, diammonium salt" OR "Tiomolibdate 

diammonium" OR "TMOIO" OR "Trichlorooxomolybdenum" OR 

"Trichlorooxomolybdenum(V)" OR "TsM1" OR "Hexaammonium heptamolybdate" OR 

"Hexammonium heptamolybdat" OR "Hexammonium tetracosaoxoheptamolybdate" 

OR "Molybdate (Mo7O24), hexammonium" OR "Molybdate (Mo7O246-), ammonium 

(1:6)" OR "Molybdate (Mo7O246-), hexaammonium" (Advanced), Search in: 

Projects Admin IC: All, Fiscal Year: Active Projects

Text Search: "Molybdate, hexaammonium" OR "Molybdic acid (H6Mo7O24), 

hexaammonium salt" OR "Molybdic acid, hexaammonium salt" OR "Diammonium 

molybdate" OR "Diammonium tetraoxomolybdate(2-)" OR "Molybdate (MoO42-), 

ammonium (1:2), (T-4)-" OR "Molybdate (MoO42-), diammonium, (beta-4)-" OR 

"Molybdate (MoO42-), diammonium, (T-4)-" OR "Molybdic acid (H2MoO4), 

diammonium salt" OR "Molybdic acid, diammonium salt" (Advanced), Search in: 

Projects Admin IC: All, Fiscal Year: Active Projects

 Text Search: molybdenum OR molybdate OR molybdic OR dimolybdate OR 

dimolybdenum OR dodecachlorohexamolybdenum OR heptamolybdate OR 

hexacarbonylmolybdenum OR hexafluoromolybdenum OR hexamolybdenum OR 

molibdate OR molibdenum OR octachlorohexamolybdenum OR paramolybdate OR 

pentachloromolybdenum OR tetraacetatodimolybdenum OR tetraoxomolybdate OR 

tetrasulfidomolybdate OR tetrathiomolybdate OR tetrathiooxomolybdate OR 

thiomolybdate OR thiomolybdic OR tiomolibdate OR trichlorooxomolybdenum OR 

pigment black 34 (Advanced), Search in: Projects AdminIC: All, Fiscal Year: 

Active Projects

Other Identified throughout the assessment process

aSeveral versions of the TSCATS database were searched, as needed, by CASRN including TSCATS1 via Toxline 

(no date limit), TSCATS2 via https://yosemite.epa.gov/oppts/epatscat8.nsf/ReportSearch?OpenForm (date restricted 

by EPA receipt date), and TSCATS via CDAT (date restricted by ‘Mail Received Date Range’), as well as google for 

recent TSCA submissions.

The 2018 results were: 

• Number of records identified from PubMed, TOXLINE, and TOXCENTER (after duplicate 

removal): 2,394

• Number of records identified from other strategies: 114

• Total number of records to undergo literature screening: 2,508

B.1.2 Literature Screening 

A two-step process was used to screen the literature search to identify relevant studies on molybdenum: 

• Title and abstract screen

• Full text screen

Title and Abstract Screen. Within the reference library, titles and abstracts were screened manually for 

relevance. Studies that were considered relevant (see Table B-1 for inclusion criteria) were moved to the 
MOLYBDENUM B-21

APPENDIX B

second step of the literature screening process. Studies were excluded when the title and abstract clearly 

indicated that the study was not relevant to the toxicological profile. 

• Number of titles and abstracts screened: 2,508

• Number of studies considered relevant and moved to the next step: 129

Full Text Screen. The second step in the literature screening process was a full text review of individual 

studies considered relevant in the title and abstract screen step. Each study was reviewed to determine 

whether it was relevant for inclusion in the toxicological profile. 

• Number of studies undergoing full text review: 129

• Number of studies cited in the pre-public draft of the toxicological profile: 235

• Total number of studies cited in the profile: 337

A summary of the results of the literature search and screening is presented in Figure B-1.
MOLYBDENUM B-22

APPENDIX B

Figure B-1. January 2018 Literature Search Results and Screen for Molybdenum
MOLYBDENUM C-1

APPENDIX C. FRAMEWORK FOR ATSDR’S SYSTEMATIC REVIEW OF 

HEALTH EFFECTS DATA FOR MOLYBDENUM

To increase the transparency of ATSDR’s process of identifying, evaluating, synthesizing, and 

interpreting the scientific evidence on the health effects associated with exposure to molybdenum, 

ATSDR utilized a slight modification of NTP’s Office of Health Assessment and Translation (OHAT) 

systematic review methodology (NTP 2013, 2015; Rooney et al. 2014). ATSDR’s framework is an eightstep process for systematic review with the goal of identifying the potential health hazards of exposure to 

molybdenum:

• Step 1. Problem Formulation

• Step 2. Literature Search and Screen for Health Effects Studies

• Step 3. Extract Data from Health Effects Studies

• Step 4. Identify Potential Health Effect Outcomes of Concern

• Step 5. Assess the Risk of Bias for Individual Studies

• Step 6. Rate the Confidence in the Body of Evidence for Each Relevant Outcome

• Step 7. Translate Confidence Rating into Level of Evidence of Health Effects

• Step 8. Integrate Evidence to Develop Hazard Identification Conclusions

C.1 PROBLEM FORMULATION

The objective of the toxicological profile and this systematic review was to identify the potential health 

hazards associated with inhalation, oral, or dermal/ocular exposure to molybdenum. The inclusion 

criteria used to identify relevant studies examining the health effects of molybdenum are presented in 

Table C-1. 

Table C-1. Inclusion Criteria for Identifying Health Effects Studies

Species

Human

Laboratory mammals

Route of exposure

Inhalation

Oral

Dermal (or ocular)

Parenteral (these studies will be considered supporting data)

Health outcome

Death

Systemic effects

Body weight effects 

Respiratory effects

Cardiovascular effects

Gastrointestinal effects

Hematological effects

Musculoskeletal effects

Hepatic effects

Renal effects
MOLYBDENUM C-2

APPENDIX C

Table C-1. Inclusion Criteria for Identifying Health Effects Studies

Dermal effects

Ocular effects

Endocrine effects

Immunological effects

Neurological effects

Reproductive effects

Developmental effects

Other noncancer effects

Cancer

Data from human and laboratory animal studies were considered relevant for addressing this objective. 

Human studies were divided into two broad categories: observational epidemiology studies and 

controlled exposure studies. The observational epidemiology studies were further divided: cohort studies 

(retrospective and prospective studies), population studies (with individual data or aggregate data), and 

case-control studies.

C.2 LITERATURE SEARCH AND SCREEN FOR HEALTH EFFECTS STUDIES

A literature search and screen was conducted to identify studies examining the health effects of 

molybdenum. The literature search framework for the toxicological profile is discussed in detail in 

Appendix B.

C.2.1 Literature Search

As noted in Appendix B, the current literature search was intended to update the draft toxicological 

profile for molybdenum released for public comment in 2017. See Appendix B for the databases 

searched and the search strategy.

A total of 2,508 records relevant to all sections of the toxicological profile were identified (after 

duplicate removal).

C.2.2 Literature Screening

As described in Appendix B, a two-step process was used to screen the literature search to identify 

relevant studies examining the health effects of molybdenum.

Title and Abstract Screen. In the Title and Abstract Screen step, 2,508 records were reviewed; 

71 documents were considered to meet the health effects inclusion criteria in Table C-1 and were moved 

to the next step in the process. 

Full Text Screen. In the second step in the literature screening process for the systematic review, a full 

text review of 92 health effects documents (documents identified in the update literature search and 

documents cited in older versions of the profile) was performed. From those 92 documents, 115 studies 

were included in the qualitative review. 
MOLYBDENUM C-3

APPENDIX C

C.3 EXTRACT DATA FROM HEALTH EFFECTS STUDIES

Relevant data extracted from the individual studies selected for inclusion in the systematic review were 

collected in customized data forms. A summary of the type of data extracted from each study is presented 

in Table C-2. For references that included more than one experiment or species, data extraction records 

were created for each experiment or species. 

Table C-2. Data Extracted From Individual Studies

Citation

Chemical form

Route of exposure (e.g., inhalation, oral, dermal)

Specific route (e.g., gavage in oil, drinking water)

Species

Strain

Exposure duration category (e.g., acute, intermediate, chronic)

Exposure duration

Frequency of exposure (e.g., 6 hours/day, 5 days/week)

Exposure length

Number of animals or subjects per sex per group 

Dose/exposure levels

Parameters monitored

Description of the study design and method

Summary of calculations used to estimate doses (if applicable)

Summary of the study results

Reviewer’s comments on the study

Outcome summary (one entry for each examined outcome)

No-observed-adverse-effect level (NOAEL) value

Lowest-observed-adverse-effect level (LOAEL) value

Effect observed at the LOAEL value

A summary of the extracted data for each study is presented in the Supplemental Document for 

Molybdenum and overviews of the results of the studies are presented in Sections 2.2–2.18 of the profile 

and in the Levels of Significant Exposures tables in Section 2.1 of the profile (Tables 2-1–2-3).

C.4 IDENTIFY POTENTIAL HEALTH EFFECT OUTCOMES OF CONCERN 

Overviews of the potential health effect outcomes for molybdenum identified in human and animal 

studies are presented in Tables C-3 and C-4, respectively. The available human studies examined a 

limited number of endpoints and reported respiratory, hepatic, endocrine, other systemic (alterations in 

uric acid levels), reproductive, and developmental effects. Animal studies examined a number of 

endpoints following inhalation and oral exposure; no dermal exposure studies were identified. These 

studies examined most systemic endpoints and reported respiratory, gastrointestinal, hematological, 

musculoskeletal, hepatic, renal, endocrine, dermal, and body weight effects. Additionally, animal studies 
MOLYBDENUM C-4 

APPENDIX C

Table C-3. Overview of the Health Outcomes for Molybdenum Evaluated In Human Studies Body weight Respiratory Cardiovascular Gastrointestinal Hematological Musculoskeletal Hepatic Renal Dermal Ocular Endocrine Immunological Neurological Reproductive Developmental

Other NoncancerCaner

Inhalation studies

Cohort 2 1 1

1 1 1

Case control

Population

Case series

Oral studies

Cohort 2 2 1

2 1 0

Case control

Population 1 1 3 1 1 1 3 2

0 0 2 0 1 1 2 0

Case series

1

0

Dermal studies

Cohort

Case control

Population

Case series

Number of studies examining endpoint 0 1 2 3 4 5-9 ≥10

Number of studies reporting outcome 0 1 2 3 4 5-9 ≥10
MOLYBDENUM C-5 

APPENDIX C

Table C-4. Overview of the Health Outcomes for Molybdenum Evaluated in Experimental Animal Studies Body weight Respiratory Cardiovascular Gastrointestinal Hematological Musculoskeletal Hepatic Renal Dermal Ocular Endocrine Immunologicala Neurologicala Reproductivea Developmental Other Noncancer

Caner

Inhalation studies

Acute-duration 5 5

5 0

Intermediate-duration 2 2 2 2 2 2 2 2 2 2

0 0 0 0 0 0 0 0 0 0

Chronic-duration 2 2 2 2 2 2 2 2 2 2 2

0 2 0 0 0 0 0 0 0 0 2

Oral studies

Acute-duration 6 1 1 5 2 2 4

2 1 0 4 1 0 3

Intermediate-duration 41 3 2 3 19 13 8 9 3 8 1 12 12 2

28 0 0 1 6 10 6 6 3 5 0 8 5 0

Chronic-duration

Dermal studies

Acute-duration 7 4 4

0 4 0

Intermediate-duration

Chronic-duration

Number of studies examining endpoint 0 1 2 3 4 5-9 ≥10

Number of studies reporting outcome 0 1 2 3 4 5-9 ≥10

aNumber of studies examining endpoint includes study evaluating histopathology, but not evaluating function.
MOLYBDENUM C-6

APPENDIX C

have reported neurological, reproductive, and developmental effects. Although animal studies have 

identified a number of affected tissues and systems, interpretation of much of the data is limited by an 

inadequate amount of copper in the diet. Studies in which the diet did not contain adequate levels of 

copper or administered ammonium tetrathiomolybdate were carried through Step 3 of the systematic 

review, but were not considered in the identification of potential health effect outcomes of concern. 

Additionally, body weight effects were not considered a primary effect especially since most studies did 

not provide data on food intake; thus, this endpoint was not considered in the assessment of potential 

human hazards. Studies examining the respiratory, hepatic, renal, uric acid, reproductive, and 

developmental outcomes were carried through to Steps 4–8 of the systematic review. There were 

115 studies (published in 92 documents) examining these potential outcomes were carried through to 

Steps 4–8 of the systematic review. 

C.5 ASSESS THE RISK OF BIAS FOR INDIVIDUAL STUDIES

C.5.1 Risk of Bias Assessment

The risk of bias of individual studies was assessed using OHAT’s Risk of Bias Tool (NTP 2015). The 

risk of bias questions for observational epidemiology studies, human-controlled exposure studies, and 

animal experimental studies are presented in Tables C-5, C-6, and C-7, respectively. Each risk of bias 

question was answered on a four-point scale:

• Definitely low risk of bias (++)

• Probably low risk of bias (+)

• Probably high risk of bias (-)

• Definitely high risk of bias (– –)

Table C-5. Risk of Bias Questionnaire for Observational Epidemiology Studies

Selection bias

Were the comparison groups appropriate?

Confounding bias

Did the study design or analysis account for important confounding and modifying variables?

Attrition/exclusion bias

Were outcome data complete without attrition or exclusion from analysis?

Detection bias

Is there confidence in the exposure characterization?

Is there confidence in outcome assessment?

Selective reporting bias

Were all measured outcomes reported?
MOLYBDENUM C-7

APPENDIX C

Table C-6. Risk of Bias Questionnaire for Human-Controlled Exposure Studies

Selection bias

Was administered dose or exposure level adequately randomized?

Was the allocation to study groups adequately concealed?

Performance bias

Were the research personnel and human subjects blinded to the study group during the study?

Attrition/exclusion bias

Were outcome data complete without attrition or exclusion from analysis?

Detection bias

Is there confidence in the exposure characterization?

Is there confidence in outcome assessment?

Selective reporting bias

Were all measured outcomes reported?

Table C-7. Risk of Bias Questionnaire for Experimental Animal Studies

Selection bias

Was administered dose or exposure level adequately randomized?

Was the allocation to study groups adequately concealed?

Performance bias

Were experimental conditions identical across study groups?

Were the research personnel blinded to the study group during the study?

Attrition/exclusion bias

Were outcome data complete without attrition or exclusion from analysis?

Detection bias

Is there confidence in the exposure characterization?

Is there confidence in outcome assessment?

Selective reporting bias

Were all measured outcomes reported? 

In general, “definitely low risk of bias” or “definitely high risk of bias” were used if the question could be 

answered with information explicitly stated in the study report. If the response to the question could be 

inferred, then “probably low risk of bias” or “probably high risk of bias” responses were typically used. 

After the risk of bias questionnaires were completed for the health effects studies, the studies were 

assigned to one of three risk of bias tiers based on the responses to the key questions listed below and the 

responses to the remaining questions. 

• Is there confidence in the exposure characterization? (only relevant for observational studies)

• Is there confidence in the outcome assessment? 

• Does the study design or analysis account for important confounding and modifying variables? 

(only relevant for observational studies)
MOLYBDENUM C-8

APPENDIX C

First Tier. Studies placed in the first tier received ratings of “definitely low” or “probably low” risk of 

bias on the key questions AND received a rating of “definitely low” or “probably low” risk of bias on the 

responses to at least 50% of the other applicable questions.

Second Tier. A study was placed in the second tier if it did not meet the criteria for the first or third tiers.

Third Tier. Studies placed in the third tier received ratings of “definitely high” or “probably high” risk of 

bias for the key questions AND received a rating of “definitely high” or “probably high” risk of bias on 

the response to at least 50% of the other applicable questions.

The results of the risk of bias assessment for the different types of molybdenum health effects studies 

(observational epidemiology, human-controlled exposure studies, and animal experimental studies) are 

presented in Tables C-8, C-9, and C-10, respectively.

C.6 RATE THE CONFIDENCE IN THE BODY OF EVIDENCE FOR EACH RELEVANT 

OUTCOME

Confidences in the bodies of human and animal evidence were evaluated independently for each potential 

outcome. ATSDR did not evaluate the confidence in the body of evidence for carcinogenicity; rather, the 

Agency defaulted to the cancer weight-of-evidence assessment of other agencies including DHHS, EPA, 

and IARC. The confidence in the body of evidence for an association or no association between exposure 

to molybdenum and a particular outcome was based on the strengths and weaknesses of individual 

studies. Four descriptors were used to describe the confidence in the body of evidence for effects or when 

no effect was found:

• High confidence: the true effect is highly likely to be reflected in the apparent relationship

• Moderate confidence: the true effect may be reflected in the apparent relationship

• Low confidence: the true effect may be different from the apparent relationship

• Very low confidence: the true effect is highly likely to be different from the apparent 

relationship

Confidence in the body of evidence for a particular outcome was rated for each type of study: casecontrol, case series, cohort, population, human-controlled exposure, and experimental animal. In the 

absence of data to the contrary, data for a particular outcome were collapsed across animal species, routes 

of exposure, and exposure durations. If species (or strain), route, or exposure duration differences were 

noted, then the data were treated as separate outcomes.
MOLYBDENUM C-9

APPENDIX C

Table C-8. Summary of Risk of Bias Assessment for Molybdenum—Observational Epidemiological Studies

Reference

Risk of bias criteria and ratings

Selection bias

Confounding 

bias

Attrition / 

exclusion bias Detection bias

Selective 

reporting bias

Were the comparison groups appropriate?Did the study design or analysis account for important confoundingand modifying variables?*Were outcome data complete without attrition or Risk of bias tier

exclusion from analysis?

Is there confidence in the exposure characterization?*Is there confidence in the outcome assessment?*Were all measured outcomes reported?

Outcome: Respiratory effects

Cohort studies

Ott et al. 2004 – – + na + ++ Second

Walravens et al. 1979 – – + + – + Second

Outcome: Hepatic effects

Cross-sectional studies

Mendy et al. 2012 + + + + – + Second

Outcome: Alterations in uric acid levels 

Cross-sectional studies

Koval’sky et al. 1961 – – + – + + Second

Cohort studies

Walravens et al. 1979 – – + + – + Second

Outcome: Reproductive effects

Cross-sectional studies

Lewis and Meeker 2015 na - + + + + First

Meeker et al. 2008 + + + ++ ++ ++ First

Meeker et al. 2010 + + ++ + ++ ++ First
MOLYBDENUM C-10

APPENDIX C

Table C-8. Summary of Risk of Bias Assessment for Molybdenum—Observational Epidemiological Studies

Reference

Risk of bias criteria and ratings

Selection bias

Confounding 

bias

Attrition / 

exclusion bias Detection bias

Selective 

reporting bias

Were the comparison groups appropriate?Did the study design or analysis account for important confoundingand modifying variables?*Were outcome data complete without attrition or Risk of bias tier

exclusion from analysis?

Is there confidence in the exposure characterization?*Is there confidence in the outcome assessment?*Were all measured outcomes reported?

Outcome: Developmental effects

Cross-sectional studies

Vazquez-Salas et al. 2014 + + + + ++ + First

Shirai et al. 2010 na – + + + + Second

++ = definitely low risk of bias; + = probably low risk of bias; – = probably high risk of bias; – – = definitely high risk of bias; na = not applicable

*Key question used to assign risk of bias tier.
MOLYBDENUM C-11

APPENDIX C

Table C-9. Summary of Risk of Bias Assessment for Molybdenum—Human-Controlled Exposure Studies

Reference

Risk of bias criteria and ratings

Selection bias

Performance 

bias

Attrition/ 

exclusion bias Detection bias

Selective 

reporting bias

Was administered dose or exposure level adequately randomized?Was the allocation to study groups adequately concealed?Were the research personnel blinded to the study group during the study?Were outcome data complete without attrition or exclusion from analysis?Is there confidence in the exposure characterization?Is there confidence in the outcomeassessment?*Were all measured outcomes reported?Risk of bias tier

Outcome: Alterations in uric acid levels

Oral acute exposure

Deosthale and Gopalan 1974 na + + + + + ++ First

++ = definitely low risk of bias; + = probably low risk of bias; – = probably high risk of bias; – – = definitely high risk of bias; na = not applicable

*Key question used to assign risk of bias tier.
MOLYBDENUM C-12

APPENDIX C

Table C-10. Summary of Risk of Bias Assessment for Molybdenum—Experimental Animal Studies

Reference

Risk of bias criteria and ratings

Selection bias Performance bias

Attrition/ 

exclusion 

bias Detection bias

Selective 

reporting 

bias Other bias

Was administered dose or exposure level adequately randomized?Was the allocation to study groups adequately concealed?Were experimental conditions identical across study groups?Were the research personnel blinded to the study group during the study?Were outcome data complete without attrition or exclusion from analysis?Is there confidence in the exposure characterization?Is there confidence in the outcome assessment?*Were all measured outcomes reported?Did the study design or analysis account for important confounding and modifying variables?Risk of bias tier

Outcome: Respiratory effects

Inhalation acute exposure

NTP 1997 (rat) ++ + ++ + ++ ++ ++ ++ + First

NTP 1997 (mouse) ++ + ++ + ++ ++ ++ ++ + First

Inhalation intermediate exposure

NTP 1997 (rat) ++ + ++ + ++ ++ ++ ++ + First

NTP 1997 (mouse) ++ + ++ + ++ ++ ++ ++ + First

Inhalation chronic exposure

NTP 1997 (rat) ++ + ++ + ++ ++ ++ ++ + First

NTP 1997 (mouse) ++ + ++ + ++ ++ ++ ++ + First

Outcome: Hepatic effects

Inhalation intermediate exposure

NTP 1997 (rat) ++ + ++ + ++ ++ ++ ++ + First

NTP 1997 (mouse) ++ + ++ + ++ ++ ++ ++ + First

Inhalation chronic exposure

NTP 1997 (rat) ++ + ++ + ++ ++ ++ ++ + First

NTP 1997 (mouse) ++ + ++ + ++ ++ ++ ++ + First
MOLYBDENUM C-13

APPENDIX C

Table C-10. Summary of Risk of Bias Assessment for Molybdenum—Experimental Animal Studies

Reference

Risk of bias criteria and ratings

Selection bias Performance bias

Attrition/ 

exclusion 

bias Detection bias

Selective 

reporting 

bias Other bias

Was administered dose or exposure level adequately randomized?Was the allocation to study groups adequately concealed?Were experimental conditions identical across study groups?Were the research personnel blinded to the study group during the study?Were outcome data complete without attrition or exclusion from analysis?Is there confidence in the exposure characterization?Is there confidence in the outcome assessment?*Were all measured outcomes reported?Did the study design or analysis account for important confounding and modifying variables?Risk of bias tier

Oral acute exposure

Bersenyi et al. 2008 (rabbit) – + + – ++ – + + + First

Bersenyi et al. 2008 (rabbit) – + + – ++ – + + + First

Oral intermediate exposure

Murray et al. 2014a (rat) ++ + ++ – ++ ++ ++ ++ ++ First

Rana and Chauhan 2000 

(rat) – + + – ++ + – ++ – Second

Rana and Kumar 1980b (rat) – + + – ++ – – + – Third

Rana and Kumar 1980c (rat) + + – – ++ – + ++ – First

Rana and Kumar 1983 (rat) + + – – ++ + + ++ – First

Rana and Prakash 1986 (rat) – + + – ++ – + + + First

Rana et al. 1980 (rat) – + + – + – + + + First

Rana et al. 1985 (rat) + + + – ++ + + + + First

Outcome: Renal effects

Inhalation intermediate exposure

NTP 1997 (rat) ++ + ++ + ++ ++ ++ ++ + First

NTP 1997 (mouse) ++ + ++ + ++ ++ ++ ++ + First
MOLYBDENUM C-14

APPENDIX C

Table C-10. Summary of Risk of Bias Assessment for Molybdenum—Experimental Animal Studies

Reference

Risk of bias criteria and ratings

Selection bias Performance bias

Attrition/ 

exclusion 

bias Detection bias

Selective 

reporting 

bias Other bias

Was administered dose or exposure level adequately randomized?Was the allocation to study groups adequately concealed?Were experimental conditions identical across study groups?Were the research personnel blinded to the study group during the study?Were outcome data complete without attrition or exclusion from analysis?Is there confidence in the exposure characterization?Is there confidence in the outcome assessment?*Were all measured outcomes reported?Did the study design or analysis account for important confounding and modifying variables?Risk of bias tier

Inhalation chronic exposure

NTP 1997 (rat) ++ + ++ + ++ ++ ++ ++ + First

NTP 1997 (mouse) ++ + ++ + ++ ++ ++ ++ + First

Oral acute exposure

Bersenyi et al. 2008 (rabbit, 

males) – + + – ++ – + + + First

Bersenyi et al. 2008 (rabbit, 

females) – + + – ++ – + + + First

Oral intermediate exposure

Bandyopadhyay et al. 1981 

(rat) – + + – ++ – + ++ ++ First

Bompart et al. 1990 (rat) + + + – ++ + + ++ + First

Murray et al. 2014a (rat) ++ + ++ – ++ ++ ++ ++ ++ First

Rana et al. 1980 (rat) – + + – + – + + + First

Rana and Kumar 1980c + + – – ++ – + ++ – First

Rana and Kumar 1983 (rat) + + – – ++ + + ++ – First
MOLYBDENUM C-15

APPENDIX C

Table C-10. Summary of Risk of Bias Assessment for Molybdenum—Experimental Animal Studies

Reference

Risk of bias criteria and ratings

Selection bias Performance bias

Attrition/ 

exclusion 

bias Detection bias

Selective 

reporting 

bias Other bias

Was administered dose or exposure level adequately randomized?Was the allocation to study groups adequately concealed?Were experimental conditions identical across study groups?Were the research personnel blinded to the study group during the study?Were outcome data complete without attrition or exclusion from analysis?Is there confidence in the exposure characterization?Is there confidence in the outcome assessment?*Were all measured outcomes reported?Did the study design or analysis account for important confounding and modifying variables?Risk of bias tier

Outcome: Alterations in uric acid levels

Oral intermediate exposure

Murray et al. 2014a (rat) ++ + ++ - ++ ++ ++ ++ ++ First

Outcome: Reproductive effects

Inhalation intermediate exposure

NTP 1997 (rat) ++ + ++ + ++ ++ ++ ++ + First

NTP 1997 (mouse) ++ + ++ + ++ ++ ++ ++ + First

Oral acute exposure

Zhang et al. 2013 (mouse) – + ++ – ++ – – + ++ – First

Zhai et al. 2013 (mouse) – + ++ – ++ – – + ++ + First

Bersenyi et al. 2008 (rabbit, 

males) – + + – ++ – + + + First

Bersenyi et al. 2008 (rabbit, 

females) – + + – ++ – + + + First

Oral intermediate exposure

Fungwe et al. 1990 (rat) + + + – ++ – + + – – First

Jeter and Davis 1954 (rat, 

adults) – + + – ++ – + + – First
MOLYBDENUM C-16

APPENDIX C

Table C-10. Summary of Risk of Bias Assessment for Molybdenum—Experimental Animal Studies

Reference

Risk of bias criteria and ratings

Selection bias Performance bias

Attrition/ 

exclusion 

bias Detection bias

Selective 

reporting 

bias Other bias

Was administered dose or exposure level adequately randomized?Was the allocation to study groups adequately concealed?Were experimental conditions identical across study groups?Were the research personnel blinded to the study group during the study?Were outcome data complete without attrition or exclusion from analysis?Is there confidence in the exposure characterization?Is there confidence in the outcome assessment?*Were all measured outcomes reported?Did the study design or analysis account for important confounding and modifying variables?Risk of bias tier

Jeter and Davis 1954 (rat, 

weanling) – + + – ++ – + + – – First

Murray et al. 2014a (rat) ++ + ++ – ++ ++ ++ ++ ++ First

Murray et al. 2019 (rat) ++ + ++ – ++ ++ ++ ++ ++ First

Pandey and Singh 2002 (rat) – + ++ – ++ + + ++ – First

Pandey and Singh 2002 (rat 

fertility study) – + ++ – ++ + + ++ – First

Outcome: Developmental effects

Oral intermediate exposure

Jeter and Davis 1954 (rat, 

weanling) – + + – ++ – + + – – First

Murray et al. 2014b (rat) ++ + + – ++ ++ + ++ + First

Pandey and Singh 2002 (rat) – + ++ – ++ + + ++ – First

++ = definitely low risk of bias; + = probably low risk of bias; – = probably high risk of bias; – – = definitely high risk of bias

*Key question used to assign risk of bias tier.
MOLYBDENUM C-17

APPENDIX C

C.6.1 Initial Confidence Rating

In ATSDR’s modification to the OHAT approach, the body of evidence for an association (or no 

association) between exposure to molybdenum and a particular outcome was given an initial confidence 

rating based on the key features of the individual studies examining that outcome. The presence of these 

key features of study design was determined for individual studies using four “yes or no” questions, 

which were customized for epidemiology, human controlled exposure, or experimental animal study 

designs. Separate questionnaires were completed for each outcome assessed in a study. The key features 

for observational epidemiology (cohort, population, and case-control) studies, human controlled exposure,

and experimental animal studies are presented in Tables C-11, C-12, and C-13, respectively. The initial 

confidence in the study was determined based on the number of key features present in the study design: 

• High Initial Confidence: Studies in which the responses to the four questions were “yes”. 

• Moderate Initial Confidence: Studies in which the responses to only three of the questions 

were “yes”. 

• Low Initial Confidence: Studies in which the responses to only two of the questions were “yes”. 

• Very Low Initial Confidence: Studies in which the response to one or none of the questions 

was “yes”. 

Table C-11. Key Features of Study Design for Observational Epidemiology 

Studies

Exposure was experimentally controlled 

Exposure occurred prior to the outcome

Outcome was assessed on individual level rather than at the population level

A comparison group was used

Table C-12. Key Features of Study Design for Human-Controlled Exposure 

Studies

A comparison group was used or the subjects served as their own control

A sufficient number of subjects were tested

Appropriate methods were used to measure outcomes (i.e., clinically-confirmed outcome versus selfreported)

Appropriate statistical analyses were performed and reported or the data were reported in such a way to 

allow independent statistical analysis
MOLYBDENUM C-18

APPENDIX C

Table C-13. Key Features of Study Design for Experimental Animal Studies

A concurrent control group was used

A sufficient number of animals per group were tested

Appropriate parameters were used to assess a potential adverse effect

Appropriate statistical analyses were performed and reported or the data were reported in such a way to 

allow independent statistical analysis

The presence or absence of the key features and the initial confidence levels for studies examining The 

presence or absence of the key features and the initial confidence levels for studies examining respiratory, 

gastrointestinal, renal, dermal, and ocular effects observed in the observational epidemiology, humancontrolled exposure, and animal experimental studies are presented in Tables C-14, C-15, and C-16, 

respectively.

Table C-14. Presence of Key Features of Study Design for Molybdenum—

Observational Epidemiology Studies

Key features 

Reference

Controlled exposureExposure prior to outcomeOutcomes assessed on an individual level Comparison group

Initial study 

confidence

Outcome: Respiratory effects

Cohort studies

Ott et al. 2004 No Yes Yes No Low

Walravens et al. 1979 No No No No Very Low

Outcome: Hepatic effects

Cross-sectional studies

Mendy et al. 2012 No No Yes Yes Low

Outcome: Alterations in uric acid levels

Cross-sectional studies

Koval’sky et al. 1961 No Yes Yes No Low

Cohort studies

Walravens et al. 1979 No No No No Very Low

Outcome: Reproductive effects

Cross-sectional studies

Lewis and Meeker 2015 No No Yes Yes Low

Meeker et al. 2008 No No Yes Yes Low

Meeker et al. 2010 No No Yes Yes Low
MOLYBDENUM C-19

APPENDIX C

Table C-14. Presence of Key Features of Study Design for Molybdenum—

Observational Epidemiology Studies

Key features 

Reference

Controlled exposureExposure prior to outcomeOutcomes assessed on an individual level Comparison group

Initial study 

confidence

Outcome: Developmental effects

Cross-sectional studies

Vazquez-Salas et al. 2014 No No Yes Yes Low

Shirai et al. 2010 No No Yes Yes Low

Table C-15. Presence of Key Features of Study Design for Molybdenum—

Human-Controlled Exposure Studies

Key feature

Reference

Concurrent control group or self-controlSufficient number of subjects testedAppropriate methods to measure outcomeAdequate data for statistical analysis

Initial study 

confidence

Outcome: Alterations in uric acid levels

Oral acute exposure

Deosthale and Gopalan 1974 Yes No Yes No Low
MOLYBDENUM C-20

APPENDIX C

Table C-16. Presence of Key Features of Study Design for Molybdenum—

Experimental Animal Studies

Key feature

Reference

Concurrent control groupSufficient number of animals per groupAppropriate parameters to assess potential effectAdequate data for statistical analysis

Initial study 

confidence

Outcome: Respiratory effects

Inhalation acute exposure

NTP 1997 (rat) Yes Yes Yes Yes High

NTP 1997 (mouse) Yes Yes Yes Yes High

Inhalation intermediate exposure

NTP 1997 (rat) Yes Yes Yes Yes High

NTP 1997 (mouse) Yes Yes Yes Yes High

Inhalation chronic exposure

NTP 1997 (rat) Yes Yes Yes Yes High

NTP 1997 (mouse) Yes Yes Yes Yes High

Outcome: Hepatic effects

Inhalation intermediate exposure

NTP 1997 (rat) Yes Yes Yes Yes High

NTP 1997 (mouse) Yes Yes Yes Yes High

Inhalation chronic exposure

NTP 1997 (rat) Yes Yes Yes Yes High

NTP 1997 (mouse) Yes Yes Yes Yes High

Oral acute exposure

Bersenyi et al. 2008 (rabbit, males) Yes No Yes Yes Moderate

Bersenyi et al. 2008 (rabbit, females) Yes No Yes Yes Moderate

Oral intermediate exposure

Murray et al. 2014a (rat) Yes Yes Yes Yes High

Rana and Chauhan 2000 (rat) Yes Yes No Yes Moderate

Rana and Kumar 1980b (rat) Yes Yes No Yes Moderate

Rana and Kumar 1980c (rat) Yes Yes No Yes Moderate

Rana and Kumar 1983 (rat) Yes Yes No Yes Moderate

Rana and Prakash 1986 (rat) Yes Yes No Yes Moderate

Rana et al. 1980 (rat) Yes Yes No No Low

Rana et al. 1985 (rat) Yes Yes No Yes Moderate

Outcome: Renal effects

Inhalation intermediate exposure

NTP 1997 (rat) Yes Yes Yes Yes High

NTP 1997 (mouse) Yes Yes Yes Yes High
MOLYBDENUM C-21

APPENDIX C

Table C-16. Presence of Key Features of Study Design for Molybdenum—

Experimental Animal Studies

Key feature

Reference

Concurrent control groupSufficient number of animals per groupAppropriate parameters to assess potential effectAdequate data for statistical analysis

Initial study 

confidence

Inhalation chronic exposure

NTP 1997 (rat) Yes Yes Yes Yes High

NTP 1997 (mouse) Yes Yes Yes Yes High

Oral acute exposure

Bersenyi et al. 2008 (rabbit, males) Yes No Yes Yes Moderate

Bersenyi et al. 2008 (rabbit, females) Yes No Yes Yes Moderate

Oral intermediate exposure

Bandyopadhyay et al. 1981 (rat) Yes No Yes No Low

Bompart et al. 1990 (rat) Yes No Yes Yes Moderate

Murray et al. 2014a (rat) Yes Yes Yes Yes High

Murray et al. 2019 (rat) Yes Yes Yes Yes High

Rana et al. 1980 (rat) Yes Yes No No Low

Rana and Kumar 1980c Yes Yes No Yes Moderate

Rana and Kumar 1983 (rat) Yes Yes No Yes Moderate

Outcome: Alterations in uric acid levels

Oral intermediate exposure

Murray et al. 2014a (rat) Yes Yes Yes Yes High

Outcome: Reproductive effects

Inhalation intermediate exposure

NTP 1997 (rat) Yes Yes Yes Yes High

NTP 1997 (mouse) Yes Yes Yes Yes High

Oral acute exposure

Zhang et al. 2013 (mouse) Yes Yes No Yes Moderate

Zhai et al. 2013 (mouse) Yes Yes No Yes Moderate

Bersenyi et al. 2008 (rabbit, males) Yes No No Yes Low

Bersenyi et al. 2008 (rabbit, females) Yes No No No Very Low

Oral intermediate exposure

Fungwe et al. 1990 (rat) Yes No Yes Yes Moderate

Jeter and Davis 1954 (rat, adult) Yes No No No Very Low

Murray et al. 2014a (rat) Yes Yes Yes Yes High

Murray et al. 2019 (rat) Yes Yes Yes Yes High

Pandey and Singh 2002 (rat) Yes Yes No Yes Moderate

Pandey and Singh 2002 (rat, fertility study) Yes Yes Yes Yes High
MOLYBDENUM C-22

APPENDIX C

Table C-16. Presence of Key Features of Study Design for Molybdenum—

Experimental Animal Studies

Key feature

Reference

Concurrent control groupSufficient number of animals per groupAppropriate parameters to assess potential effectAdequate data for statistical analysis

Initial study 

confidence

Outcome: Developmental effects

Oral intermediate exposure

Jeter and Davis 1954 (rat, weanling) Yes No No No Very Low

Murray et al. 2014b (rat) Yes Yes Yes Yes High

Murray et al. 2019 (rat) Yes Yes Yes Yes High

Pandey and Singh 2002 (rat) Yes Yes Yes Yes High

A summary of the initial confidence ratings for each outcome is presented in Table C-17. If individual 

studies for a particular outcome and study type had different study quality ratings, then the highest 

confidence rating for the group of studies was used to determine the initial confidence rating for the body 

of evidence; any exceptions were noted in Table C-17.

Table C-17. Initial Confidence Rating for Molybdenum Health Effects Studies

Finding

Initial study 

confidence

Initial 

confidence 

rating

Outcome: Respiratory effects (inhalation only)

Inhalation acute exposure

Animal studies

NTP 1997 (rat) No effect High High NTP 1997 (mouse) No effect High

Inhalation intermediate exposure

Animal studies

NTP 1997 (rat) No effect High High NTP 1997 (mouse) No effect High

Inhalation chronic exposure

Human studies

Observational studies

Ott et al. 2004 Effect Low

Low Walravens et al. 1979 Effect Very Low
MOLYBDENUM C-23

APPENDIX C

Table C-17. Initial Confidence Rating for Molybdenum Health Effects Studies

Finding

Initial study 

confidence

Initial 

confidence 

rating

Animal studies

NTP 1997 (rat) Effect High High NTP 1997 (mouse) Effect High

Outcome: Hepatic effects

Inhalation intermediate exposure

Animal studies

NTP 1997 (rat) No effect High High NTP 1997 (mouse) No effect High

Inhalation chronic exposure

Animal studies

NTP 1997 (rat) No effect High High NTP 1997 (mouse) No effect High

Oral acute exposure

Animal studies

Bersenyi et al. 2008 (rabbit, males) Effect Moderate

Moderate Bersenyi et al. 2008 (rabbit, females) Effect Moderate

Oral intermediate exposure

Animal studies

Murray et al. 2014a (rat) No effect High High

Rana and Chauhan 2000 (rat) Effect Moderate

Low

Rana and Kumar 1980b (rat) Effect Moderate

Rana and Kumar 1980c (rat) Effect Moderate

Rana and Kumar 1983 (rat) Effect Moderate

Rana and Prakash 1986 (rat) Effect Moderate

Rana et al. 1980 (rat) Effect Low

Rana et al. 1985 (rat) Effect Moderate

Oral chronic exposure

Human studies

Observational studies

Mendy et al. 2012 Effect Low Low

Outcome: Renal effects

Inhalation intermediate exposure

Animal studies

NTP 1997 (rat) No effect High High NTP 1997 (mouse) No effect High

Inhalation chronic exposure

Animal studies

NTP 1997 (rat) No effect High High NTP 1997 (mouse) No effect High

Oral acute exposure

Animal studies

Bersenyi et al. 2008 (rabbit, males) No effect Moderate

Moderate Bersenyi et al. 2008 (rabbit, females) No effect Moderate
MOLYBDENUM C-24

APPENDIX C

Table C-17. Initial Confidence Rating for Molybdenum Health Effects Studies

Finding

Initial study 

confidence

Initial 

confidence 

rating

Oral intermediate exposure

Animal studies

Bandyopadhyay et al. 1981 (rat) Effect Low

High

Bompart et al. 1990 (rat) Effect Moderate

Murray et al. 2014a (rat) Effect High

Rana et al. 1980 (rat) Effect Low

Rana and Kumar 1980c Effect Moderate

Rana and Kumar 1983 (rat) Effect Moderate

Murray et al. 2019 (rat) No effect High High

Outcome: Alterations in uric acid levels

Inhalation chronic exposure

Human studies

Observational studies

Walravens et al. 1979 Effect Very Low Very Low

Oral acute exposure

Human studies

Controlled exposure 

Deosthale and Gopalan 1974 No Effect Low Low

Oral intermediate exposure

Animal studies

Murray et al. 2014a (rat) No effect High High

Oral chronic exposure

Human studies

Observational studies

Koval’sky et al. 1961 Effect Low Low

Outcome: Reproductive effects

Inhalation intermediate exposure

Animal studies

NTP 1997 (rat) No effect High High NTP 1997 (mouse) No effect High

Oral acute exposure

Animal studies

Zhang et al. 2013 (mouse) Effect Moderate

Zhai et al. 2013 (mouse) Effect Moderate Moderate

Bersenyi et al. 2008 (male, rabbit) Effect Low

Bersenyi et al. 2008 (female, rabbit) No effect Very Low Very low

Oral intermediate exposure

Animal studies

Fungwe et al. 1990 (rat) Effect Moderate

High Jeter and Davis 1954 (rat, adult) Effect Very Low

Jeter and Davis 1954 (rat, weanling) Effect Very Low

Pandey and Singh 2002 (rat) Effect Moderate
MOLYBDENUM C-25

APPENDIX C

Table C-17. Initial Confidence Rating for Molybdenum Health Effects Studies

Finding

Initial study 

confidence

Initial 

confidence 

rating

Pandey and Singh 2002 (rat, fertility study) Effect High

Murray et al. 2014a (rat) No effect High High Murray et al. 2019 (rat) No effect High

Oral chronic exposure

Human studies

Observational studies

Lewis and Meeker 2015 Effect Low

Meeker et al. 2008 Effect Low Low

Meeker et al. 2010 Effect Low

Outcome: Developmental effects

Oral intermediate exposure

Animal studies

Pandey and Singh 2002 (rat) Effect High High

Jeter and Davis 1954 (rat, weanling) No effect Very Low

Murray et al. 2014b (rat) No effect High High

Murray et al. 2019 (rat) No effect High

Oral chronic exposure

Human studies

Observational studies

Vazquez-Salas et al. 2014 Effect Low Low

Shirai et al. 2010 No effect Low Low

C.6.2 Adjustment of the Confidence Rating

The initial confidence rating was then downgraded or upgraded depending on whether there were 

substantial issues that would decrease or increase confidence in the body of evidence. The nine properties 

of the body of evidence that were considered are listed below. The summaries of the assessment of the 

confidence in the body of evidence for respiratory, hepatic, renal, alterations in uric acid levels, 

reproductive, and developmental effects are presented in Table C-18. If the confidence ratings for a 

particular outcome were based on more than one type of human study, then the highest confidence rating 

was used for subsequent analyses. An overview of the confidence in the body of evidence for all health 

effects associated with molybdenum exposure is presented in Table C-19.

Five properties of the body of evidence were considered to determine whether the confidence rating 

should be downgraded: 

• Risk of bias. Evaluation of whether there is substantial risk of bias across most of the studies 

examining the outcome. This evaluation used the risk of bias tier groupings for individual studies 

examining a particular outcome (Tables C-14, C-15, and C-16). Below are the criteria used to 

determine whether the initial confidence in the body of evidence for each outcome should be 

downgraded for risk of bias:

o No downgrade if most studies are in the risk of bias first tier

o Downgrade one confidence level if most studies are in the risk of bias second tier

o Downgrade two confidence levels if most studies are in the risk of bias third tier
MOLYBDENUM C-26

APPENDIX C

Table C-18. Adjustments to the Initial Confidence in the Body of Evidence 

Initial confidence Adjustments to the initial confidence rating Final confidence

Outcome: Respiratory effects

Observational studies (effect) Low -1 risk of bias; -1 imprecision Very low

Animal studies (effect) High None High

Animal studies (no effect) High +1 magnitude High

Outcome: Hepatic effects

Observational studies (effect) Low -1 risk of bias Very low

Animal studies (effect) Moderate -1 indirectness (secondary outcomes); Moderate

Animal studies (no effect) High None High

Outcome: Renal effects

Animal studies High None High

Animal studies High None High

Outcome: Alterations in uric acid levels

Observational studies (effect) Low -1 risk of bias Very low

Controlled exposure studies (no effect) Low None Low

Animal studies (no effect) High None High

Outcome: Reproductive effects

Observational studies (effect) Low None Low

Animal studies (effect) High -1 inconsistency Moderate

Animal studies (no effect) High None High

Outcome: Developmental effects

Observational studies (effect) Low None Low

Observational studies (no effect) Low None Low

Animal studies High -1 inconsistency Moderate

Animal studies High None High
MOLYBDENUM C-27

APPENDIX C

Table C-19. Confidence in the Body of Evidence for Molybdenum

Outcome

Confidence in body of evidence

Human studies Animal studies

Respiratory effects Very low (effect) High (effect)

High (no effect)

Hepatic effects Very low (effect) Moderate (effect)

High (no effect)

Renal effects No data High (effect)

High (no effect)

Alterations in uric acid levels Very low (effect)

Low (no effect)

High (effect)

Reproductive Effects Low (effect) Moderate (effect)

High (no effect)

Developmental effects Low (effect)

Low (no effect)

Moderate (effect)

High (no effect)

• Unexplained inconsistency. Evaluation of whether there is inconsistency or large variability in 

the magnitude or direction of estimates of effect across studies that cannot be explained. Below 

are the criteria used to determine whether the initial confidence in the body of evidence for each 

outcome should be downgraded for unexplained inconsistency:

o No downgrade if there is little inconsistency across studies or if only one study evaluated 

the outcome

o Downgrade one confidence level if there is variability across studies in the magnitude or 

direction of the effect

o Downgrade two confidence levels if there is substantial variability across studies in the 

magnitude or direct of the effect

• Indirectness. Evaluation of four factors that can affect the applicability, generalizability, and 

relevance of the studies: 

o Relevance of the animal model to human health—unless otherwise indicated, studies in 

rats, mice, and other mammalian species are considered relevant to humans 

o Directness of the endpoints to the primary health outcome—examples of secondary 

outcomes or nonspecific outcomes include organ weight in the absence of histopathology 

or clinical chemistry findings in the absence of target tissue effects

o Nature of the exposure in human studies and route of administration in animal studies—

inhalation, oral, and dermal exposure routes are considered relevant unless there are 

compelling data to the contrary 

o Duration of treatment in animal studies and length of time between exposure and 

outcome assessment in animal and prospective human studies—this should be considered 

on an outcome-specific basis

Below are the criteria used to determine whether the initial confidence in the body of evidence for 

each outcome should be downgraded for indirectness:

o No downgrade if none of the factors are considered indirect 

o Downgrade one confidence level if one of the factors is considered indirect 

o Downgrade two confidence levels if two or more of the factors are considered indirect
MOLYBDENUM C-28

APPENDIX C

• Imprecision. Evaluation of the narrowness of the effect size estimates and whether the studies 

have adequate statistical power. Data are considered imprecise when the ratio of the upper to 

lower 95% CIs for most studies is ≥10 for tests of ratio measures (e.g., odds ratios) and ≥100 for 

absolute measures (e.g., percent control response). Adequate statistical power is determined if 

the study can detect a potentially biologically meaningful difference between groups (20% 

change from control response for categorical data or risk ratio of 1.5 for continuous data). Below 

are the criteria used to determine whether the initial confidence in the body of evidence for each 

outcome should be downgraded for imprecision:

o No downgrade if there are no serious imprecisions 

o Downgrade one confidence level for serious imprecisions 

o Downgrade two confidence levels for very serious imprecisions 

• Publication bias. Evaluation of the concern that studies with statistically significant results are 

more likely to be published than studies without statistically significant results. 

o Downgrade one level of confidence for cases where there is serious concern with 

publication bias

Four properties of the body of evidence were considered to determine whether the confidence rating 

should be upgraded: 

• Large magnitude of effect. Evaluation of whether the magnitude of effect is sufficiently large 

so that it is unlikely to have occurred as a result of bias from potential confounding factors. 

o Upgrade one confidence level if there is evidence of a large magnitude of effect in a few 

studies, provided that the studies have an overall low risk of bias and there is no serious 

unexplained inconsistency among the studies of similar dose or exposure levels; 

confidence can also be upgraded if there is one study examining the outcome, provided 

that the study has an overall low risk of bias

• Dose response. Evaluation of the dose-response relationships measured within a study and 

across studies. Below are the criteria used to determine whether the initial confidence in the body 

of evidence for each outcome should be upgraded:

o Upgrade one confidence level for evidence of a monotonic dose-response gradient

o Upgrade one confidence level for evidence of a non-monotonic dose-response gradient 

where there is prior knowledge that supports a non-monotonic dose-response and a nonmonotonic dose-response gradient is observed across studies

• Plausible confounding or other residual biases. This factor primarily applies to human studies 

and is an evaluation of unmeasured determinants of an outcome such as residual bias towards the 

null (e.g., “healthy worker” effect) or residual bias suggesting a spurious effect (e.g., recall bias). 

Below is the criterion used to determine whether the initial confidence in the body of evidence for 

each outcome should be upgraded:

o Upgrade one confidence level for evidence that residual confounding or bias would 

underestimate an apparent association or treatment effect (i.e., bias toward the null) or 

suggest a spurious effect when results suggest no effect

• Consistency in the body of evidence. Evaluation of consistency across animal models and 

species, consistency across independent studies of different human populations and exposure 

scenarios, and consistency across human study types. Below is the criterion used to determine 

whether the initial confidence in the body of evidence for each outcome should be upgraded:

o Upgrade one confidence level if there is a high degree of consistency in the database
MOLYBDENUM C-29

APPENDIX C

C.7 TRANSLATE CONFIDENCE RATING INTO LEVEL OF EVIDENCE OF HEALTH 

EFFECTS

In the seventh step of the systematic review of the health effects data for molybdenum, the confidence in 

the body of evidence for specific outcomes was translated to a level of evidence rating. The level of 

evidence rating reflected the confidence in the body of evidence and the direction of the effect (i.e., 

toxicity or no toxicity); route-specific differences were noted. The level of evidence for health effects 

was rated on a five-point scale: 

• High level of evidence: High confidence in the body of evidence for an association between 

exposure to the substance and the health outcome

• Moderate level of evidence: Moderate confidence in the body of evidence for an association 

between exposure to the substance and the health outcome

• Low level of evidence: Low confidence in the body of evidence for an association between 

exposure to the substance and the health outcome

• Evidence of no health effect: High confidence in the body of evidence that exposure to the 

substance is not associated with the health outcome

• Inadequate evidence: Low or moderate confidence in the body of evidence that exposure to the 

substance is not associated with the health outcome OR very low confidence in the body of 

evidence for an association between exposure to the substance and the health outcome

A summary of the level of evidence of health effects for molybdenum is presented in Table C-20.

Table C-20. Level of Evidence of Health Effects for Molybdenum

Outcome

Confidence in body 

of evidence

Direction of health 

effect

Level of evidence for 

health effect

Human studies

Respiratory effects

(inhalation only)

Very low Health effect Inadequate

Hepatic effects Very low Health effect Inadequate

Renal effects No data No data No data

Alterations in uric acid 

levels

Low Health effect Inadequate

Reproductive effects Low Health effect Low

Developmental effects Low Health effect Low

Animal studies

Respiratory effects

(inhalation only)

High Health effect 

No health effect

High

High

Hepatic effects Moderate Health effect

No health effect

Moderate

High

Renal effects High Health effect High

Alterations in uric acid 

levels

High No effect Evidence of no health effect
MOLYBDENUM C-30

APPENDIX C

Table C-20. Level of Evidence of Health Effects for Molybdenum

Outcome

Confidence in body 

of evidence

Direction of health 

effect

Level of evidence for 

health effect

Reproductive effects Moderate Health effect 

No health effect

Moderate

High

Developmental effectsa Moderate Health effect 

No health effect

High

Evidence of no health effect

C.8 INTEGRATE EVIDENCE TO DEVELOP HAZARD IDENTIFICATION CONCLUSIONS

The final step involved the integration of the evidence streams for the human studies and animal studies 

to allow for a determination of hazard identification conclusions. For health effects, there were four 

hazard identification conclusion categories:

• Known to be a hazard to humans

• Presumed to be a hazard to humans 

• Suspected to be a hazard to humans 

• Not classifiable as to the hazard to humans 

The initial hazard identification was based on the highest level of evidence in the human studies and the 

level of evidence in the animal studies; if there were no data for one evidence stream (human or animal), 

then the hazard identification was based on the one data stream (equivalent to treating the missing 

evidence stream as having low level of evidence). The hazard identification scheme is presented in 

and described below:

• Known: A health effect in this category would have:

o High level of evidence for health effects in human studies AND a high, moderate, or low 

level of evidence in animal studies.

• Presumed: A health effect in this category would have:

o Moderate level of evidence in human studies AND high or moderate level of evidence in 

animal studies OR

o Low level of evidence in human studies AND high level of evidence in animal studies

• Suspected: A health effect in this category would have:

o Moderate level of evidence in human studies AND low level of evidence in animal 

studies OR

o Low level of evidence in human studies AND moderate level of evidence in animal 

studies

• Not classifiable: A health effect in this category would have:

o Low level of evidence in human studies AND low level of evidence in animal studies
MOLYBDENUM C-31

APPENDIX C

Figure C-1. Hazard Identification Scheme

Other relevant data such as mechanistic or mode-of-action data were considered to raise or lower the level 

of the hazard identification conclusion by providing information that supported or opposed biological 

plausibility. 

Two hazard identification conclusion categories were used when the data indicated that there may be no 

health effect in humans:

• Not identified to be a hazard in humans

• Inadequate to determine hazard to humans

If the human level of evidence conclusion of no health effect was supported by the animal evidence of no 

health effect, then the hazard identification conclusion category of “not identified” was used. If the 

human or animal level of evidence was considered inadequate, then a hazard identification conclusion 

category of “inadequate” was used. As with the hazard identification for health effects, the impact of 

other relevant data was also considered for no health effect data. 

The hazard identification conclusions for molybdenum are listed below and summarized in Table C-21. 
MOLYBDENUM C-32

APPENDIX C

Table C-21. Hazard Identification Conclusions for Molybdenum

Outcome Hazard identification 

Respiratory effects Presumed health effect following long-term inhalation 

exposure

Hepatic effects Not classifiable as a hazard to humans

Renal effects Presumed health effect 

Alterations in uric acid levels Not classifiable as a hazard to humans

Reproductive effects Suspected health effect 

Developmental effects Not classifiable as a hazard to humans 

Presumed Health Effects

• Respiratory effects following long-term inhalation exposure to molybdenum oxides

o Inadequate evidence from studies of molybdenum oxide workers (Ott et al. 2004; Walravens 

et al. 1979). 

o High level of evidence from chronic studies in rats and mice exposed to molybdenum trioxide

(NTP 1997). Respiratory effects were not observed following acute- or intermediate-duration 

inhalation exposure.

• Renal effects

o No data in humans.

o High level of evidence of histological alterations in kidneys, alterations in renal function, 

and/or increased lipid levels in the kidneys in orally exposed rats (Bandyopadhyay et al. 

1981; Bompart et al. 1990; Murray et al. 2014a; Rana and Kumar 1980c, 1983; Rana et al. 

1980).

Not Classifiable as a Hazard to Humans

• Hepatic effects

o Inadequate evidence of increased risk of self-reported liver conditions from a cross-sectional 

study (Mendy et al. 2012).

o High evidence of no histological alterations following intermediate or chronic inhalation 

exposure of rats and mice to molybdenum trioxide (NTP 1997), acute oral exposure of rabbits 

to ammonium heptamolybdate (Bersenyi et al. 2008), or intermediate oral exposure of rats to 

sodium molybdate (Murray et al. 2014a;). 

o Moderate evidence of increases in clinical chemistry parameters and/or liver lipid levels in 

rabbits following acute oral exposure (Bersenyi et al. 2008) or rats exposed orally exposed to 

high doses (Rana and Chauhan 2000; Rana and Kumar 1980b, 1980c, 1983; Rana and 

Prakash 1986; Rana et al. 1980, 1985). 

o The hazard identification for hepatic effects was downgraded to Not Classifiable because the 

toxicological significance of the alterations in serum enzyme levels and lipid levels were not 

known and well-designed inhalation and oral laboratory animal studies have not reported 

histological alterations. 

• Alterations in uric acid levels

o Low evidence of an effect in cross-sectional studies (Koval’skiy et al. 1961; Walravens et al. 

1979).

o High confidence in an animal study not finding an effect (Murray et al. 2014a).

• Reproductive effects

o Low level of evidence of male reproductive effects in cross-sectional studies (Lewis and 

Meeker 2015; Meeker et al. 2008, 2010).
MOLYBDENUM C-33

APPENDIX C

o Two high-quality, intermediate-duration (Murray et al. 2014a) and 2-generation (Murray et 

al. 2019) studies have not reported reproductive effects.

o There is a moderate level of evidence of male and/or female reproductive effects in orally 

exposed rats (Fungwe et al. 1990; Pandey and Singh 2002), mice (Zhai et al. 2013; Zhang et 

al. 2013), and rabbits (Bersenyi et al. 2008).

• Developmental effects

o Low evidence of an effect in a cross-sectional study. Two cross-sectional studies reported no 

alterations in newborn body weight (Shirai et al. 2010; Vazquez-Salas et al. 2014); one study 

reported decreases in psychomotor development indices (Vazquez-Salas et al. 2014).

o Three studies in rats did not find alterations in resorptions, post-implantation losses, or fetal 

body weights (Jeter and Davis 1954; Murray et al. 2014b, 2019); the initial confidence levels 

for two of these studies were high and the third study was very low. A fourth study (initial 

high confidence level) involving male-only exposure found decreases in number of live 

fetuses and fetal body weights (Pandey and Singh 2002). The animal studies had different 

study designs (male only, female only, male and female exposure) making a comparison 

across studies difficult. Additionally, none of the animal studies evaluated potential 

neurodevelopmental effects, which were observed in an epidemiology study. Thus, the 

available data were not considered adequate for drawing a conclusion on the potential 

developmental toxicity of molybdenum in humans.
MOLYBDENUM D-1

APPENDIX D. USER'S GUIDE

Chapter 1. Relevance to Public Health

This chapter provides an overview of U.S. exposures, a summary of health effects based on evaluations of 

existing toxicologic, epidemiologic, and toxicokinetic information, and an overview of the minimal risk 

levels. This is designed to present interpretive, weight-of-evidence discussions for human health 

endpoints by addressing the following questions:

1. What effects are known to occur in humans?

2. What effects observed in animals are likely to be of concern to humans?

3. What exposure conditions are likely to be of concern to humans, especially around hazardous 

waste sites?

Minimal Risk Levels (MRLs)

Where sufficient toxicologic information is available, ATSDR derives MRLs for inhalation and oral 

routes of entry at each duration of exposure (acute, intermediate, and chronic). These MRLs are not 

meant to support regulatory action, but to acquaint health professionals with exposure levels at which 

adverse health effects are not expected to occur in humans.

MRLs should help physicians and public health officials determine the safety of a community living near 

a hazardous substance emission, given the concentration of a contaminant in air or the estimated daily 

dose in water. MRLs are based largely on toxicological studies in animals and on reports of human 

occupational exposure.

MRL users should be familiar with the toxicologic information on which the number is based. 

Section 1.2, Summary of Health Effects, contains basic information known about the substance. Other 

sections, such as Section 3.2 Children and Other Populations that are Unusually Susceptible and 

Section 3.4 Interactions with Other Substances, provide important supplemental information.

MRL users should also understand the MRL derivation methodology. MRLs are derived using a 

modified version of the risk assessment methodology that the Environmental Protection Agency (EPA) 

provides (Barnes and Dourson 1988) to determine reference doses (RfDs) for lifetime exposure. 

To derive an MRL, ATSDR generally selects the most sensitive endpoint which, in its best judgement, 

represents the most sensitive human health effect for a given exposure route and duration. ATSDR 

cannot make this judgement or derive an MRL unless information (quantitative or qualitative) is available 

for all potential systemic, neurological, and developmental effects. If this information and reliable 

quantitative data on the chosen endpoint are available, ATSDR derives an MRL using the most sensitive 

species (when information from multiple species is available) with the highest no-observed-adverse-effect 

level (NOAEL) that does not exceed any adverse effect levels. When a NOAEL is not available, a 

lowest-observed-adverse-effect level (LOAEL) can be used to derive an MRL, and an uncertainty factor 

of 10 must be employed. Additional uncertainty factors of 10 must be used both for human variability to 

protect sensitive subpopulations (people who are most susceptible to the health effects caused by the 

substance) and for interspecies variability (extrapolation from animals to humans). In deriving an MRL, 

these individual uncertainty factors are multiplied together. The product is then divided into the 

inhalation concentration or oral dosage selected from the study. Uncertainty factors used in developing a 
MOLYBDENUM D-2

APPENDIX D

substance-specific MRL are provided in the footnotes of the levels of significant exposure (LSE) tables

that are provided in Chapter 2. Detailed discussions of the MRLs are presented in Appendix A.

Chapter 2. Health Effects

Tables and Figures for Levels of Significant Exposure (LSE)

Tables and figures are used to summarize health effects and illustrate graphically levels of exposure 

associated with those effects. These levels cover health effects observed at increasing dose 

concentrations and durations, differences in response by species and MRLs to humans for noncancer 

endpoints. The LSE tables and figures can be used for a quick review of the health effects and to locate 

data for a specific exposure scenario. The LSE tables and figures should always be used in conjunction 

with the text. All entries in these tables and figures represent studies that provide reliable, quantitative 

estimates of NOAELs, LOAELs, or Cancer Effect Levels (CELs).

The legends presented below demonstrate the application of these tables and figures. Representative 

examples of LSE tables and figures follow. The numbers in the left column of the legends correspond to 

the numbers in the example table and figure.

TABLE LEGEND

See Sample LSE Table (page D-5)

(1) Route of exposure. One of the first considerations when reviewing the toxicity of a substance 

using these tables and figures should be the relevant and appropriate route of exposure. 

Typically, when sufficient data exist, three LSE tables and two LSE figures are presented in the 

document. The three LSE tables present data on the three principal routes of exposure

(i.e., inhalation, oral, and dermal). LSE figures are limited to the inhalation and oral routes. Not 

all substances will have data on each route of exposure and will not, therefore, have all five of the 

tables and figures. Profiles with more than one chemical may have more LSE tables and figures.

(2) Exposure period. Three exposure periods—acute (<15 days), intermediate (15–364 days), and 

chronic (≥365 days)—are presented within each relevant route of exposure. In this example, two 

oral studies of chronic-duration exposure are reported. For quick reference to health effects 

occurring from a known length of exposure, locate the applicable exposure period within the LSE 

table and figure.

(3) Figure key. Each key number in the LSE table links study information to one or more data points 

using the same key number in the corresponding LSE figure. In this example, the study 

represented by key number 51 identified NOAELs and less serious LOAELs (also see the three

"51R" data points in sample LSE Figure 2-X).

(4) Species (strain) No./group. The test species (and strain), whether animal or human, are identified 

in this column. The column also contains information on the number of subjects and sex per 

group. Chapter 1, Relevance to Public Health, covers the relevance of animal data to human 

toxicity and Section 3.1, Toxicokinetics, contains any available information on comparative 

toxicokinetics. Although NOAELs and LOAELs are species specific, the levels are extrapolated 

to equivalent human doses to derive an MRL.

(5) Exposure parameters/doses. The duration of the study and exposure regimens are provided in 

these columns. This permits comparison of NOAELs and LOAELs from different studies. In 

this case (key number 51), rats were orally exposed to “Chemical X” via feed for 2 years. For a 
MOLYBDENUM D-3

APPENDIX D

more complete review of the dosing regimen, refer to the appropriate sections of the text or the 

original reference paper (i.e., Aida et al. 1992).

(6) Parameters monitored. This column lists the parameters used to assess health effects. Parameters 

monitored could include serum (blood) chemistry (BC), behavioral (BH), biochemical changes 

(BI), body weight (BW), clinical signs (CS), developmental toxicity (DX), enzyme activity (EA), 

food intake (FI), fetal toxicity (FX), gross necropsy (GN), hematology (HE), histopathology 

(HP), lethality (LE), maternal toxicity (MX), organ function (OF), ophthalmology (OP), organ 

weight (OW), teratogenicity (TG), urinalysis (UR), and water intake (WI).

(7) Endpoint. This column lists the endpoint examined. The major categories of health endpoints 

included in LSE tables and figures are death, body weight, respiratory, cardiovascular, 

gastrointestinal, hematological, musculoskeletal, hepatic, renal, dermal, ocular, endocrine, 

immunological, neurological, reproductive, developmental, other noncancer, and cancer. "Other

noncancer" refers to any effect (e.g., alterations in blood glucose levels) not covered in these 

systems. In the example of key number 51, three endpoints (body weight, hematological, and 

hepatic) were investigated.

(8) NOAEL. A NOAEL is the highest exposure level at which no adverse effects were seen in the 

organ system studied. The body weight effect reported in key number 51 is a NOAEL at

25.5 mg/kg/day. NOAELs are not reported for cancer and death; with the exception of these two 

endpoints, this field is left blank if no NOAEL was identified in the study.

(9) LOAEL. A LOAEL is the lowest dose used in the study that caused an adverse health effect. 

LOAELs have been classified into "Less Serious" and "Serious" effects. These distinctions help 

readers identify the levels of exposure at which adverse health effects first appear and the 

gradation of effects with increasing dose. A brief description of the specific endpoint used to 

quantify the adverse effect accompanies the LOAEL. Key number 51 reports a less serious 

LOAEL of 6.1 mg/kg/day for the hepatic system, which was used to derive a chronic exposure, 

oral MRL of 0.008 mg/kg/day (see footnote "c"). MRLs are not derived from serious LOAELs. 

A cancer effect level (CEL) is the lowest exposure level associated with the onset of 

carcinogenesis in experimental or epidemiologic studies. CELs are always considered serious 

effects. The LSE tables and figures do not contain NOAELs for cancer, but the text may report 

doses not causing measurable cancer increases. If no LOAEL/CEL values were identified in the 

study, this field is left blank.

(10) Reference. The complete reference citation is provided in Chapter 8 of the profile.

(11) Footnotes. Explanations of abbreviations or reference notes for data in the LSE tables are found 

in the footnotes. For example, footnote "c" indicates that the LOAEL of 6.1 mg/kg/day in key 

number 51 was used to derive an oral MRL of 0.008 mg/kg/day.

FIGURE LEGEND

See Sample LSE Figure (page D-6)

LSE figures graphically illustrate the data presented in the corresponding LSE tables. Figures help the 

reader quickly compare health effects according to exposure concentrations for particular exposure 

periods.

(13) Exposure period. The same exposure periods appear as in the LSE table. In this example, health 

effects observed within the chronic exposure period are illustrated.
MOLYBDENUM D-4

APPENDIX D

(14) Endpoint. These are the categories of health effects for which reliable quantitative data exist. 

The same health effect endpoints appear in the LSE table.

(15) Levels of exposure. Concentrations or doses for each health effect in the LSE tables are 

graphically displayed in the LSE figures. Exposure concentration or dose is measured on the log 

scale "y" axis. Inhalation exposure is reported in mg/m3 or ppm and oral exposure is reported in 

mg/kg/day.

(16) LOAEL. In this example, the half-shaded circle that is designated 51R identifies a LOAEL 

critical endpoint in the rat upon which a chronic oral exposure MRL is based. The key number 

51 corresponds to the entry in the LSE table. The dashed descending arrow indicates the 

extrapolation from the exposure level of 6.1 mg/kg/day (see entry 51 in the sample LSE table) to 

the MRL of 0.008 mg/kg/day (see footnote "c" in the sample LSE table).

(17) CEL. Key number 59R is one of studies for which CELs were derived. The diamond symbol 

refers to a CEL for the test species (rat). The number 59 corresponds to the entry in the LSE 

table.

(18) Key to LSE figure. The key provides the abbreviations and symbols used in the figure.
MOLYBDENUM D-5

APPENDIX D

1
MOLYBDENUM D-6

APPENDIX D

1
MOLYBDENUM E-1

APPENDIX E. QUICK REFERENCE FOR HEALTH CARE PROVIDERS

Toxicological Profiles are a unique compilation of toxicological information on a given hazardous 

substance. Each profile reflects a comprehensive and extensive evaluation, summary, and interpretation 

of available toxicologic and epidemiologic information on a substance. Health care providers treating 

patients potentially exposed to hazardous substances may find the following information helpful for fast 

answers to often-asked questions.

Primary Chapters/Sections of Interest

Chapter 1: Relevance to Public Health: The Relevance to Public Health Section provides an overview 

of exposure and health effects and evaluates, interprets, and assesses the significance of toxicity 

data to human health. A table listing minimal risk levels (MRLs) is also included in this chapter.

Chapter 2: Health Effects: Specific health effects identified in both human and animal studies are 

reported by type of health effect (e.g., death, hepatic, renal, immune, reproductive), route of 

exposure (e.g., inhalation, oral, dermal), and length of exposure (e.g., acute, intermediate, and 

chronic). 

NOTE: Not all health effects reported in this section are necessarily observed in the clinical 

setting. 

Pediatrics: 

Section 3.2 Children and Other Populations that are Unusually Susceptible

Section 3.3 Biomarkers of Exposure and Effect

ATSDR Information Center 

Phone: 1-800-CDC-INFO (800-232-4636) or 1-888-232-6348 (TTY)

Internet: http://www.atsdr.cdc.gov

The following additional materials are available online:

Case Studies in Environmental Medicine are self-instructional publications designed to increase primary 

health care providers’ knowledge of a hazardous substance in the environment and to aid in the 

evaluation of potentially exposed patients (see https://www.atsdr.cdc.gov/csem/csem.html). 

Managing Hazardous Materials Incidents is a three-volume set of recommendations for on-scene 

(prehospital) and hospital medical management of patients exposed during a hazardous materials 

incident (see https://www.atsdr.cdc.gov/MHMI/index.asp). Volumes I and II are planning guides 

to assist first responders and hospital emergency department personnel in planning for incidents 

that involve hazardous materials. Volume III—Medical Management Guidelines for Acute 

Chemical Exposures—is a guide for health care professionals treating patients exposed to 

hazardous materials.

Fact Sheets (ToxFAQsTM) provide answers to frequently asked questions about toxic substances (see 

https://www.atsdr.cdc.gov/toxfaqs/Index.asp).
MOLYBDENUM E-2

APPENDIX E

Other Agencies and Organizations

The National Center for Environmental Health (NCEH) focuses on preventing or controlling disease, 

injury, and disability related to the interactions between people and their environment outside the 

workplace. Contact: NCEH, Mailstop F-29, 4770 Buford Highway, NE, Atlanta, GA 

30341-3724 • Phone: 770-488-7000 • FAX: 770-488-7015 • Web Page: 

https://www.cdc.gov/nceh/.

The National Institute for Occupational Safety and Health (NIOSH) conducts research on occupational 

diseases and injuries, responds to requests for assistance by investigating problems of health and 

safety in the workplace, recommends standards to the Occupational Safety and Health 

Administration (OSHA) and the Mine Safety and Health Administration (MSHA), and trains 

professionals in occupational safety and health. Contact: NIOSH, 395 E Street, S.W., Suite 9200, 

Patriots Plaza Building, Washington, DC 20201 • Phone: 202-245-0625 or 1-800-CDC-INFO 

(800-232-4636) • Web Page: https://www.cdc.gov/niosh/.

The National Institute of Environmental Health Sciences (NIEHS) is the principal federal agency for 

biomedical research on the effects of chemical, physical, and biologic environmental agents on 

human health and well-being. Contact: NIEHS, PO Box 12233, 104 T.W. Alexander Drive, 

Research Triangle Park, NC 27709 • Phone: 919-541-3212 • Web Page: 

https://www.niehs.nih.gov/.

Clinical Resources (Publicly Available Information)

The Association of Occupational and Environmental Clinics (AOEC) has developed a network of clinics 

in the United States to provide expertise in occupational and environmental issues. Contact: 

AOEC, 1010 Vermont Avenue, NW, #513, Washington, DC 20005 • Phone: 202-347-4976 

• FAX: 202-347-4950 • e-mail: AOEC@AOEC.ORG • Web Page: http://www.aoec.org/.

The American College of Occupational and Environmental Medicine (ACOEM) is an association of 

physicians and other health care providers specializing in the field of occupational and 

environmental medicine. Contact: ACOEM, 25 Northwest Point Boulevard, Suite 700, Elk 

Grove Village, IL 60007-1030 • Phone: 847-818-1800 • FAX: 847-818-9266 • Web Page: 

http://www.acoem.org/.

The American College of Medical Toxicology (ACMT) is a nonprofit association of physicians with 

recognized expertise in medical toxicology. Contact: ACMT, 10645 North Tatum Boulevard, 

Suite 200-111, Phoenix AZ 85028 • Phone: 844-226-8333 • FAX: 844-226-8333 • Web Page: 

http://www.acmt.net.

The Pediatric Environmental Health Specialty Units (PEHSUs) is an interconnected system of specialists 

who respond to questions from public health professionals, clinicians, policy makers, and the 

public about the impact of environmental factors on the health of children and reproductive-aged 

adults. Contact information for regional centers can be found at http://pehsu.net/findhelp.html.

The American Association of Poison Control Centers (AAPCC) provide support on the prevention and 

treatment of poison exposures. Contact: AAPCC, 515 King Street, Suite 510, Alexandria VA 

22314 • Phone: 701-894-1858 • Poison Help Line: 1-800-222-1222 • Web Page: 

http://www.aapcc.org/.
MOLYBDENUM F-1

APPENDIX F. GLOSSARY

Absorption—The process by which a substance crosses biological membranes and enters systemic 

circulation. Absorption can also refer to the taking up of liquids by solids, or of gases by solids or liquids.

Acute Exposure—Exposure to a chemical for a duration of ≤14 days, as specified in the Toxicological 

Profiles.

Adsorption—The adhesion in an extremely thin layer of molecules (as of gases, solutes, or liquids) to the 

surfaces of solid bodies or liquids with which they are in contact.

Adsorption Coefficient (Koc)—The ratio of the amount of a chemical adsorbed per unit weight of 

organic carbon in the soil or sediment to the concentration of the chemical in solution at equilibrium.

Adsorption Ratio (Kd)—The amount of a chemical adsorbed by sediment or soil (i.e., the solid phase) 

divided by the amount of chemical in the solution phase, which is in equilibrium with the solid phase, at a 

fixed solid/solution ratio. It is generally expressed in micrograms of chemical sorbed per gram of soil or 

sediment.

Benchmark Dose (BMD) or Benchmark Concentration (BMC)—is the dose/concentration 

corresponding to a specific response level estimate using a statistical dose-response model applied to 

either experimental toxicology or epidemiology data. For example, a BMD10 would be the dose 

corresponding to a 10% benchmark response (BMR). The BMD is determined by modeling the doseresponse curve in the region of the dose-response relationship where biologically observable data are 

feasible. The BMDL or BMCL is the 95% lower confidence limit on the BMD or BMC. 

Bioconcentration Factor (BCF)—The quotient of the concentration of a chemical in aquatic organisms 

at a specific time or during a discrete time period of exposure divided by the concentration in the 

surrounding water at the same time or during the same period.

Biomarkers—Indicators signaling events in biologic systems or samples, typically classified as markers 

of exposure, effect, and susceptibility.

Cancer Effect Level (CEL)—The lowest dose of a chemical in a study, or group of studies, that 

produces significant increases in the incidence of cancer (or tumors) between the exposed population and 

its appropriate control.

Carcinogen—A chemical capable of inducing cancer.

Case-Control Study—A type of epidemiological study that examines the relationship between a 

particular outcome (disease or condition) and a variety of potential causative agents (such as toxic 

chemicals). In a case-control study, a group of people with a specified and well-defined outcome is 

identified and compared to a similar group of people without the outcome.

Case Report—A report that describes a single individual with a particular disease or exposure. These 

reports may suggest some potential topics for scientific research, but are not actual research studies.

Case Series—Reports that describe the experience of a small number of individuals with the same 

disease or exposure. These reports may suggest potential topics for scientific research, but are not actual 

research studies.
MOLYBDENUM F-2

APPENDIX F

Ceiling Value—A concentration that must not be exceeded.

Chronic Exposure—Exposure to a chemical for ≥365 days, as specified in the Toxicological Profiles.

Clastogen—A substance that causes breaks in chromosomes resulting in addition, deletion, or 

rearrangement of parts of the chromosome.

Cohort Study—A type of epidemiological study of a specific group or groups of people who have had a 

common insult (e.g., exposure to an agent suspected of causing disease or a common disease) and are 

followed forward from exposure to outcome, and who are disease-free at start of follow-up. Often, at 

least one exposed group is compared to one unexposed group, while in other cohorts, exposure is a 

continuous variable and analyses are directed towards analyzing an exposure-response coefficient.

Cross-sectional Study—A type of epidemiological study of a group or groups of people that examines 

the relationship between exposure and outcome to a chemical or to chemicals at a specific point in time.

Data Needs—Substance-specific informational needs that, if met, would reduce the uncertainties of 

human health risk assessment.

Developmental Toxicity—The occurrence of adverse effects on the developing organism that may result 

from exposure to a chemical prior to conception (either parent), during prenatal development, or 

postnatally to the time of sexual maturation. Adverse developmental effects may be detected at any point 

in the life span of the organism.

Dose-Response Relationship—The quantitative relationship between the amount of exposure to a 

toxicant and the incidence of the response or amount of the response.

Embryotoxicity and Fetotoxicity—Any toxic effect on the conceptus as a result of prenatal exposure to 

a chemical; the distinguishing feature between the two terms is the stage of development during which the 

effect occurs. Effects include malformations and variations, altered growth, and in utero death.

Epidemiology—The investigation of factors that determine the frequency and distribution of disease or 

other health-related conditions within a defined human population during a specified period. 

Excretion—The process by which metabolic waste products are removed from the body.

Genotoxicity—A specific adverse effect on the genome of living cells that, upon the duplication of 

affected cells, can be expressed as a mutagenic, clastogenic, or carcinogenic event because of specific 

alteration of the molecular structure of the genome.

Half-life—A measure of rate for the time required to eliminate one-half of a quantity of a chemical from 

the body or environmental media.

Health Advisory—An estimate of acceptable drinking water levels for a chemical substance derived by 

EPA and based on health effects information. A health advisory is not a legally enforceable federal 

standard, but serves as technical guidance to assist federal, state, and local officials.

Immediately Dangerous to Life or Health (IDLH)—A condition that poses a threat of life or health, or 

conditions that pose an immediate threat of severe exposure to contaminants that are likely to have 

adverse cumulative or delayed effects on health.
MOLYBDENUM F-3

APPENDIX F

Immunotoxicity—Adverse effect on the functioning of the immune system that may result from 

exposure to chemical substances. 

Incidence—The ratio of new cases of individuals in a population who develop a specified condition to 

the total number of individuals in that population who could have developed that condition in a specified 

time period. 

Intermediate Exposure—Exposure to a chemical for a duration of 15–364 days, as specified in the 

Toxicological Profiles.

In Vitro—Isolated from the living organism and artificially maintained, as in a test tube.

In Vivo—Occurring within the living organism.

Lethal Concentration(LO) (LCLO)—The lowest concentration of a chemical in air that has been reported 

to have caused death in humans or animals.

Lethal Concentration(50) (LC50)—A calculated concentration of a chemical in air to which exposure for 

a specific length of time is expected to cause death in 50% of a defined experimental animal population.

Lethal Dose(LO) (LDLo)—The lowest dose of a chemical introduced by a route other than inhalation that 

has been reported to have caused death in humans or animals.

Lethal Dose(50) (LD50)—The dose of a chemical that has been calculated to cause death in 50% of a 

defined experimental animal population.

Lethal Time(50) (LT50)—A calculated period of time within which a specific concentration of a chemical 

is expected to cause death in 50% of a defined experimental animal population.

Lowest-Observed-Adverse-Effect Level (LOAEL)—The lowest exposure level of chemical in a study, 

or group of studies, that produces statistically or biologically significant increases in frequency or severity 

of adverse effects between the exposed population and its appropriate control.

Lymphoreticular Effects—Represent morphological effects involving lymphatic tissues such as the 

lymph nodes, spleen, and thymus.

Malformations—Permanent structural changes that may adversely affect survival, development, or 

function.

Metabolism—Process in which chemical substances are biotransformed in the body that could result in 

less toxic and/or readily excreted compounds or produce a biologically active intermediate.

Minimal Risk Level (MRL)—An estimate of daily human exposure to a hazardous substance that is 

likely to be without an appreciable risk of adverse noncancer health effects over a specified route and 

duration of exposure.

Modifying Factor (MF)—A value (greater than zero) that is applied to the derivation of a Minimal Risk 

Level (MRL) to reflect additional concerns about the database that are not covered by the uncertainty 

factors. The default value for a MF is 1.
MOLYBDENUM F-4

APPENDIX F

Morbidity—The state of being diseased; the morbidity rate is the incidence or prevalence of a disease in 

a specific population.

Mortality—Death; the mortality rate is a measure of the number of deaths in a population during a 

specified interval of time.

Mutagen—A substance that causes mutations, which are changes in the DNA sequence of a cell’s DNA. 

Mutations can lead to birth defects, miscarriages, or cancer.

Necropsy—The gross examination of the organs and tissues of a dead body to determine the cause of 

death or pathological conditions.

Neurotoxicity—The occurrence of adverse effects on the nervous system following exposure to a 

hazardous substance.

No-Observed-Adverse-Effect Level (NOAEL)—The dose of a chemical at which there were no 

statistically or biologically significant increases in frequency or severity of adverse effects seen between 

the exposed population and its appropriate control. Although effects may be produced at this dose, they 

are not considered to be adverse.

Octanol-Water Partition Coefficient (Kow)—The equilibrium ratio of the concentrations of a chemical 

in n-octanol and water, in dilute solution.

Odds Ratio (OR)—A means of measuring the association between an exposure (such as toxic substances 

and a disease or condition) that represents the best estimate of relative risk (risk as a ratio of the incidence 

among subjects exposed to a particular risk factor divided by the incidence among subjects who were not 

exposed to the risk factor). An odds ratio that is greater than 1 is considered to indicate greater risk of 

disease in the exposed group compared to the unexposed group.

Permissible Exposure Limit (PEL)—An Occupational Safety and Health Administration (OSHA) 

regulatory limit on the amount or concentration of a substance not to be exceeded in workplace air 

averaged over any 8-hour work shift of a 40-hour workweek.

Pesticide—General classification of chemicals specifically developed and produced for use in the control 

of agricultural and public health pests (insects or other organisms harmful to cultivated plants or animals).

Pharmacokinetics—The dynamic behavior of a material in the body, used to predict the fate 

(disposition) of an exogenous substance in an organism. Utilizing computational techniques, it provides 

the means of studying the absorption, distribution, metabolism, and excretion of chemicals by the body.

Pharmacokinetic Model—A set of equations that can be used to describe the time course of a parent 

chemical or metabolite in an animal system. There are two types of pharmacokinetic models: data-based 

and physiologically-based. A data-based model divides the animal system into a series of compartments, 

which, in general, do not represent real, identifiable anatomic regions of the body, whereas the 

physiologically-based model compartments represent real anatomic regions of the body.

Physiologically Based Pharmacodynamic (PBPD) Model—A type of physiologically based doseresponse model that quantitatively describes the relationship between target tissue dose and toxic 

endpoints. These models advance the importance of physiologically based models in that they clearly 

describe the biological effect (response) produced by the system following exposure to an exogenous 

substance. 
MOLYBDENUM F-5

APPENDIX F

Physiologically Based Pharmacokinetic (PBPK) Model—A type of physiologically based doseresponse model that is comprised of a series of compartments representing organs or tissue groups with 

realistic weights and blood flows. These models require a variety of physiological information, including 

tissue volumes, blood flow rates to tissues, cardiac output, alveolar ventilation rates, and possibly 

membrane permeabilities. The models also utilize biochemical information, such as blood:air partition 

coefficients, and metabolic parameters. PBPK models are also called biologically based tissue dosimetry 

models.

Prevalence—The number of cases of a disease or condition in a population at one point in time. 

Prospective Study—A type of cohort study in which a group is followed over time and the pertinent 

observations are made on events occurring after the start of the study. 

Recommended Exposure Limit (REL)—A National Institute for Occupational Safety and Health 

(NIOSH) time-weighted average (TWA) concentration for up to a 10-hour workday during a 40-hour 

workweek.

Reference Concentration (RfC)—An estimate (with uncertainty spanning perhaps an order of 

magnitude) of a continuous inhalation exposure to the human population (including sensitive subgroups) 

that is likely to be without an appreciable risk of deleterious noncancer health effects during a lifetime. 

The inhalation RfC is expressed in units of mg/m3 or ppm.

Reference Dose (RfD)—An estimate (with uncertainty spanning perhaps an order of magnitude) of the 

daily oral exposure of the human population to a potential hazard that is likely to be without risk of 

deleterious noncancer health effects during a lifetime. The oral RfD is expressed in units of mg/kg/day. 

Reportable Quantity (RQ)—The quantity of a hazardous substance that is considered reportable under 

the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA). RQs are 

(1) ≥1 pound or (2) for selected substances, an amount established by regulation either under CERCLA or 

under Section 311 of the Clean Water Act. Quantities are measured over a 24-hour period.

Reproductive Toxicity—The occurrence of adverse effects on the reproductive system that may result 

from exposure to a hazardous substance. The toxicity may be directed to the reproductive organs and/or 

the related endocrine system. The manifestation of such toxicity may be noted as alterations in sexual 

behavior, fertility, pregnancy outcomes, or modifications in other functions that are dependent on the 

integrity of this system.

Retrospective Study—A type of cohort study based on a group of persons known to have been exposed 

at some time in the past. Data are collected from routinely recorded events, up to the time the study is 

undertaken. Retrospective studies are limited to causal factors that can be ascertained from existing 

records and/or examining survivors of the cohort.

Risk—The possibility or chance that some adverse effect will result from a given exposure to a hazardous 

substance.

Risk Factor—An aspect of personal behavior or lifestyle, an environmental exposure, existing health 

condition, or an inborn or inherited characteristic that is associated with an increased occurrence of 

disease or other health-related event or condition.
MOLYBDENUM F-6

APPENDIX F

Risk Ratio/Relative Risk—The ratio of the risk among persons with specific risk factors compared to the 

risk among persons without risk factors. A risk ratio that is greater than 1 indicates greater risk of disease 

in the exposed group compared to the unexposed group.

Short-Term Exposure Limit (STEL)—A STEL is a 15-minute TWA exposure that should not be 

exceeded at any time during a workday. 

Standardized Mortality Ratio (SMR)—A ratio of the observed number of deaths and the expected 

number of deaths in a specific standard population.

Target Organ Toxicity—This term covers a broad range of adverse effects on target organs or 

physiological systems (e.g., renal, cardiovascular) extending from those arising through a single limited 

exposure to those assumed over a lifetime of exposure to a chemical.

Teratogen—A chemical that causes structural defects that affect the development of an organism.

Threshold Limit Value (TLV)—An American Conference of Governmental Industrial Hygienists 

(ACGIH) concentration of a substance to which it is believed that nearly all workers may be repeatedly 

exposed, day after day, for a working lifetime without adverse effect. The TLV may be expressed as a 

Time-Weighted Average (TLV-TWA), as a Short-Term Exposure Limit (TLV-STEL), or as a ceiling 

limit (TLV-C).

Time-Weighted Average (TWA)—An average exposure within a given time period. 

Toxicokinetic—The absorption, distribution, metabolism, and elimination of toxic compounds in the 

living organism.

Toxics Release Inventory (TRI)—The TRI is an EPA program that tracks toxic chemical releases and 

pollution prevention activities reported by industrial and federal facilities. 

Uncertainty Factor (UF)—A factor used in operationally deriving the Minimal Risk Level (MRL),

Reference Dose (RfD), or Reference Concentration (RfC) from experimental data. UFs are intended to 

account for (1) the variation in sensitivity among the members of the human population, (2) the 

uncertainty in extrapolating animal data to the case of human, (3) the uncertainty in extrapolating from 

data obtained in a study that is of less than lifetime exposure, and (4) the uncertainty in using lowestobserved-adverse-effect level (LOAEL) data rather than no-observed-adverse-effect level (NOAEL) data. 

A default for each individual UF is 10; if complete certainty in data exists, a value of 1 can be used; 

however, a reduced UF of 3 may be used on a case-by-case basis (3 being the approximate logarithmic 

average of 10 and 1).

Xenobiotic—Any substance that is foreign to the biological system.
MOLYBDENUM G-1

APPENDIX G. ACRONYMS, ABBREVIATIONS, AND SYMBOLS

AAPCC American Association of Poison Control Centers

ACGIH American Conference of Governmental Industrial Hygienists

ACOEM American College of Occupational and Environmental Medicine

ACMT American College of Medical Toxicology

ADI acceptable daily intake

ADME absorption, distribution, metabolism, and excretion

AEGL Acute Exposure Guideline Level

AIC Akaike’s information criterion 

AIHA American Industrial Hygiene Association 

ALT alanine aminotransferase

AOEC Association of Occupational and Environmental Clinics

AP alkaline phosphatase

AST aspartate aminotransferase

atm atmosphere

ATSDR Agency for Toxic Substances and Disease Registry

AWQC Ambient Water Quality Criteria

BCF bioconcentration factor

BMD/C benchmark dose or benchmark concentration

BMDX dose that produces a X% change in response rate of an adverse effect

BMDLX 95% lower confidence limit on the BMDX

BMDS Benchmark Dose Software

BMR benchmark response

BUN blood urea nitrogen 

C Celsius

CAA Clean Air Act

CAS Chemical Abstract Services

CDC Centers for Disease Control and Prevention

CEL cancer effect level

CERCLA Comprehensive Environmental Response, Compensation, and Liability Act

CFR Code of Federal Regulations

Ci curie

CI confidence interval

cm centimeter

CPSC Consumer Products Safety Commission

CWA Clean Water Act

DNA deoxyribonucleic acid

DOD Department of Defense

DOE Department of Energy

DWEL drinking water exposure level

EAFUS Everything Added to Food in the United States

ECG/EKG electrocardiogram

EEG electroencephalogram

EPA Environmental Protection Agency

ERPG emergency response planning guidelines 

F Fahrenheit

F1 first-filial generation

FDA Food and Drug Administration

FIFRA Federal Insecticide, Fungicide, and Rodenticide Act
MOLYBDENUM G-2

APPENDIX G

FR Federal Register

FSH follicle stimulating hormone

g gram

GC gas chromatography

gd gestational day

GGT γ-glutamyl transferase 

GRAS generally recognized as safe

HEC human equivalent concentration 

HED human equivalent dose 

HHS Department of Health and Human Services 

HPLC high-performance liquid chromatography

HSDB Hazardous Substance Data Bank 

IARC International Agency for Research on Cancer

IDLH immediately dangerous to life and health

IRIS Integrated Risk Information System 

Kd adsorption ratio

kg kilogram

kkg kilokilogram; 1 kilokilogram is equivalent to 1,000 kilograms and 1 metric ton

Koc organic carbon partition coefficient

Kow octanol-water partition coefficient

L liter

LC liquid chromatography

LC50 lethal concentration, 50% kill

LCLo lethal concentration, low

LD50 lethal dose, 50% kill

LDLo lethal dose, low

LDH lactic dehydrogenase

LH luteinizing hormone

LOAEL lowest-observed-adverse-effect level

LSE Level of Significant Exposure

LT50 lethal time, 50% kill

m meter

mCi millicurie

MCL maximum contaminant level

MCLG maximum contaminant level goal

MF modifying factor

mg milligram

mL milliliter

mm millimeter

mmHg millimeters of mercury

mmol millimole

MRL Minimal Risk Level

MS mass spectrometry

MSHA Mine Safety and Health Administration

Mt metric ton

NAAQS National Ambient Air Quality Standard

NAS National Academy of Science

NCEH National Center for Environmental Health

ND not detected

ng nanogram
MOLYBDENUM G-3

APPENDIX G

NHANES National Health and Nutrition Examination Survey

NIEHS National Institute of Environmental Health Sciences

NIOSH National Institute for Occupational Safety and Health

NLM National Library of Medicine

nm nanometer

nmol nanomole

NOAEL no-observed-adverse-effect level

NPL National Priorities List

NR not reported

NRC National Research Council

NS not specified

NTP National Toxicology Program

OR odds ratio

OSHA Occupational Safety and Health Administration

PAC Protective Action Criteria

PAH polycyclic aromatic hydrocarbon

PBPD physiologically based pharmacodynamic 

PBPK physiologically based pharmacokinetic 

PEHSU Pediatric Environmental Health Specialty Unit

PEL permissible exposure limit

PEL-C permissible exposure limit-ceiling value

pg picogram

PND postnatal day

POD point of departure

ppb parts per billion

ppbv parts per billion by volume

ppm parts per million

ppt parts per trillion

REL recommended exposure level/limit

REL-C recommended exposure level-ceiling value

RfC reference concentration

RfD reference dose

RNA ribonucleic acid

SARA Superfund Amendments and Reauthorization Act

SCE sister chromatid exchange

SD standard deviation

SE standard error

SGOT serum glutamic oxaloacetic transaminase (same as aspartate aminotransferase or AST)

SGPT serum glutamic pyruvic transaminase (same as alanine aminotransferase or ALT)

SIC standard industrial classification

SMR standardized mortality ratio

sRBC sheep red blood cell

STEL short term exposure limit

TLV threshold limit value

TLV-C threshold limit value-ceiling value

TRI Toxics Release Inventory

TSCA Toxic Substances Control Act

TWA time-weighted average

UF uncertainty factor

U.S. United States
MOLYBDENUM

G-4

APPENDIX G

USDA United States Department of Agriculture

USGS United States Geological Survey

USNRC U.S. Nuclear Regulatory Commission

VOC volatile organic compound

WBC white blood cell

WHO World Health Organization > greater than ≥ greater than or equal to = equal to < less than ≤ less than or equal to % percent α alpha β beta γ gamma δ delta

μm micrometer

μg microgram q1* cancer slope factor – negative + positive

(+) weakly positive result (–) weakly negative result