Outdoor air quality testing at the Wisconsin State Laboratory of Hygiene (WSLH) began in the 1970s with the advent of the federal Clean Air Act. Since then, air quality has emerged as a major environmental issue equal to or more important, from a public health standpoint, than water pollution. Epidemiological studies have shown that health issues are not limited to highly urbanized areas. In Wisconsin, local climatic conditions caused by the Great Lakes can result in unhealthy conditions even in relatively pristine areas. Air pollution affects thousands of Wisconsin residents each year, and it can be severe enough to require a physician's care. Although certain air testing (e.g., ozone) is done regionally by in situ monitors, the WSLH analyzes a variety of air pollutants including ozone precursor hydrocarbons, air particulates, and toxic metals. Exposure to aerosols containing metals may not follow typical patterns of air pollution based on routinely monitored particle mass.