A survey of recent literature on the effects of low levels of air pollutants on plants, and the results of our experiments with low levels of ozone (< 0.1 ppm) on bean (Phaseolus vulgaris), barley (Hordeum vulgare), and smartweed (Polygonum lapathifolium) indicate stimulations of various growth parameters when compared with growth in clean, filtered air. Increasing knowledge of the composition of air in various regions of the world has been interpreted by some as evidence of increased air pollution. We argue that such a conclusion may not necessarily be valid, and that the observed levels of individual constituents may be indicative of long-term air composition. The similarity between the ambient levels of pollutants reported from numerous locations and those which result in stimulations of plant growth when compared with clean air, suggests that plants exhibiting such stimulations have adapted to low pollution levels, and hence are at a disadvantage when grown in clean air. We argue that the use of filtered air controls in experiments on the effects of air pollutants on plants is therefore unrealistic and misleading, and should be replaced by the use of ambient levels of pollution.