Management of multi-media pollution to protect human health and the environment requires the establishment of standards. A variety of standards exist that include emission (mass based) standards for stationary and mobile sources. The implementation of these emission standards still results in residual concentrations in air and potentially other environmental media. Adverse health effects from such residual concentrations depend on the amount of exposure to a chemical and the inherent toxicity of the chemical. Exposure can occur by ingestion of contaminated media (soil, water, fish. vegetables, etc.), inhalation (ambient air or indoor air) or dermal contact (soil or water). Under the USEPA’s decision making models,  adverse health effects are broadly divided into carcinogenic and non-carcinogenic effects.   Chemical specific quantitative toxicity factors are typically developed based on animal experiments and the extrapolation of such results to humans using a variety of assumptions and structure activity relationships and supported by epidemiological studies, as appropriate. This paper will present the science and policies required to develop risk-based standards for various environmental media.