A number of air pollutants are being found in low concentrations in indoor and outdoor air. Several studies have measured these concentrations for a large number of compounds in hundreds of residences. The concentrations range over several orders of magnitude over time and space. The log-normal distributions that are found make it possible to assess the impact on the health of populations, especially for genotoxic effects for which a threshold concentration is not expected to exist. Spreadsheets can be constructed which incorporate the unit risk of a substance for cancer, the distribution of concentrations of the substance in the population, and the size of the population into an assessment of the expected excess cases of cancer that can be attributed to the population exposure to such a toxicant. This approach allows for the evaluation of population risk for different substances as found in different environments, and for the evaluation of the relative efficacy of different strategies for population risk reduction.