This papers introduces the concept of air pollution risk assessment (APRA) or health risk assessment (AP-HRA), describes in broad terms how the health risks of outdoor air pollution and its sources are estimated, and gives an overview of the general principles for the proper conduct of an AP-HRA for various scenarios and purposes  The main purpose of an AP-HRA is to estimate and communicate the health impact of exposure to air pollution or changes in air pollution in different socioeconomic, environmental, and policy circumstances. The first step of an AP-HRA is to assess the exposure of the target population to specific air pollutants. Monitoring data may be used to estimate the past and current exposure to air pollution for populations living near the monitoring site. In addition, air quality modelling is often used to estimate differences in exposure for different socioeconomic and environmental conditions within the geographical area of interest, and to predict changes in exposure in future policy scenarios. The second step of an AP-HRA is to estimate the health risk associated with the exposure to air pollution. This requires the use of concentration–response functions (CRFs), which quantify the health impact per concentration unit of a particular air pollutant. The third step of an AP-HRA is to quantify and express the uncertainty of the generated estimate of health impact. This is an important and integral component of the results, and it is vital to ensure both that the main message is not lost and that the results produced are understandable by policy-makers and others who do not necessarily have a technical background or expertise in AP-HRA. Results of AP-HRAs are often reported in terms of numbers of attributable deaths or cases of disease, years of life lost, disability-adjusted life years, or change in life expectancy attributable to exposure or a change in exposure to air pollution. These health impacts can then be used to evaluate costs and benefits of policy change in monetary terms.