Trichloroethylene (TCE) is a chlorinated solvent used as a degreasing agent, as a chemical intermediate in refrigerant manufacture, and as a component of products such as spot removers and adhesives (ATSDR 2014). Its primary use as a degreaser for metal parts in the automotive industry and industrial settings has declined over recent decades because of environmental regulations governing TCE emissions. The U.S. Environmental Protection Agency (EPA) has proposed to prohibit TCE for use in vapor degreasing (82 Fed. Reg. 7432 [January 19, 2017]) and aerosol degreasing and for spot cleaning in dry cleaning facilities (81 Fed. Reg. 91592 [December 16, 2016]). Occupational exposure to TCE occurs in settings where the solvent is used or manufactured, and has become an exposure issue in less obvious workplace settings where the air has become contaminated as the result of vapor intrusion. Vapor intrusion occurs when volatile contaminants in groundwater and soil migrate under buildings, evaporate, and enter the indoor air through the floors and walls (EPA 2015). Because TCE is a common environmental contaminant at many industrial and government facilities, including those of the U.S. Department of Defense (DOD), vapor intrusion as a pathway of exposure is a concern at workplaces near contaminated sites.
TCE is a human carcinogen and has noncancer effects on the liver, kidneys, neurological system, immune system, reproduction, and development (EPA 2011; IARC 2014). To protect workers, occupational exposure guidelines have been established by the Occupational Safety and Health Administration (OSHA), the National Institute for Occupational Safety and Health (NIOSH), and the American Conference of Governmental Industrial Hygienists (ACGIH). These organizations differ in their recommendations; the permissible exposure limit set by OSHA (2019) is 100 parts per million (ppm) (8-hour [h] time weighted average [TWA]), NIOSH (2007) recommends an exposure limit of 25 ppm (10-h TWA), and ACGIH (2017) has a threshold limit value of 10 ppm (8-h TWA). These guidelines are for use in occupational settings where TCE is routinely used and stored, and not for the vapor intrusion pathway. DOD is concerned that none of the occupational regulations or guidelines are adequate to protect military or civilian personnel working at DOD installations who might be exposed to TCE through direct or indirect means. Thus, the U.S. Army Public Health Center was tasked with developing an occupational exposure level (OEL) for TCE, documented in a predecisional draft titled Trichloroethylene: Occupational Exposure Level for the
To facilitate the review of the large body of scientific literature on TCE, DOD relied on a 2011 assessment of the toxicity of TCE conducted by EPA as a starting point, and developed an approach to update the hazard assessment and conduct the analyses needed to develop an OEL. DOD’s approach involved using systematic review techniques to evaluate the scientific literature, and implemented relatively novel evaluation methods (e.g., study applicability tool, Bayesian approaches to address uncertainty) designed to fit its needs (Sussan et al. 2019). A physiologically-based pharmacokinetic model (PBPK) for TCE was also used (Covington et al. 2019).
Given the approaches and techniques that are being piloted for the OEL for TCE, DOD asked the National Academies of Sciences, Engineering, and Medicine to convene a committee to review the scientific and technical basis of its proposed OEL (see the Appendix for biographical information on the members). The committee was given the following task:
review the scientific and technical basis of DOD’s proposed approach to developing an OEL documented in the report Trichloroethylene: Occupational Exposure Level for the Department of Defense. The committee will
- Provide an analysis of the overall approach and suggestions on individual components of the report that may lead to improvements in the accuracy of the proposed process. Individual components include the literature review, evidence synthesis based on weight of evidence [WOE], point-of-departure derivation, use of physiologically based pharmacokinetic modeling, use of extrapolation tools, and other elements of the process.
- Determine if the process in deriving an OEL for TCE, including the WOE approach to determine relevance of controlled laboratory studies and overall approach corroborating alternative lines of evidence, is scientifically sound.
- Determine if the derived OEL value is supported by the toxicity information and has followed the WOE approach outlined in the report and provide a summary opinion of the approach and the scientific support for the derivation of the OEL.
- Determine whether the development of a range of cancer risk levels was appropriately supported.
- Due to the controversial nature of the evidence on developmental effects, determine whether DOD’s report considered this evidence in an unbiased manner that was consistent with its use of other toxicological evidence and used sound professional judgment in its evaluation of the evidence.
Two meetings were held to review the DOD documents. The first meeting on April 18, 2019, included a public data-gathering session that involved obtaining background information on the study request, hearing presentations on technical aspects of DOD’s approach to deriving an OEL, and obtaining perspectives on the approach from stakeholders.
As specified in the Statement of Task, the committee focused on evaluating the methodology and evaluations presented in DOD’s draft report. The committee did not perform an independent review of the literature on TCE. Similarly, DOD’s description of the PBPK model and its associated code were evaluated, but the committee did not run any model simulations.
DOD found that the existing approaches to conducting literature reviews and deriving exposure values did not fit its needs, and took elements from various methods to create its own approach. To understand what elements were adapted or modified, the committee reviewed DOD’s approaches in the context of more established approaches; this contextual review is presented in Chapter 2. Detailed evaluations of how DOD performed its hazard assessment are provided in Chapters 3 and 4. DOD’s approach to conducting quantitative dose-response assessments and the steps taken to deriving an OEL for TCE and determining cancer risk levels are reviewed in Chapter 5. Chapter 6 presents the committee’s overall conclusions about DOD’s proposed process for deriving an OEL for TCE.
ACGIH (American Conference of Governmental Industrial Hygienists). 2017. TLVs and BEIs Based on the Documentation of the Threshold Limit Values for Chemical Substances and Physical Agents & Biological Exposure Indices. Cincinnati, OH: ACGIH.
ATSDR (Agency for Toxic Substances and Disease Registry). 2014. Draft Toxicological Profile for Trichloroethylene. Atlanta, GA: ATSDR, Public Health Service, U.S. Department of Health and Human Services [online]. Available: https://www.atsdr.cdc.gov/toxprofiles/TP.asp?id=173&tid=30 [accessed April 23, 2019].
Covington, T.R., J.M. Gearhart, T.R. Sterner, D.R. Mattie, H.A. Pangburn, and D.K. Ott. 2019. Translation of a Physiologically Based Pharmacokinetic (PBPK) Model Used to Develop Health Protective Levels for Trichloroethylene. Air Force Research Laboratory, Wright-Patterson AFB, OH. Report No. AFRL-SA-WP-TR-2019-0006. February 2019.
EPA (U.S. Environmental Protection Agency). 2011. Toxicological Review of Trichloroethylene (CAS No. 79-01-6) in Support of Summary Information on the Integrated Risk Information System (IRIS), September 11, 2011. EPA/635/R-09/011F. Washington, DC: EPA [online]. Available: https://www.epa.gov/iris/supporting-documents-trichloroethylene [accessed April 24, 2019].
EPA. 2015. OSWER Technical Guide for Assessing and Mitigating the Vapor Intrusion Pathway from Subsurface Vapor Sources into Indoor Air. OSWER Publication 9200.2-154. September 2015. Washington, DC: Office of Solid Waste and Emergency Response, EPA [online]. Available: https://www.epa.gov/vaporintrusion/tech
nical-guide-assessing-and-mitigating-vapor-intrusion-pathway-subsurface-vapor [accessed April 24, 2019].
IARC (International Agency for Research on Cancer). 2014. Trichloroethylene. Pp. 35-217 in IARC Monographs on the Evaluation of Carcinogenic Risks to Humans, Trichloroethylene, Tetrachloroethylene, and Some Other Chlorinated Agents: Volume 106. Lyon, France: IARC [online]. Available: https://monographs.iarc.fr/iarc-monographs-on-the-evaluation-of-carcinogenic-risks-to-humans-10 [accessed April 24, 2019].
NIOSH (National Institute for Occupational Safety and Health). 2007. NIOSH Pocket Guide to Chemical Hazards. DHHS (NIOSH) Publication No. 2005-149. Cincinnati, OH: NIOSH, Centers for Disease Control and Prevention, Department of Health and Human Services [online]. Available: https://www.cdc.gov/niosh/npg [accessed April 24, 2019].
OSHA (Occupational Safety and Health Administration). 2019. Toxic and Hazardous Substances. Occupational Safety and Health Standards. 29 CFR 1910.1000, Table Z-2 [online]. Available: https://www.osha.gov/pls/oshaweb/owadisp.show_document?p_id=9993&p_table=STANDARDS [accessed April 24, 2019].
Sussan, T.E., G.J. Leach, T.R. Covington, J.M. Gearhart, and M.S. Johnson. 2019. Trichloroethylene: Occupational Exposure Level for the Department of Defense. January 2019. U.S. Army Public Health Center, Aberdeen Proving Ground, MD.