How Does TSCA Define “unreasonable risk of injury to health or the environment”?

Perhaps the most important term in all of the Toxic Substances Control Act (TSCA) is the term “unreasonable risk of injury to health or the environment” (or simply “unreasonable risk”). Under section 6 of TSCA, the U.S. Environmental Protection Agency (EPA) evaluates the risk of chemicals to determine whether uses present “unreasonable risk.” A determination of unreasonable risk triggers risk management regulations to ensure that the chemical no longer presents unreasonable risk. Thus, knowing what is and isn’t unreasonable risk is critical to effectively eliminating that risk. Yet, TSCA does not define unreasonable risk. Even in amending TSCA in 2016, Congress avoided a definition.   
In finalizing the rule on procedures for chemical risk evaluation under TSCA, EPA could have defined the term “unreasonable risk,” but like Congress, it opted not to. Instead, EPA delineated a list of “relevant factors” it will consider, including the following: 

• The effects of the chemical substance on health and human exposure to such substance under the conditions of use (including cancer and non-cancer risks); the effects of the chemical substance on the environment and environmental exposure under the conditions of use; the population exposed (including any susceptible populations), the severity of hazard (the nature of the hazard, the irreversibility of hazard), and uncertainties.

Given this rather lengthy list of “relevant factors,” one might assume that unreasonable risk would defy a succinct and easily understood definition. But if unreasonable risk is to be eliminated, then the regulated community needs to know precisely when a workplace, for example, poses unreasonable risk.     

The risk evaluations necessarily move a step closer toward elucidating unreasonable risk. After all, it is within the risk evaluations themselves that EPA renders unreasonable risk determinations. Table 5-1 of the methylene chloride risk evaluation, for example, provides the supporting basis for the unreasonable risk determination for occupational uses of methylene chloride and notes in particular: 

• “The checkmarks indicate the type of effect and the exposure route to the population evaluated for each condition of use that support the revised unreasonable risk determination for methylene chloride. This table is based on Table 4-2 of this Risk Evaluation.”  

But what is still needed for methylene chloride is a more definitive description of unreasonable risk.

Enter the proposed risk management rule for methylene chloride in which EPA establishes an 8-hour time-weighted average (TWA) existing chemical exposure limit (ECEL) of 2 ppm and a 15-minute short-term exposure limit (STEL) of 16 ppm. These are the benchmarks of unreasonable risk. Above these numbers = unreasonable risk; at or below these values = no unreasonable risk. Indeed, as stated in the proposed rule, as a matter of risk management policy, ensuring exposures remain at or below the ECEL will eliminate any unreasonable risk of injury to health.  

Not only does EPA quantitatively define unreasonable risk, but it also qualitatively defines it as little, if any, risk of adverse effects: 

• The ECEL represents the concentration at or below which an adult human, including a member of a potentially exposed or susceptible subpopulation, would be unlikely to suffer adverse effects if exposed for a working lifetime.  

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