Methods were developed for the accurate analysis of an expanded list of hazardous organic chemicals in the ambient air. On-site analysis using an instrumented mobile laboratory was performed for a total of 44 organic chemicals. Twenty of these are suspected mutagens or carcinogens. Toxicity studies for several others are currently pending. Six important meteorological parameters were also measured. Four field studies, each about two-weeks duration, were conducted in Houston, Texas; St. Louis, Missouri; Denver, Colorado; and Riverside, California. An around-the-clock measurement schedule (24 hours per day, seven days a week) was followed at all sites, permitting extensive data collection. Widely varying weather conditions facilitated observations of pollutant accumulation and wide variabilities in concentrations of pollutants at a given site. Concentrations, variabilities, and human exposure (daily dosages) were determined for all measured pollutants. The diurnal behavior of pollutants was studied. Average daily outdoor exposure levels of all four sites were determined to be 197 micrograms/day for halomethanes (excluding chlorofluorocarbons), 140 micrograms/day for haloethanes and halopropanes, 89 micrograms/day for chloroalkenes, 32 micrograms/day for chloroaromatics, 1,394 micrograms/day for aromatic hydrocarbons, and 479 micrograms/day for secondary organics. Exposure levels at Houston, Denver, and Riverside were comparable, but levels were significantly lower at St. Louis.