A concise assessment of the risks to human health and the environment posed by exposure to 2- and 4-nitrophenol. These isomers are used as intermediates in the synthesis of a number of organophosphorus pesticides and some medical products. Environmental releases are primarily to air, water, and soil, and result from diffuse sources, including vehicle traffic and hydrolytic and photolytic degradation of pesticides. Further releases into the hydrosphere and the geosphere are caused by the dry and wet deposition of airborne nitrophenols from the atmosphere. Concerning behaviour within the environment, volatilization from water to air is expected to be slow or negligible. The report found no evidence that nitrophenols contribute directly to depletion of the stratospheric ozone layer or to global warming. Studies show that nitrophenols released to soil are biodegraded under aerobic conditions, with infiltration into groundwater occurring only under conditions unfavourable to biodegradation. Limited studies of kinetics and metabolism suggest rapid metabolism and excretion. Concerning toxic effects in laboratory animals, the formation of methaemoglobin is the most critical end-point following exposure by inhalation and, presumably, via the oral route as well. Other toxic effects include decreases in body weight gain, differences in organ weights, focal fatty degeneration of the liver, and haematological changes. Adequate data on carcinogenicity and mutagenicity were not available. Exposure of the general population to nitrophenol isomers occurs mainly through ambient air and drinking-water. Data on health effects in humans are confined to patch tests demonstrating skin sensitization in some subjects. In view of the extremely limited data, no conclusions concerning effects on human health could be reached.