Methylene Chloride Expected to be Model for EPA Regulation of Styrene
The recently released final risk evaluation and upcoming issuance of a risk management regulation for methylene chloride will show how EPA intends to process styrene and other substances through EPA’s Toxic Substances Control Act program, according to comments by agency officials.
EPA’s program under the Toxic Substances Control Act for the “existing substances” like styrene that were already in commerce when the original TSCA statute passed in 1970. EPA will announce in the Federal Register when a chemical is added to the pool of substances to be considered for prioritization.
To assess non-cancer health risks associated with exposure to methylene chloride, EPA primarily used data from studies of laboratory animals to calculate the No Observed Adverse Effect Levels (NOAEL), the highest human-equivalent exposures not expected to be associated with health impacts such as organ toxicity, nervous system effects, and irritation. To estimate non-cancer risk, data from measurements or modeling of a workplace, consumer or general population exposure scenario are compared to the NOAEL divided by an uncertainty factor that can range from 30 to 100.
EPA assesses cancer risk by using data from studies on laboratory animals to draw a dose response curve, extrapolated to the origin, which is then used to estimate the human-equivalent exposures that would cause more than one additional cancer case per 1 million people exposed at that level (referred to as a risk of 1/106) and the exposure level that would cause more than one additional cancer case per 10,000 exposed people (1/104 risk). EPA considers these risk levels to be the maximum acceptable cancer risks for, respectively, general population and workplace exposures. The cancer risk in a specific workplace or general population scenario can be estimated applying the dose response curve to the concentration level in that scenario.
A dose response curve derived using data from animal studies (red dots) is used to estimate human-equivalent exposures associated with cancer incidence above background of 1 per 1,000,000 and 1 per 10,000 exposed people, which are the maximum cancer rates considered acceptable for, respectively, general population and workplace exposures.
In 52 of the 53 methylene chloride workplace and consumer exposure scenarios considered by EPA, the measured or modeled exposures exceeded either the non-cancer NOAEL-derived maximum exposure levels or the exposures associated with maximum acceptable cancer risk derived from the dose response curve. In other words, EPA found unacceptable levels of risk in 52 out of 53 scenarios.
In the methylene chloride risk management standard, planned for release in proposed form in 2023, EPA is expected to eliminate unacceptable human health risk by banning or limiting use of the substance or requiring adoption of workpractices or control technologies to prevent exposures associated with unacceptable risk in each of the 52 scenarios.
(EPA is still developing an approach for assessing general population exposures and risks and expects to update the methylene chloride risk evaluation to include fenceline exposure scenarios in 2023. For styrene, we should anticipate that the agency will model fenceline exposures for composites manufacturing operations and then assess risks using the same methods used for workplace and consumer exposures.)
The methylene chloride risk management standard will provide important insight into several issues critical to the composites industry’s ability to use styrene under the eventual TSCA standard for that substance, including how EPA will regulate a small-business intensive industry with significant exposures and few technically or economically feasible emission reduction options.
ACMA members can contact John Schweitzer for more information about regulation of the composites industry under TSCA.