Despite a steady increase in life expectancy and a general improvement in the health of the population, we are still not all equal in the face of death and/or illness. Health inequalities persist between different population groups and, in particular, between different social classes: overall, people in the least advantaged socioeconomic groups live shorter lives and are in poorer health than those in the most advantaged socioeconomic groups. This issue is currently a major focus of study and research in public health, regularly fueling scientific and political debates. Numerous determinants have already been identified to explain the existence of these inequalities. Yet, today, some of them remain unexplained. It is precisely in this context that environmental hazards, and in particular exposure to air pollution, have been suspected of contributing to social inequalities in health (SIH). Two main mechanisms, documented in the literature, help explain how the environment might play a role in SHE: (1) disadvantaged populations may be exposed to a greater number of environmental hazards and/or higher levels of exposure (exposure differential) and/or (2) more disadvantaged populations may be more vulnerable to environmental effects (vulnerability/susceptibility differential) due to a more fragile health status compared to more advantaged populations. The purpose of this article is to present the current state of knowledge regarding the role that exposure to air pollution may play in ISS. (R.A.)