Based on the limited existing data of ambient PM 2.5 concentrations, it appears likely that many areas of the country may exceed these new standards. These gaps are due to the fact that PM is a complex mixture of multicomponent particles whose size distribution, composition, and morphology can vary significantly in space and time. Atmospheric aerosol size ranges from a few nanometers to tens of micrometers. PM is emitted directly from sources such as diesel engines and is also formed in the atmosphere from gaseous precursors. The development of effective control strategies requires a better understanding of the link between PM 2.5 and the observed health effects.
Significant uncertainties exist in the penetration of outdoor aerosol to the indoor environment.
If exposures are not adequately characterized, then causal relationships between ambient PM and health effects may be erroneously attributed. Understanding the relationship between the outdoor and indoor PM concentrations in a typical house in the area will be one of the objectives of the Pittsburgh Supersite Program. Such a data set does not exist. The difficulty and cost of PM measurements have impaired our ability to characterize their temporal and spatial variability, understand the processes that control their formation and removal, and quantify the exposure of populations to them. Overcoming these difficulties requires the evaluation of existing PM measurement methodologies and development of new technologies which will allow the cost-effective, accurate measurement of PM characteristics.