What Is PM2.5, and Why Does It Matter?

PM2.5 is fine particle pollution. The EPA says particulate matter is a mixture of solid particles and liquid droplets found in the air, and that PM2.5 means fine inhalable particles with diameters generally 2.5 micrometers and smaller, with the average human hair about 30 times larger than the largest fine particle. It says fine particulate matter is the biggest threat to your health in wildfire smoke, a mixture of gases and particles mainly from burning trees and plants.

The EPA says exposure to these particles can affect both the lungs and the heart, and that numerous scientific studies have linked particle pollution to premature death in people with heart or lung disease, nonfatal heart attacks, irregular heartbeat, aggravated asthma, decreased lung function and increased respiratory symptoms such as irritation of the airways, coughing or difficulty breathing.

On February 7, 2024 the EPA strengthened the national standards, setting the primary, health-based annual PM2.5 standard at 9.0 micrograms per cubic meter. After initially defending that rule, the EPA asked the court to vacate it, and on June 26, 2026 the US Court of Appeals for the District of Columbia Circuit denied both the petitions challenging the rule and the EPA’s motion to vacate it. Smog is related, the EPA says, a term first used around 1950 for the combination of smoke and fog in London that today refers to a mixture of pollutants made up mostly of ground level ozone.

Sources checked September 15, 2026. Every point below links the page it came from.

The sources behind this answer

  • Government sourceThe EPA says particulate matter is a mixture of solid particles and liquid droplets in the air, and that PM2.5 means fine inhalable particles with diameters generally 2.5 micrometers and smaller. US EPA ↗
  • Government sourceThe EPA says wildfire smoke is a mixture of gases and particles primarily from burning trees and plants, and that fine particulate matter, or PM2.5, is the biggest threat to your health among its pollutants. US EPA ↗
  • Government sourceThe EPA says studies have linked particle pollution to premature death in people with heart or lung disease, nonfatal heart attacks, irregular heartbeat, aggravated asthma, decreased lung function and increased respiratory symptoms. US EPA ↗
  • Regulatory filingOn February 7, 2024 the EPA strengthened the particulate matter standards, setting the primary annual PM2.5 standard at 9.0 micrograms per cubic meter. US EPA ↗
  • Legal caseIn Commonwealth of Kentucky v. EPA, decided June 26, 2026, the DC Circuit noted the EPA had moved to vacate its 2024 rule after first defending it, and denied both the petitions for review and the motion for vacatur. US Court of Appeals for the District of Columbia Circuit ↗
  • Government sourceThe EPA says the term smog was first used around 1950 to describe the combination of smoke and fog in London, and today refers to a mixture of pollutants made up mostly of ground level ozone. US EPA ↗

Sources checked September 15, 2026. A brand statement is the company’s own claim, which we report but have not verified. Our grades are model-based estimates, not lab tests.

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