Review of health effects driven by aerosol acidity: Occurrence and implications for air pollution control.

Acidity, generally expressed as pH, plays a crucial role in atmospheric processes and ecosystem evolution. Atmospheric acidic aerosol, triggering severe air pollution in the industrialization process (e.g., London Great Smoke in 1952), has detrimental effects on human health. Despite global endeavors to mitigate air pollution, the variation of aerosol acidity remains unclear and further restricts the knowledge of the acidity-driven toxicity of fine particles (PM_2.5) in the atmosphere. Here, we summarize the toxicological effects and mechanisms of inhalable acidic aerosol and its response to air pollution control. The acidity could adjust toxic components (e.g., metals, quinones, and organic peroxides) bonded in aerosol and synergize with oxidant gaseous pollutants (e.g., O₃ and NO₂) in epithelial lining fluid to induce oxidative stress and inflammation. The inhaled aerosol from the ambient air with higher acidity might elevate airway responsiveness and cause worse pulmonary dysfunction. Furthermore, historical observation data and model simulation indicate that PM_2.5 can retain its acidic property despite considerable reductions in acidifying gaseous pollutants (e.g., SO₂ and NO_x) from anthropogenic emissions, suggesting its continuing adverse impacts on human health. The study highlights that aerosol acidity could partially offset the health benefits of emission reduction, indicating that acidity-related health effects should be considered for future air pollution control policies.