A Background concentrations: is In this context, EPA uses background concentrations to mean the contributions to outdoor Air Toxics concentrations resulting from natural sources, persistence in the environment of past years' emissions and long-range transport from distant sources. Background concentrations could be levels of pollutants that would be found in 1996 even if there had been no recent manmade emissions. To accurately estimate outdoor concentrations, it is necessary to account for the background concentrations by adding them to the modeled concentrations. In this assessment, except for diesel PM, background concentrations are based on values identified in the Cumulative Exposure Project (study which estimated 1990 ambient concentrations of air toxics). From that study, EPA used background concentration values reported in the technical literature for 13 of the air toxic pollutants, and for the rest, EPA assumed a value of zero. For diesel PM, instead of using monitored air quality data to estimate background concentrations, a modeling-based approach was used. For more detailed information, see Background Concentrations.