PM25

PM2.5, also known as PM25, refers to atmospheric fine particulate matter with an aerodynamic diameter of 2.5 micrometers or smaller. It is a key component of air pollution and is monitored in air quality assessments worldwide.

PM2.5 particles are a heterogeneous mix of solids and liquid droplets. They include inorganic compounds such as sulfates and nitrates, organic compounds, black carbon, metals, and secondary particles formed in the atmosphere from precursor gases like sulfur dioxide, nitrogen oxides, and ammonia. Because of their small size, PM2.5 can be inhaled deeply into the lungs and may enter the bloodstream.

Concentrations are typically expressed as micrograms per cubic meter (µg/m³). Measurements are collected by regulatory air quality networks using gravimetric, optical, and beta-attenuation methods, and are reported alongside health guidance and air quality indices.

Health effects are linked to both short-term exposures (hours to days) and long-term exposures (months to years). They include respiratory irritation, worsened asthma symptoms, reduced lung function, and increased risk of cardiovascular events such as heart attacks and strokes. Vulnerable groups include children, the elderly, and people with preexisting heart or lung disease.

Major sources include motor vehicle emissions, power generation, industrial processes, residential burning, wildfires, and natural dust. PM2.5 also forms secondarily in the atmosphere from precursor gases.

Regulatory standards and guidelines for PM2.5 vary by country. Many jurisdictions set limits on annual and short-term averages and publish air quality indices to communicate risk levels. The WHO provides guideline values to minimize health risks, while national agencies such as the U.S. EPA and the European Union establish location-specific targets and compliance requirements.

Mitigation measures focus on reducing emissions from transport, industry, and energy production, as well as improving monitoring networks and public health advisories.