What Is PM2.5, and What Level Is Safe?

PM2.5 is the mass of airborne particles 2.5 micrometres across or smaller, in micrograms per cubic metre of air (µg/m³). They’re small enough to reach deep into the lungs and into the bloodstream, and long-term exposure is linked to heart disease, stroke, lung cancer, and early death. There’s no level known to be entirely safe: the World Health Organization (WHO) guideline is 5 µg/m³ as an annual average and 15 µg/m³ over 24 hours.

This article is general information, not medical advice.

The Sizes

• PM10: particles up to 10 µm, including dust and pollen, which mostly settle in the nose and upper airways.
• PM2.5: fine particles, from combustion (traffic, wood burning, cooking, and wildfire smoke) and from chemical reactions in the air.
• PM1 and PM4: finer and intermediate size cuts some monitors report as well.

Each larger measure includes the smaller particles, so PM10 is at least as high as PM2.5.

The Guidelines and Standards

Source	Annual average	24-hour average
WHO air quality guidelines (2021), PM2.5	5 µg/m³	15 µg/m³
US EPA standard (2024), PM2.5	9.0 µg/m³	35 µg/m³
WHO air quality guidelines (2021), PM10	15 µg/m³	45 µg/m³

The WHO’s 2021 guidelines halved the previous annual PM2.5 guideline, because the evidence showed harm at lower levels than before. The US Environmental Protection Agency (EPA) tightened its annual standard to 9.0 µg/m³ in 2024 and kept the 24-hour standard. National standards balance health with feasibility; the WHO’s guidelines are purely health-based.

Indoors

Indoor PM2.5 comes from outdoor air leaking in and from what you do inside: cooking, especially frying and searing, candles, toasting, and wood stoves. A short spike while cooking is normal; what matters most is the long-term average. A range hood vented outside and a portable air cleaner with a HEPA filter reduce indoor levels.

Low-cost monitors use laser light scattering. They aren’t reference instruments, so compare a monitor with itself over time rather than precisely with a guideline.

Following It in Kaizora

PM2.5 Mass Concentration, PM10 Mass Concentration, PM1 Mass Concentration, and PM4 Mass Concentration are air quality metrics on Air Quality, from your monitors’ files or logged by hand, each reading at a known position. Your exposure joins them with your location history: at each of your location fixes, your value is the newest reading within 100 metres in the hour before. The page also shows outdoor PM2.5 from the nearest public monitoring station, from the EPA’s AirNow, labelled preliminary. See How to Read the US Air Quality Index.

The Short Version

• PM2.5 are fine particles that reach deep into the lungs and bloodstream.
• The WHO guideline is 5 µg/m³ a year and 15 µg/m³ over 24 hours; the EPA’s annual standard is 9.0 µg/m³.
• Indoors, cooking and candles are common sources; follow the long-term average.

Sources

• World Health Organization, WHO global air quality guidelines: particulate matter (PM2.5 and PM10), ozone, nitrogen dioxide, sulfur dioxide and carbon monoxide (2021).
• US Environmental Protection Agency, National Ambient Air Quality Standards (NAAQS) for PM.