A simple method of weighting emissions data to provide a relation between source mass emissions and the resulting impact on air pollution is proposed. The importance of the following factors are considered: chemical severity of the pollutant species, source effectiveness, population exposure, and area severity. The selection of these factors is described with particular attention to the obtaining of chemical severity factors from air quality standards. Qualifications upon the use of this weighting scheme are outlined. Application to the Los Angeles and San Francisco bay areas show that weighted emissions of transportation sources are less than their mass emissions, while weighted emissions of all other pollutant sources are greater than their mass emissions. In both cases, transportation remains the dominant pollution source category.