Updated 1,4-dioxane risk evaluation reviews new exposure pathways

Regulatory background

• Under the Toxic Substances Control Act (TSCA), the Environmental Protection Agency (EPA) is required to evaluate the risks posed by existing and new chemicals. Beginning in December 2016, the EPA was tasked with reviewing a group of 10 chemicals that were prioritized for the potential risks and adverse health effects generated by exposure to the chemical.
• According to these risk evaluations, the EPA will begin by determining the proper scope of the evaluation, and review the hazards, exposures, conditions of use, and the expected populations exposed to a chemical. After the agency determines the scope of the evaluation, it will begin a hazard assessment to review the adverse health or environmental effects of exposure to the chemical. The hazard assessment is followed by an exposure assessment, which reviews the different pathways that the general population are exposed as well as exposure for susceptible subpopulations and occupational users.
• The next phase focuses on characterizing the risks determined from the hazard and exposure assessments, reviewing both the combined information as well as determining any data gaps or information quality issues from the assessments. At the end of the risk evaluation process, the EPA will make a risk determination that reviews what conditions of use present an unreasonable risk to health or the environment.
• For the purposes of risk management, the EPA now reviews the chemical as a “whole” rather than by conditions of use. What this has meant practically is that if a chemical has any conditions of use that represent unreasonable risks, all conditions of use will functionally be viewed as posing an unreasonable risk to health or the environment. During the risk management rulemaking, organizations can explain via comments or meetings to the EPA that a specific use does not pose an unreasonable risk. As most of the current proposed risk management rules ban almost all conditions of use for targeted chemicals, these explanations for why a use does not pose an unreasonable risk have been some of the only uses that have been allowed in proposed risk management rules.
• 1,4-dioxane (CAS RN 123-91-1) is a solvent used in many industrial processes as a stabilizer for chlorinated solvents as well as in paint strippers, greases, and waxes. It is also a byproduct of other paint strippers, greases, dyes, antifreeze, and some consumer products. 1,4-dioxane is completely miscible in water, meaning that it can easily move through ground or surface water. The chemical is relatively resistant to biodegradation in soil or water, but it has a relatively short half-life in the atmosphere of one to three days. 1,4-dioxane also does not bioaccumulate or biomagnify in the food chain.
• 1,4-dioxane is one of the first ten chemicals reviewed under the new TSCA risk evaluation framework. The risk evaluation process began in earnest in December 2016, and the document that reviews the scope of the risk evaluation was released in June 2017. The EPA followed the scoping document with a secondary document that reviewed problem formulation to better define the scope of the risk evaluation and clarify the chemical uses the agency expected to evaluate.
• In June 2019, the EPA released the draft risk evaluation for 1,4-dioxane, and followed it with supplemental analysis in November 2020 that targeted eight additional uses of 1,4-dioxane as a byproduct in consumer uses, where 1,4-dioxane worked as a breakdown product of other chemicals as well as reviewing risks of 1,4-dioxane in surface water.
• The agency released the final risk evaluation for 1,4-dioxane in December 2020, finding unreasonable risks to workers and occupational non-users in 13 conditions of use, but found no unreasonable risks to the environment, consumers, bystanders, or the general population.

Draft Supplement

• On July 7, 2023, the EPA published a 2023 Draft Supplement to the 2020 Risk Evaluation for 1,4-Dioxane which included additional human exposure pathways to the chemical. The agency explained that the final risk evaluation in 2020 had excluded certain known exposure pathways that were important for “understanding the health implications of exposure to 1,4-dioxane.” The supplement is intended to be used with the 2020 final risk evaluation to form a more comprehensive understanding of the risks posed by 1,4-dioxane for the purposes of risk management.
• The 2020 risk evaluation did not evaluate risks in the general population from 1,4-dioxane exposure in drinking water or air as well as the full range of exposure that may occur with 1,4-dioxane as a byproduct. The 2020 risk evaluation also did not explore some occupational-related exposures for 1,4-dioxane as a byproduct.
• The agency utilized high-end exposure scenarios for both occupational users as wellas the general population. For occupational users, the agency estimated high-end and central tendency exposures for both inhalation and dermal absorption pathways. For the general population, the EPA developed risk estimates based on exposure scenarios, exposed populations, and life stages with the highest levels of expected exposure. This included reviewing air exposures for fenceline communities within 10 kilometers of a 1,4-dioxane release site. All hazard values were derived from the 2020 risk evaluation without change, except for when the exposure duration was changed.
• Cancer continues to be the primary risk driver in the supplemental evaluations, and the agency chose to present its finding as cancer risk estimates. The agency explained that it has medium- to high-confidence in the underlying points of departure (PODs) used as the basis for the risk characterization.
• For occupational users, the agency determined that cancer risk estimates ranged from 8.1×10⁻⁷ to 8.6×10⁻⁴ for central tendency exposure and from 5.0×10⁻⁶ to 1.5×10⁻² for high-end exposures across different conditions of use. Inhalation exposures were set between 8.3×10⁻¹² to 1.8×10⁻³ for central tendency exposures and from 5.4×10⁻¹¹ to 2.3×10⁻² for high-end exposures across conditions of use.
• These occupational uses come from a variety of different industries including polyethylene terephthalate (PET) plastic manufacturing, where workers may inhale 1,4-dioxane generated as a byproduct of plastic manufacturing. Other occupational exposure scenarios include hydraulic fracturing (fracking) operations, ethoxylation processes, and industrial uses of dish soap or detergents.
• For the general population, the EPA reviewed cancer and non-cancer risks for exposures to 1,4-dioxane through surface water used as drinking water. The chemical is not readily removed through typical wastewater or drinking water processing, meaning that it can make its way to the tap from an industrial release into surface water. The agency reviewed concentrations of 1,4-dioxane in water to model potential risks and found a wide range of possible options. The agency determined that risk estimates from the models were greater than 1 in 100,000 in certain circumstances, particularly for industrial releases to surface water. The agency found lower overall risks in groundwater, which included exposure from landfill leachate, down-the-drain disposal, and other scenarios.
• The agency also reviewed the risks associated with 1,4-dioxane exposure in the air. The EPA determined that the highest estimated risks for 1,4-dioxane are within 1,000 meters of a facility that releases the chemical, and that aggregate exposures can result from multiple facilities releasing 1,4-dioxane in proximity to fenceline communities. The agency determined that cancer risk estimates for 95^th percentile modeled air concentrations at the highest risk facilities in each condition of use ranged from 1.0×10⁻¹⁰ to 1.1×10⁻⁴.

Next steps

• Comments on the draft publication will be open until September 8, 2023, via the link here. The EPA will also convene a Science Advisory Committee on Chemicals (SACC) meeting for peer review of the draft supplement. SACC will review the document prior to meetings scheduled for September 12-15, 2023. SACC had previously raised concerns about the EPA’s approach for the 2020 final risk evaluation based on evaluation and approach for the document, specifically zeroing in on the EPA omitting “well-known exposure routes.” Therefore, the draft supplement will likely be supported by SACC (with peer review edits on methodology or calculations also likely).
• The general thrust of risk management actions proposed over the last two years have been to restrict almost all uses of a target substance. The risk management rule for 1,4-dioxane is not likely to be very different, and the EPA may propose to prohibit all consumer and commercial uses but allow some specific industrial uses. However, in a recent speaking engagement at an Environmental Law Institute (ELI) event, Dr. Michal Freedhoff, Assistant Administrator for the Office of Chemical Safety and Pollution Prevention (OCSPP) recommended that companies that are able to demonstrate safe uses of any of the chemicals targeted for risk management under TSCA should contact the EPA to explain how the use does not pose an unreasonable risk. Dr. Freedhoff explained that these uses, when supported by data, are more likely to be allowed under the new risk management rules.

To contact the author of this analysis, please email Walker Livingston.
To contact the editor of this analysis, please email Patricia Iscaro.

Key Documents and Dates

• • 2023 Draft Supplement to the 2020 Risk Evaluation for 1,4-Dioxane (July 7, 2023)
  • Final Risk Evaluation for 1,4-Dioxane (December 2020)
  • Problem Formulation for 1,4-Dioxane (June 2018)
  • Scope Document for 1,4-Dioxane (June 2017)