PM2.5 refers to fine inhalable particles that are 2.5 micrometers or less in width. These particles are so small that they can be inhaled and cause serious health problems. Scientific studies have linked increases in daily PM2.5 exposure with higher cardiovascular and respiratory hospital admissions, emergency department visits, and deaths. Long-term exposure to fine particles causes increased mortality from heart disease and may be associated with increased rates of chronic bronchitis, reduced lung function, and lung cancer.

PM2.5 is a type of particle pollution, which is a mixture of solid particles and liquid droplets found in the air. Other types of particle pollution include PM10, which are inhalable particles with diameters that are generally 10 micrometers and smaller. PM2.5 is associated with the greatest proportion of adverse health effects related to air pollution, both in the United States and worldwide.

The Environmental Protection Agency (EPA) has established national ambient air quality standards for PM2.5 in outdoor air. The national annual average PM2.5 standard was most recently revised in 2012 following an exhaustive review of new literature that pointed to evidence for increased risk of premature mortality at lower PM2.5 concentrations than the existing standard. Air quality monitors measure concentrations of PM throughout the country, and EPA, state, tribal, and local agencies use that data to ensure that PM in the air is at levels that protect public health and the environment.