The focus of public health concern and research in regard to environmental lung diseases has changed across the century. Illustrative agents include radon, indoor asbestos, environmental tobacco smoke, acidic aerosols, and oxidant gases. Tremendous progress has been made in understanding and preventing environmental lung diseases. However, we remain concerned about adverse consequences of breathing polluted outdoor and indoor air. In the persistent concerns about adverse effects of polluted air on the lung, a new emphasis is pervasive; the focus has shifted from avoiding clinical disease among highly exposed individuals to protecting the population from an unacceptable burden of risk. The technique of quantitative risk assessment has become increasingly important for characterizing the safety of environmental agents. The resulting emphasis on the final risk projection and attendant uncertainties may overly emphasize gaps in our knowledge.