As the sensitivity and precision of functional tests improves, we become increasingly able to measure responses to pollutant exposures with little, if any, demonstrable health significance. Proper interpretation of such functional responses generally requires an ability to evaluate them in the context of a larger body of knowledge of biochemical, toxicological, morphometric, and pathological effects data for the particular pollutant following various short- and long-term exposures. There are few, if any, generic rules or guidelines for interpreting the significance of functional responses to air pollutants at this time, especially when their magnitude is small and they are transient. The paper presents brief descriptive summaries of functional tests which can be used for characterizing pollutant responses in field and laboratory studies, and discusses how such responses can be examined for their health significance. The specific pollutants to be discussed will be SO2, NO2, O3, and H2SO4. The scientific and ethical limitations of epidemiologic, laboratory, and animal studies are discussed.