Thanks to a large number of epidemiological studies conducted over the past few decades, the health effects of air pollution are now better understood. A causal link is now recognized between exposure to particulate matter and ozone, and mortality and morbidity. While this impact may seem small at the individual level compared to other risk factors, it becomes significant when considering the entire population. Health impact assessments (HIAs) provide an objective estimate of the potential health benefits that could result from improved air quality for a population. They serve as a decision-making tool, enabling stakeholders to plan and implement measures to protect public health more effectively. This practical guide is intended for public health professionals wishing to conduct a health impact assessment of urban air pollution using the counterfactual approach, that is, estimating the hypothetical impact of a reduction in air pollution, all other things being equal. It details the main steps required to conduct an HIA and includes simple calculation tools that can be used with routine environmental and health data. The results of the HIA are expressed in terms of the number of events prevented and gains in life expectancy. An economic impact assessment is also provided. (R.A.)