“Unreasonable risk of injury to health or the environment” (or simply “unreasonable risk”) stands out as one of the most important terms in the Toxic Substances Control Act (TSCA). Yet, neither Congress nor the U.S. Environmental Protection Agency (EPA) defined the term. Even in amending TSCA in 2016, Congress avoided a definition. In 2017, and more recently in 2024, EPA promulgated regulations on procedures for chemical risk evaluation under TSCA section 6. EPA could have defined the term unreasonable risk, but like Congress, it opted not to.

So, how has EPA been in practice defining and applying unreasonable risk? In the previous article on unreasonable risk, we turned to the risk management rule for methylene chloride, in which EPA calculated a so-called existing chemical evaluation limit (ECEL) of 2 ppm for methylene chloride. Above 2 ppm, exposure to methylene chloride presents unreasonable risk; at or below that number, there is no unreasonable risk.

EPA defines the ECEL as the concentration at which an adult would be unlikely to suffer adverse effects if exposed for a working lifetime (250 days/year for 40 years). In calculating the ECEL, EPA selected liver effects as the most sensitive non-cancer endpoint, to protect sensitive individuals. This is the same endpoint that EPA’s IRIS program selected when it calculated a reference concentration (RfC) for methylene chloride.
 
The ECEL is strikingly similar to the RfC, defined as an estimate (with uncertainty spanning perhaps an order of magnitude) of a continuous inhalation exposure to the human population (including sensitive subgroups) that is likely to be without an appreciable risk of deleterious effects during a lifetime. Indeed, the calculated RfC for methylene chloride is within a factor of 3 of the ECEL.

Is an ECEL akin to an MCLG?

Under the Safe Drinking Water Act, a Maximum Contaminant Level Goal (MCLG) is the level of a contaminant in drinking water below which there is no known or expected risk to health. MCLGs allow for a margin of safety. For non-carcinogens, the MCLG is based on a reference dose (RfD) (which is the same as an RfC, except the route of exposure is different). An RfD, like an RfC (except for the RfC, the route of exposure is inhalation), is an estimate of a daily oral exposure to the human population (including sensitive populations) that is likely to be without an appreciable risk of deleterious effects during a lifetime.

So, what does this all boil down to?

If an ECEL is akin to an RfC; and an RfD, which is akin to an RfC, is the basis for a determination of “no known or expected risk to health” + “a margin of safety,” then doesn’t that mean an ECEL is akin to an MCLG? Does unreasonable risk = no known or expected risk to health plus a margin of safety?

This is not a trivial exercise. After all, if EPA finds unreasonable risk, then under TSCA 6(a), “[EPA] shall by rule, and subject to section 18 [preemption], and in accordance with subsection (c)(2) [which delineates factors for EPA to consider in selecting risk management requirements] apply one or more of the following requirements […] to the extent necessary so that the chemical substance no longer presents such risk.” (emphasis added)

What is and isn’t unreasonable risk, therefore, has profound implications on the chemical uses EPA ultimately regulates, which could include bans or prohibitions.