Criteria air pollutants, including particulate matter, sulfur dioxide, nitrogen oxides, volatile organic compounds (precursors of ground-level ozone), carbon monoxide, and lead, and toxic air pollutants are a global concern. Historically, the Clean Air Act was concerned only with criteria air pollutants, the six common air pollutants for which the U.S. Environmental Protection Agency (EPA) had set standards. In 1990, the Clean Air Act was amended to address 188 chemical classes of air toxics.

A particular scenario that is receiving increased attention in the research and regulatory community is exposure to these compounds in near-road settings. Mobile source air toxics (MSATs) are emitted by vehicles either directly from exhaust systems or indirectly, such as from reentrainment of particle matter from roads. Addressing MSATs requires the combined expertise of engineering (for example, civil and mechanical engineering as applied to highway design and vehicle performance, respectively), the physical sciences (for example, particulate and gas phase partitioning of chemical compounds), and the social sciences (for example, decision theory as applied to selecting sites for representative samples from which to infer possible exposures).