PM2.5 refers to fine particulate matter with a diameter of 2.5 micrometers or smaller — about 30 times smaller than a human hair. Despite their tiny size, these particles pose serious health risks and are one of the most monitored air pollutants in the US.
Where Does PM2.5 Come From?
PM2.5 has both primary and secondary sources:
Primary sources (directly emitted):
- Vehicle exhaust and tire wear
- Wildfire and residential wood burning
- Industrial processes and power plants
- Construction dust
Secondary sources (formed in the atmosphere):
- Chemical reactions between gases like sulfur dioxide, nitrogen oxides, and ammonia
- These reactions form sulfates, nitrates, and ammonium compounds
Why PM2.5 Is So Dangerous
Because PM2.5 particles are so small, they bypass the nose and throat's natural filters and penetrate deep into the alveoli (air sacs) of the lungs. From there, the finest particles can enter the bloodstream and travel to other organs.
Short-term exposure (hours to days) can cause:
- Aggravated asthma and respiratory symptoms
- Decreased lung function
- Irregular heartbeat
- Increased respiratory and cardiovascular hospital admissions
Long-term exposure (years) is linked to:
- Reduced lung development in children
- Chronic obstructive pulmonary disease (COPD)
- Lung cancer
- Premature death from heart and lung disease
EPA Standards for PM2.5
The EPA has set two standards for PM2.5:
- Annual standard: 9 µg/m³ (updated in 2024, reduced from 12 µg/m³)
- 24-hour standard: 35 µg/m³
Many US cities still exceed these standards regularly, particularly in the western US where wildfires are increasingly severe.
Protecting Yourself from PM2.5
- Check your local AQI daily, especially during wildfire season
- Use a True HEPA air purifier indoors
- Wear an N95 or KN95 respirator when outdoor PM2.5 levels are high
- Avoid exercising outdoors when AQI exceeds 100
- Keep car windows up and use recirculate mode in traffic