PM2.5 (fine particulate matter)

PM2.5 refers to airborne particles with an aerodynamic diameter of 2.5 micrometres or less, arising predominantly from combustion sources — vehicle exhaust, power generation, wood burning and industrial processes — as well as secondary formation from gaseous precursors such as sulphur dioxide and nitrogen oxides. Their small size allows deep deposition in the alveolar region and translocation into systemic circulation, where they trigger oxidative stress, endothelial dysfunction and inflammatory cytokine release; carriers of the APOE ε4 allele may face heightened neurological risk through blood-brain barrier penetration. Dose-response analyses by Pope, Burnett and colleagues in large American cohorts have demonstrated continuous associations between long-term PM2.5 exposure and all-cause, cardiovascular and lung-cancer mortality without an apparent safe threshold. The WHO 2021 Air Quality Guidelines tightened the recommended annual mean limit to 5 µg/m³ — a level most European and North American cities still exceed.