Airborne particles and ozone are measured by health officials when determining levels of pollution in cities. The American Lung Association (AMA) recently released their "State of the Air" 2004 report that lists ozone and particle pollution across the U.S. by state. Officials generally monitor "course" particles in the 2.5um (micron) and 10um diameter range.
Andre Nel, a professor of Clinical Immunlogy and Allergy in the Department of Medicine at UCLA provides an overview of some of the research on airborne particles in a recent issue of Science Front Page (May 6, 2005). Nel classifies particles into three categories; coarse, fine and ultrafine. He reports that it is the fine (.1-2.5um) and ultrafine (less than .1um) particles that are unmeasured that are potentially the most dangerous. Fine and ultrafine particles are especially prevalent in cities as a result of vehicle emissions. Coarse particles are more like salts in ocean spray. The finer particles are not only difficult to measure, they are most likely to persist in the environment and to penetrate the lungs at the alveoli level.
Airborne particles in polluted air are thought to cause hundreds of thousands of premature deaths according to the World Health Organization (WHO) (here's a PDF report). As well as mortality increases, morbidity is affected by ozone. Cardiac and respiratory problems are associated with ozone pollution. Additionally health problems resulting from tissue inflammation, decreased cell clearance, oxidation stress and the release of cytokines are also suspected. More research is needed and we should be paying more attention to pollution producers.