Skin Notation (SK) Profile

Morpholine

[CAS No. 110-91-8]

Department of Health and Human Services

Centers for Disease Control and Prevention

National Institute for Occupational Safety and Health 
ii

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Suggested Citation

NIOSH [20XX]. NIOSH skin notation profile: Morpholine. By Hudson NL, Dotson GS. 

Cincinnati, OH: U.S. Department of Health and Human Services, Centers for Disease Control 

and Prevention, National Institute for Occupational Safety and Health, DHHS (NIOSH) 

Publication No. XXX

DHHS (NIOSH) Publication No. XXX
iii

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information quality guidelines. It has not been formally disseminated by the National Institute for 

Occupational Safety and Health. It does not represent and should not be construed to represent any 

agency determination or policy.

Foreword

As the largest organ of the body, the skin performs multiple critical functions, such as serving as 

the primary barrier to the external environment. For this reason, the skin is often exposed to 

potentially hazardous agents, including chemicals, which may contribute to the onset of a 

spectrum of adverse health effects ranging from localized damage (e.g., irritant contact 

dermatitis and corrosion) to induction of immune-mediated responses (e.g., allergic contact 

dermatitis and pulmonary responses), or systemic toxicity (e.g., neurotoxicity and hepatoxicity). 

Understanding the hazards related to skin contact with chemicals is a critical component of 

modern occupational safety and health programs. 

In 2009, the National Institute for Occupational Safety and Health (NIOSH) published 

Current Intelligence Bulletin (CIB) 61 – A Strategy for Assigning New NIOSH Skin Notations

[NIOSH 2009-147]. This document provides the scientific rationale and framework for the 

assignment of multiple hazard-specific skin notations (SK) that clearly distinguish between the 

systemic effects, direct (localized) effects, and immune-mediated responses caused by skin 

contact with chemicals. The key step within assignment of the hazard-specific SK is the 

determination of the hazard potential of the substance, or its potential for causing adverse health 

effects as a result of skin exposure. This determination entails a health hazard identification 

process that involves use of the following:

 Scientific data on the physicochemical properties of a chemical

 Data on human exposures and health effects

 Empirical data from in vivo and in vitro laboratory testing

 Computational techniques, including predictive algorithms and mathematical models that 

describe a selected process (e.g., skin permeation) by means of analytical or numerical 

methods. 

This Skin Notation Profile provides the SK assignments and supportive data for morpholine. In 

particular, this document evaluates and summarizes the literature describing the hazard potential 

of the substance and its assessment according to the scientific rationale and framework outlined 

in CIB 61. In meeting this objective, this Skin Notation Profile intends to inform the audience—

mostly occupational health practitioners, researchers, policy- and decision-makers, employers, 

and workers in potentially hazardous workplaces—so that improved risk-management practices 

may be developed to better protect workers from the risks of skin contact with the chemicals of 

interest.

John Howard, M.D.

Director

National Institute for Occupational Safety and Health

Centers for Disease Control and Prevention
iv

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information quality guidelines. It has not been formally disseminated by the National Institute for 

Occupational Safety and Health. It does not represent and should not be construed to represent any 

agency determination or policy.

Contents

Foreword................................................................................................................................................ iii

Abbreviations.......................................................................................................................................... v

Glossary..................................................................................................................................................vii

Acknowledgments.................................................................................................................................viii

1.0 Introduction....................................................................................................................................... 1

1.1 General Substance Information ........................................................................................................... 1

1.2 Purpose ................................................................................................................................................ 1

1.3 Overview of SK Assignment.................................................................................................................. 1

2.0 Systemic Toxicity from Skin Exposure (SK: SYS).................................................................................. 2

3.0 Direct Effects on Skin (SK: DIR)........................................................................................................... 3

4.0 Immune-mediated Responses (SK: SEN)............................................................................................. 4

5.0 Summary............................................................................................................................................ 4

References............................................................................................................................................... 6

Appendix: Calculation of the SI Ratio for Morpholine.............................................................................. 8

Overview .................................................................................................................................................... 8

Calculation ............................................................................................................................................... 10

Appendix References................................................................................................................................ 11
v

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information quality guidelines. It has not been formally disseminated by the National Institute for 

Occupational Safety and Health. It does not represent and should not be construed to represent any 

agency determination or policy.

Abbreviations

ACGIH American Conference of Governmental Industrial Hygienists 

ATSDR Agency for Toxic Substances and Disease Registry

CIB Current Intelligence Bulletin

cm2squared centimeter(s)

cm/hr centimeter(s) per hour

cm/s centimeter(s) per second

DEREK Deductive Estimation of Risk from Existing Knowledge

DIR skin notation indicating the potential for direct effects to the skin following 

contact with a chemical

EC European Commission 

g gram(s)

g/L gram(s)/liter

GHS Globally Harmonized System for Classification and Labelling of Chemicals

GPMT guinea pig maximization test

hr hour(s)

IARC International Agency for Research on Cancer

IPCS International Program for Chemical Safety 

(IRR) subnotation of SK: DIR indicating the potential for a chemical to be a skin irritant 

following exposure to the skin

kaq coefficient in the watery epidermal layer

kp skin permeation coefficient 

kpol coefficient in the protein fraction of the stratum corneum

kpsc permeation coefficient in the lipid fraction of the stratum corneum 

LD50 dose resulting in 50% mortality in the exposed population

LDLo dermal lethal dose

LLNA local lymph node assay

LOAEL lowest-observed-adverse-effect level 

log KOW base-10 logarithm of a substance’s octanol–water partition

M molarity

m

3

cubic meter(s)

mg milligram(s)

mg/cm2/hr milligram(s) per square centimeter per hour

mg/kg milligram(s) per kilogram body weight

mg/m3 milligram(s) per cubic meter

mL milliliter(s)

mL/kg milliliter(s) per kilogram body weight

MW molecular weight

NIOSH National Institute for Occupational Safety and Health

NOAEL no-observed-adverse-effect level
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information quality guidelines. It has not been formally disseminated by the National Institute for 

Occupational Safety and Health. It does not represent and should not be construed to represent any 

agency determination or policy.

NTP National Toxicology Program

OEL occupational exposure limit

OSHA Occupational Safety and Health Administration

ppm parts per million

REL recommended exposure limit

RF retention factor 

SEN skin notation indicating the potential for immune-mediated reactions following 

exposure of the skin

SI ratio ratio of skin dose to inhalation dose

SK skin notation

SW solubility in water

SYS skin notation indicating the potential for systemic toxicity following exposure of 

the skin

USEPA United States Environmental Protection Agency 

μg microgram(s)

μg/cm2 microgram(s) per square centimeter

μg/cm2/hr microgram(s) per square centimeter per hour

μL microliter(s)

μmol micromole(s)
vii

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information quality guidelines. It has not been formally disseminated by the National Institute for 

Occupational Safety and Health. It does not represent and should not be construed to represent any 

agency determination or policy.

Glossary 

Absorption—The transport of a chemical from the outer surface of the skin into both the skin 

and systemic circulation (including penetration, permeation, and resorption). 

Acute exposure—Contact with a chemical that occurs once or for only a short period of time. 

Cancer—Any one of a group of diseases that occurs when cells in the body become abnormal 

and grow or multiply out of control. 

Contaminant—A chemical that is (1) unintentionally present within a neat substance or mixture 

at a concentration less than 1.0% or (2) recognized as a potential carcinogen and present within a 

neat substance or mixture at a concentration less than 0.1%. 

Cutaneous (or percutaneous)—Referring to the skin (or through the skin). 

Dermal—Referring to the skin. 

Dermal contact—Contact with (touching) the skin. 

Direct effects—Localized, non-immune-mediated adverse health effects on the skin, including 

corrosion, primary irritation, changes in skin pigmentation, and reduction/disruption of the skin 

barrier integrity, occurring at or near the point of contact with chemicals. 

Immune-mediated responses—Responses mediated by the immune system, including allergic 

responses. 

Sensitization—A specific immune-mediated response that develops following exposure to a 

chemical, which, upon re-exposure, can lead to allergic contact dermatitis (ACD) or other 

immune-mediated diseases such as asthma, depending on the site and route of re-exposure. 

Substance—A chemical. 

Systemic effects—Systemic toxicity associated with skin absorption of chemicals after exposure 

of the skin.
viii

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information quality guidelines. It has not been formally disseminated by the National Institute for 

Occupational Safety and Health. It does not represent and should not be construed to represent any 

agency determination or policy.

Acknowledgments

This document was developed by the Education and Information Division (Paul Schulte, Ph.D., 

Director). G. Scott Dotson, Ph.D., was the project officer for this document, assisted in great part 

by Naomi Hudson, Dr.P.H., MPH, Todd Niemeier, M.Sc., and Berran Yucesoy, Ph.D. The basis 

for this document was a report (Toxicology Excellence for Risk Assessment [TERA]) contracted 

by NIOSH and prepared by Bernard Gadagbui, Ph.D., and Andrew Maier, Ph.D. 

For their contribution to the technical content and review of this document, special 

acknowledgment is given to the following NIOSH personnel:

Denver Field Office

Eric Esswein, M.Sc.

Division of Applied Research and Technology 

Clayton B’Hymer, Ph.D. 

John Snawder, Ph.D.

Mark Toraason, Ph.D.

Division of Respiratory Disease Studies 

Gregory A. Day, Ph.D. 

Aleksander Stefaniak, Ph.D. 

Division of Surveillance, Hazard Evaluations, and Field Studies

Matt Dahm, M.Sc.

Aaron Sussell, Ph.D.

Loren Tapp, M.D. 

Education and Information Division 

Devin Baker, M.Ed.

Charles L. Geraci, Ph.D.

Thomas J. Lentz, Ph.D.

Richard Niemeier, Ph.D.

Ralph Zumwalde, M.Sc.

Health Effects Laboratory Division

Stacey Anderson, Ph.D.

H. Fredrick Frasch, Ph.D. 

Michael Luster, Ph.D. 

Anna Shvedova, Ph.D.

Paul Siegel, Ph.D.
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information quality guidelines. It has not been formally disseminated by the National Institute for 

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agency determination or policy.

National Personal Protective Technology Laboratory

Heinz Ahlers, J.D., M.Sc.

Angie Shepherd

For their contribution to the technical content and review of this document, special 

acknowledgment is given to the following CDC personnel:

Office of Surveillance, Epidemiology and Laboratory Services/Epidemiology and Analysis 

Program Office 

Barbara Landreth, M.A.

In addition, special appreciation is expressed to the following individuals for serving as 

independent, external reviewers and providing comments that contributed to the development or 

improvement of this document:

 G. Frank Gerberick, Ph.D., The Procter and Gamble Company, Cincinnati, OH

 Dori Germolec, Ph.D., National Toxicology Program, National Institute for 

Environmental Health Sciences, Research Triangle, NC

 Ben Hayes, M.D., Ph.D., Division of Dermatology, Vanderbilt School of Medicine, 

Nashville, TN 

 Jennifer Sahmel, M.Sc., CIH, ChemRisk, Boulder, CO

 James Taylor, M.D., Industrial Dermatology, The Cleveland Clinic, Cleveland, OH
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information quality guidelines. It has not been formally disseminated by the National Institute for Occupational 

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policy.

1.0 Introduction 

1.1 General Substance Information 

Chemical: Morpholine

CAS No: 110-91-8

Molecular weight (MW): 87.1

Molecular formula: C4H9ON

Structural formula:

Synonyms: Diethylene imidoxide; Diethylene oximide; Tetrahydro-1,4-oxazine; Tetrahydro-p-oxazine

Uses: Morpholine is used primarily as a rubber accelerator, boiler water additive, and solvents; an 

estimated 44 million pounds of the chemical was used in 1992 [HSDB 2014]. 

1.2 Purpose 

This skin notation profile presents (1) a brief summary of epidemiological and toxicological data 

associated with skin contact with morpholine and (2) the rationale behind the hazard-specific skin 

notation (SK) assignment for morpholine. The SK assignment is based on the scientific rationale and 

logic outlined in the Current Intelligence Bulletin (CIB) 61: A Strategy for Assigning New NIOSH Skin 

Notations [NIOSH 2009]. The summarized information and health hazard assessment are limited to an 

evaluation of the potential health effects of dermal exposure to morpholine. A literature search was 

conducted through February 2013 to identify information on morpholine, including but not limited to 

data relating to its toxicokinetics, acute toxicity, repeated-dose systemic toxicity, carcinogenicity, 

biological system/function–specific effects (including reproductive and developmental effects and 

immunotoxicity), irritation, and sensitization. Information was considered from studies of humans, 

animals, or appropriate modeling systems that are relevant to assessing the effects of dermal exposure to 

morpholine. 

1.3 Overview of SK Assignment

Morpholine is potentially capable of causing numerous adverse health effects following skin contact. A 

critical review of available data has resulted in the following SK assignment for morpholine: SK: SYSDIR (COR). Table 1 provides an overview of the critical effects and data used to develop the SK 

assignment for morpholine. 
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policy.

Table 1. Summary of the SK Assignment for morpholine

Skin Notation Critical

Effect

Available

Data

SK: SYS Liver, kidney, spleen Sufficient animal data

SK: DIR (COR) Skin corrosivity Sufficient animal data

2.0 Systemic Toxicity from Skin Exposure (SK: SYS)

No toxicokinetic data following dermal exposure to morpholine were identified. The potential of 

morpholine to pose a skin absorption hazard was also evaluated, with use of a predictive algorithm for 

estimating and evaluating the health hazards of dermal exposure to substances [NIOSH 2009]. The 

evaluation method compares an estimated dose accumulated in the body from skin absorption and an 

estimated dose from respiratory absorption associated with a reference occupational exposure limit. On 

the basis of this algorithm, a ratio of the skin dose to the inhalation dose (SI ratio) of 1.76 was calculated 

for morpholine. An SI ratio of ≥0.1 indicates that skin absorption may significantly contribute to the 

overall body burden of a substance [NIOSH 2009]; therefore, morpholine is considered to be absorbed 

through the skin following dermal exposure. Additional information on the SI ratio and the variables 

used in its calculation are included in the appendix.

No human dermal lethal dose (LDLo) estimates were identified. A minimum lethal dose of 0.9 grams per 

kilogram body weight [g/kg; 900 milligrams per kilogram body weight (mg/kg)] was reported in guinea 

pigs [Shea 1939]. The reported dermal LD50 value (the dose resulting in 50% mortality in the exposed 

animals) was 0.5 milliliters per kilogram (mL/kg) [corresponding to 503.5 mg/kg] in the rabbit [Smyth 

et al. 1954]. Because the reported acute dermal LD50 value for rats and the minimum lethal dose for 

guinea pigs were lower than the critical dermal LD50 value of 2,000 mg/kg that identifies chemical 

substances with the potential for acute toxicity [NIOSH 2009], morpholine is considered acutely toxic 

following dermal exposure.

 

No epidemiological studies or human case reports or repeated-dose, sub-chronic, or chronic dermal 

toxicity studies in animals were identified. However, Shea [1939] reported a dermal toxicity study in 

rabbits and guinea pigs exposed to morpholine for up to 2 weeks to 900 mg/kg-day of diluted (aqueous) 

(rabbits) or undiluted (guinea pigs). In this study, all animals died before the eleventh (rabbits) or 

thirteenth (guinea pigs) dose. Morpholine at this dose caused necrosis of the liver and kidney tubules 

(rabbits and guinea pigs) and congestion of the spleen (rabbits). Although prolonged dermal exposure 

studies were not identified, the Shea [1939] study suggests the potential for effects at doses lower than 

the critical dermal No-Observed-Adverse-Effect-Level (NOAEL) value of 1000 mg/kg for repeat-dose 

toxicity that identifies chemical substances with the potential for repeated-dose dermal toxicity [NIOSH 

2009]. Therefore, morpholine is considered to be systemically toxic, causing liver, kidney and spleen 

effects, following repeated dermal exposure. 
3

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No standard toxicity or specialty studies were identified that evaluated biological system/function 

specific effects (including reproductive and developmental effects and immunotoxicity) following 

dermal exposure to morpholine. No studies were identified that evaluated the potential of morpholine to 

be a carcinogen in humans or animals following dermal exposure. Table 2 summarizes carcinogenic 

designations of multiple governmental and nongovernmental organizations for morpholine. 

Table 2. Summary of the carcinogenic designations*for morpholine by numerous governmental 

and nongovernmental organizations

Organization Carcinogenic designation 

NIOSH [2005] No designation

NTP [2014] No designation

USEPA [2015] No designation 

European Parliament [2008] No GHS designation

IARC [2012] Group 3: Not classifiable as to its carcinogenicity to 

humans

EC [2013]† No designation

ACGIH [2001] A4: Not classifiable as carcinogenic to humans

ACGIH = American Conference of Governmental Industrial Hygienists; EC = European Commission, Joint Research, 

Institute for Health and Consumer Protection; GHS = Globally Harmonized System for Classification and Labelling of 

Chemicals; IARC = International Agency for Research on Cancer; NIOSH = National Institute for Occupational Safety and 

Health; NTP = National Toxicology Program; USEPA = United States Environmental Protection Agency.

* The listed cancer designations were based on data from nondermal (such as oral or inhalation) exposure since studies using 

the dermal route of exposure were unavailable.

†Date accessed.

Although no toxicokinetic data were identified to evaluate the potential of morpholine to be absorbed 

through the skin, data from acute dermal toxicity study in rabbits [Smyth et al. 1954]

1

and a short-term 

dermal toxicity study in guinea pigs and rabbits [Shea 1939] are sufficient to suggest that that 

morpholine can be absorbed by the skin, systemically available, and toxic, with the potential to cause 

liver, kidney and spleen effects following repeated exposure. Therefore, on the basis of the data for this 

assessment, morpholine is assigned the SK: SYS notation.

3.0 Direct Effects on Skin (SK: DIR)

Only one study was identified that reported the potential of morpholine to have direct effects on human 

skin. Shea [1939] reported that undiluted morpholine applied to the human fingertip caused cracking of 

the cuticle and epithelium of the nail bed and burning sensations. Evidence of skin corrosivity of 

morpholine is well-documented in experimental animals. Occluded topical application of 900 mg/kg of 

the undiluted or un-neutralized morpholine as a single dose or same dose of diluted morpholine applied 

daily for up to two weeks caused necrosis of the skin, inflammation, edema, and severe burning in 

rabbits [Shea 1939]. Guinea pigs receiving occluded, daily topical application of 900 mg/kg undiluted 

morpholine for up two weeks also exhibited edema and necrosis of the skin [Shea 1939]. Smyth [1954] 

 

1References in bold text indicate studies that serve as the basis of the SK assignments.
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reported that topical application of 0.001 mL of undiluted morpholine [corresponding to 1 mg] resulted 

in skin necrosis in rabbits within 24 hours. Wang and Suskind [1988] reported 0.1 g applications of 0.5, 

2, 5 and 10% morpholine concentrations produced no irritation when applied to 1.5 squared centimeter 

(cm2) of the shaved skin of guinea pigs under occlusive conditions. 

Evidence of direct skin effects of morpholine in humans and animals are also provided by in vitro

studies. Gay et al. [1992] applied morpholine to living skin equivalent (LSE), collagen-fibroblast lattice 

from which human epidermal keratinocytes forms a stratified epidermis. Concentrations greater than 1% 

morpholine (by weight) were required to produce inflammation, indicative of irritation in both rabbit 

and human LSE. In a similar study conducted by van de Sandt and Rutten [1995], rabbit cell cultures 

administered 5% morpholine solution exhibited no changes. The structure-activity relationship model, 

Deductive Estimation of Risk from Existing Knowledge (DEREK) for Windows, predicted morpholine 

to be negative for skin irritation.

Based on the results of the studies that evaluated the direct effects of undiluted morpholine in animals 

[Shea 1939; Smyth 1954], there is sufficient information that suggests that morpholine is corrosive to 

the skin. Therefore, on the basis of the data for this assessment, morpholine is assigned the SK: DIR 

(COR) notation.

4.0 Immune-mediated Responses (SK: SEN)

No evidence was identified in human studies to suggest that morpholine has the potential to produce 

skin sensitization. Animal data was limited to a single study. In a modified Buehler test [Wang and 

Suskind 1988], 10 guinea pigs were administered 0.1 g morpholine in 5% petrolatum under occlusion on 

clipped napes for 1 hour for 3 days/week for 2 weeks. Two weeks after the final dose, the guinea pigs 

were challenged with 0.1, 0.5, and 2 % morpholine concentrations, and no sensitization was observed; 

however, Wang and Suskind [1988] reported cross-sensitization reactions when animals were initially 

sensitized with 4,4-dithiodimorpholine (DTDM) or morpholinyl-mercaptobenzothiazole (MMBT) and 

then challenged with morpholine. The structure activity relationship model, DEREK for Windows, 

predicted morpholine to be negative for skin sensitization.

The modified Buehler test identified suggests that morpholine is not a skin sensitizer, but can cross-react 

with structurally similar chemicals. However, this information is considered insufficient as the basis for 

assigning a SK-SYS notation for morpholine. Therefore, on the basis of the data for this assessment, 

morpholine is not assigned the SK: SEN notation.

5.0 Summary

No toxicokinetic data were identified to evaluate the potential of morpholine to be absorbed through the 

skin. However, data from acute dermal toxicity study in rabbits [Smyth et al. 1954] and a short-term 

dermal toxicity study in guinea pigs and rabbits [Shea 1939] are sufficient to indicate that morpholine 

has the potential to be absorbed through the skin, be systemically available and toxic, with the potential 

to cause liver, kidney and spleen effects following repeated exposure. Available studies in animals 
5

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indicate that undiluted morpholine is corrosive to the skin [Shea 1939; Smyth 1954]. Results from a 

modified Buehler test suggest that morpholine is not likely to cause skin sensitization, but can crossreact with structurally similar chemicals. Therefore, on the basis of these assessments, morpholine is 

assigned a composite skin notation of SK: SYS-DIR (COR). 

Table 3 summarizes the skin hazard designations for morpholine previously issued by NIOSH and other 

organizations. The equivalent dermal designation for morpholine, according to the Global 

Harmonization System (GHS) of Classification and Labelling of Chemicals, is Acute Toxicity Category 

4 (Harmful in contact with the skin) and Skin Corrosion Category 1B (Causes severe skin burns and eye 

damage) [European Parliament 2008].

Table 3. Summary of previous skin hazard designations for morpholine

Organization Skin hazard designation 

NIOSH [2005] [skin]: Potential for dermal absorption

OSHA [2015]*[skin]: Potential for dermal absorption

ACGIH [2001] [skin]: Based on report of human dermal penetration and a dermal 

LD50 for 24-hour skin contact of 0.5 mL/kg in rats

EC [2013]* R21: Harmful if in contact with skin

R34: Causes burns

ACGIH = American Conference of Governmental Industrial Hygienists; EC = European Commission, Joint Research, 

Institute for Health and Consumer Protection; NIOSH = National Institute for Occupational Safety and Health; OSHA = 

Occupational Safety and Health Administration.

*Date accessed.
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policy.

References

Note: Asterisks (*) denote sources cited in text; daggers (†) denote additional resources.

*ACGIH [2001]. Morpholine. In: Documentation of threshold limit values and biological exposure 

indices 7th ed., Vol. 2. Cincinnati, OH: American Conference of Governmental Industrial Hygienists.

*EC (European Commission) [ND]. Morpholine. In: EINICS (European Inventory of Existing 

Commercial Chemical Substances), http://esis.jrc.ec.europa.eu/. Accessed: 05-14-13.

*European Parliament, Council of the European Union [2008]. Regulation (EC) No 1272/2008 of the 

European Parliament and of the Council of 16 December 2008 on classification, labelling and packaging 

of substances and mixtures, amending and repealing Directives 67/548/EEC and 1999/45/EC, and 

amending Regulation (EC) No 1907/2006. OJEU, Off J Eur Union L353:1–1355, http://eurlex.europa.eu/LexUriServ/LexUriServ.do?uri=OJ:L:2008:353:0001:1355:EN:PDF. Accessed: 01-28-15. 

†Fisher AA. [1978]. Reports from the Second International Contact Dermatitis Symposium. Part III 

[proceedings]. Cutis 22: 272-289. 

*Gay R, Swiderek M, Nelson D, Ernesti A. [1992]. The living skin equivalent as a model in vitro for 

ranking the toxic potential of dermal irritants. Toxic in Vitro 6(4): 303-315. 

†Harbison RD, Marino DJ, Conaway CC, Rubin LF, Gandy J. [1989]. Chronic morpholine exposure of 

rats. Fund. Appl. Toxicol. 12:491–507. 

*HSDB (Hazardous Substances Data Bank) [2014]. Morpholine. In: HSDB (Hazardous Substances Data 

Bank), http://toxnet.nlm.nih.gov/cgi-bin/sis/search2/f?./temp/~iHMM77:1. Accessed: 01-28-15.

*IARC (International Agency for Research on Cancer) [2012]. Agents reviewed by the IARC 

monographs. In: IARC monographs on the evaluation of carcinogenic risks to humans, 

http://monographs.iarc.fr/ENG/Monographs/PDFs/index.php. Accessed: 01-28-15.

*NIOSH [2005]. NIOSH pocket guide to chemical hazards. Cincinnati, OH: U.S. Department of Health 

and Human Services, Centers for Disease Control and Prevention, National Institute for Occupational 

Safety and Health, DHHS (NIOSH) Publication No. 2005-149, 

http://www.cdc.gov/niosh/npg/npgd0211.html. Accessed: 01-28-15.

*NIOSH [2009]. Current intelligence bulletin 61: a strategy for assigning new NIOSH skin notations. 

Cincinnati, OH: U.S. Department of Health and Human Services, Centers for Disease Control and 

Prevention, National Institute for Occupational Safety and Health, DHHS (NIOSH) Publication No. 

2009-147 [http://www.cdc.gov/niosh/docs/2009-147/pdfs/2009-147.pdf]. Accessed 01-28-15.
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This information is distributed solely for the purpose of pre-dissemination peer review under applicable 

information quality guidelines. It has not been formally disseminated by the National Institute for Occupational 

Safety and Health. It does not represent and should not be construed to represent any agency determination or 

policy.

*NTP [2014]. Report on Carcinogens. Thirteenth Edition; U.S. Department of Health and Human 

Services, Public Health Service. National Toxicology Program, 

http://ntp.niehs.nih.gov/pubhealth/roc/roc13/index.html. Accessed: 01-28-15.

*OSHA [ND]. Morpholine. In: OSHA occupational chemical database, 

http://www.osha.gov/chemicaldata/chemResult.html?recNo=120. Accessed: 01-28-15.

*Shea TE [1939]. The acute and sub-acute toxicity of morpholine. J. Ind. Hyg. Toxicol., 21(7): 236-245.

*Smyth Jr. HF, Carpenter CP, Weil CS, Pozzani UC [1954]. Range-Finding Toxicity Data. List V. Arch. 

Ind. Hyg. Occup. Med. 10:61–68. 

†UNECE (United Nations Economic Comission for Europe) [2007]. Part 3: health hazards. In: Globally 

harmonized system of classification and labeling of chemical (GHS): third revised edition, 

http://www.unece.org/trans/danger/publi/ghs/ghs_rev02/English/03e_part3.pdf. Accessed: 01-28-15.

*USEPA [2015]. Morpholine. In: Integrated Risk Information System, http://www.epa.gov/iris/. 

Accessed: 01-28-15.

*Van De Sandt JJM, Rutten AAJJL [1995]. Differential effects of chemical irritants in rabbit and human 

skin organ cultures. Toxic in Vitro 9(2): 157-168. 

*Wang X, Suskind RR. [1988]. Comparative Studies of the Sensitization Potential of Morpholine, 2-

Mercaptobenzothiazole and 2 of Their Derivatives in Guinea Pigs. Contact Dermatitis 19 (1): 11-5. 
8

This information is distributed solely for the purpose of pre-dissemination peer review under applicable 

information quality guidelines. It has not been formally disseminated by the National Institute for Occupational 

Safety and Health. It does not represent and should not be construed to represent any agency determination or 

policy.

Appendix: Calculation of the SI Ratio for Morpholine

This appendix presents an overview of the SI ratio and a summary of the calculation of the SI ratio for 

morpholine. Although the SI ratio is considered in the determination of a substance’s hazard potential 

following skin contact, it is intended only to serve as supportive data during the assignment of the 

NIOSH SK. An in-depth discussion on the rationale and calculation of the SI ratio can be found in 

Appendix B of the Current Intelligence Bulletin (CIB) 61: A Strategy for Assigning New NIOSH Skin 

Notations [NIOSH 2009]. 

Overview 

The SI ratio is a predictive algorithm for estimating and evaluating the health hazards of skin exposure 

to substances. The algorithm is designed to evaluate the potential for a substance to penetrate the skin 

and induce systemic toxicity [NIOSH 2009]. The goals for incorporating this algorithm into the 

proposed strategy for assigning SYS notation are as follows:

(1) Provide an alternative method to evaluate substances for which no clinical reports or animal 

toxicity studies exist or for which empirical data are insufficient to determine systemic effects.

(2) Use the algorithm evaluation results to determine whether a substance poses a skin absorption 

hazard and should be labeled with the SYS notation.

The algorithm evaluation includes three steps:

(1) determining a skin permeation coefficient (kp) for the substance of interest,

(2) estimating substance uptake by the skin and respiratory absorption routes, and 

(3) evaluating whether the substance poses a skin exposure hazard.

The algorithm is flexible in the data requirement and can operate entirely on the basis of the 

physicochemical properties of a substance and the relevant exposure parameters. Thus, the algorithm is 

independent of the need for biologic data. Alternatively, it can function with both the physicochemical 

properties and the experimentally determined permeation coefficient when such data are available and 

appropriate for use.

The first step in the evaluation is to determine the kp for the substance to describe the transdermal 

penetration rate of the substance [NIOSH 2009]. The kp, which represents the overall diffusion of the 

substance through the stratum corneum and into the blood capillaries of the dermis, is estimated from 

the compound’s molecular weight (MW) and base-10 logarithm of its octanol–water partition coefficient 

(log KOW). In this example, kp is determined for a substance with use of Equation 1. A self-consistent set 

of units must be used, such as centimeters per hour (cm/hr), outlined in Table A1. Other model-based 

estimates of kp may also be used [NIOSH 2009].
9

This information is distributed solely for the purpose of pre-dissemination peer review under applicable 

information quality guidelines. It has not been formally disseminated by the National Institute for Occupational 

Safety and Health. It does not represent and should not be construed to represent any agency determination or 

policy.

Equation 1: Calculation of Skin Permeation Coefficient (kp)

psc pol aq

p

k k k

k

1 1

1







where kpsc is the permeation coefficient in the lipid fraction of the stratum corneum, kpol is the coefficient 

in the protein fraction of the stratum corneum, and kaq is the coefficient in the watery epidermal layer.

These components are individually estimated by

log kpsc = −1.326 + 0.6097 × log Kow − 0.1786 × MW0.5

kpol = 0.0001519 × MW−0.5

kaq = 2.5 × MW−0.5

The second step is to calculate the biologic mass uptake of the substance from skin absorption (skin 

dose) and inhalation (inhalation dose) during the same period of exposure. The skin dose is calculated as 

a mathematical product of the kp, the water solubility (SW) of the substance, the exposed skin surface 

area, and the duration of exposure. Its units are milligrams (mg). Assume that the skin exposure 

continues for 8 hours to unprotected skin on the palms of both hands (a surface area of 360 squared 

centimeters [cm2]). 

Equation 2: Determination of Skin Dose

Skin dose = kp × Sw × Exposed skin surface area × Exposure time

 = kp(cm/hr) × Sw (mg/cm3) × 360 cm2× 8 hr

The inhalation dose (in mg) is derived on the basis of the occupational exposure limit (OEL) of the 

substance—if the OEL is developed to prevent the occurrence of systemic effects rather than 

sensory/irritant effects or direct effects on the respiratory tract. Assume a continuous exposure of 8 

hours, an inhalation volume of 10 cubic meters (m3) inhaled air in 8 hours, and a factor of 75% for 

retention of the airborne substance in the lungs during respiration (retention factor, or RF).

Equation 3: Determination of Inhalation Dose

Inhalation dose = OEL × Inhalation volume × RF

 = OEL (mg/m3) × 10 m3× 0.75

The final step is to compare the calculated skin and inhalation doses and to present the result as a ratio of 

skin dose to inhalation dose (the SI ratio). This ratio quantitatively indicates (1) the significance of 

dermal absorption as a route of occupational exposure to the substance and (2) the contribution of 

dermal uptake to systemic toxicity. If a substance has an SI ratio greater than or equal to 0.1, it is 

considered a skin absorption hazard.
10

This information is distributed solely for the purpose of pre-dissemination peer review under applicable 

information quality guidelines. It has not been formally disseminated by the National Institute for Occupational 

Safety and Health. It does not represent and should not be construed to represent any agency determination or 

policy.

Calculation 

Table A1 summarizes the data applied in the previously described equations to determine the SI ratio for 

morpholine. The calculated SI ratio was 1.76. On the basis of these results, morpholine is predicted to 

represent a skin absorption hazard.

Table A1. Summary of Data used to Calculate the SI Ratio for Morpholine 

Variables Used in Calculation Units Value 

Skin permeation coefficient

Permeation coefficient of stratum corneum lipid path(kpsc) cm/hr 0.0003

Permeation coefficient of the protein fraction of the stratum 

corneum (kpol) cm/hr 1.628 × 10-5

Permeation coefficient of the watery epidermal layer (kaq) cm/hr 0.2679

Molecular weight (MW)

*

amu 87.1

Base-10 logarithm of its octanol–water partition coefficient 

(Log Kow)

* None -0.86

Calculated skin permeation coefficient (kp) cm/hr 0.0003

Skin dose

Water solubility (Sw)

* mg/cm3

1000

Calculated skin permeation coefficient (kp) cm/hr 0.0003

Estimated skin surface area (palms of hand) cm2360

Exposure time hr 8

Calculated skin dose mg 921.2

Inhalation Dose

Occupational exposure limit (OEL)† mg/m370

Inhalation volume m

3

10

Retention factor (RF) None 0.75

Inhalation dose mg 525

Skin dose–to–inhalation dose (SI) ratio None 1.755

*

Variables identified from SRC [ND].

†

The OEL used in calculation of the SI ratio for morpholine was the NIOSH recommended exposure limit (REL) 

[NIOSH 2005].
11

This information is distributed solely for the purpose of pre-dissemination peer review under applicable 

information quality guidelines. It has not been formally disseminated by the National Institute for Occupational 

Safety and Health. It does not represent and should not be construed to represent any agency determination or 

policy.

Appendix References 

NIOSH [2005]. NIOSH pocket guide to chemical hazards. Cincinnati, OH: U.S. Department of Health 

and Human Services, Centers for Disease Control and Prevention, National Institute for Occupational 

Safety and Health, DHHS (NIOSH) Publication No. 2005–149, http://www.cdc.gov/niosh/npg/. 

Accessed: 01-28-15. 

NIOSH [2009]. Current intelligence bulletin 61: a strategy for assigning new NIOSH skin notations. 

Cincinnati, OH: U.S. Department of Health and Human Services, Centers for Disease Control and 

Prevention, National Institute for Occupational Safety and Health, DHHS (NIOSH) Publication No. 

2009-147, http://www.cdc.gov/niosh/docs/2009-147/pdfs/2009-147.pdf. Accessed: 01-28-15.

SRC [2009]. Interactive PhysProp database demo, 

http://esc.syrres.com/fatepointer/webprop.asp?CAS=110918. Accessed: 01-28-15.