AQI Calculator: Convert PM2.5 to AQI and Read the AQI Levels

AQI levels and the PM2.5 breakpoints

The Air Quality Index runs from 0 to 500, and each band has a name and a color. For fine particles (PM2.5, the smoke and soot small enough to reach deep into the lungs), the EPA ties each band to a range of 24-hour average concentrations. These are the breakpoints in force since May 2024, when the EPA tightened the top of the Good band from 12.0 to 9.0 µg/m³:

AirNow's AQI basics page gives the colors and the general health message for each level.

The conversion formula

The method comes from the EPA's technical assistance document for reporting the AQI. First the concentration is truncated (not rounded) to one decimal place. Then the calculator finds the band it falls in and draws a straight line between that band's ends:

AQI = (I_hi - I_lo) ÷ (C_hi - C_lo) × (C - C_lo) + I_lo

C is your concentration, C_lo and C_hi are the band's concentration breakpoints, and I_lo and I_hi are the AQI values at those breakpoints. The result is rounded to the nearest whole number. Above 325.4 µg/m³ the calculator shows 500+, the top of the scale.

Worked example: 24.5 micrograms

The calculator opens at 24.5 µg/m³, a common reading on a hazy summer afternoon or a day with smoke far upwind.

That value sits in the Moderate band, which runs from 9.1 to 35.4 µg/m³ and from 51 to 100 on the index. So: (100 - 51) ÷ (35.4 - 9.1) × (24.5 - 9.1) + 51 = 1.863 × 15.4 + 51 = 79.7, which rounds to AQI 80, Moderate. The tool's note for that level: unusually sensitive people may want to take it easier outdoors.

Watch the edges. A reading of 35.4 is AQI 100; 35.5 jumps to 101 and the Unhealthy for sensitive groups level, the point at which people with heart or lung disease, older adults, children and pregnant people should cut back on long or heavy exertion outside.

A sensor reading is not the official AQI

The AQI is built on health studies of longer exposures, which is why the official number uses a 24-hour average. For current conditions, AirNow uses the NowCast, which weights the most recent hours more heavily. The EPA advises against applying AQI values to minute-by-minute readings from low-cost sensors. Those sensors can also read high in humid air. The AirNow Fire and Smoke Map corrects sensor data before showing it.

So treat a single reading run through this calculator as a rough comparison, useful for deciding whether to open a window, not as an official value. If your indoor monitor and the AirNow number disagree, trust the trend over the exact figure.

What to do with the number

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At Unhealthy for sensitive groups and above, the useful move is to make one room as clean as possible. The EPA's clean room guidance says to pick a room you can close off, shut the windows and run a portable air cleaner sized for it. A 20-inch box fan with a MERV 13 furnace filter strapped to the back is the do-it-yourself version; the EPA suggests it when a commercial cleaner is not available or affordable. An indoor PM2.5 monitor tells you whether the room is actually getting cleaner.

If you have to be outside on a smoky day, a well-fitted N95 respirator filters fine particles; cloth and surgical masks do little against smoke. Keep exertion low and outdoor time short.

Clean air should not depend on owning a purifier. Many counties open clean air centers (libraries, community centers and senior centers with filtered air) during smoke events, and some health departments and utilities lend or give away air cleaners. Your local health department or 2-1-1 will know. Official monitors are also spread thin in many neighborhoods near highways and industry, which is why community sensor networks matter; our story on the Toxic Wastes and Race report traces how residents learned to measure their own exposure.

Everything above was picked by reading maker specs, published reviews and owner experience; none of it went through hands-on trials by us.

Smoke and heat often arrive together, and closed windows make a hot apartment hotter. Check the heat index calculator before you seal up for the day.

Questions

What are the AQI levels?

The Air Quality Index has six levels: Good (0 to 50), Moderate (51 to 100), Unhealthy for sensitive groups (101 to 150), Unhealthy (151 to 200), Very unhealthy (201 to 300) and Hazardous (301 and above). For PM2.5, Good covers 24-hour averages up to 9.0 µg/m³ under the breakpoints the EPA has used since May 2024.

How do I convert PM2.5 to AQI?

Truncate the 24-hour PM2.5 concentration to one decimal, find the EPA band it falls in, and interpolate on a straight line between the concentration and index breakpoints of that band, then round. For example, 24.5 µg/m³ falls in the Moderate band (9.1 to 35.4) and converts to an AQI of 80.

What AQI is unhealthy for outdoor exercise?

At 101 and above, people with heart or lung disease, older adults, children and pregnant people should reduce long or heavy exertion outdoors. At 151 and above, everyone should cut back, and at 201 and above everyone should avoid long or heavy exertion outside.

Why does my air sensor show a different AQI than AirNow?

Official AQI values use 24-hour averages, or the NowCast for current conditions, from regulatory monitors, while many home sensors report short averages that swing quickly. Low-cost sensors can also read high in humid air unless a correction is applied, as the AirNow Fire and Smoke Map does.

Do N95 masks protect against wildfire smoke?

A properly fitted N95 respirator filters most fine smoke particles, while cloth and surgical masks offer little protection against them. Fit matters: gaps around the edges or facial hair under the seal let smoky air in.