OVERVIEW AND SUMMARY Finely divided particulate matter is found in ambient air everywhere. It has been a subject of concern as as potential hazard to human health@ a factor in visibility Impairment@ material deterionation@ and acidic precipitation. Total suspended particulate matter (TSP) has been identified as a criteria air pollutant to be regulated under the Clear Air Act of 1968. The definition of TSP is sampler dependent@ which has caused considerable controversy for its association with disease and other effects. As a part of the criteria pollutant review process@ the present National Ambient Air Quality Standards for TSP are currently being reevaluated. This process is required at regular intervals under the Clear Air Act. In the early 1970's@ two significant factors emerged from research on airborne particulate matter@ which bear on the NAAQS. The first was the identification of sulfate as a major chemical componment of TSP@ which had potential adverse health effects (e.g. US EPA 1975). The second was the recognition that TSP has two or more distinct mass contributions by particle size and by chemical character (e.g. Whitby and Sverdrup 1980). This segregation can be done by several criteria; the one which is most often cited refers to a coarse fraction between ~ 3 ?? and ~ 30 ?? diameter@ and a finely divided fraction ??3 ?? diameter. In the late 1970s@ a definition of inhalable particulate matter (IPM) of material less than ~ 15 ?? diameter also has been proposed. The basis for this is the definition of material which is likely to enter and be retained in the human respiratory system with high efficiency (Miller@ et al. 1979). Alternately@ a similar concept@ defining thoracic particles (TP) has been proposed@ where the upper size cut point is approximately 10 ?? diameter.