PM2.5
Also called: fine particulate matter, fine particles
PM2.5 is fine particulate matter: airborne particles 2.5 micrometers or smaller in diameter. An average human hair is about 30 times wider than the largest of them. The particles are small enough to reach deep into the lungs, and some enter the bloodstream, which makes PM2.5 the particle pollution with the greatest health risk.
Why It Matters
PM2.5 is the particle pollution the AQI tracks during smoke and haze. EPA notes that when wildfires affect air quality, the 2024 breakpoints may shift some days from Unhealthy to Very Unhealthy, or from Very Unhealthy to Hazardous. Some particles are emitted directly, from fires, smokestacks, construction sites, and unpaved roads. Many more form in the air from chemical reactions involving sulfur dioxide and nitrogen oxides from power plants, industry, and vehicles.
PM2.5 Concentration and AQI Category
| PM2.5, 24-hour average (µg/m³) | AQI category | AQI |
|---|---|---|
| 0.0 to 9.0 | Good | 0 to 50 |
| 9.1 to 35.4 | Moderate | 51 to 100 |
| 35.5 to 55.4 | Unhealthy for Sensitive Groups | 101 to 150 |
| 55.5 to 125.4 | Unhealthy | 151 to 200 |
| 125.5 to 225.4 | Very Unhealthy | 201 to 300 |
| 225.5 and higher | Hazardous | 301 and higher |
Breakpoints in effect since May 6, 2024. EPA's annual PM2.5 standard is 9.0 µg/m³ and its 24-hour standard is 35 µg/m³.
Source: U.S. Environmental Protection Agency: Final Updates to the Air Quality Index for Particulate Matter
Frequently Asked Questions
What is a safe level of PM2.5?
The Good AQI category covers 24-hour average PM2.5 up to 9.0 micrograms per cubic meter, which is also the level of EPA's annual health standard. EPA's 24-hour standard is 35 micrograms per cubic meter, the top of the Moderate range.
What is the difference between PM2.5 and PM10?
Size. PM10 is inhalable particles 10 micrometers or smaller, and PM2.5 is the fine fraction, 2.5 micrometers or smaller. Particles under 10 micrometers can get deep into the lungs, and the smallest pose the greatest health risk.
Where does PM2.5 come from?
Some is emitted directly, from fires, smokestacks, construction sites, unpaved roads, and fields. Much of it forms in the atmosphere from chemical reactions involving sulfur dioxide and nitrogen oxides emitted by power plants, industry, and vehicles.
Related Terms
AQI
The Air Quality Index (AQI) is the U.S. Environmental Protection Agency's color-coded scale for reporting air pollution. It has six categories, from Good (0 to 50) to Hazardous (301 and higher). Values of 101 and above are unhealthy, first for sensitive groups and then, as the number rises, for everyone.
Red Flag Warning
A Red Flag Warning is a National Weather Service alert that weather conditions could cause extreme fire behavior. It is issued when those conditions are occurring or expected within 24 hours. Typical criteria combine dry fuels with sustained winds of 15 mph or more and relative humidity of 25% or less, and exact thresholds vary by state and local office.
Temperature Inversion
A layer of the atmosphere where temperature increases with altitude instead of decreasing. Inversions trap pollutants and moisture near the surface, causing fog, smog, and haze. They also cap thunderstorm development.
Visibility
The horizontal distance at which objects can be seen and identified. U.S. aviation reports express prevailing visibility in statute miles. Below 3 statute miles falls in the IFR or LIFR weather category, but legal visual-flight minimums vary with airspace, altitude, and day or night under 14 CFR 91.155.
PM2.5: Guides and Explainers
These explainers connect this definition to real-world forecasting and decision-making.
Helpful Tools and Hubs
Use these pages to apply the concept, compare conditions, or follow related weather workflows.
Tool
AQI Exposure Planner
A free tool for working this out with your own numbers.
Feature
Air Quality
Track AQI, wildfire smoke, pollen, and pollutant breakdowns for your saved places.
Feature
Wildfire Tracking
Follow active wildfires, smoke forecasts, and fire weather conditions.
Feature
Conversational AI
Ask about rain timing, alerts, and what the forecast means in plain language.
Sources
Explore Other Terms
Tornado
A tornado is a violently rotating column of air in contact with the ground, extending from a thunderstorm. Tornadoes are rated from EF0 to EF5 on the Enhanced Fujita scale by the damage they cause. Most last less than 10 minutes, but the strongest can stay on the ground for more than an hour.
Sleet
Frozen raindrops that bounce when hitting the ground. Sleet forms when snowflakes partially melt falling through a warm layer, then refreeze in a cold layer near the surface. Unlike freezing rain, sleet is frozen before it hits the ground.
Humidity
The amount of water vapor present in the air. Absolute humidity measures the actual water content, while relative humidity expresses how close the air is to saturation at its current temperature, shown as a percentage.
Velocity
Doppler radar measurement of how fast precipitation is moving toward or away from the radar. Velocity data reveals wind patterns, rotation in thunderstorms, and potential tornado signatures that reflectivity alone cannot show.
Lifted Index
A stability index comparing the temperature of a rising air parcel to the environment at 500mb (about 18,000 feet). Negative values indicate instability. Values below -6 suggest strong instability and severe thunderstorm potential.
VFR
Visual Flight Rules allow flight using visual references when the applicable visibility and cloud-clearance minimums are met. Those minimums vary by airspace class, altitude, and day or night under 14 CFR 91.155. Separately, the VFR weather category means a ceiling above 3,000 feet and visibility greater than 5 statute miles.
Browse by Category
In the WeatherAI App
WeatherAI's air quality view includes AQI and pollutant detail where the source supplies it, for the U.S. and Canada on the free tier and worldwide with Standard or Pro.
Availability, coverage, update frequency, and alert delivery vary by data source, region, subscription plan, device permissions, and service conditions.