Conceptual methods of setting national ambient air quality standards in Japan are discussed in comparison with case of occupational exposure limits in industrial health. The air quality standards are set by the national government, based on recommendations of the Central Council on Countermeasures for Environmental Pollution, in conformity with the Fundamental Act on Countermeasures for Environmental Pollution. In setting the standard values, thresholds and the existence of specific groups in the people with high susceptibility to a certain pollutant are taken into account. The standards are set following minimum effect reports in three branches of health effect studies on air pollution: animal experiments, human exposure, and epidemiology. Safe levels take into account the absence of effects, suspicion of irreversible effects, dwellings of the elderly/infant/sick, epidemiologic growth of nonspecific diseases such as asthma, and so on. Under these conditions, air quality standards become far severer than in the case of industrial health limits.