Created by
Michael R. Meuser,


Air toxics concentrations for 1990 were estimated in the Cumulative Exposure Project with the use of computer dispersion modeling, rather than by measurement. The best way to assess the accuracy of the modeled pollutant concentrations is to compare them with available real-world measurements.

To evaluate the modeled 1990 concentrations, EPA gathered more than 700 measurements of annual average air toxics concentrations for 1990, representing 19 air toxics in up to 81 locations each. At each monitor location, the modeled air toxics concentration was compared to the monitored concentration by calculating a model-monitor ratio (modeled concentration divided by monitored concentration). This analysis found that: 1) about two-thirds of the time, the modeled concentration is within a factor of three of the monitored concentration (i.e., the model-monitor ratio is between 0.33 and 3.0); 2) when the modeled concentration and monitored concentration differ by more than a factor of three, the model is almost always too low, rather than too high; and 3) the model underestimates monitored concentrations about three times as frequently as it overestimates. Several state air pollution agencies have reached similar conclusions in their own comparisons of the modeled concentrations to air toxics monitoring data. Some of these states do not have air toxics monitoring data for 1990, and therefore used measurements available for more recent years. Examples of these analyses and their findings include:

It should be noted that these results represent comparisons for a limited number of air toxics in a limited number of geographical areas. While these results cannot necessarily be extrapolated to other air toxics in other areas, they provide the best indication available of how well the model results relate to real-world data. We recommend directly comparing ambient monitoring data, if available, when interpreting modeled outputs for a particular area.

Contact Daniel Axelrad at for more information about EPA's CEP. Also, please visit their website (below).

Visit EPA's Cumulative Exposure Project website to learn more..

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Silicon Valley Toxics Coalition

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Michael R. Meuser, Clary-Meuser Research Associates

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