The immediacy of the need for pollution-free vehicles is investigated by projecting to 2010 vehicle emission densities for eight urban areas and measures of population exposure to pollution for four cities. Three degrees of emission control are applied. Application of 50% control to 1968 automobiles maintains emission densities below present levels for the next 20 years, but is less effective in reducing population exposure to pollution, in some cases only slowing the rate of growth. Application of 50% control in 1968 and 80% in 1973 fails to reduce emission densities below 50% of the present. Reduction below 50% will eventually require an essentially pollution-free vehicle in many areas.