The Toxic Substances Control Act (TSCA) allows for the regulation of new industrial chemicals if a chemical may present an unreasonable risk towards the environment or if a chemical has significant exposure towards the environment. As part of this regulatory process, an environmental hazard assessment is used to identify the effects of a chemical towards organisms in the environment, and their populations, communities, and ecosystems. In the United States Environmental Protection Agency's (USEPA) Office of Pollution Prevention and Toxics (OPPT), the aquatic toxicity of chemicals has been predicted using structure activity relationships (SAR) because of a lack of measured toxicity data. A SAR is the relationship between a chemical's toxicity and its chemical structure. Quantitative and qualitative SARs have been developed for dozens of chemical classes and their validation has been an ongoing process in OPPT since 1981.
The object of SAR validation is to test a SAR's accuracy and consists of comparing the predicted toxicity values of chemicals with the measured toxicity values. When predicted and measured values are similar, the SAR is assumed to be accurate. When the predicted and measured values are significantly different, either the SAR is reformulated or a new SAR is identified for another chemical class. The goal of SAR validation is to increase the accuracy of SARs. Newly measured toxicity data are either ntegrated into existing SARs or used to develop new SARs.
A summary and discussion of this validation process for several chemical class will be presented: neutral organic chemicals, organic chemicals which show excess toxicity relative to neutral organic chemicals with similar structure, anionic surfactants, cationic surfactants, polycationic polymers, cationic dyes, acid dyes, polyanionic monomers which are strong chelators of nutrient elements, and compounds which undergo hydrolysis (e.g., acid chlorides and alkyloxysilanes).