Investigations have been made on the quantitative relationship between air pollution and human mortality. While primary focus has been on suspended particulates and sulfates from stationary sources of pollution, the evidence relating to air pollutants attributed to mobile sources was also examined. Using statistical analyses for a large number of US metropolitan areas, it was concluded that the benefits associated with a substantial abatement of air pollution from stationary sources are greater than the costs of such abatement. In contrast, the situation for mobile sources-chiefly cars and trucks is less clear-cut. That is, the costs of implementing the currently mandated US standards for automobile emissions probably exeed their potential health benefits.