A substantial body of evidence has accumulated on the existence of health inequalities and the socioeconomic factors that explain them, showing that a significant proportion of these inequalities is attributable to environmental problems. These factors are generally, though not exclusively, linked to social, economic and gender-related variables. The living environment influences socioeconomic and health inequalities mainly through two interrelated mechanisms: differential exposure, whereby socioeconomically disadvantaged population groups experience greater exposure to environmental hazards such as air pollution, and differential susceptibility, whereby these same groups suffer more severe adverse health effects due to their greater vulnerability.

Air pollution affects health through both short-term and long-term exposures. Acute exposure is associated with increases in mortality, hospital admissions and healthcare visits for respiratory and cardiovascular diseases, as well as acute symptoms, physiological changes, work and school absenteeism, and reduced performance. Prolonged exposure, by contrast, is linked to higher mortality from respiratory and cardiovascular causes, increased incidence and prevalence of chronic conditions such as asthma or chronic obstructive pulmonary disease (COPD), lung cancer, and chronic cardiovascular diseases. Methodologically, short-term effects are typically assessed using ecological designs based on time-series analyses, whereas long-term effects are mainly examined through individual-level spatial or spatio-temporal study designs.