1–800–CDC–INFOoccupational safety and health topics, contact NIOSH atTo receive NIOSH documents or more information about 

 (1–800–232–4636)

or visit the NIOSH Web site at E-mail: cdcinfo@cdc.govTTY: 1–888–232–6348

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NIOSH For a monthly update on news at NIOSH, subscribe to 

eNews by visiting www.cdc.gov/niosh/eNews.

safer • healthier • peopleDHHS (NIOSH) Publication No. 2011–160

tm

through research and preventionsafety and health at work for all peopleDelivering on the Nation’s promise:

DEPARTMENT OF HEALTH AND HUMAN SERVICES

Centers for Disease Control and Prevention

National Institute for Occupational Safety and Health

4676 Columbia Parkway

Cincinnati, Ohio 45226–1998

Official Business

Penalty for Private Use $300
On the cover left to right: (1) Scanning electron microscopy (SEM) image of agglomerated particles of pigment-grade rutile TiO2

; (2) SEM image of agglomerated ultrafine-sized particles of rutile TiO2

. Images courtesy of Bill Fox, Altairnano, 

Inc., and Dr. Aleks Stefaniak and Dr. Mark Hoover, NIOSH Nanotechnology 

Field Research Team. Used with permission.
Current Intelligence Bulletin 63

Occupational Exposure

to Titanium Dioxide 

DEPARTMENT OF HEALTH AND HUMAN SERVICES

Centers for Disease Control and Prevention

National Institute for Occupational Safety and Health 
ii

This document is in the public domain and may be freely copied or 

reprinted.

Disclaimer 

Mention of any company or product does not constitute endorsement by the National Institute for Occupational Safety and Health (NIOSH). In addition, citations 

to Web sites external to NIOSH do not constitute NIOSH endorsement of the sponsoring organizations or their programs or products. Furthermore, NIOSH is not 

responsible for the content of these Web sites. 

Ordering Information 

To receive NIOSH documents or other information about occupational safety and 

health topics, contact NIOSH at 

Telephone: 1–800–CDC–INFO (1–800–232–4636)

TTY: 1–888–232–6348

E-mail: cdcinfo@cdc.gov

or visit the NIOSH Web site at www.cdc.gov/niosh.

For a monthly update on news at NIOSH, subscribe to NIOSH eNews by visiting 

www.cdc.gov/niosh/eNews. 

DHHS (NIOSH) Publication No. 2011–160

April 2011 

Safer • Healthier • PeopleTM
iii

Foreword 

The purpose of the Occupational Safety and Health Act of 1970 (Public Law 91–596) 

is to assure safe and healthful working conditions for every working person and to 

preserve our human resources. In this Act, the National Institute for Occupational 

Safety and Health (NIOSH) is charged with recommending occupational safety and 

health standards and describing exposures that are safe for various periods of employment, including (but not limited to) the exposures at which no worker will suffer diminished health, functional capacity, or life expectancy as a result of his or her 

work experience. 

Current Intelligence Bulletins (CIBs) are issued by NIOSH to disseminate new scientific information about occupational hazards. A CIB may draw attention to a formerly unrecognized hazard, report new data on a known hazard, or disseminate information about hazard control. CIBs are distributed to representatives of academia, 

industry, organized labor, public health agencies, and public interest groups as well 

as to federal agencies responsible for ensuring the safety and health of workers. 

Titanium dioxide (TiO2), an insoluble white powder, is used extensively in many 

commercial products, including paint, cosmetics, plastics, paper, and food, as an 

anticaking or whitening agent. It is produced and used in the workplace in varying 

particle-size fractions, including fine and ultrafine sizes. The number of U.S. workers currently exposed to TiO2

 dust is unknown. 

This NIOSH CIB, based on our assessment of the current available scientific information about this widely used material, (1) reviews the animal and human data 

relevant to assessing the carcinogenicity and other adverse health effects of TiO2,(2) 

provides a quantitative risk assessment using dose-response information from the 

rat and human lung dosimetry modeling and recommended occupational exposure 

limits for fine and ultrafine (including engineered nanoscale) TiO2, and(3) describes 

exposure monitoring techniques, exposure control strategies, and research needs. 

This report only addresses occupational exposures by inhalation, and conclusions 

derived here should not be inferred to pertain to nonoccupational exposures.

NIOSH recommends exposure limits of 2.4 mg/m3 for fine TiO2 and 0.3 mg/m3 for 

ultrafine (including engineered nanoscale) TiO2, as time-weighted average (TWA) 

concentrations for up to 10 hours per day during a 40-hour work week. NIOSH has 

determined that ultrafine TiO2 is a potential occupational carcinogen but that there 

are insufficient data at this time to classify fine TiO2 as a potential occupational carcinogen. However, as a precautionary step, NIOSH used all of the animal tumor response data when conducting dose-response modeling and determining separate 
iv

RELs for ultrafine and fine TiO2. These recommendations represent levels that 

over a working lifetime are estimated to reduce risks of lung cancer to below 1 in 

1,000. NIOSH realizes that knowledge about the health effects of nanomaterials is 

an evolving area of science. Therefore, NIOSH intends to continue dialogue with the 

scientific community and will consider any comments about nano-size titanium dioxide for future updates of this document. (Send comments to nioshdocket@cdc.gov.)

NIOSH urges employers to disseminate this information to workers and customers 

and requests that professional and trade associations and labor organizations inform their members about the hazards of occupational exposure to respirable TiO2

. 

John Howard, M.D.

Director, National Institute for Occupational

 Safety and Health

Centers for Disease Control and Prevention 
v

Executive Summary 

In this Current Intelligence Bulletin, the National Institute for Occupational Safety 

and Health (NIOSH) reviews the animal and human data relevant to assessing the 

carcinogenicity of titanium dioxide (TiO2)(Chapters 2 and 3), presents a quantitative risk assessment using dose-response data in rats for both cancer (lung tumors) 

and noncancer (pulmonary inflammation) responses and extrapolation to humans 

with lung dosimetry modeling (Chapter 4), provides recommended exposure limits 

(RELs) for fine and ultrafine (including engineered nanoscale) TiO2 (Chapter 5), 

describes exposure monitoring techniques and exposure control strategies (Chapter 

6), and discusses avenues of future research (Chapter 7). This report only addresses 

occupational exposures by inhalation, and conclusions derived here should not be 

inferred to pertain to nonoccupational exposures.

TiO2 (Chemical Abstract Service [CAS] Number 13463–67–7) is a noncombustible, 

white, crystalline, solid, odorless powder. TiO2 is used extensively in many commercial products, including paints and varnishes, cosmetics, plastics, paper, and food as 

an anticaking or whitening agent. Production in the United States was an estimated 

1.45 million metric tons per year in 2007 [DOI 2008]. The number of U.S. workers 

currently exposed to TiO2 dust is not available. 

TiO2 is produced and used in the workplace in varying particle size fractions including fine (which is defined in this document as all particle sizes collected by respirable particle sampling) and ultrafine (defined as the fraction of respirable particles 

with a primary particle diameter of <0.1 μm [<100 nm]). Particles <100 nm are also 

defined as nanoparticles. 

The Occupational Safety and Health Administration (OSHA) permissible exposure 

limit for TiO2 is 15 mg/m3, based on the airborne mass fraction of total TiO2 dust 

(Chapter 1). In 1988, NIOSH recommended that TiO2 be classified as a potential occupational carcinogen and that exposures be controlled as low as feasible [NIOSH 

2002]. This recommendation was based on the observation of lung tumors (nonmalignant) in a chronic inhalation study in rats at 250 mg/m3

 of fine TiO2 [Lee et al. 

1985, 1986a] (Chapter 3). 

Later, a 2-year inhalation study showed a statistically significant increase in lung 

cancer in rats exposed to ultrafine TiO2 at an average concentration of 10 mg/m3

[Heinrich et al. 1995]. Two recent epidemiologic studies have not found a relationship between exposure to total or respirable TiO2

 and lung cancer [Fryzek et al. 

2003; Boffetta et al. 2004], although an elevation in lung cancer mortality was ob-
vi

served among male TiO2 workers in the latter study when compared to the general population (standardized mortality ratio [SMR] 1.23; 95% confidence interval 

[CI] = 1.10–1.38) (Chapter 2). However, there was no indication of an exposureresponse relationship in that study. Nonmalignant respiratory disease mortality was 

not increased significantly (P <0.05) in any of the epidemiologic studies. 

In 2006, the International Agency for Research on Cancer (IARC) reviewed TiO2 

and concluded that there was sufficient evidence of carcinogenicity in experimental 

animals and inadequate evidence of carcinogenicity in humans (Group 2B), “possibly carcinogenic to humans” [IARC 2010]. 

TiO2 and other poorly soluble, low-toxicity (PSLT) particles of fine and ultrafine 

sizes show a consistent dose-response relationship for adverse pulmonary responses 

in rats, including persistent pulmonary inflammation and lung tumors, when dose 

is expressed as particle surface area. The higher mass-based potency of ultrafine 

TiO2 compared to fine TiO2 is associated with the greater surface area of ultrafine 

particles for a given mass. The NIOSH RELs for fine and ultrafine TiO2 reflect this 

mass-based difference in potency (Chapter 5). NIOSH has reviewed and considered 

all of the relevant data related to respiratory effects of TiO2. This includes results 

from animal inhalation studies and epidemiologic studies. NIOSH has concluded 

that TiO2 is not a direct-acting carcinogen, but acts through a secondary genotoxicity mechanism that is not specific to TiO2

 but primarily related to particle size and 

surface area. The most relevant data for assessing the health risk to workers are results from a chronic animal inhalation study with ultrafine (<100 nm) TiO2

 in which 

a statistically significant increase in adenocarcinomas was observed [Heinrich et al. 

1995]. This is supported by a pattern of TiO2 induced responses that include persistent pulmonary inflammation in rats and mice [Everitt et al. 2000; Bermudez et al. 

2004] and cancer responses for PSLT particles related to surface area. Therefore, on 

the basis of the study by Heinrich et al. [1995] and the pattern of pulmonary inflammatory responses, NIOSH has determined that exposure to ultrafine TiO2

 should be 

considered a potential occupational carcinogen. 

For fine size (pigment grade) TiO2 (>100 nm), the data on which to assess carcinogenicity are limited. Generally, the epidemiologic studies for fine TiO2

 are inconclusive because of inadequate statistical power to determine whether they replicate 

or refute the animal dose-response data. This is consistent for carcinogens of low 

potency. The only chronic animal inhalation study [Lee et al. 1985], which demonstrated the development of lung tumors (bronchioalveolar adenomas) in response 

to inhalation exposure of rats to fine sized TiO2 did so at a dose of 250 mg/m3 but 

not at 10 or 50 mg/m3. The absence of lung tumor development for fine TiO2 was 

also reported by Muhle et al. [1991] in rats exposed at 5 mg/m3. However, the responses observed in animal studies exposed to ultrafine and fine TiO2

 are consistent 

with a continuum of biological response to TiO2 that is based on particle surface 

area. In other words, all the rat tumor response data on inhalation of TiO2 (ultrafine 

and fine) fit on the same dose-response curve when dose is expressed as total particle surface area in the lungs. However, exposure concentrations greater than 100 
vii

mg/m3 are generally not considered acceptable inhalation toxicology practice today. 

Consequently, in a weight-of-evidence analysis, NIOSH questions the relevance of 

the 250 mg/m3 dose for classifying exposure to TiO2 as a carcinogenic hazard to 

workers and therefore, concludes that there are insufficient data at this time to classify fine TiO2

 as a potential occupational carcinogen. Although data are insufficient 

on the cancer hazard for fine TiO2, the tumor-response data are consistent with that 

observed for ultrafine TiO2 when converted to a particle surface area metric. Thus 

to be cautious, NIOSH used all of the animal tumor response data when conducting 

dose-response modeling and determining separate RELs for ultrafine and fine TiO2. 

NIOSH also considered the crystal structure as a modifying factor in TiO2 carcinogenicity and inflammation. The evidence for crystal-dependent toxicity is from 

observed differences in reactive oxygen species (ROS) generated on the surface of 

TiO2 of different crystal structures (e.g., anatase, rutile, or mixtures) in cell-free 

systems, with differences in cytotoxicity in in vitro studies [Kawahara et al. 2003; 

Kakinoki et al. 2004; Behnajady et al. 2008; Jiang et al. 2008, Sayes et al. 2006] and 

with greater inflammation and cell proliferation at early time points following 

intratracheal instillation in rats [Warheit et al. 2007]. However, when rats were 

exposed to TiO2 in subchronic inhalation studies, no difference in pulmonary 

inflammation response to fine and ultrafine TiO2 particles of different crystal 

structure (i.e., 99% rutile vs. 80% anatase/20% rutile) was observed once dose was 

adjusted for particle surface area [Bermudez et al. 2002, 2004]; nor was there a 

difference in the lung tumor response in the chronic inhalation studies in rats at 

a given surface area dose of these fine and ultrafine particles (i.e., 99% rutile vs. 

80% anatase/20% rutile) [Lee et al. 1985; Heinrich et al. 1995]. Therefore, NIOSH 

concludes that the scientific evidence supports surface area as the critical metric 

for occupational inhalation exposure to TiO2. 

NIOSH also evaluated the potential for coatings to modify the toxicity of TiO2, as 

many industrial processes apply coatings to TiO2 particles. TiO2 toxicity has been 

shown to increase after coating with various substances [Warheit et al. 2005]. However, the toxicity of TiO2

 has not been shown to be attenuated by application of coatings. NIOSH concluded that the TiO2

 risk assessment could be used as a reasonable 

floor for potential toxicity, with the notion that toxicity may be substantially increased by particle treatment and process modification. These findings are based on 

the studies in the scientific literature and may not apply to other formulations, surface coatings, or treatments of TiO2

 for which data were not available. An extensive 

review of the risks of coated TiO2 particles is beyond the scope of this document. 

NIOSH recommends airborne exposure limits of 2.4 mg/m3 for fine TiO2 and 0.3 

mg/m3 for ultrafine (including engineered nanoscale) TiO2, as time-weighted average (TWA) concentrations for up to 10 hr/day during a 40-hour work week. These 

recommendations represent levels that over a working lifetime are estimated to reduce risks of lung cancer to below 1 in 1,000. The recommendations are based on 

using chronic inhalation studies in rats to predict lung tumor risks in humans.
viii

In the hazard classification (Chapter 5), NIOSH concludes that the adverse effects of 

inhaling TiO2 may not be material-specific but appear to be due to a generic effect 

of PSLT particles in the lungs at sufficiently high exposure. While NIOSH concludes 

that there is insufficient evidence to classify fine TiO2 as a potential occupational 

carcinogen, NIOSH is concerned about the potential carcinogenicity of ultrafine 

and engineered nanoscale TiO2 if workers are exposed at the current mass-based 

exposure limits for respirable or total mass fractions of TiO2. NIOSH recommends 

controlling exposures as low as possible, below the RELs. Sampling recommendations based on current methodology are provided (Chapter 6). 

Although sufficient data are available to assess the risks of occupational exposure to 

TiO2, additional research questions have arisen. There is a need for exposure assessment for workplace exposure to ultrafine TiO2

 in facilities producing or using TiO2. 

Other research needs include evaluation of the (1) exposure-response relationship 

of TiO2 and other PSLT particles and human health effects, (2) fate of ultrafine 

particles in the lungs and the associated pulmonary responses, and (3) effectiveness of engineering controls for controlling exposures to fine and ultrafine TiO2

. 

(Research needs are discussed further in Chapter 7). 
ix

Contents 

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

Executive Summary. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . v

Abbreviations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xii 

Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xv

1 Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.1 Composition. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.2 Uses. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.3 Production and number of workers potentially exposed. . . . . . . . . . . 2 

1.4 Current exposure limits and particle size definitions. . . . . . . . . . . . . . 3 

2 Human Studies. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 

2.1 Case Reports. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 

2.2 Epidemiologic Studies. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 

2.2.1 Chen and Fayerweather [1988]. . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2.2.2 Fryzek et al. [2003]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

2.2.3 Boffetta et al. [2001]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

2.2.4 Boffetta et al. [2004]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

2.2.5 Ramanakumar et al. [2008]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

2.3 Summary of Epidemiologic Studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

3 Experimental Studies in Animals and Comparison to Humans. . . . . . . . . . . . . 23

3.1 In Vitro Studies. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

3.1.1 Genotoxicity and Mutagenicity. . . . . . . . . . . . . . . . . . . . . . . . . . . 23

3.1.2 Oxidant Generation and Cytotoxicity. . . . . . . . . . . . . . . . . . . . . . 24

3.1.3 Effects on Phagocytosis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

3.2 In Vivo Studies in Rodent Lungs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

3.2.1 Intratracheal Instillation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

3.2.2 Acute or Subacute Inhalation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

3.2.3 Short-term Inhalation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

3.2.4 Subchronic Inhalation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

3.2.5 Chronic Inhalation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 

3.3 In Vivo Studies: Other Routes of Exposure. . . . . . . . . . . . . . . . . . . . . . . 35

3.3.1 Acute Oral Administration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
x

3.3.2 Chronic Oral Administration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

3.3.3 Intraperitoneal Injection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

3.4 Particle-Associated Lung Disease Mechanisms. . . . . . . . . . . . . . . . . . . 36

3.4.1 Role of Pulmonary Inflammation. . . . . . . . . . . . . . . . . . . . . . . . . 36

3.4.2 Dose Metric and Surface Properties. . . . . . . . . . . . . . . . . . . . . . . 37

3.5 Particle-Associated Lung Responses. . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 

3.5.1 Rodent Lung Responses to Fine and Ultrafine TiO2. . . . . . . . . . 42

3.5.2 Comparison of Rodent and Human Lung Responses to

 PSLT including TiO2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

3.6 Rat Model in Risk Assessment of Inhaled Particles . . . . . . . . . . . . . . . 48

4 Quantitative Risk Assessment. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

4.1 Data and Approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

4.2 Methods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

4.2.1 Particle Characteristics. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

4.2.2 Critical Dose. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

4.2.3 Estimating Human Equivalent Exposure. . . . . . . . . . . . . . . . . . . 53

4.2.4 Particle Dosimetry Modeling. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54 

4.3 Dose-Response Modeling of Rat Data and Extrapolation

 to Humans. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

4.3.1 Pulmonary Inflammation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

4.3.2 Lung Tumors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

4.3.3 Alternate Models and Assumptions. . . . . . . . . . . . . . . . . . . . . . . . 63

4.3.4 Mechanistic Considerations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

4.4 Quantitative Comparison of Risk Estimates from Human and

 Animal Data. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68

4.5 Possible Bases for an REL. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68

4.5.1 Pulmonary Inflammation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68

4.5.2 Lung Tumors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69

4.5.3 Comparison of Possible Bases for an REL. . . . . . . . . . . . . . . . . . . 70 

5 Hazard Classification and Recommended Exposure Limits. . . . . . . . . . . . . . . . . 73

5.1 Hazard Classification. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73

5.1.1 Mechanistic Considerations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74

5.1.2 Limitations of the Rat Tumor Data. . . . . . . . . . . . . . . . . . . . . . . . 75

5.1.3 Cancer Classification in Humans. . . . . . . . . . . . . . . . . . . . . . . . . . 76

5.2 Recommended Exposure Limits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77

6 Measurement and Control of TiO2 Aerosol in the Workplace. . . . . . . . . . . . . . . 79

6.1 Exposure Metric. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79

6.2 Exposure Assessment. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80

6.3 Control of Workplace Exposures to TiO2. . . . . . . . . . . . . . . . . . . . . . . . 80 
xi

7 Research Needs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85

7.1 Workplace Exposures and Human Health. . . . . . . . . . . . . . . . . . . . . . . 85 

7.2 Experimental Studies. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85

7.3 Measurement, Controls, and Respirators. . . . . . . . . . . . . . . . . . . . . . . . 85 

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87

Appendices

A. Statistical Tests of the Rat Lung Tumor Models. . . . . . . . . . . . . . . . . . . . 105

B. Threshold Model for Pulmonary Inflammation in Rats. . . . . . . . . . . . . 111

C. Comparison of Rat- and Human-based Excess Risk Estimates

 for Lung Cancer Following Chronic Inhalation of TiO2. . . . . . . . . . . 113

D. Calculation of Upper Bound on Excess Risk of Lung Cancer

 in an Epidemiologic Study of Workers Exposed to TiO2. . . . . . . . . . 117
xii

Abbreviations 

ACGIH American Conference of Governmental Industrial Hygienists

BAL bronchoalveolar lavage

BALF bronchoalveolar lavage fluid

BAP benzo(a)pyrene

BaSO4 barium sulfate

BET Brunauer, Emmett, and Teller

BMD benchmark dose

BMDL benchmark dose lower bound

BMDS benchmark dose software

°C degree(s) Celsius

CAS Chemical Abstract Service

CFR Code of Federal Regulations

CI confidence interval

cm centimeter(s)

DNA deoxyribonucleic acid

E expected

EDS energy dispersive spectroscopy

g gram(s)

g/cm3 grams per cubic centimeter

g/ml gram per milliliter

GSD geometric standard deviation

hprt hypoxanthine-guanine phosphoribosyl transferase

hr hour(s)

IARC International Agency for Research on Cancer

ICRP International Commission on Radiological Protection

IR incidence ratio

IT intratracheal instillation

kg kilogram

L liter(s)

LCL lower confidence limit

LDH lactate dehydrogenase

m meter(s)

MA model average
xiii

MAK Federal Republic of Germany maximum concentration value

 in the workplace

MCEF mixed cellulose ester filter

mg milligram(s)

mg/kg milligram per kilogram body weight

mg/m3 milligrams per cubic meter 

mg/m3 • yr milligrams per cubic meter times years

min minute(s)

ml milliliter(s)

ML maximum likelihood

MLE maximum likelihood estimate

mm millimeter(s)

MMAD mass median aerodynamic diameter

MPPD multiple-path particle dosimetry

n number

NAICS North American Industry Classification System

NCI National Cancer Institute

NIOSH National Institute for Occupational Safety and Health

nm nanometer(s)

NOAEL no-observed-adverse-effect level

O observed

OR odds ratio

OSHA Occupational Safety and Health Administration

P probability

PBS phosphate buffered saline

PEL permissible exposure limit

PH proportional hazards

PKT pigmentary potassium titinate 

PMN polymorphonuclear leukocytes

PNOR/S particles not otherwise regulated or specified

PNOR particles not otherwise regulated

PNOS particles not otherwise specified

PSLT poorly soluble, low toxicity

REL recommended exposure limit

ROS reactive oxygen species

RNS reactive nitrogen species

RR relative risk

SiO2 silicon dioxide

SMR standardized mortality ratio

TEM transmission electron microscopy
xiv

TiCl4 titanium tetrachloride

TiO2 titanium dioxide

TWA time-weighted average

UCL upper confidence limit

U.K. United Kingdom

UV ultraviolet

U.S. United States

wk week(s)

μg microgram(s)

μm micrometer(s)

% percent 
xv

Acknowledgments 

This Current Intelligence Bulletin (CIB) was prepared by the Education and Information Division (EID), Paul Schulte, Director; Risk Evaluation Branch, Christine 

Sofge, Chief; Document Development Branch, T.J. Lentz, Chief. Faye Rice (EID) 

managed the preparation of the final CIB and the responses to external review 

comments. The document was authored by the EID Titanium Dioxide Document 

Development Team and interdivisional authors who developed first drafts of some 

chapters and sections. 

EID Document Development Team

David Dankovic and Eileen Kuempel (primary authors), and in alphabetical order, 

Charles Geraci, Stephen Gilbert, Faye Rice, Paul Schulte, Randall Smith, Christine 

Sofge, Matthew Wheeler, Ralph Zumwalde 

Division of Applied Research and Technology (DART) 

Andrew Maynard (currently with the University of Michigan School of Public Health, 

Risk Science Center) 

Division of Respiratory Disease Studies (DRDS) 

Michael Attfield, Germania Pinheiro (currently with the National Center for HIV/

AIDS, Viral Hepatitis, STD, and TB Prevention) 

Division of Surveillance, Hazard Evaluations, and Field Studies (DSHEFS) 

Avima Ruder 

Health Effects Laboratory Division (HELD) 

Ann Hubbs 

Cross-divisional team to evaluate data on carcinogenicity of TiO2 

David Dankovic (EID)

Heinz Ahlers (EID), (currently with the National Personal Protective Technology

 Laboratory [NPPTL])

Vincent Castranova (HELD)

Eileen Kuempel (EID)

Dennis Lynch (DART)

Avima Ruder (DSHEFS)

Mark Toraason (DART)

Val Vallyathan (HELD)

Ralph Zumwalde (EID) 
xvi

Other NIOSH reviewers who provided critical feedback important to the 

preparation of the document 

Paul Middendorf (Office of the Director (OD), Sid Soderholm (OD), Jimmy Stephens 

(OD), Frank Hearl (OD), John Piacentino (OD), Roger Rosa (OD), John Decker 

(OD), Paul Baron (DART), Cynthia Striley (DART), G. Scott Earnest (DART), Jennifer Topmiller (DART), Patricia Sullivan (DRDS), John McKernan (DSHEFS), Mary 

Schubauer-Berigan (DSHEFS), Richard Niemeier (EID), Roland BerryAnn (NPPTL) 

EID Editorial and Document Assistance 

Devin Baker, Anne Hamilton, Norma Helton, Laurel Jones, John Lechliter, Alma 

McLemore, Jessica Porco, Brenda Proffitt, Cathy Rotunda, Stella Stephens, Jane Weber, Linda Worley 

Document Design and Layout 

Vanessa B. Williams (EID) 

NIOSH acknowledges the contribution of the following scientists who provided 

data and/or additional information on their studies to aid the NIOSH risk assessment and comparative analysis: Dr. Tran of IOM, Dr. Bermudez of the Hamner Institute, Dr. Fryzek, currently at MedImmune, and Dr. Boffetta, currently at the Tisch 

Cancer Institute at Mount Sinai School of Medicine.

NIOSH expresses appreciation to the following independent, external reviewers for 

providing comments that contributed to the development of this document: 

External Expert Peer Review Panel

Chao W. Chen, Ph.D.

Senior Statistician

National Center for Environmental Assessment 

U.S. Environmental Protection Agency 

Harvey Clewell, Ph.D.

Director, Center for Human Health Assessment

Centers for Health Research

Chemical Industry Institute of Toxicology (CIIT) 

Prof. Dr. med. Helmut Greim

Institute of Toxicology and Environmental Hygiene

Technical University of Munich 

Franklin E. Mirer, Ph.D., CIH

Director, Health and Safety Department

International Union, UAW (at time of review) 

Current title and affiliation:

Professor

Environmental and Occupational Health Sciences

Urban Public Health Program

Hunter School of Health Sciences
xvii

Jonathan M. Samet, MD, MS

Chairman, Department of Epidemiology

Bloomberg School of Public Health

Johns Hopkins University

1

1 Introduction

1.1 Composition

Titanium dioxide (TiO2), Chemical Abstract 

Service [CAS] (CAS Number 13463–67–7), is a 

noncombustible, white, crystalline, solid, odorless powder [NIOSH 2002; ACGIH 2001a]. 

TiO2 isinsoluble in water, hydrochloric acid, nitric acid, or alcohol, and it is soluble in hot concentrated sulfuric acid, hydrogen fluoride, or 

alkali [ACGIH 2001a]. TiO2 has several naturally occurring mineral forms, or polymorphs, 

which have the same chemical formula and 

different crystalline structure. Common TiO2

polymorphs include rutile (CAS Number 1317–

80–2) and anatase (CAS Number 1317–70–0). 

While both rutile and anatase belong to the tetragonal crystal system, rutile has a denser arrangement of atoms (Figure 1). 

At temperatures greater than 915oC, anatase 

reverts to the rutile structure [http://mineral.

galleries.com/minerals/oxides/anatase/anatase.

htm]. The luster and hardness of anatase and 

rutile are also similar, but the cleavage differs. The density (specific gravity) of rutile is 

4.25 g/ml [http://webmineral.com/data/Rutile.

shtml], and that of anatase is 3.9 g/ml [http://

webmineral.com/data/Anatase.shtml]. Common impurities in rutile include iron, tantalum, niobium, chromium, vanadium, and 

tin [http://www.mindat.org/min-3486.html],

while those in anatase include iron, tin, vanadium, and niobium [http://www.mindat.org/

min-213.html].

The sulfate process and the chloride process are 

two main industrial processes that produce TiO2

Rutile Anatase

Figure 1. Rutile and anatase TiO2 crystal structure (Image courtesy of Cynthia Striley, NIOSH) 
2 NIOSH CIB 63 • Titanium Dioxide

pigment [IARC 1989; Boffetta et al. 2004]. In 

the sulfate process, anatase or rutile TiO2 is produced by digesting ilmenite (iron titanate) or 

titanium slag with sulfuric acid. In the chloride 

process, natural or synthetic rutile is chlorinated at temperatures of 850 to 1,000o

C [IARC 

1989], and the titanium tetrachloride (TiCl4) 

is converted to the rutile form by vapor-phase 

oxidation [Lewis 1993]. Both anatase and rutile are used as white pigment. Rutile TiO2

 is 

the most commonly used white pigment because of its high refractive index and relatively 

low absorption of light [Wicks 1993]. Anatase 

is used for specialized applications (e.g., in paper and fibers). TiO2

 does not absorb visible 

light, but it strongly absorbs ultraviolet (UV) 

radiation. Commercial rutile TiO2 is prepared 

with an average particle size of 0.22 μm to 

0.25 μm [Wicks 1993]. Pigment-grade TiO2

refers to anatase and rutile pigments with a 

median particle size that usually ranges from 

0.2 μm to 0.3 μm [Aitken et al. 2004]. Particle 

size is an important determinant of the properties of pigments and other final products 

[Wicks 1993]. 

1.2 Uses

TiO2 is used mainly in paints, varnishes, lacquer, paper, plastic, ceramics, rubber, and 

printing ink. TiO2 is also used in welding rod 

coatings, floor coverings, catalysts, coated fabrics and textiles, cosmetics, food colorants, 

glassware, pharmaceuticals, roofing granules, 

rubber tire manufacturing, and in the production of electronic components and dental impressions [Lewis 1993; ACGIH 2001a; IARC 

1989; DOI 2005]. Both the anatase and rutile 

forms of TiO2 are semiconductors [Egerton 

1997]. TiO2 white pigment is widely used due to 

its high refractive index. Since the 1960s, TiO2 

has been coated with other materials (e.g., silica, 

alumina) for commercial applications [Lee et 

al. 1985].

1.3 Production and Number 

of Workers Potentially

Exposed 

An estimate of the number of U.S. workers 

currently exposed to TiO2 dust is not available. The only current information is an unreferenced estimate submitted by industry to 

NIOSH in response to request for public 

comment on the draft document. Industry 

estimates that the number of U.S. workers in 

the “so-called ‘white’ end of TiO2 production 

plants” is “approximately 1,100 workers nationwide” and that there is no reliable estimate 

of the number of workers involved in the “initial compounding of downstream products” 

[American Chemistry Council 2006]. 

In 2007, an estimated 1.45 million metric tons 

of TiO2 pigment were produced by four U.S. 

companies at eight facilities in seven states 

that employed an estimated 4,300 workers 

(jobs not described) [DOI 2008]. The paint 

(includes varnishes and lacquers), plastic and 

rubber, and paper industries accounted for an 

estimated 95% of TiO2 pigment used in the 

United States in 2004 [DOI 2005]. In 2006, the 

U.S. Bureau of Labor Statistics estimated that 

there were about 68,000 U.S. workers in all occupations (excluding self-employed workers) 

in paint, coating, and adhesive manufacturing 

(North American Industry Classification System [NAICS] code 325500), 803,000 in plastics 

and rubber products manufacturing (NAICS 

code 326000), and about 138,000 employed 

in pulp, paper, and paperboard mills (NAICS 

code 322100) [BLS 2006]. In 1991, TiO2 was 

the 43rd highest-volume chemical produced in 

the United States [Lewis 1993]. 
NIOSH CIB 63 • Titanium Dioxide 3

1.4 Current Exposure 

Limits and Particle Size 

Definitions

Occupational exposure to TiO2 is regulated by 

the Occupational Safety and Health Administration (OSHA) under the permissible exposure limit (PEL) of 15 mg/m3

 for TiO2 as total 

dust (8-hr time-weighted average [TWA] concentration) [29 CFR*

 1910.1000; Table Z–1]. 

The OSHA PEL for particles not otherwise regulated (PNOR) is 5 mg/m3

 as respirable dust 

[29 CFR 1910.1000; Table Z–1]. These and other exposure limits for TiO2

 and PNOR or particles not otherwise specified (PNOS) are listed 

in Table 1. PNOR/S are defined as all inert or 

nuisance dusts, whether mineral, inorganic or 

organic, not regulated specifically by substance 

name by OSHA (PNOR) or classified by the 

American Conference of Governmental Industrial Hygienists (ACGIH)(PNOS). The same 

exposure limits are often given for TiO2 and 

PNOR/PNOS (Table 1). OSHA definitions for 

the total and respirable particle size fractions 

refer to specific sampling methods and devices 

[OSHA 2002], while the maximum concentration value in the workplace (MAK) and the 

ACGIH definitions for respirable and inhalable particle sizes are based on the internationally developed definitions of particle size selection sampling [CEN 1993; ISO 1995; ACGIH 

1984, 1994]. NIOSH also recommends the use 

of the international definitions [NIOSH 1995]. 

Aerodynamic diameter affects how a particle 

behaves in air and determines the probability 

of deposition at locations within the respiratory tract. Aerodynamic diameter is defined 

as the diameter of a spherical particle that has 

the same settling velocity as a particle with a 

* See CFR in references.

density of 1 g/cm3 (the density of a water droplet) [Hinds 1999]. 

“Respirable” is defined as the aerodynamic size 

of particles that, when inhaled, are capable of 

depositing in the gas-exchange (alveolar) region 

of the lungs [ICRP 1994]. Sampling methods 

have been developed to estimate the airborne 

mass concentration of respirable particles [CEN 

1993; ISO 1995; ACGIH 1994; NIOSH 1998]. 

“Fine” is defined in this document as all particle sizes that are collected by respirable particle 

sampling (i.e., 50% collection efficiency for particles of 4 μm, with some collection of particles 

up to 10 μm). Fine is sometimes used to refer 

to the particle fraction between 0.1 μm and approximately 3 μm [Aitken et al. 2004], and to 

pigment-grade TiO2 [e.g., Lee et al. 1985]. The 

term “fine” has been replaced by “respirable” 

by some organizations, e.g., MAK [DFG 2000], 

which is consistent with international sampling 

conventions [CEN 1993; ISO 1995]. 

“Ultrafine” is defined as the fraction of respirable particles with primary particle diameter <0.1 μm (<100 nm), which is a widely used 

definition. Particles <100 nm are also defined 

as nanoparticles. A primary particle is defined 

as the smallest identifiable subdivision of a particulate system [BSI 2005]. Additional methods 

are needed to determine if an airborne respirable particle sample includes ultrafine TiO2

(Chapter 6). In this document, the terms fine 

and respirable are used interchangeably to retain both the common terminology and the 

international sampling convention. 

In 1988, NIOSH classified TiO2 as a potential 

occupational carcinogen and did not establish 

a recommended exposure limit (REL) for TiO2

[NIOSH 2002]. This classification was based on 

the observation that TiO2 caused lung tumors 
4 NIOSH CIB 63 • Titanium Dioxide

Table 1. Occupational exposure limits and guidelines for TiO2* and PNOR/S

TiO2 PNOR/S

Agency

Single-shift TWA

(mg/m3) Comments

Single-shift TWA

(mg/m3) Comments

NIOSH [2002]† — Potential human 

carcinogen

— —

OSHA 15 Total‡ 15

5

Total

Respirable

ACGIH 

[2001a, 2001b, 

2005, 2009]

10 Category A4 (not

classifiable as a

human carcinogen); 

TiO2 is under study 

by ACGIH [ACGIH 

2009].

10§

3§

Inhalable

Respirable

MAK††

[DFG 2000, 2008]

 __ Inhalable fraction 

except for ultrafine 

particles; suspected 

carcinogen (MAK 

Category 3A)

4

1.5

Inhalable

Respirable

*Abbreviations: ACGIH = American Conference of Governmental Industrial Hygienists; MAK = Federal Republic of 

Germany Maximum Concentration Values in the Workplace; NIOSH = National Institute for Occupational Safety 

and Health; OSHA = Occupational Safety and Health Administration; PNOR/S = particles not otherwise regulated 

or specified; TiO2 = titanium dioxide; TWA = time-weighted average; TLV® = threshold limit value

†Recommendations in effect before publication of this document.

‡Total, inhalable, and respirable refer to the particulate size fraction, as defined by the respective agencies.

§PNOS guideline (too little evidence to assign TLV®). Applies to particles without applicable TLV®, insoluble or 

poorly soluble, low toxicity (PSLT) [ACGIH 2005, 2009]. Inorganic only; and for particulate matter containing no 

asbestos and < 1% crystalline silica [ACGIH 2001b].

††MAK values are long-term averages. Single shift excursions are permitted within a factor of 2 of the MAK value. 

The TiO2 MAK value has been withdrawn and pregnancy risk group C is not applicable [DFG 2008].
NIOSH CIB 63 • Titanium Dioxide 5

in rats in a long-term, high-dose bioassay 

[Lee et al. 1985]. NIOSH concluded that the 

results from this study met the criteria set 

forth in the OSHA cancer policy (29 CFR 

Part 1990, Identification, Classification, and 

Regulation of Carcinogens) by producing 

tumors in a long-term mammalian bioassay. The International Agency for Research 

on Cancer (IARC) formerly classified TiO2

in Group 3, with limited evidence of animal 

carcinogenicity and inadequate evidence for 

human carcinogenicity [IARC 1989]. In 2006, 

IARC classified TiO2 in Group 2B, with sufficient evidence of carcinogenicity in experimental animals and inadequate evidence for 

human carcinogenicity and an overall evaluation of “possibly carcinogenic to humans 

(Group 2B)” [IARC 2010]. The scientific evidence pertaining to hazard classification and 

exposure limits for TiO2 is reviewed and evaluated in this document. 

NIOSH CIB 63 • Titanium Dioxide 7

2 Human Studies 

2.1 Case Reports 

Case reports can provide information about the 

potential health effects of exposure to titanium 

dioxide (TiO2) that may lead to formal epidemiologic studies of a relationship between occupational exposure and observed cases. 

A few case reports described adverse health effects in workers with potential TiO2 exposure. 

These effects included adenocarcinoma of the 

lung and TiO2-associated pneumoconiosis in 

a male TiO2 packer with 13 years of potential 

dust exposure and a 40-year history of smoking [Yamadori et al. 1986]. Pulmonary fibrosis 

or fibrotic changes and alveolar macrophage 

responses were identified by thoracotomy or 

autopsy tissue sampling in three workers with 

6 to 9 years of dusty work in a TiO2 factory. No 

workplace exposure data were reported. Two 

workers were “moderate” or “heavy” smokers 

(pack-years not reported) and smoking habits 

were not reported for the other worker [Elo et 

al. 1972]. Small amounts of silica were present 

in all three lung samples, and significant nickel 

was present in the lung tissue of the autopsied 

case. Exposure was confirmed using sputum 

samples that contained macrophages with high 

concentrations of titanium 2 to 3 years after their 

last exposure [Määttä and Arstila 1975]. Titanium particles were identified in the lymph 

nodes of the autopsied case. The lung concentrations of titanium were higher than the 

lung concentration range of control autopsy 

specimens from patients not exposed to TiO2

(statistical testing and number of controls not 

reported). 

Moran et al. [1991] presented cases of TiO2 exposure in four males and two females. However, occupation was unknown for one male and 

one female, and the lung tissue of one worker 

(artist/painter) was not examined (skin biopsy 

of arm lesions was performed). Smoking habits were not reported. Diffuse fibrosing interstitial pneumonia, bronchopneumonia, and alveolar metaplasia were reported in three male 

patients (a TiO2 worker, a painter, and a paper 

mill worker) with lung-deposited TiO2 (rutile) 

and smaller amounts of tissue-deposited silica 

[Moran et al. 1991]. Titanium was also identified in the liver, spleen, and one peribronchial 

lymph node of the TiO2 worker, and talc was 

identified in the lungs of that patient and the 

paper mill worker. 

A case of pulmonary alveolar proteinosis (i.e., 

deposition of proteinaceous and lipid material 

within the airspaces of the lung) was reported 

in a worker employed for more than 25 years as 

a painter, with 8 years of spray painting experience. He smoked two packs of cigarettes per 

day until he was hospitalized. Titanium was 

the major type of metallic particle found in his 

lung tissues [Keller et al. 1995]. 

According to a four-sentence abstract from the 

Toxic Exposure Surveillance System (TESS) of the 

American Association of Poison Control Centers, 

death occurred suddenly in a 26-year-old male 

worker while pressure-cleaning inside a tank 

containing TiO2; death was “felt to be due to 

inhalation of this particulate chemical” [Litovitz et al. 2002; Litovitz 2004]. There was no 

other information about the cause of death 

or indication that an autopsy was conducted. 
8 NIOSH CIB 63 • Titanium Dioxide

TESS data are used for hazard identification, 

education, and training [Litovitz et al. 2002; 

Litovitz 2004]. 

In pathology studies of TiO2 workers, tissuedeposited titanium was often used to confirm 

exposure. In many cases, titanium, rather than 

TiO2, was identified in lung tissues; the presence of TiO2

 was inferred when a TiO2-exposed 

worker had pulmonary deposition of titanium 

(e.g., Ophus et al. [1979]; Rode et al. [1981]; 

Määttä and Arstila [1975]; Elo et al. [1972]; 

Humble et al. [2003]). In other case reports, 

X-ray crystallography identified TiO2 (i.e., anatase) in tissue digests [Moran et al. 1991], and 

X-ray diffraction distinguished rutile from anatase [Rode et al. 1981]. Similarly, with the exception of one individual in whom talc was identified [Moran et al. 1991], pathology studies (i.e., 

Elo et al. [1972]; Moran et al. [1991]) identified 

the silica as “SiO2” (silicon dioxide) or “silica” in 

tissue and did not indicate whether it was crystalline or amorphous. 

In summary, few TiO2-related health effects 

were identified in case reports. None of the 

case reports provided quantitative industrial 

hygiene information about workers’ TiO2 dust 

exposure. Lung particle analyses indicated that 

workers exposed to respirable TiO2 had particle retention in their lungs that included titanium, silica (form not specified), and other 

minerals sometimes years after cessation of 

exposure. The chronic tissue reaction to lungdeposited titanium is distinct from chronic silicosis. Most cases of tissue-deposited titanium 

presented with a local macrophage response 

with associated fibrosis that was generally mild, 

but of variable severity, at the site of deposition. 

More severe reactions were observed in a few 

cases. The prevalence of similar histopathologic 

responses in other TiO2-exposed populations 

is not known. The effects of concurrent or 

sequential exposure to carcinogenic particles, 

such as crystalline silica, nickel, and tobacco 

smoke, were not determined. 

2.2 Epidemiologic Studies 

A few epidemiologic studies have evaluated 

the carcinogenicity of TiO2 in humans; they are 

described here and in Table 2–1. Epidemiologic studies of workers exposed to related compounds, such as TiCl4

 or titanium metal dust 

(i.e., Fayerweather et al. [1992] and Garabrant 

et al. [1987]) were not included because those 

compounds may have properties and effects 

that differ from those of TiO2 and discussion 

of those differences is beyond the scope of this 

document.

2.2.1 Chen and Fayerweather [1988]

Chen and Fayerweather [1988] conducted a 

mortality, morbidity, and nested case-control 

study of 2,477 male wage-grade workers employed for more than 1 year before January 1, 

1984 in two TiO2 production plants in the 

United States. The objectives of the study were 

to determine if workers potentially exposed to 

TiO2 had higher risks of lung cancer, chronic 

respiratory disease, pleural thickening/plaques, 

or pulmonary fibrosis than referent groups. 

Of the 2,477 male workers, 1,576 were potentially exposed to TiO2

. Other exposures included TiCl4

, pigmentary potassium titinate (PKT), 

and asbestos. (The TiCl4-exposed workers were 

evaluated in Fayerweather et al. [1992]). Quantitative results from exposure monitoring or 

sampling performed after 1975 may have been 

included in the study; however, it was unclear 

what exposure measurements were available 

after 1975 and how they were used. Committees (not described) were established at the 

plants to estimate TiO2 exposures for all jobs. 
NIOSH CIB 63 

• Titanium Dioxide

9

Reference

and country

Study design, cohort,

and follow-up Subgroup

Number of

deaths or cases

in subgroup Risk measure 95% CI

Adjusted for

smoking Comments

Boffetta et al. [2001], Canada

Population-based casecontrol study of 857 cases of histologically confirmed lung cancer diagnosed from 1979 to 1985 in men aged 35–70. Controls were randomly selected healthy residents (n = 533) and persons with cancers of other organs (n = 533).†

Ever exposed to TiO2Substantial exposure to TiO2Level of exposure:

LowMediumHigh

Duration of exposure:

1–21 years≥ 22 years

Exposed toTiO2 fumes

33

8

25

62

1716

5

OR = 0.9OR = 1.0OR = 0.9OR = 1.0OR = 0.3OR = 1.0OR = 0.8OR = 9.1

0.5–1.50.3–2.70.5–1.70.3–3.3

0.07–1.9

0.5–2.00.4–1.6

0.7–118

Yes TiO2 exposures were estimated by industrial hygienists based on occupational 

histories collected by Siemiatycki et al. [1991] and other sources. “Substantial” exposure defined as exposure for >5 years at a medium or high frequency and concentration.Lung cancer ORs were adjusted for age, family income, ethnicity, respondent (i.e., self or proxy), and smoking.Small number of cases ever exposed to TiO2 (n = 33).Limitations include self- or proxy-reporting of occupational exposures. Most TiO2 fume-exposed cases (n = 5) and controls (n = 1) were also exposed to chromium and nickel.

Table 2–1. Summary of epidemiologic studies of workers exposed to TiO2*

See footnotes at end of table. (Continued)
10 NIOSH CIB 63 • Titanium Dioxide Reference and country Study design, cohort, and follow-up Subgroup Number of deaths or cases in subgroup Risk measure 95% CI Adjusted for smoking Comments Boffetta et al. [2004], Finland, France, Germany, Italy, Norway, United Kingdom Retrospective cohort mortal- ity study of 15,017 workers (14,331 men) employed ≥ 1 month in 11 TiO2 production facilities and followed for mortality from 1950–1972 until 1997–2001 (follow-up period varied by country). Employment records were complete from 1927–1969 until 1995–2001. Male lung cancer: Cumulative respirable TiO2 dust exposure (mg/m3 ∙ year): 0–0.73 0.73–3.43 3.44–13.19 13.20+ Male nonmalignant respiratory diseases: Cumulative respirable TiO2 dust exposure (mg/m3 ∙ year): 0–0.8 0.9–3.8 3.9–16.1 16.2+ 5353525340394039 RR = 1.00 RR = 1.19 RR = 1.03 RR = 0.89 RR = 1.00 RR = 1.23 RR = 0.91 RR = 1.12 Reference category 0.80–1.77 0.69–1.55 0.58–1.35 Reference category 0.76–1.99 0.56–1.49 0.67–1.86 Smoking data were available for 5,378 work- ers, but “since most available smoking data refer to recent years, no direct adjustment of risk estimates was attempted” [Boffetta et al. 2004]. No evidence of increased mortality risk with increas- ing cumulative TiO2 dust exposure. (

P-values for tests 

of linear trend were 0.5 and 0.6 for lung cancer mortality and nonmalignant respiratory disease mortality, respectively).Estimated cumulative TiO2dust exposure was derived from job title and work history. Exposure indices were not calculated when > 25% of the occupational history or > 5 years were missing.SMRs were not significantly increased for any cause of death except male lung cancer (SMR = 1.23; 95% CI 1.10–1.38; 306.5 deaths observed). Female workers were not included in most statistical analyses because of small number of deaths (n = 33).

Table 2–1 (Continued). Summary of epidemiologic studies of workers exposed to TiO2

*

See footnotes at end of table. (Continued)
NIOSH CIB 63 

• Titanium Dioxide 11

Reference

and country

Study design, cohort,

and follow-up Subgroup

Number of

deaths or cases

in subgroup Risk measure 95% CI

Adjusted for

smoking Comments

Chen and Fayerweather [1988], United States

Mortality, morbidity, and nested case-control study of male, wage-grade employees of two TiO2 production plants. Of 2,477 male employees, 1,576 were potentially exposed to TiO2. Study subjects worked >1 year before January 1, 1984. Mortality was followed from 1935 through 1983 and compared with U.S. white male mortality rates or company rates. Cancer and chronic respiratory disease incidence cases from 1956–1985 were available from company insurance registry. Case-control methods were applied to findings from 398 chest X-ray films from current male employees as of January 1, 1984.

Lung cancer deaths 1935–1983Lung cancer deaths 1957–1983Lung cancer cases 1956–1985Lung cancer casescase-control study)Chronic respiratory disease cases (casecontrol study)Pleural thickening/plaque cases (casecontrol study)

998

168822

O/E = 0.52

(national rates)

O/E = 0.59

(company rates)

O/E = 1.04

(company rates)

OR = 0.6OR = 0.8

OR = 1.4§

11–24‡

9–22‡3–13‡

Not reportedNot reportedNot reported 

Smoking histories were available for current workers; only use in X-ray case-control study was reported.

No statistically significant association or trends were reported. However, study has limitations (see text).Unclear source and exposure history of 898 controls in nested case-control study—may have been from company disease registry rather than entire worker population.Lung cancer OR was adjusted for age and exposure to TiCl4, potassium titinate, and asbestos. “Chronic respiratory disease” was not defined.Controls (n = 372) for pleural thickening case-control study were active employees with normal chest X-ray findings. ORs were adjusted for age, current cigarette smoking habits, and exposure to known respiratory hazards (not defined).

Table 2–1 (Continued). Summary of epidemiologic studies of workers exposed to TiO2*

See footnotes at end of table. (Continued)
12 NIOSH CIB 63 • Titanium Dioxide Reference and country Study design, cohort, and follow-up Subgroup Number of deaths or cases in subgroup Risk measure 95% CI Adjusted for smoking Comments Fryzek [2004], Fryzek et al. [2003, 2004a], United States Retrospective cohort mortal- ity study of 409 female and 3,832 male workers employed ≥ 6 months on or after Janu- ary 1, 1960, at four TiO2 pro- duction facilities. The cohort was followed for mortality until the end of 2000. Mor- tality rates by sex, age, race, time period, and State where plant was located were used for numbers of expected deaths. Thirty-five percent (n = 1,496) of workers were employed in jobs with high potential TiO2 dust exposure (i.e., packers, micronizers, and addbacks). Trachea, bronchus, lung cancer deaths High potential TiO2 exposure jobs Nonmalignant respira- tory disease deaths High potential TiO2 exposure jobs All causes of death High potential TiO2 exposure jobs 611131

3

533112

SMR = 1.0SMR = 1.0SMR = 0.8SMR = 0.4SMR = 0.8SMR = 0.7

0.8–1.30.5–1.70.6–1.20.1–1.30.8–0.90.6–0.9

No Lung cancer and nonmalignant respiratory disease 

SMRs not elevated significantly with exception of lung cancer death in the subgroup of shortest-term workers (0–9 years worked) with >20 years since first hire (SMR for cancer of trachea, bronchus, lung = 1.5; 95% CI 1.0–2.3; 

P < 

0.05; number of deaths in subgroup not reported). Internal analyses with models found no significant exposure-response trends for those diseases or total cancers.Study limitations:(1) short follow-up period (avg. 21 years) and about half the cohort born after 1940;(2) more than half worked fewer than 10 years;(3) limited data on nonoccupational factors (e.g., smoking).

Table 2–1 (Continued). Summary of epidemiologic studies of workers exposed to TiO2

*

See footnotes at end of table. (Continued)
NIOSH CIB 63 

• Titanium Dioxide 13

Reference

and country

Study design, cohort,

and follow-up Subgroup

Number of

deaths or cases

in subgroup Risk measure 95% CI

Adjusted for

smoking Comments

Fryzek [2004], Fryzek et al. [2003, 2004a], United States(Continued)

Study authors responded to a letter to the editor about modeling results, particularly the negative lung cancer exposure-response trend for cumulative TiO2

exposure category and duration of exposure [Beaumont et al. 2004]. Response included additional analyses that yielded hazard ratios similar to original ones [Fryzek 2004; Fryzek et al. 2003, 2004a].914 full-shift or near fullshift personal air samples for TiO2 dust were used in the analysis. Mean TiO2 dust concentrations declined from 13.7 mg/m3 ±17.9 (21 samples) in 1976–1980 to 3.1 mg/m3 ± 6.1 (357 samples) in 1996–2000. They were 6.2 ± 9.4 mg/m3 (686 samples) in jobs with high potential for TiO2 exposure.

Table 2–1 (Continued). Summary of epidemiologic studies of workers exposed to TiO2*

See footnotes at end of table. (Continued)
14 NIOSH CIB 63 • Titanium Dioxide Reference and country Study design, cohort, and follow-up Subgroup Number of deaths or cases in subgroup Risk measure 95% CI Adjusted for smoking Comments Ramanakumar et al. [2008], Canada Two population-based lung cancer case-control studies: Study I (1979–1986): 857 histologically confirmed lung cancer cases 1979 to 1985 in men aged 35–70. Controls were randomly se- lected healthy residents (n = 533) and 1,349 persons with cancers of other organs (n = 533). Study II (1995–2001): 765 histologically confirmed male and 471 female lung cancer cases with 899 male and 613 female population-based controls. Pooled data from Studies I and II: Any TiO2 exposure Nonsubstantial TiO2 exposure Substantial TiO2 exposure 7668

8

OR = 1.0OR = 1.0OR = 1.2

0.8–1.50.6–1.70.4–3.6

Yes Study I similar to Boffetta 

et al. [2001] (above).TiO2 exposures were estimated by industrial hygienists based on occupational histories collected by Siemiatycki et al. [1991] and other sources.“Substantial” exposure defined as exposure for >5 years at a medium or high frequency and concentration, excluding the 5 years before diagnosis or interview, with confidence of probable or definite occurrence.Lung cancer ORs were adjusted for age, family income, ethnicity, respondent (i.e., self or proxy), years of schooling, tobacco smoking (3 variables), and exposure to at least one of the other job hazards (i.e., cadmium compounds, asbestos, silica).Small number of cases in “substantial” TiO2 exposure category (n = 8 males; 0 females).Occupational exposures reported by self- or surrogate respondents. 

Table 2–1 (Continued). Summary of epidemiologic studies of workers exposed to TiO2

*

See footnotes at end of table. (Continued)
NIOSH CIB 63 

• Titanium Dioxide 15

Reference

and country

Study design, cohort,

and follow-up Subgroup

Number of

deaths or cases

in subgroup Risk measure 95% CI

Adjusted for

smoking Comments

Siemiatycki et al. [1991], Canada

Population-based casecontrol study of 3,730 histologically confirmed cases of 20 types of cancer diagnosed from September 1979 to June 1985 in men aged 35–70.140 cases had some occupational TiO2 exposure. Two control groups were used: 533 population-based controls and a group of cancer patients.

Lung cancer cases with any occupational TiO2exposureLung cancer cases with “substantial” occupational TiO2 exposureSquamous cell lung cancer cases with any occupational TiO2exposure (populationbased controls)Squamous cell lung cancer cases with “substantial” occupational TiO2 exposure Bladder cancer cases with any occupational TiO2 exposure (cancer patient controls)Substantial occupational TiO2 exposure

38

5

20

2

28

3

OR = 1.0OR = 2.0OR = 1.6OR = 1.3OR = 1.7OR = 4.5

0.7–1.5**0.6–7.4**0.9–3.0**0.2–9.8**1.1–2.6**

 0.9–22.0**

Yes Results provide little information about TiO2-specific 

effects because this study evaluated 293 exposures, including TiO2.Exposure was estimated by “chemist-hygienists” based on occupational histories.“Substantial” exposure defined as >10 years in the industry or occupation up to 5 years before onset [Siemiatycki et al. 1991, p. 122].

Table 2–1 (Continued). Summary of epidemiologic studies of workers exposed to TiO2*

*Abbreviations: CI = confidence interval; O/E = observed number of deaths or cases divided by expected number of deaths or cases; OR = odds ratio; RR = relative risk; SMR = standardized mortality ratio; TiO2 = 

titanium dioxide.

†Number of controls in Boffetta et al. [2001] subgroups: 43 ever exposed, 9 substantial exposure; 29 low exposure; 9 medium exposure; 5 high exposure; 22 worked 1–21 years; 21 worked ≥ 22 years.‡90% acceptance range for the expected number of deaths or cases§Reported as “not statistically significantly elevated.”**90% CI.
16 NIOSH CIB 63 • Titanium Dioxide

A cumulative exposure index, duration of exposure, and TWA exposure were derived and 

used in the analyses (details not provided).

Chest radiographic examination was used to 

detect fibrosis and pleural abnormalities, and 

the most recent chest X-ray of active employees (on 1/1/1984) was read blindly by two Breaders. Chest X-ray films were not available 

for retired and terminated workers. 

Observed numbers of cancer morbidity cases 

(i.e., incident cases) were compared to expected numbers based on company rates. Observed numbers of deaths were compared to 

expected numbers from company rates and 

national rates. Ninety percent (90%) “acceptance ranges” were calculated for the expected 

numbers of cases or deaths. The nested casecontrol study investigated decedent lung cancer and chronic respiratory disease, incident 

lung cancer and chronic respiratory disease 

(not described), and radiographic chest abnormalities. Incidence data from the company’s 

insurance registry were available from 1956 to 

1985 for cancer and chronic respiratory disease. Mortality data from 1957 to 1983 were 

obtained from the company mortality registry. 

The study reported the number of observed 

deaths for the period 1935–1983; the source 

for deaths prior to 1957 is not clear. Vital status 

was determined for “about 94%” of the study 

cohort and death certificates were obtained for 

“about 94%” of workers known to be deceased. 

The observed number of deaths from all cancers was lower than the expected number based 

on U.S. mortality rates; however, the observed 

number of deaths from all causes was greater 

than the expected number when based on company mortality rates (194 deaths observed; 175.5 

expected; 90% acceptance range for the expected number of deaths = 154–198). Lung cancer 

deaths were lower than the expected number 

based on national rates (9 deaths observed/17.3 

expected = 0.52; 90% acceptance range for the 

expected number of deaths = 11–24) and company rates (9 deaths observed/15.3 deaths expected = 0.59; 90% acceptance range for the

expected number of deaths = 9–22). Lung cancer morbidity was not greater than expected 

(company rates; 8 cases observed; 7.7 expected; 

90% acceptance range for the expected number of cases = 3–13). 

Nested case-control analyses found no association between TiO2

 exposure and lung cancer 

morbidity after adjusting for age and exposure 

to TiCl4, PKT, and asbestos (16 lung cancer 

cases; 898 controls; TiO2 odds ratio [OR] = 0.6). 

The OR did not increase with increasing average exposure, duration of exposure, or cumulative exposure index. No statistically significant positive relationships were found between 

TiO2 exposure and cases of chronic respiratory 

disease (88 cases; 898 noncancer, nonrespiratory disease controls; TiO2

 OR = 0.8). Chest 

X-ray findings from 398 films showed few 

abnormalities—there were four subjects with 

“questionable nodules” but none with fibrosis. 

Pleural thickening or plaques were present in 

5.6% (n = 19) of the workers potentially exposed to TiO2 compared with 4.8% (n = 3) in 

the unexposed group. Case-control analyses 

of 22 cases and 372 controls with pleural abnormalities found a nonstatistically significant 

OR of 1.4 for those potentially exposed and no 

consistent exposure-response relationship.

This study did not report statistically significant 

increased mortality from lung cancer, chronic 

respiratory disease, or fibrosis associated with 

titanium exposure. However, it has limitations: 

(note: the study component or information affected by the limitation is mentioned, when 

possible) (1) Existence of quantitative exposure 
NIOSH CIB 63 • Titanium Dioxide 17

data for respirable TiO2 after 1975 is implied; 

the type of measurement (e.g., total, respirable, 

or submicrometer), type of sample (e.g., area 

or personal), number of samples, sampling location and times, nature of samples (e.g., epidemiologic study or compliance survey), and 

breathing zone particle sizes were not reported. 

(Exposure data were used in the nested casecontrol analyses of morbidity and mortality.) 

(2) The report did not describe the number of 

workers, cases, or deaths in each exposure duration quartile which could contribute to understanding of all component results. (3) The presence of other chemicals and asbestos could have 

acted as confounders. (4) Incidence and mortality data were not described in detail and could 

have been affected by the healthy worker effect. 

(5) Company registries were the only apparent 

source for some incidence and mortality information (e.g., company records may have been 

based on those workers eligible for pensions 

and thus not typical of the general workforce). 

2.2.2 Fryzek et al. [2003]

Fryzek et al. [2003] conducted a retrospective 

cohort mortality study of 4,241 workers with 

potential exposure to TiO2 employed on or after 1/1/1960 for at least 6 months at four TiO2

production plants in the United States. 

The plants used either a sulfate process or a chloride process to produce TiO2

 from the original 

ore. Nearly 2,400 records of air sampling measurements of sulfuric acid mist, sulfur dioxide, 

hydrogen sulfide, hydrogen chloride, chlorine, 

TiCl4, and TiO2 were obtained from the four 

plants. Most were area samples and many were 

of short duration. Full-shift or near full-shift 

personal samples (n = 914; time-weighted averaging not reported) for total TiO2 dust were 

used to estimate relative exposure concentrations between jobs over time. Total mean 

TiO2 dust levels declined from 13.7 mg/m3 in 

1976–1980 to 3.1 mg/m3 during 1996–2000. 

Packers, micronizers, and addbacks had about 

3 to 6 times higher exposure concentrations 

than other jobs. Exposure categories, defined 

by plant, job title, and calendar years in the job, 

were created to examine mortality patterns in 

those jobs where the potential for TiO2 exposure was greatest. 

Mortality of 409 female workers and 3,832 

male workers was followed until 12/31/2000 

(average follow-up time = 21 years; standard 

deviation = 11 years). The number of expected 

deaths was based on mortality rates by sex, age, 

race, time period, and the state where the plant 

was located, and standardized mortality ratios 

(SMRs) and confidence intervals (CIs) were 

calculated. Cox proportional hazards (PH) 

models, which adjusted for effects of age, sex, 

geographic area, and date of hire, were used to 

estimate relative risks (RR) of TiO2 exposure 

(i.e., average intensity, duration, and cumulative exposure) in medium or high exposure 

groups versus the lowest exposure group. 

Of the 4,241 workers (58% white, 22% nonwhite, 20% unknown race, 90% male), 958 did 

not have adequate work history information 

and were omitted from some plant analyses. 

Some company records from the early period 

may have been lost or destroyed; however, 

the study authors “found no evidence to support such an assumption” [Fryzek et al. 2003]. 

Wage status (i.e., hourly or salaried) was not 

described. Thirty-five percent of workers had 

been employed in jobs with the highest potential for TiO2

 exposure. Workers experienced a 

significantly low overall mortality (533 deaths, 

SMR = 0.8, 95% CI = 0.8–0.9; P < 0.05). No 

significantly increased SMRs were found for 

any specific cause of death and there were no 

trends with exposure. (Results were not reported 
18 NIOSH CIB 63 • Titanium Dioxide

by category of race). The number of deaths 

from trachea, bronchus, or lung cancer was 

not greater than expected (i.e., 61 deaths; SMR 

= 1.0; 95% CI = 0.8–1.3). However, there was 

a significant 50% elevated SMR for workers 

employed 9 years or less with at least 20 years 

since hire (SMR = 1.5 [cancer of trachea, bronchus, or lung]; 95% CI = 1.0–2.3; number of 

deaths not reported). SMRs for this cancer did 

not increase with increasing TiO2 concentrations (i.e., as evidenced by job category in Table 6 of the study). Workers in jobs with greatest TiO2

 exposure had significantly fewer than 

expected total deaths (112 deaths; SMR = 0.7; 

95% CI = 0.6–0.9) and mortality from cancers 

of trachea, bronchus, or lung was not greater 

than expected (11 deaths; SMR = 1.0; 95% CI = 

0.5–1.7). Internal analyses (i.e., Cox PH models) revealed no significant trends or exposureresponse associations for total cancers, lung 

cancer, or other causes of death. No association between TiO2 exposure and increased risk 

of cancer death was observed in this study (i.e., 

Fryzek et al. [2003]). 

Limitations of this study include (1) about 

half the cohort was born after 1940, lung cancer in these younger people would be less frequent and the latency from first exposure to 

TiO2 would be short, (2) duration of employment was often quite short, (3) no information about ultrafine exposures (probably because collection methods were not available 

over the course of the study), and (4) limited 

data on nonoccupational factors (e.g., smoking). Smoking information abstracted from 

medical records from 1960 forward of 2,503 

workers from the four plants showed no imbalance across job groups. In all job groups, the 

prevalence of smoking was about 55% and it 

declined over time by decade of hire. However, 

the information was inadequate for individual 

adjustments for smoking [Fryzek et al. 2003]. 

Fryzek et al. [Fryzek 2004; Fryzek et al. 2004a] 

performed additional analyses in response to a 

suggestion that the RRs may have been artificially low, especially in the highest category of 

cumulative exposure, because of the statistical 

methods used [Beaumont et al. 2004]. These 

analyses yielded hazard ratios similar to those 

in the original analysis and found no significant exposure-response relationships for lung 

cancer mortality and cumulative TiO2 exposure (i.e., “low,” “medium,” “high”) with either a 

time-independent exposure variable or a timedependent exposure variable and a 15-year 

exposure lag (adjusted for age, sex, geographic 

area, and date of hire) [Fryzek 2004; Fryzek et 

al. 2004a]. The hazard ratio for trachea, bronchus, and lung cancer from “medium” cumulative TiO2 exposure (15-year lag) was greater 

than 1.0 (hazard ratio for “medium” cumulative exposure, time-dependent exposure variable and 15-year lag = 1.3; 95% CI = 0.6–2.8) 

and less than 1.0 for “high”: (hazard ratio = 

0.7; 95% CI = 0.2–1.8; “low” was the referent 

group) [Fryzek 2004; Fryzek et al. 2004a]. 

2.2.3 Boffetta et al. [2001]

Boffetta et al. [2001] reevaluated lung can- [2001] reevaluated lung cancer risk from exposure to TiO2

 in a subset of 

a population-based case-control study of 293 

substances including TiO2 (i.e., Siemiatycki et 

al. [1991]; see Table 2–1 for description of Siemiatycki et al. [1991]). 

Histologically confirmed lung cancer cases 

(n = 857) from hospitals and noncancer referents were randomly selected from the population of Montreal, Canada. Cases were male, 

aged 35 to 70, diagnosed from 1979 to 1985, 

and controls were 533 randomly selected 

healthy residents and 533 persons with cancer 

in other organs. 
NIOSH CIB 63 • Titanium Dioxide 19

Job information was translated into a list of potential exposures, including all Ti compounds 

and TiO2 as dust, mist, or fumes. Using professional judgment, industrial hygienists assigned 

qualitative exposure estimates to industry and 

job combinations worked by study subjects, 

based on information provided in interviews 

with subjects, proxies, and trained interviewers and recorded on a detailed questionnaire. 

The exposure assessment was conducted blindly 

(i.e., case or referent status not known). Duration, likelihood (possible, probable, definite), 

frequency (<5%, 5%–30%, >30%), and extent 

(low, medium, high) of exposure were assessed. 

Those with probable or definite exposure for at 

least 5 years before the interview were classified 

as “exposed.” Boffetta et al. [2001] classified exposure as “substantial” if it occurred for more 

than 5 years at a medium or high frequency and 

level. (Siemiatycki et al. [1991] used a different 

definition and included five workers exposed 

to titanium slag who were excluded by Boffetta 

et al. [2001]; see Table 2–1). Only 33 cases and 

43 controls were classified as ever exposed to 

TiO2 (OR = 0.9; 95% CI = 0.5–1.5). Results of 

unconditional logistic models were adjusted 

for age, socioeconomic status, ethnicity, respondent status (i.e., self or proxy), tobacco 

smoking, asbestos, and benzo(a)pyrene (BAP) 

exposure. No trend was apparent for estimated 

frequency, level, or duration of exposure. The 

OR was 1.0 (95% CI = 0.3–2.7) for medium or 

high exposure for at least 5 years. Results did 

not depend on choice of referent group, and no 

significant associations were found with TiO2

exposure and histologic type of lung cancer. The 

likelihood of finding a small increase in lung 

cancer risk was limited by the small number of 

cases assessed. However, the study did find an 

excess risk for lung cancer associated with both 

asbestos and BAP, indicating that the study 

was able to detect risks associated with potent 

carcinogens. The study had a power of 86% to 

detect an OR of 2 at the 5% level, and 65% power for an OR of 1.5. 

Limitations of this study include (1) selfreporting or proxy reporting of exposure information; (2) use of surrogate indices for 

exposure; (3) absence of particle size characterization; and (4) the nonstatistically significant 

lung cancer OR for exposure to TiO2 fumes, 

which was based on a small group of subjects 

and most were also exposed to nickel and chromium (5 cases; 1 referent; OR = 9.1; 95% CI = 

0.7–118). In addition, exposures were limited 

mainly to those processes, jobs, and industries 

in the Montreal area. For example, the study 

probably included few, if any, workers who 

manufactured TiO2. Most workers classified as 

TiO2-exposed were painters and motor vehicle 

mechanics and repairers with painting experience; the highly exposed cases mixed raw materials for the manufacture of TiO2

-containing 

paints and plastics. 

2.2.4 Boffetta et al. [2004]

Boffetta et al. [2004] conducted a retrospective 

cohort mortality study of lung cancer in 15,017 

workers (14,331 men, 686 women) employed 

at least 1 month in 11 TiO2 production facilities in six European countries. The factories 

produced mainly pigment-grade TiO2. Estimated cumulative occupational exposure to 

respirable TiO2 dust was derived from job title 

and work history. Observed numbers of deaths 

were compared with expected numbers based 

on national rates; exposure-response relationships within the cohort were evaluated using 

the Cox PH model. Few deaths occurred in female workers (n = 33); therefore, most analyses 

did not include female deaths. The follow-up 

period ranged from 1950–1972 until 1997–

2001; 2,619 male and 33 female workers were 
20 NIOSH CIB 63 • Titanium Dioxide

reported as deceased. (The follow-up periods 

probably have a range of years because the 

follow-up procedures varied with the participating countries.) The cause of death was not 

known for 5.9% of deceased cohort members. 

Male lung cancer was the only cause of death 

with a statistically significant SMR (SMR = 

1.23; 95% CI = 1.10–1.38; 306.5 deaths [not a 

whole number because of correction factors for 

missing deaths]). However, the Cox regression 

analysis of male lung cancer mortality found 

no evidence of increased risk with increasing cumulative respirable TiO2

 dust exposure 

(P-value for test of linear trend = 0.5). There 

was no consistent and monotonic increase in 

SMRs with duration of employment, although 

workers with more than 15 years of employment had slightly higher SMRs than workers 

with 5 to 15 years of employment and an effect 

of time since first employment was suggested 

for workers employed more than 10 years. (The 

authors indicated that the increase in lung cancer mortality with increasing time since first 

employment could be “explained by the large 

contribution of person-years to the categories 

with longest time since first employment from 

countries such as Germany, with increased 

overall lung cancer mortality” [Boffetta et al. 

2004].) For male nonmalignant respiratory 

disease mortality, the number of observed 

deaths was lower than the expected number 

(SMR = 0.88; 95% CI = 0.77–1.02; 201.9 deaths 

observed; 228.4 expected), and there was no 

evidence of an exposure-response relationship. 

The authors suggested that lack of exposureresponse relationships may have been related 

to a lack of (1) statistical power or (2) inclusion of workers who were employed before 

the beginning of the follow-up period when 

exposure concentrations tended to be high. 

(Regarding the latter point, the authors stated 

that this phenomenon could have occurred 

and resulted in a loss of power, but “the results 

of the analysis on the inception cohort, composed of workers whose employment is entirely 

covered by the follow-up, are remarkably similar to the results of the whole cohort, arguing 

against survival bias” [Boffetta et al. 2004]). The 

authors also suggested that the statistically significant SMR for male lung cancer could represent (1) heterogeneity by country, which the 

authors thought should be explained by chance 

and differences in effects of confounders (see 

next item), rather than factors of TiO2 dust exposure; (2) differences in the effects of potential 

confounders, such as smoking or occupational 

exposure to lung carcinogens; or (3) use of national reference rates instead of local rates. 

2.2.5 Ramanakumar et al. [2008]

Ramanakumar et al. [2008] analyzed data from 

two large, population-based case-control studies conducted in Montreal, Canada, and focused on lung cancer risk from occupational 

exposure to four agents selected from a large 

set of agents and mixtures: carbon black, TiO2, 

industrial talc, and cosmetic talc. Results from 

the first study (i.e., Study I by Boffetta et al. 

[2001]) involving TiO2 were published and are 

described above (see Section 2.2.3). Subject 

interviews for Study I were conducted from 

1982–1986 and included 857 lung cancer cases 

in men aged 35–70 years. Study II’s interviews 

were conducted from 1995–2001 and included men and women aged 35–75 years (765 

male lung cancer cases, 471 female lung cancer cases). In both studies, lung cancer cases 

were obtained from 18 of the largest hospitals 

in the metropolitan Montreal area and confirmed histologically. Controls were randomly 

sampled from the population. Study I had an 

additional control group of persons with nonlung cancers (Study I: 533 population controls 
NIOSH CIB 63 • Titanium Dioxide 21

and 1,349 cancer controls; Study II: 899 male 

controls and 613 female controls). Exposure 

assessment methods were similar to those described in Boffetta et al. [2001] and Siemiatycki 

et al. [1991] and included estimates by industrial hygienists based on job histories (see Section 2.2.3 and Table 2–1). Major occupations 

of cases and controls with TiO2 exposure (n = 

206) were painting, paper hanging, and related occupations (37%); construction laborer, 

grinder chipper and related occupations; motor-body repairmen; and paint plant laborer. 

Unconditional multivariate logistic regression 

models estimated the association between the 

exposure and lung cancer and included potential confounders of age, ethnicity, education, income, type of respondent (i.e., self or 

surrogate), smoking history, and exposure to 

at least one other known occupational hazard 

(i.e., cadmium compounds, silica, or asbestos). 

Association of substance with cell type (i.e., 

squamous cell, adenocarcinoma, small cell) 

was also assessed. ORs for lung cancer and exposure to TiO2

 (i.e., “any,” “nonsubstantial,” or 

“substantial”) were not statistically significantly increased, exposure-response trends were 

not apparent, and there was no evidence of a 

confounding effect of smoking or other confounder or an association with histologic type 

(results by histologic type were not shown). In 

the pooled analysis of cases, controls, and sexes 

from both studies, the ORs were 1.0 (95% CI = 

0.8–1.5), 1.0 (95% CI = 0.6–1.7), and 1.2 (95% 

CI = 0.4–3.6) for TiO2 exposure categories of 

“any,” “nonsubstantial,” and “substantial,” respectively. ORs were adjusted for the possible 

confounders mentioned above [Ramanakumar 

et al. 2008]. 

Limitations of this study are similar to those 

for the Boffetta et al. [2001] study and include 

(1) self-reporting or proxy reporting of exposure information, (2) use of surrogate indices 

for exposure, (3) absence of particle size characterization, (4) few lung cancer cases in the 

“substantial exposure” to TiO2 category (n = 8, 

both studies combined) and no female cases in 

that category, and (5) exposures limited mainly 

to those processes, jobs, and industries in the 

Montreal area. 

2.3 Summary of 

Epidemiologic Studies

In general, the five epidemiologic studies of 

TiO2-exposed workers represent a range of 

environments, from industry to populationbased, and appear to be reasonably representative of worker exposures over several decades. 

One major deficiency is the absence of any cohort studies of workers who handle or use TiO2

(rather than production workers). 

Overall, these studies provide no clear evidence of elevated risks of lung cancer mortality 

or morbidity among those workers exposed to 

TiO2 dust. 

Nonmalignant respiratory disease mortality 

was not increased significantly (i.e., P < 0.05) 

in any of the three epidemiologic studies that 

investigated it. Two of the three retrospective 

cohort mortality studies found small numbers 

of deaths from respiratory diseases other than 

lung cancer, and the number of pneumoconiosis deaths within that category was not reported, indicating that these studies may have 

lacked the statistical power to detect an increased risk of mortality from TiO2

-associated 

pneumoconiosis (i.e., Chen and Fayerweather 

[1988]: 11 deaths from nonmalignant diseases 

of the respiratory system; Fryzek et al. [2003]: 

31 nonmalignant respiratory disease deaths). 

The third study had a larger number of male 

deaths from nonmalignant respiratory disease 
22 NIOSH CIB 63 • Titanium Dioxide

and found no excess mortality (SMR = 0.88; 

201.9 deaths observed; 95% CI = 0.77–1.02) 

[Boffetta et al. 2004]. None of the studies reported an SMR for pneumoconiosis mortality; 

Boffetta et al. [2004] did discuss four pleural 

cancer deaths, although the number of observed deaths was lower than expected number, based on national rates. Boffetta et al. 

[2004] suggested that “mortality data might 

not be very sensitive to assess risks of chronic 

respiratory diseases.” 

In addition to the methodologic and epidemiologic limitations of the studies, they were 

not designed to investigate the relationship between TiO2

 particle size and lung cancer risk, 

an important question for assessing the potential occupational carcinogenicity of TiO2

. Further research is needed to determine whether 

such epidemiologic studies of TiO2-exposed 

workers can be designed and conducted and 

to also study workers who manufacture or use 

products that contain TiO2 (see Chapter 7 Research Needs). 
NIOSH CIB 63 • Titanium Dioxide 23

3 Experimental Studies in Animals and

 Comparison to Humans 

3.1 In Vitro Studies 

3.1.1 Genotoxicity and Mutagenicity

Titanium dioxide (TiO2) did not show genotoxic activity in several standard assays: cellkilling in deoxyribonucleic acid (DNA)-repair 

deficient Bacillus subtilis, mutagenesis in Salmonella typhimurium or E. coli, or transformation 

of Syrian hamster embryo cells (particle size and 

crystal form not specified) [IARC 1989]. TiO2

was not genotoxic in a Drosphila wing spot test 

[Tripathy et al. 1990] or mutagenic in mouse 

lymphoma cells [Myhr and Caspary 1991] (particle size and crystal structure not provided in 

either study). More recent genotoxicity studies have shown that TiO2

 induced chromosomal changes, including micronuclei in Chinese hamster ovary cells (particularly when a 

cytokinesis-block technique was employed) 

[Lu et al. 1998] and sister chromatid exchanges 

and micronuclei in lymphocytes [Türkez and 

Geyikoğlu 2007] (particle size and crystal form 

not provided in either study). Photo-illumination 

of TiO2 (anatase/rutile samples of various ratios; particle size not known) catalyzed oxidative DNA damage in cultured human fibroblast 

cells, which the assay indicated was due to hydroxyl radicals [Dunford et al. 1997]. Ultrafine 

TiO2 (particle diameter < 100 nm; crystal structure not provided) induced apoptosis in Syrian 

hamster embryo cells [Rahman et al. 2002] and 

in cultured human lymphoblastoid cells [Wang 

et al. 2007a]. Sanderson et al. [2007] provided 

additional physical-chemical data on the ultrafine TiO2

 material studied in Wang et al. 

[2007a] (anatase > 99%; mean particle diameter = 6.57; surface area = 147.9 m2

/g). DNA 

damage (micronuclei) was produced in human 

lymphoblastoid cells at a 65 μg/ml dose without excessive cell killing (20%) [Sanderson et 

al. 2007; Wang et al. 2007a]. 

Ultrafine TiO2 (80% anatase, 20% rutile) was 

not genotoxic without UV/vis light irradiation 

in treated cells but did show dose-dependent 

increase in chromosome aberrations in a Chinese hamster cell line with photoactivation 

[Nakagawa et al. 1997]. Greater photocatalytic 

activity by mass was observed for anatase than 

for rutile TiO2 (specific surface areas: 14.6 and 

7.8 m2/g, respectively) [Kakinoki et al. 2004] 

and for anatase-rutile mixtures (e.g., 80% anatase, 20% rutile [Behnajady et al. 2008]) compared to either particle type alone (0.5% wt 

anatase due to photoinduced interfacial electron transfer from anatase to rutile [Kawahara 

et al. 2003]). Particle size influenced oxidative 

DNA damage in cultured human bronchial 

epithelial cells, which was detected for 10- and 

20-nm but not 200-nm diameter anatase TiO2

[Gurr et al. 2005]. In the absence of photoactivation, an anatase-rutile mixture (50%, 50%; 

diameter = 200 nm) induced higher oxidative 

DNA damage than did the pure anatase or rutile (200 nm each) [Gurr et al. 2005]. Overall, 

these studies indicate that TiO2 exhibits genotoxicity (DNA damage) under certain conditions but not mutagenicity (genetic alteration) 

in the assays used. 
24 NIOSH CIB 63 • Titanium Dioxide

3.1.2 Oxidant Generation and

Cytotoxicity

TiO2 is considered to be of relatively low inherent toxicity, although the crystal phase can 

influence the particle surface properties and 

cytotoxicity in vitro. Sayes et al. [2006] reported that nano-anatase produced more ROS 

and was more cytotoxic than nano-rutile, but 

only after UV irradiation. ROS generation by 

cells in vitro (mouse BV2 microglia) treated 

with P25 ultrafine TiO2 (70% anatase and 30% 

rutile) was suggested as the mechanism for 

damaging neurons in complex grain cell cultures [Long et al. 2007]. In contrast, Xia et al. 

[2006] reported that TiO2 (80% anatase, 20% 

rutile; ~25 nm diameter) did not induce oxidative stress or increase the heme oxygenase 1 

(HO-1) expression in phagocytic cells (RAW 

264.7), which the authors suggested may be 

due to passivation of the particle surfaces by 

culture medium components or neutralization 

by available antioxidants. In a study comparing 

in vitro cellular responses to P25 ultrafine TiO2

(21 nm particle size) and fine TiO2 (1 μm particle size) at exposure concentrations of 0.5–200 

μg/ml, the generation of ROS was significantly 

elevated relative to controls after 4-hr exposure to either fine or ultrafine TiO2

, although 

the ROS induced by ultrafine TiO2 was greater 

than that of fine TiO2 at each exposure concentration; no cytotoxicity was observed 24 hours 

after treatment at any of these doses [Kang et 

al. 2008]. Thus, photoactivation appears to be an 

important mechanism for increasing cytotoxicity of TiO2

, especially for formulations containing anatase. However, TiO2

 cytotoxicity is low 

relative to more inherently cytotoxic particles 

such as crystalline silica [Duffin et al. 2007]. 

3.1.3 Effects on Phagocytosis

Renwick et al. [2001] reported that both fine 

and ultrafine TiO2 particles (250 and 29 nm 

mean diameter; 6.6 and 50 m2/g surface area, respectively) reduced the ability of J774.2 mouse 

alveolar macrophages to phagocytose 2 μm 

latex beads at doses of 0.39 and 0.78 μg/mm2. 

Ultrafine particles (TiO2 and carbon black) impaired macrophage phagocytosis at lower mass 

doses than did their fine particle counterparts, 

although this effect was primarily seen with ultrafine carbon black (254 m2

/g surface area). 

Möller et al. [2002] found that ultrafine TiO2

(20 nm diameter), but not fine TiO2 (220 nm 

diameter), significantly retarded relaxation 

and increased cytoskeletal stiffness in mouse 

alveolar macrophages (J774A.1 cell line) at a 

dose of 320 μg/ml. Ultrafine TiO2 inhibited 

proliferation in J774A.1 cells to a greater extent than did fine TiO2

 (100 μg/ml dose; 50% 

and 90% of control proliferation, respectively). 

In primary alveolar macrophages (BD-AM, 

isolated from beagle dogs by bronchoalveolar 

lavage [BAL*]), either fine or ultrafine TiO2

(100 μg/ml dose) caused moderate retardation 

of relaxation. Neither fine nor ultrafine TiO2

caused impaired phagocytosis of latex micropheres in BD-AM cells, but both particle sizes 

significantly impaired phagocytosis in J774A.1 

cells (~65% of control level by fine or ultrafine 

TiO2). Ultrafine TiO2 reduced the fraction of 

viable cells (either J774A.1 or BD-AM) to a 

greater extent than did fine TiO2 (at 100 μg/ml 

dose). 

These in vitro studies provide mechanistic information about how particle-macrophage 

interactions may influence cell function and 

disease processes in vivo. Overall, ultrafine 

TiO2 impairs alveolar macrophage function 

to a greater extent than does fine TiO2, which 

*Bronchoalveolar lavage (BAL) is a procedure for 

washing the lungs to obtain BAL fluid (BALF), which 

contains cellular and biochemical indicators of lung 

health and disease status.
NIOSH CIB 63 • Titanium Dioxide 25

may also relate to the greater inflammatory response to ultrafine TiO2

 at a given mass dose. 

3.2 In Vivo Studies in Rodent

Lungs

3.2.1 Intratracheal Instillation

3.2.1.1 Short-term follow-up

Studies with male Fischer 344 rats instilled 

with 500 μg of TiO2 of four different particle 

sizes and two crystal structures (rutile: 12 and 

230 nm; anatase: 21 and 250 nm) indicate 

that the ultrafine TiO2 particles (~20 nm) are 

translocated to the lung interstitium (interstitialized) to a greater extent and cleared from 

the lungs more slowly than fine TiO2 particles 

(~250 nm) [Ferin et al. 1992]. Other intratracheal instillation (IT) studies conducted by the 

same laboratory showed that ultrafine TiO2

particles produced a greater pulmonary inflammation response than an equal mass dose 

of fine TiO2 particles. The greater toxicity of the 

ultrafine particles was related to their larger 

particle surface area dose and their increased 

interstitialization [Oberdörster et al. 1992]. 

In a study of four types of ultrafine particles 

(TiO2, nickel, carbon black, and cobalt; particle 

sizes 14–20 nm), male Wistar rats (aged 10–12 

weeks) were administered IT doses of 125 μg 

of particles, and BAL was performed after 4 or 

18 hours. Ultrafine TiO2 was the least toxic and 

inflammogenic, although it was still significantly greater than the saline-treated controls 

[Dick et al. 2003]. The adverse pulmonary 

responses were associated with the free radical 

activity of the particles, which was very low for 

ultrafine TiO2. 

Höhr et al. [2002] observed that, for the same 

surface area, the inflammatory response (as 

measured by BAL fluid (BALF) markers of 

inflammation) in female Wistar rats to uncoated TiO2

 particles covered with surface 

hydroxyl groups (hydrophilic surface) was 

similar to that of TiO2 particles with surface 

OCH3-groups (hydrophobic surface) replacing 

OH-groups. The IT doses in this study were 1 

or 6 mg fine (180 nm particle size; 10 m2/g 

specific surface area) or ultrafine (20–30 nm; 

40 m2/g specific surface area) TiO2, and BAL 

was performed 16 hours after the IT. 

Ultrafine TiO2 was more damaging than fine 

TiO2 in the lungs of male Wistar rats [Renwick 

et al. 2004]. The particle mean diameters of the 

ultrafine and fine TiO2 were 29 and 250 nm, respectively, and the specific surface areas were 

50 and 6.6 m2/g; the crystal structure was not 

specified [Renwick et al. 2004]. Twenty-four 

hours after instillation of ultrafine or fine 

TiO2, rats treated with ultrafine TiO2, but not 

fine TiO2, had BALF elevations in the percentage of neutrophils (indicating inflammation), 

γ-glutamyl transpeptidase concentration (a 

measure of cell damage), protein concentration (a measure of epithelium permeability), 

and lactate dehydrogenase (LDH) (an indicator of cytotoxicity and cell death) [Renwick et 

al. 2004]. The 125 μg IT dose of either ultrafine or fine TiO2

 did not cause any significant 

adverse lung response in 24 hours. At 500 μg, 

the phagocytic ability of the alveolar macrophages was significantly reduced by exposure 

to particles of either size. The 500 μg IT dose 

of ultrafine TiO2, but not fine TiO2, was associated with an increased sensitivity of alveolar 

macrophages to a chemotactic stimulus, an 

effect which can reduce the macrophage mobility and clearance of particles from the lungs. 

In a study that included both inhalation (see 

Section 3.2.3) and IT exposures to six different 

formulations of fine rutile TiO2 (including uncoated or alumina- or amorphous silica-coated; 

particle size 290–440 nm; specific surface area 
26 NIOSH CIB 63 • Titanium Dioxide

6–28 m2/g), male 8-week old Sprague Dawley 

rats were dosed with 2 or 10 mg/kg by IT. Pulmonary inflammation (measured by polymorphonuclear leukocytes [PMNs] in BALF) was 

statistically significantly elevated at 24 hours in 

the rats administered 10 mg/kg for each of the 

coated or uncoated formulations. The coated 

TiO2 formulations produced higher inflammation than the uncoated TiO2

 [Warheit et al. 

2005]. 

In a study of rutile TiO2 nanorods, inflammation responses were examined in BALF and 

whole blood in Wistar rats 24 hours after an 

IT dose of 1 or 5 mg/kg [Nemmar et al. 2008]. 

At both doses, the neutrophilic inflammation 

in BALF was significantly greater than the vehicle controls. The number of monocytes and 

granulocytes in blood was dose-dependently 

elevated, while the platelets were significantly 

reduced at the higher dose, indicating platelet 

aggregation. 

Mice instilled with 1 mg fine TiO2 (250 nm mean 

diameter) showed no evidence of inflammation 

at 4, 24, or 72 hrs after instillation as assessed by 

inflammatory cells in BALF and expression of a 

variety of inflammatory cytokines in lung tissue 

[Hubbard et al. 2002]. 

Adult male ICR mice (2 months old, 30 g) were 

exposed to ultrafine (nanoscale) TiO2 (rutile, 

21 nm average particle size; specific surface 

area of 50 m2/g) or fine (microscale) TiO2

(180–250 nm diameter; specific surface area 

of 6.5 m2/g) by a single IT dose of either 0.1 or 

0.5 mg per mouse [Chen et al. 2006]. One week 

later, the lungs showed “significant changes in 

morphology and histology” in the mice receiving the 0.1-mg dose of nanoscale TiO2

, including disruption of the alveolar septa and alveolar 

enlargement (indicating emphysema), type II 

pneumocyte proliferation, increased alveolar 

epithelial thickness, and accumulation of particle-laden alveolar macrophages. Nanoscale 

TiO2 elicited a significantly greater increase in 

chemokines associated with pulmonary emphysema and alveolar epithelial cell apoptosis than 

did the microscale TiO2. 

A dose-response relationship was not seen, as 

the adverse effects of the 0.1-mg dose of nanoscale TiO2

 exceeded those of the 0.5-mg 

dose. “No significant pathological changes” 

were observed at either dose of the microscale 

TiO2. 

In summary, these short-term studies show 

that while TiO2 was less toxic than several other 

particle types tested, TiO2 did elicit pulmonary 

inflammation and cell damage at sufficiently 

high surface area doses (i.e., greater response 

to ultrafine TiO2 at a given-mass dose). 

3.2.1.2 Intermediate-term follow-up

Rehn et al. [2003] observed an acute (3-day) inflammatory response to instillation of ultrafine 

TiO2 and found that the response from a single 

instillation decreased over time, returning to 

control levels by 90 days after the instillation. 

The reversibility of the inflammatory response 

to ultrafine TiO2 contrasted with the progressive increase in inflammation over 90 days 

that was seen with crystalline silica (quartz) 

in the same study. This study also compared 

a silanized hydrophobic preparation of ultrafine TiO2

 to an untreated hydrophilic form and 

concluded that alteration of surface properties 

by silanization does not greatly alter the biological response of the lung to ultrafine TiO2

. 

Three recent studies of various types of nanoscale or microscale TiO2

 used a similar experimental design, which involved IT dosing 

of male Crl:CD®(SD):IGS BR rats (approximate-
NIOSH CIB 63 • Titanium Dioxide 27

ly 8 weeks of age; 240–255 g body weight). Instilled particle doses were either 1 or 5 mg/kg, 

and BAL was performed at 24 hours, 1 week, 

1 month, and 3 months after instillation [Warheit et al. 2006a,b; 2007]. Cell proliferation assays and histopathological examination were 

also performed. Min-U-Sil quartz was used as a 

positive control, and phosphate buffered saline 

(PBS) was the instillation vehicle in controls. 

In the first study of two hydrophilic types of 

TiO2 (“R-100” or “Pigment A”), rats were administered IT doses of either 1 or 5 mg/kg of 

either type of TiO2, carbonyl iron, or Min-USil quartz. Primary average particle sizes were 

300 nm, 290 nm, ~1.2 μm, or ~1.5 μm, respectively [Warheit et al. 2006a]. Significantly 

elevated PMNs in BALF was observed for the 

two types of TiO2 or carbonyl iron at 24 hours 

postexposure, but not at the later time points. 

The second study compared two types of nanoscale TiO2

 rods (anatase, 92–233 nm length, 

20–35 nm width; 26.5 m2/g specific surface 

area), nanoscale TiO2 dots (anatase, 5.8–6.1 

nm spheres; 169 m2/g specific surface area), 

and microscale rutile TiO2 (300 nm primary 

particle diameter; 6 m2/g specific surface area) 

[Warheit et al. 2006b]. A statistically significant 

increase in the percentage of PMNs in BALF 

was seen at the 5 mg/kg dose for all three TiO2

materials tested (which was higher in the rats 

administered the nanoscale TiO2) but returned 

to control levels at the 1-week time point. There 

were no statistically significant lung responses 

(inflammation or histopathology) to either the 

fine or the ultrafine TiO2 at either dose (1 or 

5 mg/kg) compared to controls at the 1-week 

to 3-month time points. 

In the third study, comparisons were made of 

the lung inflammation, cytotoxicity, cell proliferation, and histopathological responses of 

two types of ultrafine rutile TiO2, a fine rutile TiO2

, an ultrafine anatase-rutile mixture 

(80%, 20%) TiO2, and Min-U-Sil quartz particles [Warheit et al. 2007]. Although the surface area of these particles varied from 5.8 to 

53 m2/g, the median particle sizes in the PBS 

instillation vehicle were similar (2.1–2.7 μm), 

perhaps due to agglomeration. The pulmonary 

inflammation (% PMNs) of the 5 mg/kg-dose 

group of anatase/rutile TiO2 was statistically 

significantly greater than the PBS controls 24 hrs 

and 1 wk after IT (but not at 1 or 3 months), 

while the ultrafine and fine rutile TiO2 groups 

did not differ significantly from controls at any 

time point. The tracheobronchial epithelial cell 

proliferation (% proliferating cells) at the 5 mg/

kg-dose group of anatase/rutile TiO2 was also 

statistically significantly greater than controls 

24 hrs after IT (but not at the later time points), 

while the ultrafine and fine rutile TiO2 groups 

did not differ significantly from controls at any 

time point. The two ultrafine rutile TiO2 preparations were passivated with amorphous silica 

and alumina coatings to reduce their chemical and photoreactivity to a low level similar 

to that of the fine rutile TiO2, while the ultrafine anatase/rutile TiO2

 was not passivated and 

was also more acidic. The ultrafine anatase/

rutile TiO2 was more chemically reactive in a 

Vitamin C assay measuring oxidation potential. These results suggest that the crystal phase 

and surface properties can influence the lung 

responses to TiO2. In both studies, the Min-USil quartz-instilled rats showed the expected 

persistent inflammation. 

3.2.1.3 Long-term follow-up

In a study of the role of lung phagocytic cells 

on oxidant-derived mutations, rats were dosed

by IT with either 10 or 100 mg/kg of fine TiO2

(anatase; median diameter: 180 nm; surface 

area: 8.8 m2/g) and held for 15 months [Driscoll 
28 NIOSH CIB 63 • Titanium Dioxide

et al. 1997]. Type II cells isolated from the rats 

dosed with 100 mg/kg fine TiO2 exhibited an increased hypoxanthine-guanine phosphoribosyl 

transferase (hprt) mutation frequency, but type 

II cells isolated from rats treated with 10 mg/kg 

fine TiO2 did not. Neutrophil counts were significantly elevated in the BALF isolated from 

rats instilled 15 months earlier with 100 mg/

kg fine TiO2, as well as by 10 or 100 mg/kg of 

α-quartz or carbon black. Hprt mutations could 

be induced in RLE-6TN cells in vitro by cells 

from the BALF isolated from the 100 mg/kg fine 

TiO2-treated rats. The authors concluded that 

the results supported a role for particle-elicited 

macrophages and neutrophils in the in vivo mutagenic effects of particle exposure, possibly mediated by cell-derived oxidants. 

An IT study in hamsters suggested that fine 

TiO2 (97% < 5 μm, including 51% < 0.5 μm) 

may act as a co-carcinogen [Stenbäck et al. 

1976]. When BAP and fine TiO2 (<0.5 μm particle size) were administered by IT to 48 hamsters (male and female Syrian golden hamsters, 

6–7 weeks of age), 16 laryngeal, 18 tracheal, 

and 18 lung tumors developed, compared to 

only 2 laryngeal tumors found in the BAPtreated controls. In hamsters receiving an IT 

dose of 3 mg fine TiO2 only in 0.2 ml saline 

once a week for 15 weeks, no respiratory tract 

tumors were found. The animals were kept until death, which occurred by 80 weeks in treated hamsters and by 120 weeks in controls. 

TiO2 was included in an IT study of 19 different dusts in female SPF Wistar rats [Pott and 

Roller 2005; Mohr et al. 2006]. The types of 

TiO2 tested were ultrafine hydrophilic (P25; 

“majority anatase”; ~0.025 μm mean particle 

size; 52 m2/g specific surface area), ultrafine hydrophobic (coated) (P805; 21 nm particle size; 

32.5 m2/g specific surface area), and small-fine 

anatase (hydrophilic) (200 nm particle size; 

9.9 m2/g specific surface area). Groups of 48 

rats were administered two or three IT doses, 

and then maintained for 26 weeks before terminal sacrifice. The IT doses (number of doses 

× mass per dose) and the corresponding lung 

tumor response (percentages of rats with benign or malignant tumors) included ultrafine 

hydrophilic TiO2 (5 doses of 3 mg, 5 doses of 

6 mg, and 10 doses of 6 mg—52%, 67%, 69% 

tumors, respectively), ultrafine hydrophobic 

(coated) TiO2 (15 doses of 0.5 mg, and 30 doses 

of 0.5 mg—0% and 6.7% tumors, respectively); 

and small-fine anatase TiO2 (10 doses of 6 mg, 

and 20 doses of 6 mg—30% and 64% tumors, 

respectively) [Mohr et al. 2006]. The original 

6-mg dose for hydrophobic coated TiO2 was 

reduced to 0.5 mg because of acute mortality 

at the higher dose. The TiO2 was analyzed with 

the tumor data for the other poorly soluble 

particles (1,002 rats surviving 26 weeks), and 

they found that the dose metric that provided 

the best fit to the tumor data was particle volume and particle size [Pott and Roller 2005; 

Roller and Pott 2006]. Borm et al. [2000] and 

Morfeld et al. [2006] analyzed a subset of these 

data (709 rats) for five different poorly soluble 

particles of different sizes (TiO2 of low and 

high surface area, carbon black, diesel exhaust 

particles, and amorphous silica). Morfeld et al. 

[2006] fit a multivariate Cox model to these 

pooled data (excluding silica) and reported a 

threshold dose of 10 mg and a saturation dose 

of 20 mg for lung tumors. Although ultrafine 

particles were more tumorigenic than fine particles, in their multivariate model no difference 

was seen between particle mass, volume, or 

surface area after accounting for particle type 

and rat survival time; they suggested a high degree of agglomeration in these IT preparations 

may have reduced the effective particle surface 

area relative to that estimated from Brunauer, 

Emmett, and Teller (BET) analysis [Morfeld et 
NIOSH CIB 63 • Titanium Dioxide 29

al. 2006]. Roller [2007] considered the threshold findings to be inconsistent with the statistically significant lung tumor incidences in 

three dose groups that were within the 95% 

confidence interval of the estimated threshold in Morfeld et al. [2006]. All of the analyses 

of these pooled data including TiO2 showed a 

greater tumor potency of the ultrafine versus 

fine particles, whether the dose was expressed 

as particle volume and size or as particle surface area [Borm et al. 2000; Pott and Roller 

2005; Roller and Pott 2006; Morfeld et al. 2006]. 

This was considered to be due to the greater 

translocation of ultrafine particles to the lung 

interstitium [Borm et al. 2000; Pott and Roller 

2005]. Although these IT studies used relatively high-mass doses, increasing dose-response 

relationships were observed for particles of a 

given size; greater tumor responses were also 

observed for the ultrafines compared to fine 

particles at a given mass dose. One study suggests a genotoxic mechanism involving DNA 

damage from oxidants produced by phagocytic 

and inflammatory cells in response to the TiO2

particles in the lungs. 

3.2.2 Acute or Subacute Inhalation

Nurkiewicz et al. [2008] investigated the role of 

particle size in systemic microvascular function 

in male SPF Sprague Dawley rats inhaling either 

fine TiO2 (1 μm; 2.34 m2/g) or P25 ultrafine TiO2

(21 nm; 48.08 m2/g) at airborne exposures aimed 

at achieving similar particle mass deposition in 

the lungs (ultrafine: 1.5–12 mg/m3, 240–720 min; 

fine: 3–15 mg/m3, 240–480 min). No evidence 

of pulmonary inflammation or lung damage 

(based on BALF markers) was observed at 

these exposures. However, 24 hours after exposure, the arteriolar vasodilation response 

(to Ca2+ intraluminal infusion of the spinotrapezius muscle) was found to be significantly impaired in rats exposed to ultrafine TiO2

compared to either the control rats or the rats 

exposed to fine TiO2 with the same retained 

mass dose in the lungs. On an equivalent mass 

basis, ultrafine TiO2 was approximately an order of magnitude more potent than fine TiO2

in causing systemic microvascular dysfunction. When converted to surface area dose, the 

potency of the fine TiO2 was greater, which the 

authors suggested was due to overestimation of 

ultrafine particle surface area delivered to the 

lungs due to agglomeration. Either fine or ultrafine TiO2

 caused systemic microvessel dysfunction at inhalation doses that did not cause 

marked lung inflammation. This effect was 

related to the adherence of PMNs to the microvessel walls and production of ROS in the 

microvessels. This study indicates that cardiovascular effects may occur at particle exposure 

concentrations below those causing adverse 

pulmonary effects. 

Rats (male WKY/Kyo@Rj, 246–316 g body 

weight) were exposed by inhalation (endotracheal intubation) to ultrafine TiO2

 (20 nm 

count median diameter) at an airborne mass 

concentration of approximately 0.1 mg/m3

(7.2 × 106 mean number concentration) for 1 

hour [Geiser et al. 2008]. BAL was performed 

either 1 or 24 hours after the inhalation exposure. Elemental microanalysis of the particles 

provided evidence that the alveolar macrophages did not efficiently phagocytize the particles; rather particle uptake was “sporadic” 

and “unspecific.” 

Mice (C57Bl/6 male, 6 weeks of age) were 

exposed by whole-body inhalation to TiO2

nanoparticles (2–5 nm primary particle size; 

210 m2/g specific surface area) for either 4 hours 

(acute) or 4 hr/day for 10 days (subacute) [Grassian et al. 2007]. Airborne TiO2

 concentrations 

were 0.77 or 7.22 mg/m3 for the acute exposure 

and 8.88 mg/m3 for the subacute exposures. In 
30 NIOSH CIB 63 • Titanium Dioxide

the subacute study, groups of mice were necropsied at the end of the exposure period and 

at 1, 2, and 3 weeks postexposure. No adverse 

effects were observed after the 4-hour exposure. A “significant but modest” inflammatory 

response was observed in the mice at 0, 1, or 

2 weeks after the subacute exposures, with recovery at the 3rd week postexposure (the number of alveolar macrophages in BALF was statistically significantly greater than controls in 

the 1- and 2-week postexposure groups). 

3.2.3 Short-Term Inhalation

Short-term exposure to respirable fine TiO2 has 

been shown to result in particle accumulation in 

the lungs of rodents inhaling relatively high particle concentrations. The pulmonary retention 

of these particles increased as exposure concentrations increased. In one study, after 4 weeks of 

exposure to 5 mg/m3, 50 mg/m3, and 250 mg/

m3, the fine TiO2 retention half-life in the lung 

increased (~68 days, ~110 days, and ~330 days, 

respectively) [Warheit et al. 1997], which indicates overloading of alveolar macrophage-mediated clearance of particles from the lungs.

In multiple studies, the most frequently noted 

change after 1 to 4 weeks of fine TiO2 inhalation was the appearance of macrophages laden 

with particles, which were principally localized to the alveoli, bronchus-associated lymphoid tissue, and lung-associated lymph nodes 

[Driscoll et al. 1991; Warheit et al. 1997; Huang 

et al. 2001]. Particle-laden macrophages increased in number with increasing intensity of 

exposure and decreased in number after cessation of exposure [Warheit et al. 1997]. Alveolar 

macrophages from rats inhaling 250 mg/m3

fine TiO2 for 4 weeks also appeared to be functionally impaired as demonstrated by persistently diminished chemotactic and phagocytic 

capacity [Warheit et al. 1997]. 

Inflammation in the lungs of fine rutile TiO2-

exposed rats was dependent upon exposure 

concentration and duration. Rats exposed to 

250 mg/m3 fine TiO2 6 hr/day, 5 days/week 

for 4 weeks had markedly increased numbers 

of granulocytes in BALF [Warheit et al. 1997]. 

The granulocytic response was muted after recovery, but numbers did not approach control 

values until 6 months after exposures ceased. 

Rats exposed to 50 mg/m3 fine TiO2 6 hr/day, 

5 days/wk for 4 weeks had a small but significantly increased number of granulocytes in 

the BALF that returned to control levels at 

3 months after exposures ceased [Warheit et 

al. 1997]. This study showed that high concentrations of poorly soluble, low toxicity (PSLT) 

dust (e.g., fine TiO2 and carbonyl iron) caused 

impaired pulmonary clearance and persistent 

inflammation in rats.

In a study of male Fischer 344 rats, inhaling 

22.3 mg/m3 of ultrafine (20 nm particle size, 

anatase) TiO2 6 hr/day, 5 days/wk for up to 

12 weeks, the antioxidant enzyme manganesecontaining superoxide dismutase (MnSOD) 

expression in the lungs increased dramatically 

and was correlated with pulmonary inflammation indicators [Janssen et al. 1994]. Fine TiO2

(23.5 mg/m3 of 250 nm particle size, anatase) 

did not produce this response at the exposure 

conditions in this study. Follow-up observation of the rats in this study showed that the inflammatory lesions “regressed” during a 1-year 

period following cessation of exposure [Baggs 

et al. 1997]. This observation suggests that the 

inflammatory response from short-term exposures to TiO2

 may be reversible to some degree, 

if there is a cessation of exposure.

In a separate study, rats exposed to airborne 

concentrations of 50 mg/m3 fine TiO2 7 hr/day, 

5 days/week for 75 days had significantly elevated neutrophil numbers, LDH (a measure of 
NIOSH CIB 63 • Titanium Dioxide 31

cell injury) concentration, and n-acetylglucosaminidase (a measure of inflammation) concentration in BALF [Donaldson et al. 1990]. 

However, in this study the BALF of rats inhaling 10 mg/m3

 or 50 mg/m3 fine TiO2 7 hr/day, 

5 days/week for 2 to 32 days had PMN numbers, macrophage numbers, and LDH concentrations that were indistinguishable from control values [Donaldson et al. 1990]. 

Rats exposed to airborne concentrations of

51 mg/m3 fine TiO2 (1.0 μm mass median 

aerodynamic diameter [MMAD]) 6 hr/day for 

5 days (whole body exposures) had no significant changes in BALF neutrophil number, 

macrophage number, lymphocyte number, 

LDH concentration, n-acetylglucosaminidase 

concentration, or measures of macrophage activation 1 to 9 weeks after exposure [Driscoll 

et al. 1991]. The TiO2 lung burden at the end 

of exposure was 1.8 mg/lung, and the retention was 39% 28 days after the end of exposure. 

Similarly, rats exposed to 0.1, 1, or 10 mg/m3 6 h/

day, 5 days/week for 4 weeks had no evidence 

of lung injury as assessed by BAL 1 week to 

6 months after exposure or by histopathology 6 months after exposure [Henderson et 

al. 1995]. 

Pulmonary responses to six different formulations of fine rutile TiO2

 (including uncoated 

or alumina- or amorphous silica-coated; particle size 290–440 nm; specific surface area 

6–28 m2/g) were investigated in male, 8-week 

old Sprague Dawley rats, with both IT (see 

Section 3.2.1.1) and inhalation exposures. Rats 

were exposed to very high airborne concentrations (1130–1310 mg/m3

) of the different 

formulations of fine TiO2 for 30 days (6 hr/

day, 5 days/week). The pulmonary inflammation response (assessed by histopathology) 

remained significantly elevated 1 month after 

exposure to the TiO2 coated with alumina (7%) 

and amorphous silica (8%). The coated TiO2

formulations produced higher inflammation 

than the uncoated TiO2 in the inhalation study 

(as in the IT study) [Warheit et al. 2005]. 

3.2.4 Subchronic Inhalation

In a study of two fine-sized, PSLT particles—

TiO2 and barium sulfate (BaSO4)—no-observed 

-adverse-effect levels (NOAELs) were estimated based on the relationship between the 

particle surface area dose, overloading of lung 

clearance, and neutrophilic inflammation in 

rats [Tran et al. 1999]. These two PSLT particles had similar densities (4.25 and 4.50 g/cm3

, 

respectively) but different particle sizes (2.1 

and 4.3 μm MMAD); since both of these factors influence particle deposition in the lungs, 

the exposure concentrations were adjusted to 

provide similar particle mass deposition in the 

lungs. Male Wistar rats (age 12 weeks, specificpathogen free) were exposed by whole body 

inhalation (7 hr/day, 5 days/wk) to either 

25 mg/m3 for 7.5 months (209 calendar days) 

or to 50 mg/m3 for 4 months (118 calendar 

days). The findings showed that the retardation 

of alveolar macrophage-mediated clearance, 

particle transfer to the lung-associated lymph 

nodes, and influx of PMNs were related to the 

lung burden as particle surface area dose. A 

mean airborne concentration of 3 mg/m3 finesized TiO2

 was estimated as the NOAEL, which 

was defined as a 95% probability that the lung 

responses would be below those predicted using 

the “no overload level” for the average animal.

The relationship between subchronic inhalation of TiO2

 and neutrophilic inflammation was 

also investigated in a study of Montserrat volcanic ash [Cullen et al. 2002]. Male Wistar rats 

(225 g, specific-pathogen free) were exposed 

by nose-only inhalation (6 hr/day, 5 days/wk) 

to 140 mg/m3 of TiO2 (1.2 μm MMAD) for up 
32 NIOSH CIB 63 • Titanium Dioxide

to 2 months. The concentration of ash (5.3 μm 

MMAD) was 253 mg/m3, which was predicted 

to provide the same retained lung burden as 

the TiO2. After the 8-week exposure, the histopathological examination showed relatively 

minor pathology changes in the lungs of rats 

treated with TiO2, although the pathological 

changes in the lungs of the ash-exposed rats 

were generally “more marked.” The pulmonary 

inflammation response was also greater in the 

ash-exposed rats, based on the percentage of 

PMNs in the BALF, which ranged from 19%–

53% PMNs for the ash and from 0.2%–16% for 

TiO2 at 14–56 days of exposure. 

Fine TiO2 (pigmentary; 1.4 μm MMAD) was 

studied in female rats (CDF[F344]/CrlBR), mice (B3C3F1/CrlBR), and hamsters 

[Lak:LVG(SYR)BR], 6 wk old, after a 13-week 

inhalation exposure to 10, 50, or 250 mg/m3, 

followed by up to 52 weeks without TiO2 exposure [Bermudez et al. 2002; Everitt et al. 2000]. 

Retained particle burdens in the lungs and 

lung-associated lymph nodes were measured 

at the end of exposure and at 4, 13, 26, and 52 

weeks postexposure. The lung particle retention patterns indicated that after the 13 weeks 

of exposure to 50 or 250 mg/m3, clearance 

overload had occurred in rats and mice but 

not in hamsters. In mice and rats, the numbers 

of macrophages and the percentage of neutrophils were significantly increased in BALF after 

13 weeks of exposure to 50 or 250 mg/m3 fine 

TiO2 and remained elevated through the 52-

week postexposure. These BALF cell responses 

were significantly elevated in hamsters after 

exposure to 250 mg/m3 but were no longer significantly elevated by 26 weeks postexposure. 

Alveolar cell proliferation was significantly elevated only at 52-week postexposure in the rats 

[Bermudez et al. 2002]. Histopathology showed 

alveolar hypertrophy and hyperplasia of type II 

epithelial cells in rats after the 13-week exposure 

to 50 or 250 mg/m3 fine TiO2. In mice, alveolar type II cell hypertrophy was observed (dose 

not given), while in hamsters minimal type II 

epithelial cell hypertropy and hyperplasia were 

observed at 50 or 250 mg/m3. Foci of alveolar 

epithelial cell hypertrophy and hyperplasia 

were seen to be often associated with aggregates of particle-laden alveolar macrophages 

in all three species. In rats, but not mice and 

hamsters, these foci of alveolar epithelial hypertrophy became increasingly more prominent with time, even after cessation of exposure, and at the high dose, rats progressed to 

bronchiolization of alveoli (metaplasia) and to 

fibrotic changes with focal interstitialization 

of TiO2 particles. Alveolar lipoproteinosis and 

cholesterol clefts were observed in rats 52 weeks 

after the 13-week exposure to 250 mg/m3. Although “high particle burdens” were associated with “proliferative epithelial changes associated with particle-induced inflammation” 

in all three species, only rats developed metaplastic and fibrotic lesions [Everitt et al. 2000; 

Bermudez et al. 2002]. 

P25 ultrafine TiO2 (21 nm primary particle 

size; 1.37 MMAD) was studied in female rats, 

mice, and hamsters after a 13-week inhalation 

exposure to 0.5, 2, or 10 mg/m3, followed by a 

recovery period (without TiO2 exposure) for 

up to 52 weeks [Bermudez et al. 2004] (same 

rodent strains as Bermudez et al. 2002). Pulmonary responses and retained particle burdens in the lungs and lung-associated lymph 

nodes were measured at the end of exposure 

and at 4, 13, 26, and 52 weeks postexposure. 

Retardation of pulmonary clearance following exposure to 10 mg/m3

 was observed in rats 

and mice but not in hamsters. Pulmonary inflammation was also observed at the 10 mg/m3

dose in rats and mice but not in hamsters. The 

total number of cells in BALF was significantly 
NIOSH CIB 63 • Titanium Dioxide 33

elevated in rats and mice after 13 weeks of exposure to 10 mg/m3

, and the percentages of 

neutrophils and macrophages remained statistically significantly elevated through 52 weeks 

postexposure in both species. The BALF cell 

responses were not statistically significant following exposure to 0.5 or 2 mg/m3

 in mice or 

rats (except for significantly elevated neutrophils in rats immediately after the 13-wk exposure to 2 mg/m3

) (Tables S1–S3 in Bermudez et 

al. 2004). BALF cell responses were not significantly elevated in hamsters at any exposure or 

observation time. 

The alveolar cell replication index was statistically significantly increased at 0, 4, and 13 

weeks after the 13-wk exposure to 10 mg/m3

ultrafine TiO2 in rats, while in mice it was significantly increased at 13 and 26 weeks after 

exposure cessation [Bermudez et al. 2004]. In 

rats inhaling 10 mg/m3, the histopathologic 

responses included epithelial and fibroproliferative changes, interstitial particle accumulation, and alveolar septal fibrosis. Although 

most of the epithelial proliferative lesions had 

regressed postexposure, some remaining lesions were believed to have progressed. At 52 

weeks postexposure, minimal to mild metaplastic changes and minimal to mild particleinduced alveolar septal fibroplasia were seen 

in rats. In mice inhaling 10 mg/m3, lesions 

were described as aggregations of heavily particle laden macrophages in the centriacinar 

sites. During postexposure, these cell aggregates were observed to move to the interstitial 

areas over time. No epithelial, metaplastic, 

and fibroproliferative changes were observed 

by histopathology in the mice or hamsters 

(although the mice had significantly elevated 

alveolar cell replication at 13 and 26 weeks 

postexposure in the 10 mg/m3 dose group, this 

response apparently did not result in histopathologically visible changes). The absence of 

adverse pulmonary responses in hamsters was 

considered to reflect their rapid clearance of 

particles from the lungs. 

3.2.5 Chronic Inhalation

TiO2 has been investigated in three chronic inhalation studies in rats, including fine TiO2

 in 

Lee et al. [1985] and Muhle et al. [1991] and 

ultrafine TiO2 in Heinrich et al. [1995]. These 

studies were also reported in other publications, including Lee et al. [1986a], Muhle et al. 

[1989, 1994], and Bellmann et al. [1991]. In another 2-year rat inhalation study, an increase in 

lung tumors was found in rats exposed to TiCl4

[Lee et al. 1986b]. TiCl4 is an intermediate in 

the production of pigment TiO2, including by 

hydrolysis of TiCl4 to produce TiO2 and HCl. 

However, TiCl4 is a highly volatile compound 

with different properties than TiO2, and thus it 

is not addressed further in this document. 

In Lee et al. [1985], groups of 100 male and 

100 female rats (CD, Sprague-Dawley derived; 

strain not specified) were exposed by whole 

body inhalation to fine rutile TiO2 (1.5–1.7 μm 

MMAD) for 6 hr/day, 5 days/week, for up to 

two years, to 10, 50, or 250 mg/m3 (84% respirable; < 13 μm MMAD). A fourth group (control) was exposed to air. In each group, twenty 

rats were killed at 3, 6, or 12 months; 80 rats 

were exposed for 2 years, and all surviving rats 

were killed at the end of exposure. No increase 

in lung tumors was observed at 10 or 50 mg/m3. 

At 250 mg/m3, bronchioalveolar adenomas 

were observed in 12 out of 77 male rats and 

13 out of 74 female rats. In addition, squamous 

cell carcinomas were reported in 1 male and 13 

females at 250 mg/m3. The squamous cell carcinomas were noted as being dermoid, cyst-like 

squamous cell carcinomas [Lee et al. 1985], later reclassified as proliferative keratin cysts [Carlton 1994], and later still as a continuum ranging 
34 NIOSH CIB 63 • Titanium Dioxide

from pulmonary keratinizing cysts through 

pulmonary keratinizing eptheliomas to frank 

pulmonary squamous carcinomas [Boorman 

et al. 1996]. A recent reanalysis of the 16 tumors originally classified as cystic keratinizing 

squamous cell carcinomas in Lee et al. [1985] 

had a similar interpretation: two were reclassified as squamous metaplasia, one as a poorly 

keratinizing squamous cell carcinoma, and 

13 as nonneoplastic pulmonary keratin cysts 

[Warheit and Frame 2006]. 

In both the Muhle et al. [1991] and Heinrich 

et al. [1995] studies, TiO2 was used as a negative control in 2-year chronic inhalation studies of toner and diesel exhaust, respectively. In 

Muhle et al. [1991], the airborne concentration 

of TiO2 (rutile) was 5 mg/m3 (78% respirable, 

according to 1984 ACGIH criterion). Male and 

female Fischer 344 rats were exposed for up to 

24 months by whole body inhalation and sacrificed beginning at 25.5 months. No increase 

in lung tumors was observed in TiO2-exposed 

animals; the lung tumor incidence was 2/100 

in TiO2-exposed animals versus 3/100 in nonexposed controls. 

In the Heinrich et al. [1995] study, 100 female 

Wistar rats were exposed to ultrafine TiO2

(80% anatase, 20% rutile; 15–40 nm primary 

particle size; 0.8 mm MMAD; 48 (± 2.0) m2/g 

specific surface area) at an average of approximately 10 mg/m3

, 18 h/day 5d/wk, for up to 

24 months (actual concentrations were 7.2 

mg/m3 for 4 months, followed by 14.8 mg/m3

for 4 months, and 9.4 mg/m3 for 16 months). 

Following the 2-year exposure, the rats were 

held without TiO2 exposure for 6 months 

[Heinrich et al. 1995]. At 6 months of exposure, 99/100 of the rats had developed bronchioloalveolar hyperplasia, and by 2 years 

all rats had developed slight to moderate interstitial fibrosis [Heinrich et al. 1995]. After 

24 months of exposure, four of the nine rats 

examined had developed tumors (including 

a total of two squamous cell carcinomas, one 

adenocarcinoma, and two benign squamous 

cell tumors). At 30 months (6 months after 

the end of exposure), a statistically significant 

increase in adenocarcinomas was observed 

(13 adenocarcinomas, in addition to 3 squamous cell carcinomas and 4 adenomas, in 100 

rats). In addition, 20 rats had benign keratinizing cystic squamous-cell tumors. Only 1 adenocarcinoma, and no other lung tumors, was 

observed in 217 nonexposed control rats. 

NMRI mice were also exposed to ultrafine TiO2

in Heinrich et al. [1995]. The lifespan of NMRI 

mice was significantly decreased by inhaling 

approximately 10 mg/m3 ultrafine TiO2, 18 hr/ 

day for 13.5 months [Heinrich et al. 1995]. 

This exposure did not produce an elevated tumor response in the NMRI mice, but the 30% 

lung tumor prevalence in controls may have 

decreased the sensitivity for detecting carcinogenic effects in this assay. In a study of several 

types of particles, 100 female Wistar (Crl:[WI] 

Br) rats were exposed to 10.4 mg/m3 TiO2, 

18 hr/day 5 days/wk, for 24 months (followed 

by 6 months in clean air). No information was 

provided on the particle size or crystal structure of TiO2

 used in this study. Cystic keratizing epitheliomas were observed in 16% of 

the rats. In addition, 3.0% cystic keratinizing

squamous-cell carcinomas and 1% nonkeratinizing squamous-cell carcinoma were observed [Rittinghausen et al.1997]. 

The primary data used in the dose-response 

model in the TiO2 risk assessment (Chapter 4) 

include the ultrafine TiO2 data of Heinrich et al. 

[1995] and the fine TiO2 data of Lee et al. [1985]. 

Differences in postexposure follow-up (24 and 

30 months, respectively, in Lee et al. [1985] 

and Heinrich et al. [1995]) may have increased 
NIOSH CIB 63 • Titanium Dioxide 35

the likelihood of detecting lung tumors in the 

ultrafine TiO2-exposed rats. However, the differences in the hours of exposure per day (6 

and 18 hrs, respectively, in Lee et al. [1985] and 

Heinrich et al. [1995])) were accounted for in 

the risk assessment models since the retained 

particle lung burden (at the end of 2-year inhalation exposure) was the dose metric used in 

those models. 

In summary, the chronic inhalation studies 

in rodents show dose-related pulmonary responses to fine or ultrafine TiO2

. At sufficiently 

high particle mass or surface area dose, the responses in rats include reduced lung clearance 

and increased particle retention (“overload”), 

pulmonary inflammation, oxidative stress, tissue damage, fibrosis, and lung cancer. Studies 

in mice showed impaired lung clearance and 

inflammation, but not fibrosis or lung cancer. 

Studies in hamsters found little adverse effect 

of TiO2 on either lung clearance or response. 

3.3 In Vivo Studies: Other

Routes of Exposure 

3.3.1 Acute Oral Administration

The acute toxicity of nanometer (25 and 80 nm) 

and submicron (155 nm) TiO2 was investigated 

after a large single dose of TiO2 (5 g/kg body 

weight) in male and female CD-I mice [Wang 

et al. 2007b]. The TiO2 was retained in liver, 

spleen, kidney, and lung tissues, indicating uptake by the gastrointestinal tract and systemic 

transport to the other tissues. No acute toxicity 

was observed. However, statistically significant 

changes were seen in several serum biochemical 

parameters, including LDH and alpha-hydroxybutyrate (suggesting cardiovascular damage), 

which were higher in mice treated with either 

25 and 80 nm nanoscale TiO2 compared to fine 

TiO2. Pathological evidence of hepatic injury 

and kidney damage was also observed. In female mice treated with the 80 nm nanoscale 

and the fine particles, the liver injury included 

hydropic degeneration around the central vein 

and spotty necrosis of hepatocytes; renal damage included protein-filled liquid in the renal 

tubule and swelling of the renal glomerulus. 

The tissue injury markers did not always relate to particle size; damage was often but not 

always greater in the mice treated with the 

nanoscale particles compared to the fine particles, and the 80-nm TiO2

 was more damaging than the 25-nm TiO2

 by some indicators. 

Given the single large dose in this study, it was 

not possible to evaluate the dose-response relationship or to determine whether the effects 

were specific to the high dose. 

3.3.2 Chronic Oral Administration

The National Cancer Institute conducted a bioassay of TiO2

 for possible carcinogenicity by 

the oral route. TiO2 was administered in feed 

to Fischer 344 rats and B6C3F1 mice. Groups of 

50 rats and 50 mice of each sex were fed either 

25,000 or 50,000 parts per million TiO2 (2.5% 

or 5%) for 103 weeks and then observed for an 

additional week. In the female rats, C-cell adenomas or carcinomas of the thyroid occurred at 

an incidence of 1 out of 48 in the control group, 

0 out of 47 in the low-dose group, and 6 out 

of 44 in the high-dose group. It should also be 

noted that a similar incidence of C-cell adenomas or carcinomas of the thyroid, as observed 

in the high-dose group of the TiO2 feeding 

study, has been seen in control female Fischer 

344 rats used in other studies. No significant 

excess tumors occurred in male or female mice 

or in male rats. It was concluded that, under 

the conditions of this bioassay, TiO2 is not carcinogenic by the oral route for Fischer 344 rats 

or B6C3F1 mice [NCI 1979]. 
36 NIOSH CIB 63 • Titanium Dioxide

In a study of male and female Fischer 344 rats 

fed diets containing up to 5% TiO2-coated mica 

for up to 130 weeks, no treatment-related toxicologic or carcinogenic effects were reported 

[Bernard et al. 1990]. 

3.3.3 Intraperitoneal Injection

Female Wistar rats received intraperitoneal injections of P25 ultrafine anatase TiO2

 (2 ml 

0.9% NaCl solution) of either (1) a total dose 

of 90 mg per animal (once per week for five 

weeks); (2) a single injection of 5 mg; or (3) a 

series of injections of 2, 4, and 4 mg at weekly 

intervals [Pott et al. 1987]. Controls received 

a single injection of saline alone. The average 

lifespan of rats in the three treatment groups 

were 120, 102, and 130, respectively, and 120 

weeks for controls. In the first treatment group, 

6 out of 113 rats developed sarcomas, mesotheliomas, or abdominal cavity carcinomas, compared to 2 carcinomas in controls. No intraabdominal tumors were found in the other two 

treatment groups. This study may not be relevant to inhalation exposure to TiO2

 since studies have not shown that either fine or ultrafine 

TiO2 would be likely to reach the peritoneum 

after depositing in the lungs. 

3.4 Particle-Associated Lung

Disease Mechanisms 

3.4.1 Role of Pulmonary

Inflammation

Chronic pulmonary inflammation is characterized by persistent elevation of the number of 

PMNs (measured in BALF) or by an increased 

number of inflammatory cells in interstitial 

lung tissue (observed by histopathology). Pulmonary inflammation is a defense mechanism 

against foreign material in the lungs. PMNs 

are recruited from the pulmonary vasculature 

in response to chemotactic stimuli (cytokines) 

generated by lung cells including alveolar macrophages, which patrol the lungs as part of the 

normal lung defense to phagocytose and clear 

foreign material [Lehnert 1993; Snipes 1996]. 

Additional alveolar macrophages are recruited 

from blood monocytes into the lung alveoli in 

response to particle deposition. Typically, the 

PMN response is short-lived but may become 

chronic in response to persistent stimuli, e.g., 

prolonged particle exposure [Lehnert 1993]. 

Particle-induced pulmonary inflammation, 

oxidative stress, lung tissue damage, and epithelial cell proliferation are considered to be 

the key steps leading to lung tumor development in the rat acting through a secondary 

genotoxic mechanism [Knaapen et al. 2004; 

Baan 2007]. Oxidative stress is considered 

the underlying mechanism of the proliferative and genotoxic responses to poorly soluble particles including TiO2

 and other PSLT 

[Donaldson et al. 1996; Shi et al. 1998; Vallyathan et al. 1998; Knaapen et al. 2002; Donaldson and Stone 2003]. Reactive oxygen or 

nitrogen species (ROS/RNS) are released by 

inflammatory cells (macrophages and PMNs) 

and/or by particle reactive surfaces. Oxidative stress results from an imbalance of the 

damaging oxidants and the protective antioxidants. Oxidants can damage the lung epithelial tissue and may also induce genetic damage in proliferating epithelial cells, increasing 

the probability of neoplastic transformation 

[Driscoll et al. 1997]. The mechanisms linking 

pulmonary inflammation with lung cancer 

may involve (1) induction of hprt mutations 

in DNA of lung epithelial cells by inflammatory cell-derived oxidants and increased cell 

proliferation [Driscoll et al. 1996], and (2) 

inhibition of the nucleotide excision repair 

of DNA (with adducts from other exposures 
NIOSH CIB 63 • Titanium Dioxide 37

such as polycyclic aromatic hydrocarbons) in 

the lung epithelial cells [Güngör et al. 2007]. 

Both of these mechanisms involve induction of 

cell-derived inflammatory mediators, without 

requiring particle surface-generated oxidants. 

A secondary genotoxic mechanism would, in 

theory, involve a threshold dose that triggers 

inflammation and overwhelms the body’s antioxidant and DNA repair mechanisms [Greim 

and Ziegler-Skylakakis 2007]. Antioxidant defense responses vary across species, and interindividual variability is generally greater in humans than in laboratory animals [Slade et al. 

1985, 1993]. Limited quantitative data in humans make the identification of a populationbased threshold difficult in practice [Bailer et 

al. 1997], and the quantitative aspects of the 

inflammatory response (level and duration) 

that are sufficient to cause a high probability 

of lung tumor development are not known. 

While there is a clear association between inflammation and genotoxicity, the specific linkages between key cellular processes such as 

cell cycle arrest, DNA repair, proliferation, and 

apoptosis are not well understood [Schins and 

Knaapen 2007]. 

Thus, chronic pulmonary inflammation appears to be required in the development of 

lung tumors in rats following chronic inhalation exposure to TiO2

; i.e., acting through a 

secondary genotoxic mechanism involving 

oxidative DNA damage. An implication of this 

mechanism is that maintaining exposures below those causing inflammation would also 

prevent tumor development, although the distribution of inflammatory responses in human 

populations is not known and a direct genotoxic mechanism cannot be ruled out for discrete nanoscale TiO2

 (see Section 3.5.2.1). 

3.4.2 Dose Metric and Surface

Properties

High mass or volume dose of PSLT fine particles in the lungs has been associated with 

overloading, while ultrafine particles impair 

lung clearance at lower mass or volume doses [Bellmann et al. 1991; Morrow et al. 1991; 

Oberdörster et al. 1994b; Heinrich et al. 1995]. 

The increased lung retention and inflammatory response of ultrafine PSLT particles compared to fine PSLT particles correlates better to 

the particle surface area dose [Tran et al. 2000]. 

Some evidence suggests that reduced lung 

clearance of ultrafine particles may involve 

mechanisms other than high-dose overloading, such as altered alveolar macrophage function (phagocytosis or chemotaxis) [Renwick et 

al. 2001, 2004] and greater ability to enter the 

lung interstitium [Ferin et al. 1992; Adamson 

and Bowden 1981; Oberdörster et al. 1994b; 

Adamson and Hedgecock 1995]. 

The quantitative relationships between the 

particle dose (expressed as mass or surface 

area) and the pulmonary responses of inflammation or lung tumors can be determined 

from the results of subchronic or chronic inhalation studies in rats. When the rat lung dose 

is expressed as particle mass, several different 

dose-response relationships are observed for 

pulmonary inflammation following subchronic inhalation of various types of poorly soluble 

particles (Figure 3–1). However, when dose is 

converted to particle surface area, the different 

types (TiO2 and BaSO4) and sizes (ultrafine and 

fine TiO2) of PSLT particles can be described 

by the same dose-response curve (Figure 3–2), 

while crystalline silica, SiO2 (a high-toxicity 

particle) demonstrates a more inflammogenic 

response when compared to PSLT particles of 

a given mass or surface area dose. 
38 NIOSH CIB 63 • Titanium Dioxide

Figure 3–1. Pulmonary inflammation (PMN count) of high toxicity dust (crystalline silica) particles compared to low toxicity dust (TiO2

 and BaSO4) of both fine and ultrafine size, based on 

particle mass dose in rat lungs; Particle size: F (fine), UF (ultrafine)

Data source: Porter et al. [2001], Oberdörster et al. [1994a], Tran et al. [1999] 

Figure 3–2. Pulmonary inflammation (PMN count) of high toxicity dust (crystalline silica) particles compared to low toxicity dust (TiO2

 and BaSO4) of both fine and ultrafine size, based on 

particle surface area dose in rat lungs; Particle size: F (fine), UF (ultrafine)

Data source: Porter et al. [2001], Oberdörster et al. [1994a], Tran et al. [1999]
NIOSH CIB 63 • Titanium Dioxide 39

Similarly, the rat lung tumor response to various types and sizes of respirable particles (including fine and ultrafine TiO2

, toner, coal dust, 

diesel exhaust particulate, carbon black, and 

talc) has been associated with the total particle 

surface area dose in the lungs (Figure 3–3). 

This relationship, shown by Oberdörster and 

Yu [1990], was extended by Driscoll [1996] to 

include results from subsequent chronic inhalation studies in rats exposed to PSLT particles 

and by Miller [1999] who refit these data using 

a logistic regression model. The lung tumor 

response in these analyses is based on all lung 

tumors, since some of the studies did not distinguish the keratinizing squamous cell cysts 

from the squamous cell carcinomas [Martin et 

al. 1977; Lee et al. 1985; Mauderly et al. 1987]. 

Keratinizing squamous cell cysts have been 

observed primarily in the lungs of female rats 

exposed to PSLT, including TiO2 (discussed 

further in Sections 3.2.5, 3.5.2.4, and 3.6).

Figure 3–3. Relationship between particle surface area dose in the lungs of rats after chronic inhalation to various types of poorly soluble, low toxicity (PSLT) particles and tumor proportion (all 

tumors including keratinizing squamous cell cysts) 

Data source: toner [Muhle et al. 1991], coal dust [Martin et al. 1977], diesel exhaust particulate 

[Mauderly et al. 1987; Lewis et al. 1989; Nikula et al. 1995; Heinrich et al. 1995], TiO2 [Muhle et al. 

1991; Heinrich et al. 1995; Lee et al. 1985, 1986a], carbon black [Nikula et al. 1995; Heinrich et al. 

1995], talc [NTP 1993]
40 NIOSH CIB 63 • Titanium Dioxide

Figures 3–4 and 3–5 show the particle mass 

and surface area dose-response relationships 

for fine and ultrafine TiO2 for chronic inhalation exposure and lung tumor response in rats. 

In these figures, the lung tumor response data 

are shown separately for male and female rats 

at 24 months in Lee et al. [1985] and for female rats at 24 or 30 months, including either 

all tumors or tumors without keratinizing 

cystic tumors [Heinrich et al. 1995] since this 

study distinguished these tumor types. The 

data are plotted per gram of lung to adjust for 

differences in the lung mass in the two strains 

of rats (Sprague-Dawley and Wistar). Figure 

3–4 shows that when TiO2 is expressed as mass 

dose, the lung tumor response to ultrafine TiO2

is much greater at a given dose than that for 

fine TiO2; yet when TiO2 is expressed as particle surface area dose, both fine and ultrafine 

TiO2 data fit the same dose-response curve 

(Figure 3–5). These findings indicate that, like 

other PSLT particles, TiO2 is a rat lung carcinogen, and for a given surface area dose, the 

equivalent mass dose associated with elevated 

tumor response would be much higher for fine 

particles than for ultrafine particles. 

Figure 3–4. TiO2 mass dose in the lungs of rats exposed by inhalation for 2 years and tumor proportion (either all tumors or tumors excluding keratinizing squamous cell cysts) 

Note: Spline model fits to Lee data. (Heinrich dose data are jittered, i.e., staggered)

Data source: Heinrich et al. [1995], Lee et al. [1985, 1986a], and Muhle et al. [1991] 
NIOSH CIB 63 • Titanium Dioxide 41

The difference in TiO2 crystal structure in 

these subchronic and chronic studies did not 

influence the dose-response relationships for 

pulmonary inflammation and lung tumors 

[Bermudez et al. 2002, 2004; Lee et al. 1985; 

Heinrich et al. 1995]. That is, the particle surface area dose and response relationships were 

consistent for the ultrafine (80% anatase, 20% 

rutile) and fine (99% rutile) TiO2 despite the 

differences in crystal structure. In contrast, 

differences in ROS generation and toxicity 

have been observed for different TiO2 crystal 

structures in cell-free, in vitro, and short-term 

in vivo studies. Cell-free assays have reported 

crystal structure (anatase, rutile, or mixtures) 

influences particle surface ROS generation 

[Kawahara et al. 2003; Kakinoki et al. 2004; 

Behnajady et al. 2008; Jiang et al. 2008]. In a cellfree study designed to investigate the role of surface area and crystal structure on particle ROS 

generation, Jiang et al. [2008] observed that size, 

surface area, and crystal structure all contribute 

to ROS generation. The ROS generation was 

associated with the number of defective sites 

per surface area, which was constant for many 

of the particle sizes but varied for some of the 

smaller particle sizes (10–30 nm) due to differences in particle generation methods. In an 

Figure 3–5. TiO2 surface area dose in the lungs of rats exposed by inhalation for 2 years and tumor 

proportion (either all tumors or tumors excluding keratinizing squamous cell cysts) 

Note: Spline model fits to Lee data. (Heinrich dose data are jittered, i.e., staggered)

Data source: Heinrich et al. [1995], Lee et al. [1985, 1986a], and Muhle et al. [1991] 
42 NIOSH CIB 63 • Titanium Dioxide

in vitro cell assay, cytotoxicity was associated 

with greater ROS generation from photoactivated TiO2

 [Sayes et al. 2006]. In a pulmonary 

toxicity study in rats, the surface activity of the 

TiO2 particle (related to crystal structure, passivation, and acidity) was associated with the 

inflammation and cell proliferation responses 

at early time points, but not at the later time 

points [Warheit et al. 2007]. 

Although these studies (cited in preceding 

paragraph) indicate that the particle surface 

properties pertaining to the crystal structure 

of TiO2, including photoactivation, can influence the ROS generation, cytotoxicity, and 

acute lung responses, these studies also show 

that crystal structure does not influence the 

pulmonary inflammation or tumor responses 

following subchronic or chronic exposures. 

The reason for these differences in the acute 

and longer-term responses with respect to 

TiO2 crystal structure are not known but could 

relate to immediate effects of exposure to photoactivated TiO2

 and to quenching of ROS on 

the TiO2 surfaces by lung surfactant. Unlike 

crystalline silica, which elicits a pronounced 

pulmonary inflammation response, the inflammatory response to TiO2

 of either the rutile or anatase/rutile crystal form was much 

less pronounced after subchronic exposure in 

rats [Bermudez et al. 2002, 2004] (Figure 3-2).

Reactive species (ROS/RNS) are also produced 

by alveolar macrophages and inflammatory 

cells during lung clearance and immunological responses to inhaled particles, and the 

oxidative stress that can occur when antioxidant defenses are overwhelmed is considered 

an underlying mechanism of the proliferative 

and genotoxic responses to inhaled particles 

[Donaldson et al. 1996; Shi et al. 1998; Vallyathan et al. 1998; Knaapen et al. 2002, 2004; 

Donaldson and Stone 2003; Baan 2007]. PSLT 

particles, including TiO2, have relatively low 

surface reactivity compared to the more inherently toxic particles with higher surface activity such as crystalline silica [Duffin et al. 2007]. 

These findings are based on the studies in the 

scientific literature and may not apply to other 

formulations, surface coatings, or treatments 

of TiO2 for which data were not available. 

3.5 Particle-Associated Lung

Responses 

3.5.1 Rodent Lung Responses to

Fine and Ultrafine TiO2

Like other PSLT, fine and ultrafine TiO2 can 

elicit persistent pulmonary inflammation at 

sufficiently high dose and/or duration of exposure. This occurs at doses that impair the 

normal clearance of particles from the alveolar 

(gas exchange) region of the lungs, i.e., “overloading” of alveolar macrophage-mediated 

particle clearance from the lungs [ILSI 2000; 

Mauderly 1996; Morrow 1988; Le Bouffant 

1971]. As the lung burden increases, alveolar 

macrophages become activated and release 

ROS/RNS and cellular factors that stimulate pathological events [Driscoll et al. 1990; 

Lapp and Castranova 1993; Castranova 1998; 

Driscoll 2000]. Lung overload is characterized 

in rats, and to some extent in mice and hamsters, as increased accumulation of particleladen macrophages, increased lung weight, 

infiltration of neutrophils, increased epithelial 

permeability, increased transfer of particles to 

lymph nodes, persistent inflammation, lipoproteinosis, fibrosis, alveolar epithelial cell 

hyperplasia, and (eventually in rats) metaplasia and nonneoplastic and neoplastic tumors 

[ILSI 2000; Mauderly 1996; Morrow et al. 1991; 

Muhle et al. 1991]. 
NIOSH CIB 63 • Titanium Dioxide 43

Lung inflammation and histopathological responses to TiO2

 and other PSLT particles were 

more severe in the rat than in the mouse or 

hamster strains studied [Everitt et al. 2000; 

Bermudez et al. 2002, 2004; Elder et al. 2005; 

Heinrich et al. 1995]. Qualitatively similar early 

lung responses have been observed in rodents, 

especially rats and mice, although differences in 

disease progression and severity were also seen 

[Everitt et al. 2000; Bermudez et al. 2002, 2004; 

Elder et al. 2005]. Hamsters continued to effectively clear particles from the lungs, while 

the mice and rats developed overloading of 

lung clearance and retained higher lung particle burdens postexposure [Bermudez et al. 

2002, 2004; Elder et al. 2005]. Mice and rats 

developed persistent pulmonary inflammation through 52 weeks after a 13-week exposure to 50 or 250 mg/m3

 fine TiO2 [Everitt et 

al. 2000; Bermudez et al. 2002] or to 10 mg/m3

ultrafine TiO2 [Bermudez et al. 2004]. The rat 

lung response to another PSLT (carbon black) 

was more inflammatory (e.g., greater and more 

sustained generation of ROS in lung cells and 

higher levels of some inflammatory cytokines in 

BALF) compared to mice or hamsters [Carter 

et al. 2006]. All three rodent species developed 

proliferative epithelial changes at the higher 

doses; however, only the rat developed metaplasia and fibrosis [Everitt et al. 2000; Bermudez et 

al. 2002, 2004]. Both rats and mice inhaling P25 

ultrafine TiO2 had elevated alveolar cell proliferation [Bermudez et al. 2004], but only rats 

had a significantly elevated lung tumor response 

[Heinrich et al. 1995]. 

Different strains of mice were used in the subchronic and chronic studies (B3C3F1/CrlBR 

in Bermudez et al. [2004] and NMRI in Heinrich et al. [1995]). The NMRI mice had a high 

background tumor response (in the unexposed 

control mice), which might have limited the 

ability to detect any particle-related increase in 

that study [Heinrich et al. 1995]. The B3C3F1/

CrlBR mouse strain, which showed a proliferative alveolar cell response to ultrafine TiO2

[Bermudez et al. 2004], has not been evaluated 

for tumor response to TiO2. Thus, the mouse 

data are limited as to their utility in evaluating 

a TiO2 lung tumor response. In addition, the 

rapid particle lung clearance in hamsters and 

their relative insensitivity to inhaled particles 

also limits the usefulness of that species in evaluating lung responses to TiO2

. In the rat studies, three different strains were used (SpragueDawley [Lee et al. 1985], Wistar [Heinrich et 

al. 1995], and Fischer-344 [Muhle et al. 1991]), 

although no heterogeneity was observed in 

the dose-response relationship when these 

data were pooled and the dose was expressed 

as particle surface area per gram of lung (Appendix A). Based on the available data, the rat 

is the most sensitive rodent species to the pulmonary effects (inflammation and tumors) of 

inhaling PSLT particles such as TiO2.

3.5.2 Comparison of Rodent and 

Human Lung Responses to 

PSLT Including TiO2

3.5.2.1 Particle retention and lung

response

The similarities and differences between human and rat lung structures, particle deposition, and responses to inhaled poorly soluble 

particles including TiO2 have been described 

[Green et al. 2007; Baan 2007; Hext et al. 2005; 

Nikula et al. 2001; Heinrich 1996; Watson and 

Valberg 1996]. For example, there are structural differences in the small airways in the lungs 

of rats and humans (e.g., lack of well-defined 

respiratory bronchioles in rats). However, this 

region of the lungs is a primary site of particle 

deposition in both species, and particles that 

deposit in this region can translocate into the 
44 NIOSH CIB 63 • Titanium Dioxide

interstitium where they can elicit inflammatory and fibrotic responses [Green et al. 2007]. 

Both similarities and differences in rat and human nonneoplastic responses to inhaled particles were observed in a comparative pathology study (based on lung tissues from autopsy 

studies in workers and in chronic inhalation 

studies in rats, with chronic exposure to either coal dust, silica, or talc in both species) 

[Green et al. 2007]. Humans and rats showed 

some consistency in response by type of dust; 

i.e., granulomatous nodular inflammation was 

more severe among workers and rats exposed 

to silica or talc compared to coal dust. Similarly, humans and rats showed some graded 

response by dose; e.g., more severe centriacinar fibrosis at high versus low coal dust exposure in both humans and rats. In humans, the 

centriacinar fibrotic response was more severe 

among individuals exposed to either silica or 

coal dust compared to the rat response to these 

dusts. In rats, intra-alveolar acute inflammation, lipoproteinosis, and alveolar epithelial 

hyperplasia responses were more severe following chronic exposure to silica, talc, or coal 

dust compared to those responses in humans. 

The greater inflammatory and cell proliferation 

responses in rats suggest they may be more susceptible than humans to lung tumor responses 

to inhaled particles via a secondary genotoxic 

mechanism involving chronic pulmonary inflammation, oxidative stress, and cell damage 

and proliferation. However, it is also important 

to note that alveolar epithelial hyperplasia was 

clearly observed in workers exposed to silica, 

talc, or coal dust (and this response was more 

severe in humans compared to rats at “low” 

coal dust exposure). An important consideration in interpreting these study results is 

that the quantitative dust exposures were well 

known in rats and poorly known in humans 

[Green et al. 2007]. The rat exposures to these 

dusts ranged from 2 to 18 mg/m3 for 2 years. 

In humans, the exposure concentrations and 

durations were not reported for silica and talc; 

for coal dust, the durations were not reported, 

and “high” versus “low” exposure was defined 

as having worked before or after the 2 mg/m3

standard. Since it is not possible from these 

data to determine how well the dust exposures 

and lung burdens compare between the rats 

and humans, the qualitative comparisons are 

probably more reliable than the quantitative 

comparisons, which could be due in part to 

unknown differences in the lung dose in rats 

and humans. 

Different patterns of particle retention have 

been observed in rats, monkeys, and humans 

[Nikula et al. 2001]. Although no data are 

available on TiO2 particle retention patterns, 

coal dust (fine size) or diesel exhaust particles 

(ultrafine) were retained at a higher volume 

percentage in the alveolar lumen in rats and 

in the interstitium in monkeys [Nikula et al. 

1997]. The animals had been exposed by inhalation to 2 mg/m3

 of coal dust and/or diesel exhaust particulate for 2 years [Lewis et al. 1989]. 

A greater proportion of particles was also 

retained in the interstitium in humans compared to rats [Nikula et al. 2001]. In humans, 

the proportion of particles in the interstitium 

increased as the duration of exposure and estimated coal dust concentration increased. In 

rats, the particle retention pattern did not vary 

with increasing concentration of diesel exhaust 

particulate from 0.35 to 7.0 mg/m3. The authors 

[Nikula et al. 1997, 2001] suggest that the difference in dust retention patterns may explain 

the apparent greater susceptibility of the rat to 

develop particle-related lung cancer compared 

to other species. However, these studies did not 

examine the particle retention patterns in rats at 

overloading doses, which are the doses at which 
NIOSH CIB 63 • Titanium Dioxide 45

elevated lung tumors are observed. During overload, the particle lung burden increases due to 

greater retention or sequestration of particles in 

the alveolar lumen and increased translocation 

of particles to the lung interstitium and lymph 

nodes [Lauweryns and Baert 1977; Adamson 

and Bowden 1981; Morrow 1988; Bellmann 

et al. 1991; Muhle et al. 1991; Oberdörster et 

al. 1992; Adamson and Hedgecock 1995]. The 

increased interstitialization of particles in rats 

at high doses has also been inferred by the increasing particle mass measured in the lungassociated lymph nodes [Bermudez et al. 2002] 

since the movement of particles from the alveolar region to the pulmonary lymphatics 

requires transport through the alveolar epithelium and its basement membrane [Lauweryns 

and Baert 1977]. Thus, the particle retention 

observed in rats at overloading doses may better represent the particle retention in the lungs 

of workers in dusty jobs, such as coal miners, 

where little or no particle clearance was observed among retired miners [Freedman and 

Robinson 1998]. In addition, similar particle 

retention patterns (i.e., location in lungs) have 

been observed in rats, mice, and hamsters [Bermudez et al. 2002], yet the rat lung response 

is more severe. The retained dose is clearly a 

main factor in the lung response, and the low 

response in the hamster has been attributed to 

its fast clearance and low retention of particles 

[Bermudez et al. 2002]. These findings suggest 

that the particle retention pattern is not the 

only, or necessarily the most important, factor 

influencing lung response between species including rat and human. 

Particle size and ability to translocate from the 

lung alveolar region into the interstitium may 

also influence lung tissue responses. Borm et al. 

[2000] noted that the rat lung tumor response 

increased linearly with chronic pulmonary 

inflammation (following IT of various types 

of fine-sized particles), but that the rat lung 

tumor response to ultrafine TiO2 was much 

greater relative to the inflammation response. 

They suggested that tumor development from 

ultrafine particles may be due to high interstitialization rather than to overload and its sequella as seen for the fine-sized particles. Although this hypothesis remains unproven, it 

is clear that the rat lung tumor response to ultrafine TiO2

 was greater than that of fine TiO2

on a mass basis [Lee et al. 1985; Heinrich et al. 

1995]. The rat lung tumor response was consistent across particle size on a particle surface 

area basis, suggesting that the particle surface 

is key to eliciting the response. 

The extent of particle disaggregation in the 

lungs could influence the available particle surface as well as the ability of particles to translocate from the alveolar lumen into the interstitium, each of which could also influence 

pulmonary responses, as shown with various 

sizes of ultrafine and fine particles such as TiO2

[Oberdörster 1996]. In a study using simulated 

lung surfactant, disaggregation of P25 ultrafine 

TiO2 was not observed [Maier et al. 2006]. Another nanoscale TiO2

 particle (4 nm primary 

particle diameter; 22 nm count median diameter; 330 m2

/g specific surface area) was observed inside cells and cell organelles including 

the nucleus [Geiser et al. 2005]. Based on this 

observation, it has been suggested that an alternative genotoxic mechanism for nanoscale 

particles might involve direct interaction with 

DNA [Schins and Knaapen 2007]. NIOSH is 

not aware of any studies of the carcinogenicity of discrete nanoscale TiO2

 particles that 

would provide information to test a hypothesis 

of direct genotoxicity. However, this possible 

mechanism has been considered in the risk assessment (Chapters 4 and 5). 
46 NIOSH CIB 63 • Titanium Dioxide

3.5.2.2 Inflammation in rats and humans 

In rats that have already developed particleassociated lung tumors, the percentage of PMNs 

in the lungs is relatively high (e.g., 40%–60%) 

[Muhle et al. 1991]. However, it is not known 

what sustained level of PMNs is required to 

trigger epithelial proliferation and tumorigenic responses. In a chronic inhalation study, 

an average level of approximately 4% PMNs 

in BALF was measured in rats at the first dose 

and time point that was associated with a statistically significantly increased lung clearance half-time (i.e., a relatively early stage of 

overloading) [Muhle et al. 1991]. Similarly, in 

a subchronic inhalation study, 4% PMNs was 

predicted in rats at an average lung dose of TiO2

or BaSO4 that was not yet overloaded (based 

on measured particle lung burden) [Tran et 

al. 1999]. The relatively slight differences in 

the rat responses in these studies [Muhle et al. 

1991; Tran et al. 1999] may be due in part to 

differences in rat strains (Fisher-344; Wistar) 

and measures to assess overloading (clearance 

half-time and lung burden retention). 

In humans, chronic inflammation has been associated with nonneoplastic lung diseases in 

workers with dusty jobs. Rom [1991] found a 

statistically significant increase in the percentage of PMNs in BALF of workers with respiratory impairment who had been exposed to 

asbestos, coal, or silica (4.5% PMN in cases 

vs. 1.5% PMNs in controls). Elevated levels 

of PMNs have been observed in the BALF of 

miners with simple coal workers’ pneumoconiosis (31% of total BALF cells vs. 3% in controls) [Vallyathan et al. 2000] and in patients 

with acute silicosis (also a ten-fold increase 

over controls) [Lapp and Castranova 1993; 

Goodman et al. 1992]. Humans with lung 

diseases that are characterized by chronic inflammation and epithelial cell proliferation 

(e.g., idiopathic pulmonary fibrosis, diffuse 

interstitial fibrosis associated with pneumoconiosis) have an increased risk of lung cancer 

[Katabami et al. 2000]. Dose-related increases 

in lung cancer have been observed in workers 

exposed to respirable crystalline silica [Rice et 

al. 2001; Attfield and Costello 2004], which can 

cause inflammation and oxidative tissue damage [Castranova 2000]. Chronic inflammation 

appears to be important in the etiology of dustrelated lung disease, not only in rats, but also 

in humans with dusty jobs [Castranova 1998, 

2000]. Case studies of nonmalignant lung disease in TiO2

 workers have reported lung responses indicative of inflammation, including 

pulmonary alveolar proteinosis [Keller et al. 

1995] and interstitial fibrosis [Yamadori et al. 

1986; Moran et al. 1991; Elo et al. 1972]. 

The percentage of PMNs in BALF of normal controls “rarely exceeded” 4% [Haslam et al. 1987]. 

In other human studies, “normal” PMN percentages ranged from 2%–17% [Costabel et al. 1990]. 

An average of 3% PMNs (range 0%–10%) was 

observed in the BALF of nonsmoker controls 

(without lung disease), while 0.8% (range 0.2%–

3%) was seen in smoker controls (yet the PMN 

count in smokers was higher; this was due to elevation in other lung cell populations in smokers) 

[Hughes and Haslam 1990]. An elevation of 10% 

PMNs in the BALF of an individual has also been 

cited as being clinically abnormal [Martin et al. 

1985; Crystal et al. 1981]. 

3.5.2.3 Noncancer responses to TiO2 in rats 

and humans

Case studies of workers exposed to TiO2 provide some limited information to compare the 

human lung responses to those observed in the 

rat. In both human and animal studies, TiO2

has been shown to persist in the lungs. In some 

workers, extensive pulmonary deposition was 
NIOSH CIB 63 • Titanium Dioxide 47

observed years after workplace exposure to 

TiO2 had ceased [Määttä and Arstila 1975; 

Rode et al. 1981]. This suggests that human 

lung retention of TiO2 may be more similar to 

that in rats and mice compared to hamsters, 

which continue to clear the particles effectively 

at exposure concentrations that caused overloading in rats and mice [Everitt et al. 2000; 

Bermudez et al. 2002, 2004]. 

Inflammation, observed in pathologic examination of lung tissue, was associated with 

titanium (by X-ray or elemental analysis) in 

the majority of human cases with heavy TiO2

deposition in the lung [Elo et al. 1972; Rode 

et al. 1981; Yamadori et al. 1986; Moran et al. 

1991]. Pulmonary inflammation has also been 

observed in studies in rats, mice, and hamsters 

exposed to TiO2 [Lee et al. 1985, 1986a; Everitt et al. 2000; Bermudez et al. 2002]. Continued pulmonary inflammation in the lungs of 

some exposed workers after exposure cessation 

[Määttä and Arstila 1975; Rode et al. 1981] also 

appears to be more consistent with the findings 

in rats and mice than in hamsters, where inflammation gradually resolved with cessation 

of exposure. 

In one case study, pulmonary alveolar proteinosis (lipoproteinosis) was reported in a 

painter whose lung concentrations of titanium 

(60–129 × 106 particles/cm3 lung tissue) were 

“among the highest recorded” in a database of 

similar analyses [Keller et al. 1995]. The titanium-containing particles were “consistent with 

titanium dioxide” and were the major type of 

particle found in the workers’ lungs. In this case, 

the lipoproteinosis appeared more extensive 

than the lipoproteinosis seen in TiO2-exposed 

rats [Lee et al. 1985, 1986a; Bermudez et al. 

2002]. Thus, although the rat lipoproteinosis 

response was more severe than that in workers exposed to various fine-sized dusts [Green 

et al. 2007], this case study illustrates that humans can also experience severe lipoproteinosis response which is associated with particle 

retention in the lungs. 

Mild fibrosis has been observed in the lungs of 

workers exposed to TiO2 [Elo et al. 1972; Moran et al. 1991; Yamadori et al. 1986] and in 

rats with chronic inhalation exposure to TiO2

[Heinrich et al. 1995; Lee et al. 1985, 1986a]. 

Alveolar metaplasia has been described in 

three human patients who had inhalation exposures to airborne TiO2

 [Moran et al. 1991]. 

In laboratory animals, alveolar metaplasia has 

been observed in rats, but not mice or hamsters, with subchronic inhalation to fine and 

ultrafine TiO2 [Lee et al. 1985; Everitt et al. 

2000; Bermudez et al. 2002, 2004]. 

Although these studies suggest some similarities in the lung responses reported in case studies of workers exposed to respirable TiO2

 and in 

experimental studies in mice and rats, the human studies are limited by being observational 

in nature and by lacking quantitative exposure 

data. Information was typically not available on 

various factors including other exposures that 

could have contributed to these lung responses 

in the workers. Also, systematic histopathological comparisons were not performed, for example, on the specific alveolar metaplastic changes 

of the rat and human lungs. 

3.5.2.4 Lung tumor types in rats and 

humans

Lung tumors observed in rats following chronic 

inhalation of TiO2 include squamous cell keratinizing cysts, bronchioloalveolar adenomas, squamous cell carcinomas, and adenocarcinomas 

[Heinrich et al. 1995; Lee et al. 1985] (see Section 3.2.5). The significance of the squamous 

cell keratinizing cystic tumor (a.k.a. proliferative keratin cyst) for human risk assessment 
48 NIOSH CIB 63 • Titanium Dioxide

has been evaluated [Carlton 1994; Boorman et 

al. 1996]. Squamous cell keratinizing cystic tumors are most prevalent in female rats exposed 

to high mass or surface area concentrations of 

PSLT. In a recent reanalysis of the lung tumors 

in the Lee et al. [1985] chronic inhalation study 

of fine (pigment-grade) TiO2, the 15 lesions 

originally recorded as “squamous cell carcinoma” were reclassified as 16 tumors including 

two squamous metaplasia, 13 pulmonary keratin cysts, and one squamous cell carcinoma 

[Warheit and Frame 2006]. This reevaluation 

is consistent with the earlier evaluation of the 

squamous cell keratinizing cystic tumors by 

Boorman et al. [1996]. The classification of the 

29 bronchioloalveolar adenomas in Warheit 

and Frame [2006] remained unchanged from 

that reported in Lee et al. [1985]. 

Human and rat lung cancer cell types have similarities and differences in their histopathologies 

[ILSI 2000; Maronpot et al. 2004]. The respiratory tracts of humans and rats are qualitatively 

similar in their major structures and functions, 

although there are also specific differences such 

as the absence of respiratory bronchioles in rats 

[Miller 1999; Green et al. 2007]. In humans, the 

major cell types of lung cancer worldwide are 

adenocarcinoma and squamous-cell carcinoma 

(also observed in rats) and small- and large-cell 

anaplastic carcinoma (not seen in rats) [Everitt 

and Preston 1999; ILSI 2000]. In rats exposed 

to PSLT, most cancers are adenocarcinomas or 

squamous-cell carcinomas of the alveolar ducts 

[ILSI 2000]. In recent years, there has been a 

shift in the worldwide prevalence of human 

lung tumors toward adenocarcinomas in the 

bronchoalveolar region [ILSI 2000; Maronpot et 

al. 2004; Kuschner 1995]. 

Maronpot et al. [2004] suggest that some of the 

apparent difference in the bronchioloalveolar 

carcinoma incidence in rodents and humans 

can be explained by differences in terminology, 

and that a more accurate comparison would be 

to combine the adenocarcinomas and bronchioloalveolar carcinomas in humans, which 

would significantly reduce the apparent difference. Cigarette smoking in humans is also 

likely to contribute to the difference between 

the incidences of human and rodent tumor 

types. Maronpot et al. [2004] suggest that if the 

smoking-related tumor types were eliminated 

from the comparison, then the major lung 

tumor types in humans would be adenocarcinomas and bronchioloalveolar carcinomas, 

which would correspond closely to the types of 

lung tumors occurring in rodents. 

3.6 Rat Model in Risk

Assessment of Inhaled

Particles 

The extent to which the rat model is relevant to 

predicting human lung doses and responses to 

inhaled particles including TiO2 has been the 

subject of debate [Watson and Valberg 1996; 

ILSI 2000; Maronpot et al. 2004; Hext et al. 

2005; Baan 2007]. Lung clearance of particles is 

slower in humans than in rats, by approximately an order of magnitude [Hseih and Yu 1998; 

Snipes 1996; ICRP 1994], and some humans 

(e.g., coal miners) may be exposed to concentrations resulting in doses that would overload 

particle clearance from rat lungs [Morrow et al. 

1991; Freedman and Robinson 1988]. Thus, the 

doses that cause overloading in rats appear to 

be relevant to estimating disease risk in workers given these similarities in particle retention 

in the lungs of rats at overloading doses and in 

workers with high dust exposures.

Some have stated that the inhalation doseresponse data from rats exposed to PSLT 

particles should not be used in extrapolating 
NIOSH CIB 63 • Titanium Dioxide 49

cancer risks to humans because the rat lung 

tumor response has been attributed as a ratspecific response to the overloading of particle 

clearance from the lungs [Levy 1996; Watson 

and Valberg 1996; Hext et al. 2005; Warheit and 

Frame 2006]. While some of the tumors (keratinizing cystic tumors) may not be relevant to 

humans, other types of tumors (adenomas, adenocarcinoma, squamous cell carcinoma) do 

occur in humans. 

Mice and hamsters are known to give false negatives to a greater extent than rats in bioassays 

for some particulates that have been classified 

by the IARC as human carcinogens (limited or 

sufficient evidence), including crystalline silica 

and nickel subsulfide; and the mouse lung tumor response to other known human particulate carcinogens—including beryllium, cadmium, nickel oxide, tobacco smoke, asbestos, and 

diesel exhaust—is substantially less in mice than 

that in rats [Mauderly 1997]. These particulates 

may act by various plausible mechanisms, which 

are not fully understood [Schins and Knaapen 

2007; Castranova 1998]. The risks of several 

known human particulate carcinogens would 

thus be underestimated by using dose-response 

data from rodent models other than the rat. Although the mechanism of particle-elicited lung 

tumors remains to be fully elucidated, the rat 

and human lung responses to poorly soluble 

particles of low or high toxicity (e.g., coal dust 

and silica) are qualitatively similar in many of 

the key steps for which there are data, including 

pulmonary inflammation, oxidative stress, and 

alveolar epithelial cell hyperplasia [Vallyathan 

et al. 1998; Castranova 2000; Green et al. 2007; 

Baan 2007]. Case studies of lung responses in 

humans exposed to TiO2 are consistent in 

showing some similarities with rat responses 

(see Section 3.5.2.3). Semiquantitative comparisons of the lungs of rats and humans exposed to diesel exhaust, coal dust, silica, or talc 

indicate both similarities and differences, including similar regions of particle retention in 

the lungs but at different proportions [Nikula 

et al. 2001], and greater fibrotic response in 

humans but greater inflammation and epithelial hyperplasia responses in rats [Green et al. 

2007]. These sensitive inflammatory and proliferative responses to particles in the lungs are 

considered key to the rat lung tumor response 

[ILSI 2000]. Although the data are limited for 

quantitative comparison of rat and human 

dose-response relationships to inhaled particles, the available data (e.g., crystalline silica 

and diesel exhaust particles) indicate that the 

rat-based estimates are not substantially greater, and some are lower, than the human-based 

risk estimates [Stayner et al. 1998; Kuempel et 

al. 2001c, 2002]. Collectively, these studies indicate that, while there are uncertainties about 

the rat as a model for particle-related lung cancer in humans, and specifically for TiO2

, there 

is insufficient evidence for concluding that the 

rat is not a valid model. Expert advisory panels 

have concluded that chronic inhalation studies in rats are the most appropriate tests for 

predicting the inhalation hazard and risk of fibers to humans [Vu et al. 1996] and that, in the 

absence of mechanistic data to the contrary, 

it is reasonable to assume that the rat model 

can identify potential carcinogenic hazards of 

poorly soluble particles to humans [ILSI 2000], 

including PSLT such as TiO2. 

NIOSH CIB 63 • Titanium Dioxide 51

4 Quantitative Risk Assessment

4.1 Data and Approach 

Dose-response data are needed to quantify 

risks of workers exposed to TiO2. Such data 

may be obtained either from human studies 

or extrapolated to humans from animal studies. The epidemiologic studies on lung cancer 

have not shown a dose-response relationship in 

TiO2 workers [Fryzek et al. 2003; Boffetta et al. 

2004]. However, dose-response data are available in rats, for both cancer (lung tumors) and 

early, noncancer (pulmonary inflammation) 

endpoints. The lung tumor data (see Table 4–4) 

are from chronic inhalation studies and include three dose groups for fine TiO2

 and one 

dose group for ultrafine TiO2 (in addition to 

controls). The pulmonary inflammation data 

are from subchronic inhalation studies of fine 

and ultrafine particles [Tran et al. 1999; Cullen et al. 2002; Bermudez et al. 2002; Bermudez 

et al. 2004]. Various modeling approaches are 

used to fit these data and to estimate the risk of 

disease in workers exposed to TiO2 for up to a 

45-year working lifetime. 

The modeling results from the rat dose-response 

data provide the quantitative basis for developing the RELs for TiO2

, while the mechanistic 

data from rodent and human studies (Chapter 3) 

provide scientific information on selecting the 

risk assessment models and methods. The practical aspects of mass-based aerosol sampling 

and analysis were also considered in the overall 

approach (i.e., the conversion between particle 

surface area for the rat dose-response relationships and mass for the human dose estimates 

and RELs). Figure 4–1 illustrates the risk assessment approach. 

4.2 Methods

Dose-response modeling was used to estimate 

the retained particle burden in the lungs associated with lung tumors or pulmonary inflammation. Both maximum likelihood (MLE) and 

95% lower CI estimates of the internal lung 

doses in rats were computed. Particle surface 

area was the dose metric used in these models 

because it has been shown to be a better predictor than particle mass of both cancer and 

noncancer responses in rats (Chapter 3). In the 

absence of quantitative data comparing rat and 

human lung responses to TiO2, rat and human 

lung tissue are assumed to have equal sensitivity to an equal internal dose in units of particle 

surface area per unit of lung surface area [EPA 

1994]. 

4.2.1 Particle Characteristics

Study-specific values of particle MMAD, geometric standard deviation (GSD), and specific surface area were used in the dosimetric 

modeling when available (see Tables 4–1 and 

4–4). The Heinrich et al. [1995] study reported 

a specific surface area (48 ± 2 m2/g ultrafine 

TiO2) for the airborne particulate, as measured by the BET N2

 adsorption method. For 
52 NIOSH CIB 63 • Titanium Dioxide

Figure 4–1. Risk assessment approach using rat dose-response data to derive recommended 

exposure limits for TiO2.

Dose-response model

(particle surface area

dose in lungs)

Calculate tissue

dose—BMD

Assume equal response to equivalent dose

Recommended

exposure limit

Working lifetime

exposure concentration*

Equivalent tissue dose

RAT

HUMAN

Extrapolate

(species

differences in 

lung surface 

area)

Technical feasibility

Variability/uncertainty

Human lung

dosimetry model

*Compare rat-based risk estimates with

 upper bound on risk from human studies
NIOSH CIB 63 • Titanium Dioxide 53

the Lee et al. [1985] study, the specific surface 

area (4.99 m2/g fine TiO2) reported by Driscoll 

[1996] was used; that value was based on measurement of the specific surface area of a rutile TiO2

 sample similar to that used in the Lee 

study [Driscoll 2002]. This specific surface area 

was also assumed for the fine TiO2 in the Muhle et al. [1991] study. Otherwise, fine TiO2

 was 

assumed to have the particle characteristics reported by Tran et al. [1999] and a specific surface area of 6.68 m2

/g, and ultrafine TiO2 was 

assumed to have the particle characteristics reported by Heinrich et al. [1995] and a specific 

surface area of 48 m2/g. 

4.2.2 Critical Dose

The term “critical dose” is defined as the retained particle dose in the rat lung (maximum 

likelihood estimate [MLE] or 95% lower confidence limit [LCL]) associated with a specified 

response, including either initiation of inflammation or a given excess risk of lung cancer. 

One measure of critical dose is the benchmark

dose (BMD), which has been defined as “. . . a 

statistical lower confidence limit on the dose 

corresponding to a small increase in effect over 

the background level” [Crump 1984]. In current practice, and as used in this document, the 

BMD refers to the MLE from the model; and 

the benchmark dose lower bound (BMDL) is 

the 95% LCL of the BMD [Gaylor et al. 1998], 

which is equivalent to the BMD as originally 

defined by Crump [1984]. For dichotomous 

noncancer responses the benchmark response 

level is typically set at a 5% or 10% excess risk; 

however, there is less agreement on benchmark 

response levels for continuous responses. The 

benchmark response level used in this analysis 

for pulmonary inflammation, as assessed by elevated levels of PMNs, was 4% PMNs in BALF 

[Tran et al. 1999] (see Section 3.5.2.2). Another 

measure of critical dose, used in an earlier draft 

of this current intelligence bulletin, was the estimated threshold dose derived from a piecewise 

linear model fit to the pulmonary inflammation data (Appendix B). As discussed in Section 

4.3.1.2, not all of the current data sets are compatible with a threshold model for pulmonary 

inflammation; however, the threshold estimates 

described in Appendix B have been retained in 

this document for comparative purposes. For 

lung tumors the approach to estimating critical 

lung doses was to determine the doses associated with a specified level of excess risk (e.g., 1 

excess case per 1,000 workers exposed over a 

45-year working lifetime), either estimated directly from a selected model or by model averaging using a suite of models [Wheeler and 

Bailer 2007]. The 1 per 1,000 lifetime excess 

risk level was considered to be a significant risk 

based on the “benzene” decision [U.S. Supreme 

Court 1980]. 

4.2.3 Estimating Human Equivalent 

Exposure

The critical doses were derived using particle 

surface area per gram of rat lung, which was 

estimated from the mass lung burden data, rat 

lung weights, and measurements or estimates 

of specific surface area (i.e., particle surface 

area per mass). The use of a particle surface 

area per gram of lung dose for the rat doseresponse analyses was necessary in order to 

normalize the rat lung weights to a common 

dose measure, since several strains of rats with 

varying lung weights were used in the analyses 

(see Table 4–1 and Table 4–4), and it would be 

improper to equate the total lung doses of rats 

with greatly varying lung weights (Table 4–4). 

The critical doses were then multiplied by 1.5 

in order to normalize them to rats of the size 

used as a reference for lung surface area, as 
54 NIOSH CIB 63 • Titanium Dioxide

these were estimated to have lung weights of 

approximately 1.5 grams, based on the animal’s 

body weights. The critical doses were then extrapolated to humans based on the ratio of rat 

lung to human lung surface areas, which were 

assumed to be 0.41 m2 for Fischer 344 rats, 

0.4 m2 for Sprague-Dawley rats, and 102.2 m2

for humans [Mercer et al. 1994]. These critical 

particle surface area doses were then converted 

back to particle mass dose for humans because 

the current human lung dosimetry models 

(used to estimate airborne concentration leading to the critical lung doses) are all mass-based 

and because the current occupational exposure 

limits for most airborne particulates including 

TiO2 are also mass-based. 

4.2.4 Particle Dosimetry Modeling

The multiple-path particle dosimetry model, 

version 2, (MPPD2) human lung dosimetry 

model [CIIT and RIVM 2002] was used to estimate the working lifetime airborne mass concentrations associated with the critical doses 

in human lungs, as extrapolated from the rat 

dose-response data. The specific MPPD2 module used was the Yeh/Schum Symmetric model, 

assuming 17.5 breaths per minute and a tidal 

volume of 1,143 ml, with exposures of 8 hours 

per day, 5 days per week for a working lifetime 

of 45 years. The total of alveolar TiO2 plus TiO2

in the lung-associated lymph nodes was considered to be the critical human lung dose. 

The respiratory frequency and tidal volume 

were chosen to be consistent with the International Commission on Radiological Protection (ICRP) parameter values for occupational 

exposure, which equate an occupational exposure to 5.5 hours/day of light exercise and 

2.5 hours/day of sedentary sitting, with a total 

inhalation volume of 9.6 m3 in an 8-hour day 

[ICRP 1994]. The ICRP assumes 20 breaths per 

minute and a tidal volume of 1,250 ml for light 

exercise and 12 breaths per minute and a tidal 

volume of 625 ml for sedentary sitting. The 

values assumed in this analysis for modeling 

TiO2 exposures—i.e., 17.5 breaths per minute 

and a tidal volume of 1,143 ml—are a weighted 

average of the respiratory values for the light 

exercise and sedentary sitting conditions and 

are designed to match the ICRP [1994] value 

for total daily occupational inhalation volume. 

In summary, the dose-response data in rats 

were used to determine the critical dose associated with pulmonary inflammation or lung 

tumors, as particle surface area per surface 

area of lung tissue. The working lifetime airborne mass concentrations associated with the 

human-equivalent critical lung burdens were 

estimated using human lung dosimetry models. The results of these quantitative analyses 

and the derivation of the RELs for fine and ultrafine TiO2

 are provided in the remainder of 

this chapter. 

4.3 Dose-Response Modeling 

of Rat Data and 

Extrapolation to Humans 

4.3.1 Pulmonary Inflammation

4.3.1.1 Rat data

Data from four different subchronic inhalation 

studies in rats were used to investigate the relationship between particle surface area dose and 

pulmonary inflammation response: (1) TiO2

used as a control in a study of the toxicity of 

volcanic ash [Cullen et al. 2002], (2) fine TiO2

and BaSO4 in a study of the particle surface area 

as dose metric [Tran et al. 1999], (3) fine TiO2

in a multidose study [Bermudez et al. 2002], 

and (4) ultrafine TiO2 in a multidose study 
NIOSH CIB 63 • Titanium Dioxide 55

[Bermudez et al. 2004]. Details of these studies 

are provided in Table 4–1. Individual rat data 

were obtained for PMN count in the lungs in 

each study. In the Tran et al. study and the two 

Bermudez et al. studies different groups of rats 

were used to measure lung burden and PMNs, 

and group average values were used to represent the lung burden of TiO2

 in each exposure 

group. In the Cullen et al. study, PMN and lung 

burden data were obtained for each individual 

rat. Data from the two Bermudez et al. studies 

were combined and treated as a single study for 

dose-response analysis purposes.

4.3.1.2 Critical dose estimates in rats

The TiO2 pulmonary inflammation data from 

the Tran et al. [1999] and Cullen et al. [2002] 

studies could be fitted with a piecewise linear 

model which included a threshold parameter 

(described in Appendix B), and the threshold 

parameter estimate was significantly different 

from zero at a 95% confidence level. The MLE 

of the threshold dose was 0.0134 m2 (particle 

surface area for either fine or ultrafine TiO2) 

for TiO2 alone (90% CI = 0.0109–0.0145) based 

on data from Tran et al. [1999] and 0.0409 m2

(90% CI = 0.0395–0.0484) based on data from 

Cullen et al. [2002]. However, the fine and ultrafine TiO2

 pulmonary inflammation data 

from the Bermudez et al. [2002] and Bermudez 

et al. [2004] data sets provided no indication 

of a nonzero response threshold and were not 

consistent with a threshold model. Therefore, 

critical dose estimation for the pulmonary inflammation data was carried out via a benchmark dose approach, since benchmark doses 

could be fit to all three of the data sets (Tran 

et al. [1999], Cullen et al. [2002], and the 

combined data from Bermudez et al. [2002] 

and Bermudez et al. [2004]). 

Continuous models in the benchmark dose 

software (BMDS) suite [EPA 2007] were fitted to the pulmonary inflammation data using percent neutrophils as the response and 

TiO2 surface area (m2) per gram of lung as the 

predictor. The model that fit best for all three 

sets of TiO2 data was the Hill model, which 

converged and provided an adequate fit. Since 

there appears to be an upper limit to the degree 

of physiological response, the Hill model is able 

to capture this behavior better than the linear, 

quadratic, or power models. Models were fitted using constant variance for the Cullen et 

al. [2002] data and a nonconstant variance for 

the Tran et al. [1999] data and the combined 

data from Bermudez et al. [2002] and Bermudez et al. [2004]. The model fits for these data 

sets are illustrated in Figures 4-2, 4-3, and 4-4, 

respectively. In all models the critical dose or 

BMD was defined as the particle surface area 

per gram of lung tissue associated with a 4% 

inflammatory response of neutrophils, which 

has been equated to a low level inflammatory 

response [Tran et al. 1999]; see Section 3.5.2.2. 

The benchmark dose estimates for pulmonary 

inflammation in rats are shown in Table 4–2. 

 4.3.1.3 Estimated human equivalent 

exposure 

The critical dose estimates from Table 4–2 were 

converted to mass dose and extrapolated to 

humans by adjusting for species differences in 

lung surface area, as described in Section 4.2.3. 

Pulmonary dosimetry modeling was used to 

estimate the occupational exposure concentrations corresponding to the benchmark dose 

estimates, as described in Section 4.2.4. Lifetime occupational exposure concentrations 

estimated to produce human lung burdens 

equivalent to the rat critical dose levels for 

inflammation are presented in Table 4–3, 
56 NIOSH CIB 63 • Titanium Dioxide

Table 4–1. Comparison of rat inhalation studies used to model the relationship between 

TiO2 and pulmonary inflammation

Experimental 

conditions 

Study

Tran et al. 

[1999]

Cullen et al.

[2002]

Bermudez et al.

[2002]

Bermudez et al.

[2004]

TiO2 particle size:

MMAD (GSD)

2.1 (2.2) μm 1.2 (2.2) μm 1.44 (1.71) μm 1.44 (2.60) μm

(agglomerated)

Specific surface area 6.68 m2/g 6.41 m2/g 6.68 m2/g

(estimated)

48 m2/g

(estimated)

Sex, rat strain Male, Wistar rats Male, Wistar rats Female, Fischer

344 rats

Female, Fischer

344 rats

Exposure conditions Whole body 

inhalation

7 hr/day, 5 days/

week

Nose-only 

inhalation

6 hr/day, 5 days/

week

Whole body 

inhalation

6 hr/day, 5 days/week

Whole body 

inhalation

6 hr/day, 5 days/week

TiO2 concentration, 

duration

25 mg/m3, 7.5 months 

50 mg/m3, 4 months 

140 mg/m3, 2 months 10, 50, or 250 g/m3,

13 weeks

0.5, 2, or 10 mg/m3, 

13 weeks

MMAD = mass median aerodynamic diameter; GSD = geometric standard deviation

Figure 4–2. Hill model fit to rat data on pulmonary inflammation (percent neutrophils) and particle surface area dose of TiO2

 

Data source: Cullen et al. [2002]
NIOSH CIB 63 • Titanium Dioxide 57

Figure 4–3. Hill model fit to rat data on pulmonary inflammation (percent neutrophils) and particle surface area dose of TiO2

 

Data source: Tran et al. [1999] 

Figure 4–4. Hill model fit to rat data on pulmonary inflammation (percent neutrophils) and particle surface area dose of TiO2

 

Data source: (combined) Bermudez et al. [2002], Bermudez et al. [2004] 
58 NIOSH CIB 63 • Titanium Dioxide

Table 4–2. Benchmark dose estimates for particle surface area dose (m2) per gram of lung 

associated with pulmonary inflammation in rats (as PMNs in BALF), based on a Hill model

Data modeled MLE 95% LCL

TiO2 [Tran et al. 1999] 0.0205 0.0191

TiO2 [Cullen et al. 2002] 0.1054 0.0861

TiO2 [Bermudez et al. 2002 and Bermudez et al. 2004, 

combined]

0.0159 0.0144

BALF = bronchoalveolar lavage fluid; LCL = lower confidence limit; MLE = maximum likelihood estimate; PMNs = polymorphonuclear leukocytes; TiO2

 = titanium dioxide.

Table 4–3. Estimated mean airborne mass concentrations of fine and ultrafine TiO2 in 

humans and related human lung burdens (TiO2 surface area dose) associated with 

pulmonary inflammation after a 45-year working lifetime 

Critical dose in human lungs*

MPPD (ICRP) lung 

model (mg/m3)

†

Particle surface 

area (m2/lung)

Particle mass

(g/lung)

Particle size and study MLE 95% LCL MLE 95% LCL MLE 95% LCL

Fine TiO2 (2.1 μm, 2.2 GSD; 6.68 m2/g):

Tran et al. [1999] 7.86 7.32 1.18 1.10 1.11 1.03

Cullen et al. [2002] 40.39 33.00 6.30 5.15 5.94 4.86

Bermudez et al. [2002 and 2004] 6.09 5.52 0.91 0.83 0.86 0.78

Ultrafine TiO2 (0.8 μm,§ 1.8 GSD; 48 m2

/g):

Tran et al. [1999] 7.86 7.32 0.164 0.153 0.136 0.127

Cullen et al. [2002] 40.39 33.00 0.842 0.687 0.698 0.570

Bermudez et al. [2002 and 2004] 6.09 5.52 0.127 0.115 0.105 0.095

MPPD = multiple-path particle dosimetry [CIIT and RIVM 2002] model, including ICRP [1994] clearance model; GSD = geometric standard deviation; ICRP = International Commission on Radiological Protection; LCL = lower confidence limit; MLE = 

maximum likelihood estimate; TiO2 = titanium dioxide.

*MLE and 95% LCL were determined in rats (Table 4–2) and extrapolated to humans based on species differences in lung surface 

area, as described in Section 4.2.3. 

†

Mean concentration estimates derived from the CIIT and RIVM [2002] lung model, which includes the ICRP [1994] clearance 

model. 

§

Mass median aerodynamic diameter (MMAD). Ultrafine particle size is for agglomerate [Heinrich et al. 1995].
NIOSH CIB 63 • Titanium Dioxide 59

ranging from 0.78–5.94 mg/m3 for fine TiO2

and 0.095–0.698 mg/m3 for ultrafine TiO2. 

These exposure concentrations are presented 

only for comparison to the concentrations associated with lung tumors presented in Table 

4–6. As discussed in Section 4.5.1, the inflammatory levels presented in Table 4–3 are estimates of frank-effect levels (as opposed to noeffect levels) and would normally be adjusted 

by animal-to-human and interindividual uncertainty factors before being used as the basis 

for a REL. 

4.3.2 Lung Tumors

4.3.2.1 Rat data

Dose-response data from chronic inhalation 

studies in rats exposed to TiO2 were used to 

estimate working lifetime exposures and lung 

cancer risks in humans. These studies are described in more detail in Table 4–4 and include 

fine (pigment-grade) rutile TiO2 [Lee et al. 

1985; Muhle et al. 1991] and ultrafine anatase 

TiO2 [Heinrich et al. 1995]. The doses for fine 

TiO2 were 5 mg/m3 [Muhle et al. 1991] and 10, 

50, and 250 mg/m3 [Lee et al. 1985]. For ultrafine TiO2

 there was a single dose of approximately 10 mg/m3

 [Heinrich et al. 1995]. Each 

of these studies reported the retained particle 

mass lung burdens in the rats. The internal 

dose measure of particle burden at 24 months 

of exposure was used in the dose-response 

models, either as particle mass or particle surface area (calculated from the reported or estimated particle surface area per gram). 

The relationship between particle surface area 

dose of either fine or ultrafine TiO2 and lung 

tumor response (including all tumors or tumors excluding the squamous cell keratinizing 

cysts) in male and female rats was shown in 

Chapter 3. Statistically significant increases in 

lung tumors were observed at the highest dose 

of fine TiO2 (250 mg/m3) or ultrafine TiO2 (approximately 10 mg/m3

), whether or not the 

squamous cell keratinizing cysts were included 

in the tumor counts. 

Different strains and sexes of rats were used in 

each of these three TiO2 studies. The Lee et al. 

[1985] study used male and female SpragueDawley rats (crl:CD strain). The Heinrich study 

used female Wistar rats [crl:(WI)BR strain]. The 

Muhle et al. [1991] study used male and female 

Fischer-344 rats but reported only the average 

of the male and female lung tumor proportions. 

The body weights and lung weights differed by 

rat strain and sex (Table 4–4). These lung mass 

differences were taken into account when calculating the internal doses, either as mass (mg 

TiO2/g lung tissue) or surface area (m2 TiO2/m2

lung tissue). 

4.3.2.2 Critical dose estimates in rats

Statistical models for quantal response were 

fitted to the rat tumor data, including the suite 

of models in the BMDS [EPA 2003]. The response variable used was either all lung tumors 

or tumors excluding squamous cell keratinizing cystic tumors. Figure 4–5 shows the fit of 

the various BMD models [EPA 2003] to the 

lung tumor response data (without squamous 

cell keratinizing cysts) in male and female 

rats chronically exposed to fine or ultrafine 

TiO2 [Lee et al. 1985; Heinrich et al. 1995; and 

Muhle et al. 1991]. 

The lung tumor response in male and female 

rats was significantly different for “all tumors” 

but not when squamous cell keratinizing cystic tumors were removed from the analysis 

(Appendix A, Table A–2). In other words, the 

male and female rat lung tumor responses 

were equivalent except for the squamous cell 
60 NIOSH CIB 63 

• Titanium Dioxide

Table 4–4. Summary of chronic inhalation studies in rats exposed to TiO2 

Treated rats

Particle size 

and type; 

study Rat strain

Mean body 

weight of 

controls at

24 months (g)

Mean lung 

weight of 

controls at

24 months (g)

Particle size 

MMAD (μm) 

and specific SA 

(m2/g TiO2)

Exposure 

concentration 

(mg/m3)

Retained mean dose 

(mg TiO2 /lung)*

Tumor proportion (rats with 

tumors/total rats)

Female Male Female Male Female Male Female Male Average

Fine TiO2(≥ 99% rutile):Lee et al. [1985, 1986a]

Sprague-Dawley (crl: CD)

557 780 2.35 3.25 MMAD: 

1.5 to 1.7SA: 4.99 [Driscoll 1996]

0

1050

250

0

32.3

130

545.8

0 

20.7

118.3784.8

0/770/750/7414/74

2/792/711/7512/77†

————

Muhle et al. [1989, 1991, 1994]; Bellman et al. [1991]

Fischer-344 337 403 1.05 1.38 MMAD: 1.1 

(GSD: 1.6)SA: 4.99(estimate)

05

0

2.72

——

——

——

3/100

 2/100§

(Continued)

See footnotes at end of table.
NIOSH CIB 63 

• Titanium Dioxide 61

Treated rats

Particle size 

and type; 

study Rat strain

Mean body 

weight of 

controls at

24 months (g)

Mean lung 

weight of 

controls at

24 months (g)

Particle size 

MMAD (μm) 

and specific SA 

(m2/g TiO2)

Exposure 

concentration 

(mg/m3)

Retained mean dose 

(mg TiO2 /lung)*

Tumor proportion (rats with 

tumors/total rats)

Female Male Female Male Female Male Female Male Average

Ultrafine TiO2(~80% anatase; ~20% rutile):Heinrich et al. [1995]; Muhle et al. [1994]

Wistar [crl:(WI)BR)

417 — 1.44 MMAD: 0.80 

(GSD: 1.8)(agglomerates)0.015–0.040 (individual particles)SA: 48(SD: 2)

0

~10

039.29(SD: 7.36)

At 24 months:0/10 (controls)4/9 (all tumors)At 30 months: 1/217 (controls) 19/100 (no keratinizing cysts) 32/100 (all tumors)**

GSD = geometric standard deviation; MMAD = mass median aerodynamic diameter; SA = surface area (mean or assumed mean); SD = arithmetic standard deviation; TiO2 = titanium dioxide; crl:CD and crl:(WI)BR are the rat strain names from Charles River Laboratories, Inc.  *Lung particle burdens in controls not reported; assumed to be zero.

†Tumor types: controls, male: 2 bronchioloalveolar adenomas. At 10 mg/m3, males: 1 large cell anaplastic carcinoma and 1 bronchioloalveolar adenoma. At 50 mg/m3, male: 1 bronchioloalveolar adenoma. At 250 mg/m3, females: 13 bronchioloalveolar adenomas and 1 squamous cell carcinoma; males: 12 bronchioloalveolar adenomas. In addition to the tumors listed above, 

13 keratin cysts were observed: 1 in a 10 mg/m3 male, 1 in a 250 mg/m3 male, and 11 in 250 mg/m3 females.

§Dose was averaged for male and female rats because the tumor rates were reported only for male and female rats combined. Tumor types: controls, 2 adenocarcinomas and 1 adenoma. At 5 

mg/m3: 1 adenocarcinoma and 1 adenoma.

**Tumor types: controls, at 30 months: 1 adenocarcinoma. At ~10 mg/m3: 20 benign squamous-cell tumors, 3 squamous-cell carcinomas, 4 adenomas, and 13 adenocarcinomas (includes 8 

rats with 2 tumors each).

Table 4–4 (Continued). Summary of chronic inhalation studies in rats exposed to TiO2 
62 NIOSH CIB 63 • Titanium Dioxide

keratinizing cystic tumor response, which was 

elevated only in the female rats. To account for 

the heterogeneity in the “all tumor” response 

among male and female rats [Lee et al. 1985; 

Heinrich et al. 1995], a modified logistic regression model was developed (Appendix A); this 

model also adjusted for the combined mean tumor response for male and female rats reported 

by Muhle et al. [1991]. As discussed in Chapter 3, many pathologists consider the rat lung 

squamous cell keratinizing cystic tumor to be 

Figure 4–5. BMD models and three-model average fit to the lung tumor data (without squamous 

cell keratinizing cysts) in male and female rats chronically exposed to fine or ultrafine TiO2 

Note: The specific models used in constructing the model average were the multistage, Weibull, 

and log-probit models. The confidence intervals represent 95% binomial confidence limits for the 

individual data points.

Data source: BMD models and three-model average [Wheeler and Bailer 2007], male and female 

rats exposed to TiO2 [Lee et al. 1985; Heinrich et al. 1995; Muhle et al. 1991] 

irrelevant to human lung pathology. Excess risk 

estimates of lung tumors were estimated both 

ways—either with or without the squamous cell 

keratinizing cystic tumor data. The full results 

of the analyses including squamous cell keratinizing cystic tumors can be found in Appendix A. 

Inclusion of the keratinizing cystic tumors in the 

analyses resulted in slightly higher excess risk estimates in females, but not males. Since the male 

and female rat tumor responses may be combined 

when the squamous cell keratinizing cystic 
NIOSH CIB 63 • Titanium Dioxide 63

sub-linear; the multistage model is cubic in 

form, the Weibull model is similar with a power 

of 2.94, and the log-probit model has a slope of 

1.45. Therefore the dose-response relationship is 

quite steep, and the estimated risk drops sharply 

as the dose is reduced, as shown in Table 4–7.

4.3.2.3 Estimated human equivalent 

exposure 

The critical dose estimates from Table 4–5 were 

converted to mass dose and extrapolated to 

humans by adjusting for species differences in 

lung surface area, as described in Section 4.2.3. 

Pulmonary dosimetry modeling was used to 

estimate the occupational exposure concentrations corresponding to the benchmark dose 

estimates, as described in Section 4.2.4. Lifetime occupational exposure concentrations 

estimated to produce human lung burdens 

equivalent to the rat critical dose levels for 

lung tumors are presented in Table 4–6 for fine 

and ultrafine TiO2. Estimated occupational exposure levels corresponding to various levels 

of lifetime excess risk are shown in Table 4–7. 

This table shows the risk estimates for both fine 

and ultrafine TiO2. The 95% LCL estimates of 

the occupational exposure concentrations expected to produce a given level of lifetime excess risk are shown in the right-hand column. 

The concentrations shown in bold for fine and 

ultrafine TiO2 represent 1 per 1000 risk levels, 

which NIOSH has used as the basis for establishing RELs. The REL for ultrafine TiO2

 was 

rounded from 0.29 mg/m3 to 0.3.

4.3.3 Alternate Models and 

Assumptions

The choice of dosimetry models influences the 

estimates of the mean airborne concentration. 

A possible alternative to the MPPD model of 

tumors are excluded and exclusion does not 

have a major numeric impact on the risk estimates, risk estimates for TiO2

-induced lung 

tumors are based on the combined male and 

female rat lung tumors, excluding the squamous cell keratinizing cystic tumors. All lung 

tumor-based risk estimates shown below have 

been derived on this basis. Classification of the 

squamous cell keratinizing cystic tumors in the 

Lee et al. [1985] study was based on a reanalysis 

of these lesions by Warheit and Frame [2006]. 

The estimated particle surface area dose associated with a 1/1000 excess risk of lung tumors is 

shown in Table 4–5 for lung tumors excluding 

squamous cell keratinizing cystic lesions. The 

estimated particle surface area doses—BMD 

and BMDL—associated with a 1/1000 excess 

risk of lung cancer vary considerably depending on the shape of the model in the low-dose 

region. The model-based estimates were then 

summarized using a model averaging technique 

[Wheeler and Bailer 2007], which weights the 

various models based on the model fit. Model 

averaging provides an approach for summarizing the risk estimates from the various models, 

which differ in the low-dose region which is of 

interest for human health risk estimation, and 

also provides an approach for addressing the 

uncertainty in choice of model in the BMD approach. The specific model averaging method 

used was the three-model average procedure as 

described by Wheeler and Bailer [2007], using 

the multistage, Weibull, and log-probit models. 

These received weights of 0.14, 0.382 and 0.478, 

respectively, in the averaging procedure. This 

approach was considered to be appropriate for 

the TiO2 data set because the dose-response relationship is nonlinear, and the specific models 

used in the three-model average procedure do 

not impose low-dose linearity on the model average if linearity is not indicated by the data. In 

this case the best fitting models are all strongly 
64 NIOSH CIB 63 • Titanium Dioxide

Table 4–5. BMD and BMDL estimates of TiO2 particle surface area dose in rat lungs

(m2/g-lung) associated with 1/1000 excess risk of lung cancer*

Model: BMDS [EPA 2003]

P-value

(for lack of fit)† BMD BMDL

Gamma 0.33 0.258 0.043

Logistic 0.31 0.035 0.027

Log-logistic‡ 0.17 0.223 0.026

Log-probit‡ 0.28 0.286 0.051

Multistage 0.40 0.218 0.016

Probit 0.29 0.030 0.022

Quantal-linear 0.15 0.0075 0.0058

Quantal-quadratic 0.38 0.092 0.081

Weibull 0.32 0.212 0.036

MA** — 0.244 0.029

MA = model average; BMD = maximum-likelihood estimate of benchmark dose; BMDL = benchmark dose lower bound (95% lower 

confidence limit for the benchmark dose); BMDS = benchmark dose software; TiO2 = titanium dioxide.

*Response modeled: lung tumors excluding cystic keratinizing squamous lesions—from two studies of fine TiO2 [Lee et al. 1985; 

Muhle et al. 1991] and one study of ultrafine TiO2 [Heinrich et al. 1995].

†Acceptable model fit determined by P > 0.05.

‡BMDS did not converge. Model fitting and BMD and BMDL estimation were carried out as described in Wheeler and Bailer 

[2007]. **Average model, as described by Wheeler and Bailer [2007], based on the multistage, Weibull, and log-probit models. P-values are 

not defined in model averaging because the degrees of freedom are unknown.
NIOSH CIB 63 • Titanium Dioxide 65

Table 4–6. Estimated mean airborne mass concentrations of fine and ultrafine TiO2 in 

humans and related human lung burdens (TiO2* surface area dose) associated with 1/1000 

excess risk of lung cancer after a 45-year working lifetime

Critical dose in human lungs†

Mean airborne 

exposure‡

Particle size and model fit to rat doseresponse data for lung tumors§

Particle surface area 

(m2/lung)

Particle mass

(g/lung)

MPPD (ICRP) lung 

model (mg/m3)

MLE 95% LCL MLE 95% LCL MLE 95% LCL

Fine TiO2 (2.1 μm, 2.2 GSD; 6.68 m2/g):

Gamma 98.9 16.5 14.8 2.5 14.0 2.3

Logistic 13.4 10.3 2.0 1.5 1.9 1.5

Log-logistic 85.5 10.0 12.8 1.5 12.1 1.4

Log-probit 109.6 19.5 16.4 2.9 15.5 2.8

Multistage 83.5 6.1 12.5 0.9 11.8 0.9

Probit 11.5 8.4 1.7 1.3 1.6 1.2

Quantal-linear 2.9 2.2 0.4 0.3 0.4 0.3

Quantal-quadratic 35.3 31.0 5.3 4.6 5.0 4.4

Weibull 81.2 13.8 12.2 2.1 11.5 1.9

MA¶ 93.5 17.0 14.0 2.5 13.2 2.4

Ultrafine TiO2 (0.8 μm, 1.8 GSD; 48 m2/g)**

Gamma 98.9 16.5 2.06 0.34 1.71 0.28

Logistic 13.4 10.3 0.28 0.22 0.23 0.18

Log-logistic 85.5 10.0 1.78 0.21 1.48 0.17

Log-probit 109.6 19.5 2.28 0.41 1.89 0.34

Multistage 83.5 6.1 1.74 0.13 1.44 0.11

Probit 11.5 8.4 0.24 0.18 0.20 0.15

Quantal-linear 2.9 2.2 0.06 0.05 0.05 0.04

Quantal-quadratic 35.3 31.0 0.73 0.65 0.61 0.54

Weibull 81.2 13.8 1.69 0.29 1.40 0.24

MA¶ 93.5 17.0 1.95 0.35 1.62 0.29

*Abbreviations: MA = model average; BMD = benchmark dose; GSD = geometric standard deviation, LCL = lower confidence 

limit; MLE = maximum likelihood estimate; TiO2 = titanium dioxide, MPPD = multiple-path particle dosimetry [CIIT and 

RIVM 2002] model.

†MLE and 95% LCL were determined in rats (Table 4–5) and extrapolated to humans based on species differences in lung surface 

area, as described in Section 4.2.3. 

‡Mean concentration estimates were derived from the CIIT and RIVM [2002] lung model. 

§Without keratinizing cystic lesions. 

¶Model averaging combined estimates from the multistage, Weibull, and log-probit models [Wheeler and Bailer 2007].

**Mass median aerodynamic diameter (MMAD). Agglomerated particle size for ultrafine TiO2

 was used in the deposition model 

[CIIT and RIVM 2002]. Specific surface area was used to convert from particle surface area dose to mass dose; thus airborne 

particles with different specific surface areas would result in different mass concentration estimates from those shown here.
66 NIOSH CIB 63 • Titanium Dioxide

Table 4–7. Model average estimates† of mean airborne mass concentrations of fine and 

ultrafine TiO2 in humans and related human lung burdens (TiO2* surface area dose) 

associated with various levels of excess risk of lung cancer after a 45-year working lifetime

Critical dose in human lungs‡

Mean airborne

exposure§

Particle size and lifetime added risk 

estimated from rat dose-response

data for lung tumors¶

Particle surface area 

(m2/lung)

Particle mass

(g/lung)

MPPD (ICRP) lung 

model (mg/m3)

MLE 95% LCL MLE 95% LCL MLE 95% LCL

Fine TiO2 (2.1 μm, 2.2 GSD; 6.68 m2/g):

 1 in 500 114.2 24.9 17.1 3.7 16.1 3.5

 1 in 1000 93.5 17.0 14.0 2.5 13.2 2.4††

 1 in 2000 76.3 11.1 11.4 1.7 10.8 1.6

 1 in 5000 57.5 6.2 8.6 0.9 8.1 0.9

 1 in 10,000 46.4 3.8 6.9 0.6 6.5 0.5

 1 in 100,000 21.4 0.5 3.2 0.1 3.0 0.1

Ultrafine TiO2 (0.8 μm, 1.8 GSD; 

 48 m2/g)**:

 1 in 500 114.2 24.9 2.38 0.52 1.97 0.43

 1 in 1000 93.5 17.0 1.95 0.35 1.62 0.29††

 1 in 2000 76.3 11.1 1.59 0.23 1.32 0.19

 1 in 5000 57.5 6.2 1.20 0.13 0.99 0.11

 1 in 10,000 46.4 3.8 0.97 0.08 0.80 0.07

 1 in 100,000 21.4 0.5 0.45 0.01 0.37 0.01

*Abbreviations: MA = model average; BMD = benchmark dose; GSD = geometric standard deviation, LCL = lower confidence 

limit; MLE = maximum likelihood estimate; TiO2 = titanium dioxide, MPPD = multiple-path particle dosimetry [CIIT and 

RIVM 2002] model. †

Model averaging combined estimates from the multistage, Weibull, and log-probit models [Wheeler and Bailer 2007].

‡

MLE and 95% LCL were determined in rats (Table 4–5) and extrapolated to humans based on species differences in lung surface 

area, as described in Section 4.2.3. §

Mean concentration estimates were derived from the CIIT and RIVM [2002] lung model. 

¶

Without keratinizing cystic lesions. 

**Mass median aerodynamic diameter (MMAD). Agglomerated particle size for ultrafine TiO2 was used in the deposition model 

[CIIT and RIVM 2002]. Specific surface area was used to convert from particle surface area dose to mass dose; thus airborne 

particles with different specific surface areas would result in different mass concentration estimates from those shown here. ††The exposure levels shown in boldface are the 95% LCL estimates of the concentrations of fine and ultrafine TiO2

 considered appropriate for establishment of a REL. The ultrafine exposure level of 0.29 mg/m3

 was rounded to 0.3 for the REL.
NIOSH CIB 63 • Titanium Dioxide 67

Sprague-Dawley rats, and 102.2 m2 for humans 

[Mercer et al. 1994]. Alternatively, extrapolation 

could be based on the relative lung weights of 

rat and human lungs, using a dose metric of 

particle surface area per gram of lung tissue. In 

that case, the estimates of the working lifetime 

occupational exposure levels equivalent to the 

rat critical dose levels would be higher by a 

factor of approximately four. The lung surface 

area-based approach was selected for this analysis because insoluble particles deposit and 

clear along the surface of the respiratory tract, 

so that dose per unit surface area is often used 

as a normalizing factor for comparing particle 

doses across species [EPA 1994]. 

The critical dose estimates in Table 4–5 also vary 

depending on the dose-response model used 

and on whether the MLE or the 95% LCL is 

used as the basis for estimation. The MLE dose 

estimates associated with a 1/1000 excess risk of 

lung tumors vary by a factor of 38, while the 95% 

LCL estimates vary by a factor of 14. All of the 

models provided statistically adequate fits to the 

data, and there is little basis to select one model 

over another. This uncertainty regarding model 

form has been addressed by the use of MA, as 

described by Wheeler and Bailer [2007], which 

weights the models based on the model fit as assessed by the Akaike information criterion. However, any of the models shown could conceivably 

be selected as the basis for human risk estimation. 

Use of the model-averaged 95% LCL value, as opposed to the model-averaged MLE, is intended to 

address both model uncertainty and variability in 

the rat data; however, it is possible that the 95% 

LCL may underestimate the true variability of the 

human population. 

4.3.4 Mechanistic Considerations 

The mechanism of action of TiO2 is relevant to 

a consideration of the associated risks because, 

particle deposition of CIIT and RIVM [2002], 

which was used for particle dosimetry modeling in this analysis, would be an interstitialization/sequestration model that was developed 

and calibrated using data of U.S. coal miners 

[Kuempel et al. 2001a,b] and later validated 

using data of U.K. coal miners [Tran and Buchanan 2000]. The MPPD model uses the ICRP 

alveolar clearance model, which was developed 

using data on the clearance of radiolabeled 

tracer particles in humans, and has been in 

use for many years [ICRP 1994]. More data 

are needed to evaluate the model structures 

and determine how well each model would describe the retained doses associated with low 

particle exposures in humans, particularly for 

ultrafine particles. The MPPD model was selected for use in this analysis on the grounds 

that it is widely used and well accepted for particle dosimetry modeling in general, while use of 

the interstitialization/sequestration model has to 

date been limited primarily to modeling exposures to coal dust. Nevertheless, it should be noted that the interstitialization/sequestration model predicts lung burdens of fine and ultrafine TiO2

which are approximately double those predicted 

by the MPPD model. Thus, the use of the interstitialization/sequestration model for particle dosimetry would approximately halve the estimates 

of occupational exposure levels equivalent to the 

rat critical dose levels, compared to estimates developed using the MPPD model. 

The method selected for extrapolating between 

rats and humans also influences the estimates of 

occupational exposure levels equivalent to the 

rat critical dose levels. To extrapolate the critical 

particle surface area dose in the lungs of rats to 

the lungs of humans, either the relative mass or 

surface area of the lungs in each species could be 

used. The results presented in this analysis are 

based on the relative alveolar surface area, assuming 0.41 m2

 for Fischer 344 rats, 0.4 m2 for 
68 NIOSH CIB 63 • Titanium Dioxide

as discussed earlier, the weight of evidence suggests that the tumor response observed in rats 

exposed to fine and ultrafine TiO2 results from 

a secondary genotoxic mechanism involving 

chronic inflammation and cell proliferation, 

rather than via genotoxicity of TiO2 itself. This 

effect appears related to the physical form of the 

inhaled particle (i.e., particle surface area) rather 

than the chemical compound itself. Other PSLT 

particles—such as BaSO4, carbon black, toner, 

and coal dust—also produce inflammation and 

lung tumors in proportion to particle surface 

area (Figures 3–2 and 3–3) and therefore appear 

to act via a similar mechanism.

Studies supporting this mechanism include empirical studies of the pulmonary inflammatory 

response of rats exposed to TiO2 and other PSLT 

(see Sections 3.2.4 and 3.4.1); the tumor response of TiO2

 and other PSLT, which have consistent dose-response relationships (see Section 

3.4.2); and in vitro studies, which show that inflammatory cells isolated from BALF from rats 

exposed to TiO2 released ROS that could induce 

mutations in naive cells (see Section 3.2.1.3). 

There is some evidence, though limited, that inflammation may be a factor in the initiation of 

human lung cancer as well (see Section 3.5.2.2). 

In considering all the data, NIOSH has determined that a plausible mechanism of action for 

TiO2 in rats can be described as the accumulation of TiO2

 in the lungs, overloading of lung 

clearance mechanisms, followed by increased 

pulmonary inflammation and oxidative stress, 

cellular proliferation, and, at higher doses, tumorigenesis. These effects are better described 

by particle surface area than mass dose (see 

Section 3.4.2). The best-fitting dose-response 

curves for the tumorigenicity of TiO2 are nonlinear; e.g., the multistage model in Table 4–5 

is cubic with no linear term, the quantalquadratic model is quadratic with no linear 

term, and the gamma and Weibull models 

have power terms of approximately 4 and 3, 

respectively. This nonlinearity is consistent 

with a secondary genotoxic mechanism and 

suggests that the carcinogenic potency of TiO2

would decrease more than proportionately 

with decreasing surface area dose as described 

in the best-fitting risk assessment models. 

4.4 Quantitative Comparison 

of Risk Estimates From 

Human and Animal Data

A quantitative comparison was performed 

(Appendix C) of the rat-based excess risk estimates for human lung cancer due to exposure 

to fine TiO2 to the 95% upper confidence limit 

(UCL) of excess risk from the epidemiologic 

studies (Appendix D), in order to compare 

the rat- and human-based excess risks of lung 

cancer. If the sensitivity of the rat response to 

inhaled particulates differs from that of humans, then the excess risks derived from the 

rat data would be expected to differ from the 

excess risks estimated from the human studies. 

The results of the comparison of the rat- and 

human-based excess risk estimates were used 

to assess whether or not there was adequate 

precision in the data to reasonably exclude the 

rat model as a basis for predicting the excess 

risk of lung cancer in humans exposed to TiO2. 

The results of these comparisons showed that 

the MLE and 95% UCL excess risk estimates 

from the rat studies were lower than the 95% 

UCL from the human studies for an estimated 

working lifetime (Appendix C, Table C–1). 

These results indicate that, given the variability 

in the human studies [Fryzek et al. 2003; Boffetta et al. 2004], the rat-based excess risk estimates cannot reasonably be dismissed from 

use in predicting the excess risk of lung cancer 
NIOSH CIB 63 • Titanium Dioxide 69

in humans exposed to TiO2. Thus, NIOSH 

determined that it is prudent to use these 

rat dose-response data for risk assessment in 

workers exposed to TiO2. 

4.5 Possible Bases for an REL

4.5.1 Pulmonary Inflammation

As discussed above, the evidence in rats suggests that the lung tumor mechanism associated with PSLT particles such as TiO2

 is a 

secondary genotoxic mechanism involving 

chronic inflammation and cell proliferation. 

One plausible approach to developing risk estimates for TiO2

 is to estimate exposure concentrations that would not be expected to produce 

an inflammatory response, thus preventing the 

development of responses that are secondary 

to inflammation, including cancer. A benchmark dose analysis for pulmonary inflammation in the rat was described in Section 4.3.1, 

and the results of extrapolating the rat BMDs 

to humans are presented in Table 4–3. Since 

two of the three studies available yielded 95% 

BMDLs of 0.78 and 1.03 mg/m3, a concentration of approximately 0.9 mg/m3

 is reasonable 

as the starting point for development of recommendations for human exposures for fine 

TiO2. Similarly, a concentration of approximately 0.11 mg/m3

 is appropriate as the starting point for developing recommended exposures to ultrafine TiO2

. 

As noted in Section 4.3.1.3, the human pulmonary inflammation BMDs in Table 4–3 are estimates of frank-effect levels and should be adjusted by the application of uncertainty factors 

to allow for uncertainty in animal-to-human extrapolation and interindividual variability. These 

uncertainty factors are commonly assumed to 

be ten-fold for animal-to-human extrapolation and another ten-fold for interindividual 

variability; the animal-to-human uncertainty 

may be subdivided into a factor of 4 for toxicokinetics and 2.5 for toxicodynamics (WHO 

1994). Since the rat BMDs were extrapolated to humans using a deposition/clearance model, it is reasonable to assume that the 

animal-to-human toxicokinetic subfactor of 

4 has already been accounted for; therefore, a 

total uncertainty factor of 25 (2.5 for animalto-human toxicodynamics times 10 for interindividual variability) should be applied. This 

results in estimated exposure concentrations 

designed to prevent pulmonary inflammation 

of 0.04 mg/m3 for fine TiO2 and 0.004 mg/m3

for ultrafine TiO2. 

4.5.2 Lung Tumors

Rather than estimating an exposure concentration designed to avoid secondary toxicity by 

preventing pulmonary inflammation, another 

possible basis for developing a REL is to model 

the risk of lung tumors directly. In the absence 

of mechanistic data in humans, the tumorigenic 

mechanism operative in rats cannot be ruled 

out. Therefore, one approach for estimation of 

recommended levels of occupational exposure 

to TiO2 is to estimate the pulmonary particle 

surface area dose associated with a 1/1000 increase in rat lung tumors and to extrapolate 

that dose to humans on the basis of particle 

surface area per unit of lung surface area. This 

approach was used to assess the excess risk of 

lung cancer at various working lifetime exposure concentrations of fine or ultrafine TiO2

(Table 4–6). Selection of the model for estimating risks has a significant impact on the 

risk estimates. As shown in Table 4–6, the 95% 

LCL working lifetime mean concentration of 

fine TiO2 associated with a 1/1000 excess risk 

of lung cancer is 0.3 to 4.4 mg/m3, depending 

on the model used to fit the rat lung tumor 
70 NIOSH CIB 63 • Titanium Dioxide

data. For ultrafine TiO2, the 95% LCL working 

lifetime mean concentration associated with 

a 1/1000 excess risk of lung cancer is 0.04 to 

0.54 mg/m3, depending on the model. 

Although any of the models evaluated in Table 

4–6 could conceivably be used to develop recommendations for occupational exposures to 

TiO2, the model averaging procedure is attractive since it incorporates both statistical variability and model uncertainty into confidence 

limit estimation. However, an argument could 

also be made for basing recommendations 

on the multistage model, due to its long history of use for carcinogen risk assessment or 

the quantal-linear model, on the grounds that 

it generates the lowest BMD and BMDL and 

is thus arguably the most health-protective. 

The BMD and BMDL derived via each of these 

models are shown in Table 4–6 for both fine 

and ultrafine TiO2. 

Since the various models produce different risk 

estimates and there is no clear mechanistically 

based preference for one model over another, 

it is appropriate to summarize the results by 

using a MA technique. MA, as implemented 

here, uses all the information from the various 

dose-response models, weighting each model 

by the Akaike information criterion for model 

fit and constructing an average dose-response 

model with lower bounds computed by bootstrapping. This method was described by 

Wheeler and Bailer [2007], who demonstrated 

via simulation studies that the MA method has 

superior statistical properties to a strategy of 

simply picking the best-fitting model from the 

BMDS suite. As shown in Table 4–6, the model 

average estimate of the working lifetime mean 

concentration of fine TiO2 associated with a 

1/1000 excess risk of lung cancer is 13.2 mg/m3, 

with a 95% LCL of 2.4 mg/m3. The corresponding estimates for ultrafine TiO2

 are 1.6 mg/m3, 

with a 95% LCL of 0.3 mg/m3. NIOSH believes 

that it is reasonable and prudent to use the 

95% LCL model-averaged estimates as the basis for RELs, as opposed to the MLEs, in order 

to allow for model uncertainty and statistical 

variability in the estimates. 

4.5.3 Comparison of Possible Bases 

for an REL

As discussed above, occupational exposure 

concentrations designed to prevent pulmonary inflammation, and thus prevent the development of secondary toxicity (including 

lung tumors), are 0.04 mg/m3 for fine TiO2 and 

0.004 mg/m3 for ultrafine TiO2. In comparison, 

modeling of the dose-response relationship for 

lung tumors indicates that occupational exposure concentrations of 2.4 mg/m3

 for fine TiO2

and 0.3 mg/m3 for ultrafine TiO2 would be sufficient to reduce the risk of lung tumors to a 

1/1000 lifetime excess risk level. The discrepancy between the occupational exposure concentrations estimated from modeling either 

pulmonary inflammation or lung tumors raises 

serious questions concerning the optimal basis 

for a TiO2 REL. However, it must be acknowledged that the two sets of possible RELs are 

not based on entirely comparable endpoints. 

The pulmonary inflammation-based exposure 

concentrations are expected to entirely prevent 

the development of toxicity secondary to pulmonary inflammation, resulting in zero excess 

risk of lung tumors due to exposure to TiO2. In 

contrast, the lung tumor-based exposure concentrations are designed to allow a small, but 

nonzero, excess risk of lung tumors due to occupational exposure to TiO2

. 

As discussed in Section 3.4.1, particle-induced 

pulmonary inflammation may act as a precursor for lung tumor development; however, 
NIOSH CIB 63 • Titanium Dioxide 71

pulmonary inflammation itself is not a specific 

biomarker for lung cancer. As noted in Section 3.5.2.2, the precise level of sustained 

inflammation necessary to initiate a tumorigenic response is currently unknown. It is possible that the 4% PMN response used in this 

analysis as the benchmark response level for 

pulmonary inflammation is overly protective 

and that a somewhat greater inflammatory response is required for tumor initiation. 

It is also possible that the 25-fold uncertainty factor applied to the critical dose estimate for pulmonary inflammation may be 

overly conservative, since pulmonary inflammation is an early event in the sequence 

of events leading to lung tumors. However, 

NIOSH has not previously used early events 

or secondary toxicity as a rationale for applying smaller than normal uncertainty factors. 

Given that in this case the primary objective 

of preventing pulmonary inflammation is to 

prevent the development of lung tumors, and 

given that lung tumors can be adequately controlled by exposures many-fold higher than the 

inflammation-based exposure concentrations, 

NIOSH has concluded that it is appropriate to 

base RELs for TiO2 on lung tumors rather than 

pulmonary inflammation. However, NIOSH 

notes that extremely low-level exposures to 

TiO2—i.e., at concentrations less than the pulmonary inflammation-based RELs—may pose 

no excess risk of lung tumors. 

NIOSH CIB 63 • Titanium Dioxide 73

5 Hazard Classification and Recommended

Exposure Limits 

NIOSH initiated the evaluation of titanium 

dioxide by considering it as a single substance 

with no distinction regarding particle size. 

However, a review of all the relevant scientific literature indicated that there could be a 

greater occupational health risk with smaller 

size (ultrafine) particles and therefore NIOSH 

provides separate recommendations for the ultrafine and fine categories.

NIOSH has reviewed the relevant animal and 

human data to assess the carcinogenicity of 

titanium dioxide (TiO2) and has reached the 

following conclusions. First, the weight of evidence suggests that the tumor response observed in rats exposed to ultrafine TiO2

 resulted 

from a secondary genotoxic mechanism involving chronic inflammation and cell proliferation, 

rather than via direct genotoxicity of TiO2. This 

effect appears to be related to the physical form 

of the inhaled particle (i.e., particle surface area) 

rather than to the chemical compound itself. 

Second, based on the weight of the scientific 

data (including increase in adenocarcinoma tumor incidence in a chronic inhalation study in 

rats of 10 mg/m3), NIOSH determined that inhaled ultrafine TiO2

 is a potential occupational 

carcinogen and is recommending exposure limits to minimize the cancer risk from exposure to 

ultrafine TiO2. Finally, because the tumorigenic 

dose of fine TiO2 (250 mg/m3) in the Lee et al. 

studies [1985, 1986a] was substantially higher 

than current inhalation toxicology practice—

and because there was no significant increase in 

tumors at 10 or 50 mg/m3—NIOSH did not use 

the highest dose in its hazard identification and 

concluded that there is insufficient evidence to 

classify fine TiO2 as a potential occupational 

carcinogen. Although NIOSH has determined 

that the data are insufficient for cancer hazard 

classification of fine TiO2, the particle surface 

area dose and tumor response relationship 

is consistent with that observed for ultrafine 

TiO2 and warrants that precautionary measures be taken to protect the health of workers 

exposed to fine TiO2. Therefore, NIOSH used 

all of the animal tumor response data to conduct the dose-response modeling, and developed separate mass-based RELs for ultrafine 

and fine TiO2. 

5.1 Hazard Classification

NIOSH reviewed the current scientific data on 

TiO2 to evaluate the weight of the evidence for 

the NIOSH designation of TiO2 as a “potential 

occupational carcinogen.” Two factors were 

considered in this evaluation: (1) the evidence 

in humans and animals for an increased risk of 

lung cancer from inhalation of TiO2, including 

exposure up to a full working lifetime, and (2) 

the evidence on the biologic mechanism of the 

dose-response relationship observed in rats, 

including evaluation of the particle characteristics and dose metrics that are related to the 

pulmonary effects. 

No exposure-related increase in carcinogenicity was observed in the epidemiologic studies 

conducted on workers exposed to TiO2 dust in 
74 NIOSH CIB 63 • Titanium Dioxide

the workplace [Boffetta et al. 2001, 2003, 2004; 

Fryzek 2004; Fryzek et al. 2003, 2004a]. In rats 

exposed to fine TiO2 by chronic inhalation, 

lung tumors were elevated at 250 mg/m3, but 

not at 10 or 50 mg/m3 [Lee et al. 1985; 1986a]. 

In contrast, chronic inhalation exposures to 

ultrafine TiO2 at approximately 10 mg/m3 resulted in a statistically significant increase in 

malignant lung tumors in rats, although lung 

tumors in mice were not elevated [Heinrich 

et al. 1995]. The lung tumors observed in rats 

after exposure to 250 mg/m3 of fine TiO2 were 

the basis for the original NIOSH designation of 

TiO2 as a “potential occupational carcinogen.” 

However, because this dose is considered to be 

significantly higher than currently accepted inhalation toxicology practice [Lewis et al. 1989], 

NIOSH concluded that the response at such a 

high dose should not be used in making its hazard identification. Therefore, NIOSH has come 

to different conclusions regarding the potential 

occupational carcinogenicity for fine versus 

ultrafine TiO2. NIOSH evaluated the doseresponse data in humans and animals, along 

with the mechanistic factors described below, 

in assessing the scientific basis for the current 

NIOSH designation of ultrafine but not fine 

TiO2 as a “potential occupational carcinogen.” 

In addition, NIOSH used the rat dose-response 

data in a quantitative risk assessment to develop 

estimates of excess risk of nonmalignant and 

malignant lung responses in workers over a 

45-year working lifetime. These risk estimates 

were used in the development of RELs for fine 

and ultrafine TiO2. 

5.1.1 Mechanistic Considerations

As described in detail in Chapter 3, the mechanistic data considered by NIOSH were obtained from published subchronic and chronic 

studies in rodents exposed by inhalation to 

TiO2 or other PSLT particles. These studies include findings on the kinetics of particle clearance from the lungs and on the nature of the 

relationship between particle surface area and 

pulmonary inflammation or lung tumor response. The mechanistic issues considered by 

NIOSH include the influence of particle size or 

surface area (vs. specific chemical reactivity) 

on the carcinogenicity of TiO2 in rat lungs, the 

relationship between particle surface area dose 

and pulmonary inflammation or lung tumor 

response in rats, and the mechanistic evidence 

on the development of particle-elicited lung 

tumors in rats. These considerations are discussed in detail in Chapter 3. 

NIOSH also considered the crystal structure 

as a modifying factor in TiO2 carcinogenicity and inflammation. As described in Chapter 3, some short-term studies indicate that 

the particle surface properties pertaining to 

the crystal structure of TiO2, including photoactivation, can influence the ROS generation, cytotoxicity, and acute lung responses. 

These studies also show that crystal structure 

does not influence the pulmonary inflammation or tumor responses following subchronic 

or chronic exposures [Bermudez et al. 2002, 

2004]. The reason for these differences in the 

acute and longer-term responses with respect 

to TiO2 crystal structure are not known but 

could relate to immediate effects of exposure to 

photoactivated TiO2 and to quenching of ROS 

on the TiO2 surfaces by lung surfactant. PSLT 

particles, including TiO2, have relatively low 

surface reactivity compared to the more inherently toxic particles with higher surface activity such as crystalline silica [Duffin et al. 2007]. 

These findings are based on the studies in the 

scientific literature and may not apply to other 

formulations, surface coatings, or treatments 

of TiO2 for which data were not available. 
NIOSH CIB 63 • Titanium Dioxide 75

After analysis of the issues, NIOSH concluded 

that the most plausible mechanism for TiO2 

carcinogenesis is a nonchemical specific interaction of the particle with the cells in the lung, 

characterized by persistent inflammation and 

mediated by secondary genotoxic processes. 

The dose-response relationships for both the inflammation and tumorigenicity associated with 

TiO2 exposure are consistent with those for other 

PSLT particles. Based on this evidence, NIOSH 

concluded that the adverse effects produced by 

TiO2 exposure in the lungs are likely not substance-specific, but may be due to a nonchemical-specific effect of PSLT particles in the lungs 

at sufficiently high particle surface area exposures. However, because the tumorigenic dose 

for fine TiO2 [Lee et al. 1985] of 250 mg/m3 was 

significantly higher than currently accepted inhalation toxicology practice [Lewis et al. 1989], 

NIOSH did not use the 250 mg/m3 dose in its 

hazard identification. Therefore, NIOSH concluded that there are insufficient data to classify 

fine TiO2 as a potential occupational carcinogen 

but there are sufficient data indicating that ultrafine TiO2

 has the potential to cause cancer 

after adequate occupational exposure. 

5.1.2 Limitations of the Rat

Tumor Data

NIOSH evaluated all publicly available epidemiology studies and laboratory animal inhalation studies and determined that the best data 

to support a quantitative risk assessment for 

TiO2 were from rat inhalation studies [Lee et 

al. 1985; Muhle et al. 1991; Heinrich et al. 1995; 

Cullen et al. 2002; Tran et al. 1999; Bermudez et 

al. 2002, 2004]. These studies provided exposureresponse data for both inflammation and tumorigenicity and were used as the basis of the 

quantitative risk assessment. 

NIOSH considered the scientific literature that 

supported and disputed on the rat as an appropriate model for human lung cancer after 

exposure to inhaled particles and reached the 

conclusion that the rat is an appropriate species on which to base its quantitative risk assessment for TiO2

. Although there is not extensive evidence that the overloading of lung 

clearance, as observed in rats (Chapter 3), 

occurs in humans, lung burdens consistent 

with overloading doses in rats have been observed in some humans with dusty jobs (e.g., 

coal miners) [Stöber et al. 1965; Carlberg et 

al. 1971; Douglas et al. 1986]. Rather than excluding the rat as the appropriate model, the 

lung overload process may cause the rat to attain lung burdens comparable to those that can 

occur in workers with dusty jobs. In addition, 

evidence suggests that, as in the rat, inhalation 

of particles increases the human inflammatory 

response, and increases in the inflammatory 

response may increase the risk of cancer (see 

Section 3.5.2.2). This information provides additional support for the determination that the 

rat is a reasonable animal model with which 

to predict human tumor response for particles 

such as TiO2. 

After evaluating all of the available data, NIOSH 

concluded that the appropriate dose metric in 

its risk assessment was particle surface area. 

Both tumorigenicity and inflammation were 

more strongly associated with particle surface 

area than particle mass dose. The separate risk 

estimates for fine and ultrafine TiO2 are supported by the higher lung cancer potency in 

rats of ultrafine TiO2 compared to fine TiO2, 

which was associated with the greater surface 

area of ultrafine particles for a given mass. In 

rats chronically exposed to airborne fine TiO2, 

statistically significant excess lung tumors were 

observed only in the 250 mg/m3 dose group. Although exposure concentrations greater than 
76 NIOSH CIB 63 • Titanium Dioxide

100 mg/m3 are not currently standard methodology in inhalation toxicity studies [Lewis et al. 

1989], and NIOSH questions the relevance of 

the 250 mg/m3 dose for classifying exposure to 

TiO2 as a carcinogenic hazard to workers, the 

tumor-response data are consistent with that 

observed for ultrafine TiO2 when converted to 

a particle surface area metric. Thus to be cautious, NIOSH used all of the animal tumor response data when conducting dose-response 

modeling and determining separate RELs for 

ultrafine and fine TiO2. With chronic exposure 

to airborne ultrafine TiO2, lung tumors were 

seen in rats exposed to an average of approximately 10 mg/m3

 (ranged from 7.2 mg/m3 to 

14.8 mg/m3) over the exposure period. 

It would be a better reflection of the entire body 

of available data to set RELs as the inhaled surface area of the particles rather than the mass 

of the particles. This would be consistent with 

the scientific evidence showing an increase in 

potency with increase in particle surface area 

(or decrease in particle size) of TiO2 and other 

PSLT particles. For this reason, the basis of the 

RELs for fine and ultrafine TiO2 is the rat doseresponse data for particle surface area dose and 

pulmonary response. However, current technology does not permit the routine measurement of the surface area of airborne particles; 

therefore, NIOSH recommends sampling the 

mass airborne concentration of TiO2 as two 

broad primary particle size categories: fine 

(< 10 μm) and ultrafine (< 0.1 μm). These categories reflect current aerosol size conventions, 

although it is recognized that actual particle 

size distributions in the workplace will vary. 

5.1.3 Cancer Classification in 

Humans

Since the public comment and peer review draft 

of this document was made available, NIOSH 

has learned that the IARC has reassessed 

TiO2. IARC now classifies TiO2 as an IARC 

Group 2B carcinogen, “possibly carcinogenic 

to humans” [IARC 2010]. NIOSH supports 

this decision and the underlying analysis leading to this conclusion.

Based on the animal studies described in 

Chapter 3 and the quantitative risk assessment 

in Chapter 4, NIOSH has concluded that ultrafine but not fine TiO2

 particulate matter is 

a potential occupational carcinogen. However, 

as a precautionary step, NIOSH conducted a 

quantitative risk assessment based on the tumor 

data for both fine and ultrafine TiO2.

The potency of ultrafine TiO2, which has a much 

higher surface area per unit mass than fine TiO2, 

was many times greater than fine TiO2, with 

malignant tumors observed at the lowest dose 

level of ultrafine TiO2 tested (10 mg/m3). 

The lack of an exposure-response relationship in the epidemiologic studies of workers 

exposed to TiO2 dust in the workplace should 

not be interpreted as evidence of discordance 

between the mechanism presumed to operate 

in rats and the human potential for carcinogenicity. As demonstrated by the quantitative 

comparison between the animal and human 

studies (see Section 4.4 and Appendix C), the 

responses were not statistically inconsistent: 

the epidemiologic studies were not sufficiently 

precise to determine whether they replicated or 

refuted the animal dose-response. This is not a 

surprising finding for carcinogens of lower potency, such as fine-sized TiO2

. 

The mechanistic data reviewed above, however, leave open the possibility of species differences beyond what would be anticipated for 

a genotoxic carcinogen. Although it is plausible that the secondary genotoxic mechanism 
NIOSH CIB 63 • Titanium Dioxide 77

proposed here operates in humans exposed 

to TiO2 dust, there is insufficient evidence to 

corroborate this. In addition, there is limited 

information on the kinetics or specific physiological response to TiO2

 particles in humans. 

The evidence suggests that inhalation of lower 

surface area TiO2 is not likely to result in carcinogenicity in any test species. This concept is 

reflected in the quantitative risk assessment, in 

which the curvilinear dose response predicts 

that lower exposures have disproportionally 

less risk than higher exposures. For workers, 

this suggests that exposure to concentrations 

lower than the RELs will be less hazardous and 

may pose a negligible risk. 

Although the analysis in this document is limited to consideration of the workplace hazard 

posed by TiO2, these findings suggest that other 

PSLT particles inhaled in the workplace may 

pose similar hazards, particularly nano-sized 

particles with high surface areas. NIOSH is 

concerned that other nano-sized PSLT particles 

may have similar health effects to those observed for TiO2

, in which ultrafine TiO2 particles were observed to be more carcinogenic and 

inflammogenic on a mass basis than fine TiO2

[Heinrich et al. 1995; Lee et al. 1985, 1986a]. 

5.2 Recommended Exposure

Limits

NIOSH recommends airborne exposure limits

of 2.4 mg/m3 for fine TiO2 and 0.3 mg/m3 for 

ultrafine (including engineered nanoscale) TiO2

as TWA concentrations for up to 10 hr/day 

during a 40-hour work week, using the international definitions of respirable dust [CEN 

1993; ISO 1995] and the NIOSH Method 0600 

for sampling airborne respirable particles 

[NIOSH 1998]. NIOSH selected these exposure limits for recommendation because they 

would reduce working lifetime risks for lung 

cancer to below 1/1000. Cancer risks greater 

than 1/1000 are considered significant and worthy of intervention by OSHA. NIOSH has used 

this risk level in a variety of circumstances, including citing this level as appropriate for developing authoritative recommendations in Criteria Documents and risk assessments published 

in scientific journal articles [NIOSH 1995; 

NIOSH 2006; Rice et al. 2001; Park et al. 2002; 

Stayner et al. 2000; Dankovic et al. 2007]. It is 

noted that the true risk of lung cancer due to 

exposure to TiO2 at these concentrations could 

be much lower than 1/1000. To account for the 

risk that exists in work environments where 

airborne exposures to fine and ultrafine TiO2

occur, exposure measurements to each size 

fraction should be combined using the additive formula and compared to the additive REL 

of 1 (unitless) (see Figure 6–1 Exposure assessment protocol for TiO2

). 

Because agglomerated ultrafine particles are 

frequently measured as fine-sized but behave 

biologically as ultrafine particles due to the surface area of the constituent particles, exposures 

to agglomerated ultrafine particles should be 

controlled to the ultrafine REL. 

“Respirable” is defined as particles of aerodynamic size that, when inhaled, are capable 

of depositing in the gas-exchange (alveolar) 

region of the lungs [ICRP 1994]. Sampling 

methods have been developed to estimate the 

airborne mass concentration of respirable particles [CEN 1993; ISO 1995; NIOSH 1998]. 

“Fine” is defined in this document as all particle sizes that are collected by respirable particle sampling (i.e., 50% collection efficiency for 

particles of 4 μm, with some collection of particles up to10 μm). “Ultrafine” is defined as the 

fraction of respirable particles with primary 

particle diameter < 0.1 μm (< 100 nm), which 

is a widely used definition. Additional methods 
78 NIOSH CIB 63 • Titanium Dioxide

are needed to determine if an airborne respirable particle sample includes ultrafine TiO2

(Chapter 6). 

The NIOSH RELs for fine TiO2 of 2.4 mg/m3 and 

ultrafine TiO2 of 0.3 mg/m3 were derived using 

the model averaging procedure (described in 

Sections 4.3–4.5) of the dose-response relationship for lung tumors in rats, extrapolated to occupational exposures. The ultrafine REL reflects 

NIOSH’s greater concern for the potential carcinogenicity of ultrafine TiO2

 particles. As particle size decreases, the surface area increases (for 

equal mass), and the tumor potency increases 

per mass unit of dose. NIOSH acknowledges 

the evolving state of knowledge on the hazards 

associated with nano-sized particles and invites 

additional discussion on the risks of nanoparticulate exposures in the workplace.
NIOSH CIB 63 • Titanium Dioxide 79

6 Measurement and Control of TiO2 Aerosol in

the Workplace

6.1 Exposure Metric

Based on the observed relationship between 

particle surface area dose and toxicity (Chapters 3 and 4), the measurement of aerosol 

surface area would be the preferred method 

for evaluating workplace exposures to TiO2. 

However, personal sampling devices that can 

be routinely used in the workplace for measuring particle surface area are not currently available. As an alternative, if the airborne particle 

size distribution of the aerosol is known in the 

workplace and the size distribution remains 

relatively constant with time, mass concentration measurements may be useful as a surrogate for surface area measurements. NIOSH 

is recommending that a mass-based airborne 

concentration measurement be used for monitoring workplace exposures to fine and ultrafine (including engineered nanoscale) TiO2 until more appropriate measurement techniques 

can be developed. NIOSH is currently evaluating the efficacy of various sampling techniques 

for measuring fine and ultrafine TiO2 and may 

make specific recommendations at a later date.

In the interim, personal exposure concentrations 

to fine (pigment-grade) and ultrafine (including 

engineered nanoscale) TiO2 should be determined 

with NIOSH Method 0600 using a standard 

10-mm nylon cyclone or equivalent particle sizeselective sampler [NIOSH 1998]. Measurement 

results from NIOSH Method 0600 should provide 

a reasonable estimate of the exposure concentration to fine and ultrafine (including engineered 

nanoscale) TiO2 at the NIOSH RELs of 2.4 and 

0.3 mg/m3, respectively, when the predominant 

exposure to workers is TiO2. No personal sampling devices are available at this time to specifically measure the mass concentrations of 

ultrafine aerosols; however, the use of NIOSH 

Method 0600 will permit the collection of 

most airborne ultrafine (including engineered 

nanoscale) particles and agglomerates. 

In work environments where exposure to other 

types of aerosols occur or when the size distribution of TiO2 (fine vs. ultrafine) is unknown, 

other analytical techniques may be needed to 

characterize exposures. NIOSH Method 7300 

[NIOSH 2003] can be used to assist in differentiating TiO2 from other aerosols collected on 

the filter while electron microscopy, equipped 

with X-ray energy dispersive spectroscopy 

(EDS), may be needed to measure and identify particles. In workplaces where TiO2 is purchased as a single type of bulk powder, the primary particle size of the bulk powder can be 

used to determine whether the REL for fine or 

ultrafine should be applied if adequate airborne 

exposure data exist to confirm that the airborne 

particle size has not substantially been altered 

during the handling and/or material processing of TiO2

. Although NIOSH Methods 0600 

and 7300 have not been validated in the field 

for airborne exposure to TiO2, they have been 

validated for other substances and, therefore, 

should provide results for TiO2 within the expected accuracy of the Methods. 
80 NIOSH CIB 63 • Titanium Dioxide

6.2 Exposure Assessment

A multitiered workplace exposure assessment 

might be warranted in work environments 

where the airborne particle size distribution 

of TiO2 is unknown (fine vs. ultrafine) and/

or where other airborne aerosols may interfere with the interpretation of sample results. 

Figure 6–1 illustrates an exposure assessment 

strategy that can be used to measure and identify particles so that exposure concentrations 

can be determined for fine and ultrafine (including engineered nanoscale) TiO2

. An initial 

exposure assessment should include the simultaneous collection of respirable dust samples 

with one sample using a hydrophobic filter (as 

described in NIOSH Method 0600) and the 

other sample using a mixed cellulose ester filter 

(MCEF).* If the respirable exposure concentration for TiO2 (as determined by Method 0600) 

is less than 0.3 mg/m3, then no further action 

is required. If the exposure concentration exceeds 0.3 mg/m3

, then additional characterization of the sample is needed to determine the 

particle size and percentage of TiO2 and other 

extraneous material on the sample. To assist in 

this assessment, the duplicate respirable sample 

collected on a MCEF should be evaluated using transmission electron microscopy (TEM) to 

size particles and determine the percentage of 

fine (> 0.1 μm) and ultrafine (< 0.1 μm) TiO2. 

When feasible, the percent of fine and ultrafine 

TiO2 should be determined based on the measurement of “primary” particles (includes agglomerates comprised of primary particles). 

The identification of particles (e.g., TiO2)

by TEM can be accomplished using EDS or 

*Note: The collection time for samples using a MCEF 

may need to be shorter than the duplicate samples collected and analyzed by Method 0600 to ensure that 

particle loading on the filter doesn’t become excessive 

and hinder particle sizing and identification by TEM.

energy loss spectroscopy. Once the percent 

of TiO2 (by particle size) has been determined, 

adjustments can be made to the mass concentration (as determined by Method 0600) to 

assess whether exposure to the NIOSH RELs 

for fine, ultrafine (including engineered nanoscale), or combined fine and ultrafine TiO2 

had been exceeded. 

To minimize the future need to collect samples for TEM analysis, samples collected using Method 0600 can also be analyzed using NIOSH Method 7300 or other equivalent 

methods to determine the amount of TiO2 on 

the sample. When using NIOSH Method 7300, 

it is important that steps be taken (i.e., pretreatment with sulfuric or hydrofluoric acid) to 

insure the complete dissolution and recovery 

of TiO2 from the sample. The results obtained 

using Method 7300, or other equivalent method, should be compared with the respirable 

mass concentration to determine the relative 

percentage of TiO2 in the concentration measurement. Future assessments of workplace exposures can then be limited to the collection 

and analysis of samples using Method 0600 

and Method 7300 (or equivalent) to ensure 

that airborne TiO2 concentrations have not 

changed over time. 

6.3 Control of Workplace

Exposures to TiO2

Given the extensive commercial use of fine 

(pigment grade) TiO2, the potential for occupational exposure exists in many workplaces. 

However, few data exist on airborne concentrations and information on workplaces where 

exposure might occur. Most of the available 

data for fine TiO2 are reported as total dust 

and not as the respirable fraction. Historical 

total dust exposure measurements collected 

in TiO2 production plants in the United States 
NIOSH CIB 63 • Titanium Dioxide 81

Figure 6–1. Exposure assessment protocol for TiO2.
82 NIOSH CIB 63 • Titanium Dioxide

often exceeded 10 mg/m3 [IARC 1989], while 

more contemporary exposure concentrations 

in these plants indicate that mean total inhalable dust measurements may be below 3 mg/m3

(1.1 mg/m3 median) for most job tasks [Fryzek 

et al. 2003]. Given the particle size dimensions 

of fine TiO2 (~0.1 μm to 4 μm, avg. of 0.5 μm) 

[Malvern Instruments 2004], it is reasonable 

to conclude that respirable TiO2 particles comprised a significant fraction of the total dust 

measurement. Estimates of worker exposures 

to respirable TiO2 determined in 1999 from 8 

European plants producing fine TiO2 ranged 

from 0.1 to 5.99 mg/m3 (plant with highest 

measured concentrations) to 0.1 to 0.67 mg/m3

(plant with lowest measured concentrations) 

[Boffetta et al. 2003]. The highest reported 

worker exposure concentrations to TiO2 in 

both U.S. and European production plants 

were among packers and micronizers and 

during maintenance [Sleeuwenhoek 2005]. 

Results of a NIOSH health hazard evaluation 

conducted at a facility using powder paint containing TiO2 found airborne concentrations of 

respirable TiO2 ranging from 0.01 to 1.0 mg/m3

(9 samples) [NIOSH 2009b]. TEM analysis conducted on one airborne sample found 42.4% of 

the TiO2 particles less than 0.1 μm in diameter. 

NIOSH is not aware of any extensive commercial production of ultrafine anatase TiO2 in the 

United States although it may be imported for 

use. Ultrafine rutile TiO2 is being commercially 

produced as an additive for plastics to absorb 

and scatter ultraviolet light; 10%–20% of the 

ultrafine TiO2 is reported to be < 100 nm in size 

[DuPont 2007]. Engineered TiO2 nanoparticles 

are also being manufactured, and like ultrafine 

TiO2, they are finding commercial application 

as a photocatalyst for the destruction of chemical and microbial contaminants in air and water, in light-emitting diodes and solar cells, in 

plastics, as a UV blocker, and as a “self-cleaning” 

surface coating. While a paucity of data exist 

on worker exposure to engineered TiO2, exposure measurements taken at a facility manufacturing engineered TiO2 found respirable 

exposure concentrations as high as 0.14 mg/m3 

[Berges et al. 2007]. The primary particle size 

determined by TEM analysis ranged from 25 

to 100 nm. 

The control of workplace exposures to TiO2

should be primarily accomplished through the 

use of engineering controls. Although limited 

data exist on occupational exposures to TiO2,

reducing exposures can be achieved using a variety of standard control techniques [Raterman 

1996; Burton 1997]. Standard industrial hygiene practices for controlling airborne hazards 

include engineering controls, work practice 

and administrative procedures, and personal 

protective equipment. Examples of engineering controls include process modifications and 

the use of an industrial ventilation system to 

reduce worker airborne exposures [ACGIH 

2001c]. In general, control techniques such 

as source enclosure (i.e., isolating the generation source from the worker) and local exhaust 

ventilation systems are the preferred methods 

for preventing worker exposure to aerosols. In 

light of current scientific knowledge regarding 

the generation, transport, and capture of aerosols, these control techniques should be effective for both fine and ultrafine particles [Seinfeld and Pandis 1998; Hinds 1999]. Conventional 

engineering controls, using ventilation systems 

to isolate the exposure source from workers, 

should be effective in reducing airborne exposures to fine and ultrafine (including engineered 

nanoscale) TiO2. Ventilation systems should be 

equipped with high efficiency particulate air 

(HEPA) filters which are designed to remove 

99.97% of particles 300 nm in diameter. Particles 

smaller than 200 nm are generally collected on the 

filter by diffusion, whereas particles larger than 
NIOSH CIB 63 • Titanium Dioxide 83

800 nm are deposited through impaction and 

interception [Lee and Liu 1981, 1982]. Ventilation systems must be properly designed, tested, 

and routinely maintained to provide maximum 

efficiency. 

When engineering controls and work practices 

cannot reduce worker TiO2 exposures to below the REL then a respirator program should 

be implemented. The OSHA respiratory protection standard (29 CFR 1910.134) sets out 

the elements for both voluntary and required 

respirator use. All elements of the standard 

should be followed. Primary elements of the 

OSHA respiratory protection standard include 

(1) an evaluation of the worker’s ability to perform the work while wearing a respirator, (2) 

regular training of personnel, (3) periodic environmental monitoring, (4) respirator fit-testing, and (5) respirator maintenance, inspection, 

cleaning, and storage. The program should be 

evaluated regularly and respirators should be 

selected by the person who is in charge of the 

program and knowledgeable about the workplace and the limitations associated with each 

type of respirator. 

NIOSH provides guidance for selecting an appropriate respirator in NIOSH Respirator Selection Logic 2004 [NIOSH 2004] available online 

at http://www.cdc.gov/niosh/docs/2005-100/default.html. The selection logic takes into account 

the expected exposure concentration, other 

potential exposures, and the job task. For most 

job tasks involving TiO2 exposure, a properly 

fit-tested, half-facepiece particulate respirator 

will provide protection up to 10 times the respective REL. When selecting the appropriate 

filter and determining filter change schedules, 

the respirator program manager should consider that particle overloading of the filters 

may occur because of the size and characteristics of TiO2

 particles. Studies on the filtration 

performance of N95 filtering-facepiece respirators found that the mean penetration levels 

for 40 nm particles ranged from 1.4% to 5.2%, 

indicating that N95 and higher performing 

respirator filters would be effective at capturing airborne ultrafine TiO2

 particles [Balazy et 

al. 2006; Rengasamy et al. 2007, 2008]. 

In workplaces where there is potential worker 

exposure to TiO2, employers should establish 

an occupational health surveillance program. 

Occupational health surveillance, which includes hazard surveillance (hazard and exposure assessment) and medical surveillance, is 

an essential component of an effective occupational safety and health program. An important objective of the program is the systematic 

collection and analysis of exposure and health 

data for the purpose of preventing illness. In 

workplaces where exposure to ultrafine or engineered TiO2

 occurs, NIOSH has published 

interim guidance on steps that can be taken to 

minimize the risk of exposure [NIOSH 2009a] 

and recommendations for medical surveillance and screening [NIOSH 2009c] that could 

be used in establishing an occupational health 

surveillance program.

NIOSH CIB 63 • Titanium Dioxide 85

7 Research Needs 

Additional data and information are needed to 

assist NIOSH in evaluating the occupational 

safety and health issues of working with fine 

and ultrafine TiO2. Data are particularly needed on the airborne particle size distributions 

and exposures to ultrafines in specific operations or tasks. These data may be merged with 

existing epidemiologic data to determine if 

exposure to ultrafine TiO2 is associated with 

adverse health effects. Information is needed 

about whether respiratory health (e.g., lung 

function) is affected in workers exposed to 

TiO2. Experimental studies on the mechanism 

of toxicity and tumorigenicity of ultrafine TiO2

would increase understanding of whether factors in addition to surface area may be important. Although sampling devices for all particle 

sizes are available for research purposes, practical devices for routine sampling in the workplace are needed. 

7.1 Workplace Exposures and

Human Health 

•	 Quantify the airborne particle size distribution of TiO2

 by job or process and 

obtain quantitative estimates of workers’ 

exposures to fine and ultrafine TiO2. 

•	 Conduct epidemiologic studies of workers manufacturing or using TiO2

-containing products using quantitative estimates 

of exposure by particle size, including fine 

and ultrafine fractions (see bullet above). 

•	 Evaluate the extent to which the specific 

surface area in bulk TiO2 is representative 

of the specific surface area of the airborne 

TiO2 particles that workers inhale and 

that are retained in the lungs. 

•	 Investigate the adequacy of current massbased human lung dosimetry models for 

predicting the clearance and retention of 

inhaled ultrafine particles. 

7.2 Experimental Studies

•	 Investigate the fate of ultrafine particles 

(e.g., TiO2) in the lungs and the associated pulmonary responses. 

•	 Investigate the ability of ultrafine particles (e.g., TiO2

) to enter cells and interact 

with organelle structures and DNA in mitochondria or the nucleus. 

7.3 Measurement, Controls,

and Respirators 

•	 Develop accurate, practical sampling devices for ultrafine particles (e.g., surface 

area sampling devices). 

•	 Evaluate effectiveness of engineering controls for controlling exposures to fine and 

ultrafine TiO2. 

•	 Determine effectiveness of respirators for 

ultrafine TiO2. 

NIOSH CIB 63 • Titanium Dioxide 87

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NIOSH CIB 63 • Titanium Dioxide 105

APPENDIX A: Statistical Tests of the Rat Lung 

Tumor Models 

A–1. Goodness of Fit of Models

using Mass or Surface 

Area Dose Metric

As seen in Figures 3–4 and 3–5, particle surface 

area is a much better dose metric than particle 

mass for predicting the lung tumor response in 

rats after chronic exposure to fine and ultrafine TiO2

. The statistical fit of these models is 

shown in Table A–1, using either mass or particle surface area dose. These goodness-of-fit 

tests show that particle surface area dose provides an adequate fit to models using either the 

all tumor response or tumors excluding squamous cell keratinizing cysts and that particle 

mass dose provides an inadequate fit to these 

data. The P-values are for statistical tests of the 

lack of fit; thus, P < 0.05 indicates lack of fit. 

A–2. Heterogeneity Tests (Rat

Strain or Sex; Particle Size)

Because of the observed differences in tumor 

response in males and females, when squamous cell keratinizing cystic tumors were included in the analysis (Table 4–4), it was important to test for heterogeneity in response 

by rat sex. Since the data were from different 

studies and rat strains, these factors were also 

investigated for heterogeneity (the influence 

of study and strain could not be evaluated 

separately because a different strain was used 

in each study). Finally, the possibility of heterogeneity in response to fine and ultrafine 

TiO2 after adjustment for particle surface area 

was investigated to determine whether other 

factors may be associated with particle size that 

influence lung tumor response and that may 

not have been accounted for by particle surface area dose. Table A–2 shows that there was 

statistically significant heterogeneity between 

male and female rats for the all lung tumors response but not for the tumors excluding squamous cell keratinizing cysts. No heterogeneity 

in tumor response was observed across study/

strain or for fine versus ultrafine when dose 

was expressed as particle surface area per gram 

of lung. These analyses showed that all of the 

data from the different studies, rat strains, and 

both sexes [Lee et al. 1985; Muhle et al. 1991; 

Heinrich et al. 1995] could be pooled and the 

model fit was adequate when the dose metric 

used is particle surface area per gram of lung 

and the tumor response is neoplastic tumors 

(i.e., without squamous cell keratinizing cystic 

tumors). 

In addition, a modified dose-response model 

was developed to examine the all-tumor response (by adjusting for rat sex and to include 

the averaged male/female lung tumor response 

data in the Muhle et al. [1991] study) (see Section A–3). 
106 NIOSH CIB 63 • Titanium Dioxide

A–3. Modified Logistic

 Regression Model (All

 Tumors in Male and 

 Female Rats) 

A modified logistic regression model was 

constructed to use all tumor data (including 

squamous cell keratinizing cystic tumors) to 

account for heterogeneity in tumor response 

observed between male and female rats in the 

Lee et al. [1985] and Heinrich et al. [1995] 

studies. In addition, the Muhle et al. [1991] 

study reported tumor response for males and 

females combined. For these reasons, the standard models in the BMDS [EPA 2003] could 

not be used. The BMDS models do not allow 

for covariates (e.g., sex) or for alternative model structures to account for the combined data. 

In the modified logistic regression model, the 

total tumor count was evaluated as the sum of 

tumors from two distinct binomial responses. 

This implies that the expected response can be 

modeled as 

obs += pnpnN ffmm (equation 1) 

where += nnN fm and the set ( pm pf ) are binomial probabilities of tumor response for 

males and females that are modeled using the 

same assumptions of logistic regression. For 

example, female rats would have the following 

response: 

exp(1 )

exp( )

dose

dose

p

ff

ff

f ⋅++

⋅+ = βαβα (equation 2) 

that is the same as a logistic model that investigates only female rats. Thus, to model responses across studies using male, female, and male/

female combinations, equations (1) and (2) can 

be used when nm and nf

 are known. When they 

are not known (using results reported in Muhle et al. [1991] ), these quantities are estimated 

to be 2

n . 

With pm and pf now estimable using all data, 

the benchmark dose (BMD) can be computed 

by methods described by Gaylor et al. [1998]. 

Further the benchmark dose lower bound 

(BMDL) can be computed using profile likelihoods, which are described by Crump and 

Howe [1985]. For simplicity in the calculation, 

we compute the male and female BMDL at the 

nominal level of α = 0.025, which implies a 

combined nominal coverage α = 0.05. 

This model provides a method to examine the 

dose-response relationships based on all tumors using all available rat data for fine and ultrafine TiO2

 [Lee et al. 1985; Muhle et al. 1991; 

Heinrich et al. 1995]. Table A–3 provides the 

BMD (BMDL) excess risk estimates using this 

method on the pooled data for the response 

of either all-tumors or lung tumors excluding 

cystic keratinizing squamous lesions for the 

logistic regression model, which is one of the 

models in the BMDS model suite and which 

was selected based on the statistical properties of the logistic regression model that were 

amenable to developing this modified method 

(as discussed above). Table A–4 provides a 

comparison of the BMD (BMDL) excess risk 

estimates for all-tumors in male and female rat 

data, fit separately to each of the models in the 

BMDS model suite [EPA 2003].
NIOSH CIB 63 • Titanium Dioxide 107

Table A–1. Goodness of fit of logistic regression models to pooled rat data of lung tumor 

proportion and TiO2 dose (as retained particle mass or surface area in the lungs) in rats 

after 24-month exposure*

Dose metric Tumor response

Degrees of 

freedom

P-value 

(dose only 

model)

Degrees of 

freedom

P-value (dose 

and sex 

terms)

Surface area (m2/g lung) All tumors 10 0.056 8 0.29

Mass (mg/g lung) 10 < 0.0001 8 < 0.0001

Surface area (m2/g lung) No keratinizing cysts 10 0.50 8 0.62

Mass (mg/g lung) 10 < 0.0001 8 < 0.0001

*

Pearson test for lack of fit. In the model with both dose and sex terms, the slopes and intercepts are averaged for the 

male/female combined average data from Muhle et al. [1991]. Rat data are from two studies of fine TiO2 [Lee et al. 

1985; Muhle et al. 1991] and one study of ultrafine TiO2 [Heinrich et al. 1995] (12 data points total). 

Table A–2. Tests for heterogeneity of rat sex or study/strain in dose-response relationship, 

based on likelihood ratio tests 

Test * Tumor response

Degrees of 

freedom P-value Heterogeneity

Rat sex (male vs. 

female)†,‡

All lung tumors 2 0.012 Yes

No keratinizing cysts 2 0.14 No

Study/strain†,§ All lung tumors 4 0.46 No

No keratinizing cysts 4 0.44 No

Ultrafine vs. fine (in 

females)¶,**

All lung tumors 2 0.66 No

No keratinizing cysts 2 0.22 No

*

Null model includes two terms: intercept and slope × surface area dose (m2/g lung). 

†

Data include Lee et al. [1985] (male, female); Heinrich et al. [1995] (female); and Muhle et al. [1991] (male-female 

average)—12 data points total. 

‡

Full model includes four terms: separate intercepts and slopes for male and female rats (male-female average data were 

included assigned a value of 0.5 each for male and female indicators). 

§

Full model includes six terms: intercept and slope from null model (for comparison group), and separate intercept and 

slope terms for each of the other two study/strains.

¶

Data include females from Lee et al. [1985] and Heinrich et al. [1995]—6 data points total.

**Full model includes four terms: intercept and slope from null model (for comparison group), and separate intercept 

and slope terms for the other group. 
108 NIOSH CIB 63 • Titanium Dioxide

Table A–3. All tumors or lung tumors excluding cystic keratinizing squamous lesions: Logistic 

(sex-adjusted) model used to estimate benchmark dose (BMD) and lower 95% confidence 

limit (BMDL) estimates—expressed as TiO2 particle surface area in the lungs (m2/g)—in 

pooled rat data (males, female, and male-female average)*

Rat sex DF 

P-value

(for lack of fit)

BMD (BMDL) by excess risk level

1/10† 1/1000‡

Tumors excluding cystic keratinizing squamous lesions

Male

8 0.73

1.07 (0.81) 0.011

Female 1.04 (0.93) 0.010

All tumors

Male

8 0.35

1.01 (0.78) 0.010

Female 0.85 (0.75) 0.0085

*

Data are from two studies of fine TiO2 [Lee et al. 1985; Muhle et al. 1991] and one study of ultrafine TiO2 [Heinrich 

et al. 1995]. 

†

Estimated directly from model.

‡

Estimated from linear extrapolation of BMD and BMDL at 1/10 excess risk level.
NIOSH CIB 63 • Titanium Dioxide 109

Table A–4. All tumors: Benchmark dose (BMD) and lower 95% confidence limit (BMDL) 

estimates—expressed TiO2 particle surface area in the lungs (m2/g)—by model fit separately 

to male and female rat data 

MALE rats [Lee et al. 1985]

FEMALE rats

 [Lee et al. 1985; Heinrich et al. 1995]

P-value

(for lack

of fit)

BMD (BMDL) by excess risk level

P-value 

(for lack 

of fit)

BMD (BMDL) by excess risk level

Model

(BMDS 2003) 1/10* 1/1000* 1/1000† 1/10* 1/1000* 1/1000†

Gamma 0.51 1.11 0.54 0.011 0.20 0.76 0.20 0.0076

(0.65) (0.0062) (0.0065) (0.54) (0.038) (0.0054)

Logistic 0.64 1.00 0.026 0.01 0.15 0.86 0.050 0.0086

(0.82) (0.018) (0.0082) (0.77) (0.027) (0.0077)

Multistage 0.80 1.05 0.22 0.010 0.30 0.65 0.063 0.0065

(0.65) (0.0062) (0.0065) (0.51) (0.0080) (0.0051)

Probit 0.62 0.98 0.023 0.0098 0.24 0.79 0.044 0.0079

(0.78) (0.015) (0.0078) (0.70) (0.023) (0.0070)

Quantal-linear 0.40 0.87 0.0083 0.0087 0.068 0.37 0.0035 0.0037

(0.54) (0.0051) (0.0054) (0.30) (0.0028) (0.0030)

Quantal-quadratic 0.73 0.98 0.096 0.0098 0.30 0.65 0.063 0.0065

(0.78) (0.076) (0.0078) (0.58) (0.057) (0.0058)

Weibull 0.52 1.15 0.66 0.012 0.16 0.76 0.13 0.0076

(0.65) (0.0027) (0.0065) (0.52) (0.024) (0.0052)

*

Estimated directly from each model (in multistage, degree of polynomial: 3rd, male; 2nd, female). 

†

Estimated from linear extrapolation of BMD and BMDL at 1/10 excess risk level.

NIOSH CIB 63 • Titanium Dioxide 111

APPENDIX B: Threshold Model for Pulmonary 

Inflammation in Rats 

functions of dose; these functions were then 

used to estimate the critical dose at which the 

mean neutrophil levels went above the background. For the continuous functions that did 

not include a threshold parameter, this critical level was found using the BMD method 

[Crump 1984] and software [EPA 2003]. For 

purposes of calculation, the BMD was defined 

as the particle surface area dose in the lungs 

associated with )( μ dipmn corresponding to 

the upper 5th percentile of the distribution of 

PMN counts in control rat lungs. 

For the piecewise linear model, which is a threshold model, we assumed no dose-response, and 

thus no additional risk, above background prior 

to some critical threshold γ . For points beyond 

the threshold, the dose-response was modeled 

using a linear function of dose. For example, 

 . 

As the parameter γ is an unknown term, the 

above function is nonlinear and is fit using 

maximum likelihood (ML) estimation. Very 

approximate (1-α)% CIs can be found using 

profile likelihoods [Hudson 1966]. As the confidence limits are only rough approximations, 

the limits and significance of the threshold can 

be cross validated using parametric bootstrap 

methods [Efron and Tibshirani 1998]. 

A threshold model (i.e., piecewise linear or 

“hockeystick”) was examined for its ability to 

adequately represent TiO2

-induced pulmonary 

inflammation in rat lungs [Tran et al. 1999; 

Cullen et al. 2002; Bermudez et al. 2002; Bermudez et al. 2004]. As described in Section 

4.3.1.2, the TiO2

 pulmonary inflammation data 

from the Tran et al. [1999] and Cullen et al. 

[2002] studies could be fitted with a piecewise 

linear model which included a threshold parameter, and the threshold parameter estimate 

was significantly different from zero at a 95% 

confidence level. However, the fine and ultrafine TiO2

 pulmonary inflammation data from 

the Bermudez et al. [2002] and Bermudez et 

al. [2004] data sets provided no indication of 

a nonzero response threshold and were not 

consistent with a threshold model. The piecewise linear modeling methodology is detailed 

below. 

In modeling pulmonary inflammation (as neutrophilic cell count in BAL fluid) in rat lungs, 

the response was assumed to be normally 

distributed with the mean response being a 

function of the dose and the variance proportional to a power of the mean. Thus for the ith

rat given the dose di

, the mean neutrophilic 

cell count would be with variance 

, where μ pmn is any continuous 

function of dose,α is a proportionality constant, and ρ represents a constant power. The 

mean response was modeled using a variety of 

NIOSH CIB 63 • Titanium Dioxide 113

APPENDIX C: Comparison of Rat- and HumanBased Excess Risk Estimates for Lung Cancer 

Following Chronic Inhalation of TiO2

As described in Chapter 2, the epidemiologic 

studies of workers exposed to TiO2 did not find 

a statistically significant relationship between 

the estimated exposure to total or respirable 

TiO2 and lung cancer mortality [Fryzek et al. 

2003; Boffetta et al. 2004], suggesting that the 

precision of these studies for estimating excess risks of concern for worker health (e.g., 

≤1/1000) may be limited. The exposure data 

in Fryzek et al. [2003] were based on the total dust fraction, whereas respirable dust data 

were used to estimate exposures in Boffetta et 

al. [2004]. Neither study had exposure data for 

ultrafine particles. Chronic inhalation studies in rats exposed to fine [Lee et al. 1985] and 

ultrafine TiO2 [Heinrich et al. 1995] showed 

statistically significant dose-response relationships for lung tumors (Chapter 3). However, 

the rat lung tumor response at high particle 

doses which overload the lung clearance has 

been questioned as to its relevance to humans 

[Watson and Valberg 1996; Warheit et al. 1997; 

Hext et al. 2005]. Recent studies have shown 

that rats inhaling TiO2 are more sensitive than 

mice and hamsters to pulmonary effects including inflammation [Bermudez et al. 2002, 

2004], although the hamsters had much faster 

clearance and lower retained lung burdens of 

TiO2 compared to rats and mice. Because of 

the observed dose-response data for TiO2 and 

lung cancer in rats, it is important to quantitatively compare the rat-based excess risk estimates with excess risk estimates derived from 

results of the epidemiologic studies. 

The purpose of these analyses is to quantitatively compare the rat-based excess risks of 

lung cancer with results from the human studies. If the sensitivity of the rat response to inhaled particulates differs from that of humans, 

then the excess risks derived from the rat data 

would be expected to differ from the excess 

risks estimated from the human studies. The 

results of the comparison will be used to assess whether or not the observed differences 

of excess risks have adequate precision for reasonably excluding the rat model as a basis for 

predicting the excess risk of lung cancer in humans exposed to TiO2

. 

Methods

Excess risk estimates for lung cancer in workers were derived from the epidemiologic studies (Appendix D) and from the chronic inhalation studies in rats [Heinrich et al. 1995; Lee et 

al. 1985; Muhle et al. 1991]. These excess risk 

estimates and associated standard errors were 

computed for a mean exposure concentration 

of 2.4 mg/m3 over a 45-year working lifetime. 
114 NIOSH CIB 63 • Titanium Dioxide

This exposure concentration was selected to 

correspond to a low value relative to the rat data 

(which is also the NIOSH REL, see Chapter 5). 

Excess risks were derived from the rat data 

based on the three-model average procedure 

described in Chapter 4. The Model Averaging 

for Dichotomous Response (MADR) software 

used for the model average risk estimation was 

modified to output the model coefficients and 

model weights associated with each model fit 

in a 2,000-sample bootstrap, based on the rat 

data. The model coefficients and weights were 

then used to construct the distribution of risk 

estimates from rats exposed to the equivalent 

of a lifetime occupational exposure to 2.4 mg/m3

fine TiO2. The rat-equivalent exposures were 

estimated by using the MPPD2 model to estimate the human lung burden associated with 

a 45-year occupational exposure to 2.4 mg/m3

fine TiO2. This lung burden (2545 mg TiO2) 

was then extrapolated to rats following the 

steps described in Chapter 4 for extrapolating from rats to humans, but in reverse order. 

This procedure yielded an estimated rat lung 

burden of 6.64 mg TiO2. Since the rat-based, 

dose-response modeling was based on a particle surface area dose metric, the rat lung burden was then converted to 0.0444 m2

 particle 

surface area per gram of lung tissue, assuming 

a specific surface area of 6.68 m2/g TiO2. The 

2,000 sets of model equations from the model 

average were then solved for a concentration 

of 0.0444 m2/g lung tissue, yielding a bootstrap 

estimate of the distribution of the excess risk 

associated with exposure to TiO2. 

Excess risks were estimated from each of the 

two worker cohort studies, using two different 

methods for each. For the cohort studied by 

Boffetta et al. [2004], two different values for 

representing the highest cumulative exposure 

group were separately assumed; and for the 

cohort studied by Fryzek et al. [2003], two different exposure lags (no lag, 15-year lag) were 

separately used. 

Results 

Table C shows the rat-based maximum likelihood estimate (MLE) and 95% upper confidence limit (UCL) of excess risks for lung cancer and the human-based 95% UCL on excess 

risk from exposure to TiO2. There is consistency in the estimates of the 95% UCL from these 

two independent epidemiologic studies at the 

exposure concentration evaluated for both 

studies, 2.4 mg/m3 (Boffetta: 0.038 and 0.053; 

Fryzek: 0.046 and 0.056). The rat-based MLE 

and 95% UCL excess risk estimates for fine 

TiO2 exposures are lower than the 95% UCL 

risk estimates based on the human studies in 

Table C. This result suggests that the rat-based 

risk estimates for fine TiO2 are not inconsistent 

with the human risk estimates derived from 

the epidemiological studies. 

Discussion

These two epidemiologic studies are subject to 

substantially larger variability than are the rat 

studies. The results of the epidemiologic studies of TiO2

 workers by Fryzek et al. [2003] and 

Boffetta et al. [2003, 2004] are consistent with 

a range of excess risks at given exposures, including the null exposure-response relationship (i.e., no association between the risk of 

lung cancer and TiO2 exposure) and an exposure-response relationship consistent with 

the low-dose extrapolations from the rat studies (based on the method used, a three-model 

average as described in Chapter 4). Both the 

MLE and 95% UCL excess risk estimates from 

the rat studies were lower than the 95% UCL 

from the human studies for fine TiO2. 
NIOSH CIB 63 • Titanium Dioxide 115

In conclusion, the comparison of the rat- and 

human-based excess risk estimates for lung cancer suggests that the rat-based estimates for exposure to fine TiO2

 particles are not inconsistent 

with those from the human studies. Therefore, it 

is not possible to exclude the rat model as an acceptable model for predicting lung cancer risks 

from TiO2 exposure in workers. 

Table C. Comparison of rat-based excess risk estimates for lung cancer from TiO2 (using a 

model average procedure) with the 95% upper confidence limit (95% UCL) of excess risk 

of lung cancer in workers, at 2.4 mg/m3 exposure concentration, for a 45-year working 

lifetime*

TiO2 mean 

concentration

(mg/m3) over 

45-year working 

lifetime

Human-based 

excess risk (95% 

UCL): two different 

estimates from 

Boffetta et al. 

[2003, 2004]

Human-based 

excess risk (95% 

UCL): two different 

estimates from 

Fryzek et al. [2003]

Rat-based excess risk

estimate for fine TiO2

MLE 95% UCL

2.4 0.038†

0.053‡

0.056§

0.046¶

0.000005 0.0031

*

Methods notes: The value of 2.4 mg/m3 is a low value relative to the rat study, and is also the NIOSH REL. The MPPD 

human lung model [CIIT and RIVM 2002] was first used to estimate the lung burden after 45-years of exposure to 

a given mean concentration. The estimated retained particle mass lung burden was extrapolated from human to an 

equivalent particle surface area lung burden in rats, based on species differences in the surface area of lungs, and 

using the specific surface area value of fine TiO2 (6.68 m2/g). The rat dose-response model (three-model average, see 

Chapter 4) was then used to estimate the excess risk of lung cancer at a given dose. 

†

From Boffetta et al. [2003, 2004)] assumed 78.1 mg-yr/m3 in highest cumulative exposure group (respirable TiO2). 

‡

From Boffetta et al. [2003, 2004], assumed 56.5 mg-yr/m3 in highest cumulative exposure group (respirable TiO2). 

§

From Fryzek et al. [2003, 2004a,b]; Fryzek [2004] unlagged model (total TiO2). 

¶

From Fryzek et al. [2003, 2004a,b]; Fryzek [2004] model with 15-year lag (total TiO2).

NIOSH CIB 63 • Titanium Dioxide 117

APPENDIX D: Calculation of Upper Bound on 

Excess Risk of Lung Cancer in an Epidemiologic 

Study of Workers Exposed to TiO2 

Results from two epidemiologic studies [Fryzek 2004; Fryzek et al. 2003, 2004a; Boffetta et 

al. 2003, 2004] were used to compute the upper 

bound estimates of excess lung cancer risk. The 

excess risks for lung cancer corresponding to 

the upper limit of a two-sided 95% CI on the 

RR associated with cumulative exposure to total TiO2

 dust in U.S. workers were based on results supplied by Fryzek [2004] for Cox regressions fitted to cumulative exposures viewed as a 

time-dependent variable. The provided results 

include the coefficients and standard errors for 

the continuous model for cumulative exposure 

[Fryzek 2004]. For a study of United Kingdom 

and European Union workers exposed to respirable TiO2

 [Boffetta et al. 2004], excess risks 

for lung cancer were not available and therefore were derived from the results provided in 

a detailed earlier report Boffetta et al. [2003], 

as follows. The excess risk estimates computed 

from each of these epidemiologic studies were 

then used in Appendix C for comparison to 

the rat-based excess risk estimates for humans 

(Chapter 4). 

Methods

Categorical results on exposure-response are 

reported in Tables 4–1 (SMRs) and Table 4–2 

(Cox regressions) of Boffetta et al. [2003]. 

There are four categories—i.e., 0–0.73, 0.74–

3.44, 3.45–13.19, 13.20+ (mg/m3•yr)—in these 

results, and the maximum observed exposure 

is 143 mg/m3•yr (Table 2–8 of Boffetta et al. 

[2003]). Hence, the midpoints of the categories 

are 0.365, 2.09, 8.32, 78.1 mg/m3•yr. The value 

of the highest category depends on the maximum observed value and is subject to considerable variability. An alternate value for this 

category is 56.5 mg/m3•yr. This value is based 

on estimating the conditional mean cumulative exposure given that the exposure exceeds 

13.20 using the lognormal distribution that has 

median 1.98 and 75th percentile equal to 6.88 

based on results in Table 2–8 (Overall) of Boffetta et al. [2003]. Results are generated using 

both 78.1 and 56.5 mg/m3•yr to represent the 

highest exposure group. The SMRs reported in 

Table 4–1 were modeled as follows:

E[SMR] = Alpha*(1+Beta*CumX) where 

SMR = Y/E is the ratio of the observed to the 

expected count. 

=> E[Y] = Alpha*(1+Beta*CumX)*E fitted to 

observed counts (Y) by iteratively reweighted 

least squares (IRLS) with weights proportional 

to 1/E[Y]. 

Notes: Beta describes the effect of cumulative 

exposure, CumX, and Alpha allows the cohort 
118 NIOSH CIB 63 • Titanium Dioxide

to differ from the referent population under 

unexposed conditions. 

The estimators of Alpha and Beta are based 

on iteratively reweighted least squares with 

weights proportional to the reciprocal of the 

mean. Although these estimates are equivalent to Poisson regression MLEs, the observed 

counts are not strictly Poisson. This is due to 

the adjustments made by Boffetta et al. [2003] 

for missing cause of death arising from the limited time that German death certificates were 

maintained. The reported observed counts are 

53+0.9, 53+2.3, 52+2.7, 53+2.4 where 0.9, 2.3, 

2.7, and 2.4 have been added by Boffetta et al. 

[2003] for missing cause of death that are estimated to have been lung cancer deaths. Invoking a Poisson regression model should work 

well given such small adjustments having been 

added to Poisson counts of 53, 53, 52, and 53. 

Hence, Alpha and Beta are estimated accordingly but their standard errors and CIs do not 

rely on the Poisson assumption; instead, standard errors were estimated from the data and 

CIs were based on the t distribution with 2 degrees of freedom. 

A similar approach using the results of Table 

4–2 was not attempted since these categorical 

RR estimates are correlated and information 

on the correlations was not reported by Boffetta et al. [2003]. 

Results

Results based on modeling the SMRs in Table 

4–1 of Boffetta et al. [2003] with a linear effect 

of cumulative exposure are presented in Table 

D–1. These results are sensitive to the value 

used to represent the highest cumulative exposure category, particularly the estimate of the 

effect of exposure. However, zero is contained 

in both of the 95% CIs for Beta indicating that 

the slope of the exposure-response is not significant for these data. 

Estimates of excess risk based on application of 

the results given in Table D–1 to U.S. population rates using the method given by BEIR IV 

[1988] appear in Table D–2. 

Discussion 

The exposure assessment conducted by Boffetta et al. [2003] relies heavily on tours of the factories by two occupational hygienists who first 

reconstructed historical exposures without 

using any measurements (as described in Boffetta et al. [2003]; Cherrie et al. [1996]; Cherrie [1999]; Cherrie and Schneider [1999]). The 

sole use of exposure measurements by Boffetta 

et al. [2003] was to calculate a single adjustment 

factor to apply to the previously constructed 

exposure estimates so that the average of the 

measurements coincided with the corresponding reconstructed estimates. However, Boffetta 

et al. [2003] offer no analyses of their data to 

support this approach. Also, the best value to 

use to represent the highest exposure interval 

(i.e., 13.20 + mg/m3•yr) is not known and the 

results for the two values examined suggest that 

there is some sensitivity to this value. Hence, 

these upper limits that reflect only statistical 

variability are likely to be increased if the effects of other sources of uncertainty could be 

quantified. 
NIOSH CIB 63 • Titanium Dioxide 119

Table D–1. Results on Beta from modeling the SMRs reported in Table 4–1 of Boffetta et al. 

[2003] for the model, E[SMR] = Alpha*(1+Beta*CumX)

Value representing 

highest CumX Beta* estimate Approx std error

Approximate 95% 

confidence Limits

 78.1 0.000044 0.00163 -0.00697 0.00706

 56.5 0.000109 0.00229 -0.00975 0.00996

*Beta is the coefficient for the effect of 1 mg/m3

•yr cumulative exposure to respirable TiO2 dust.

Table D–2. Lifetime excess risk after 45 years of exposure estimated by applying the above 

UCLs on Beta and the linear relative rate model of lung cancer to U.S. population rates*

Occupational 

exposure (8-hr 

TWA respirable 

mg/m3)

Background 

risk (Ro)

Beta=0.000044 

excess risk†

 (Rx-Ro)

UCL=0.00706 

excess risk†

(Rx-Ro) 

Beta=0.000109 

excess risk‡

(Rx-Ro) 

UCL=0.00996 

excess risk‡

(Rx-Ro)

 0.0 0.056 0 0 0 0

 1.5 0.0002 0.024 0.0004 0.033

 5.0 0.0005 0.076 0.0012 0.11

 15.0 0.0015 0.21 0.0037 0.27

*Based on the method given by BEIR IV using U.S. population rates given in Vital Statistics of the U.S. 1992 Vol II Part 

A [NCHS 1996]. Occupational exposure from age 20 through age 64 and excess risks subject to early removal by 

competing risks are accumulated up to age 85.

†Value representing the highest exposure category is 78.1 mg/m3

 yr based on the midpoint of the interval [13.20, 143].

‡Value representing the highest exposure category is 56.5 mg/m3

 yr based on the conditional mean given exposures 

greater than 13.20 using the conditional distribution derived from the lognormal distribution having median and 

75th percentiles equal to 1.98 and 6.88 mg/m3 yr, respectively.
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