PM2.5 Air Quality Index Calculator
US AQI from a PM2.5 concentration using the 2024 EPA breakpoints, with the category and health guidance.
What it does
Fine particles are the smallest solid bits and droplets in the air, and the fraction at or below 2.5 micrometres is small enough to reach deep into the lungs. That fraction is PM2.5, and it is the one that drives most smoke and haze health warnings. Because a concentration in micrograms per cubic metre is hard to interpret, the US Environmental Protection Agency maps it onto the Air Quality Index, a scale from 0 to 500 that everyone can read.
The mapping is piecewise linear. The EPA divides the concentration range into bands, each with a matching index range, and inside a band the index moves in step with the concentration. The 2024 revision changed the PM2.5 bands after the annual standard was tightened, so the Good band now ends at 9.0 ug/m3 rather than the older 12.0, and the upper bands were redrawn to reflect the health evidence. Old charts and old app data can disagree with the current rules, which is why the version matters.
The categories carry advice rather than measurement. Good needs no action for most people. Moderate is acceptable, though unusually sensitive people may notice effects. Unhealthy for sensitive groups means those people should cut back on long or heavy outdoor exertion. Unhealthy affects everyone, and very unhealthy and hazardous call for reducing or avoiding outdoor exertion altogether.
Two practical notes matter. The index is built on a 24-hour average, so a single smoky hour is not the same thing as a daily value, and the reported AQI for a city is the highest sub-index among its pollutants, not just PM2.5. This tool handles the PM2.5 sub-index, which is what you want when you have a particle reading in front of you.
Above 325.4 ug/m3 the published table has no ordinary upper band, and the rule in Appendix G of the air monitoring regulations is to extend the top segment. The tool follows that rule, so a reading of 500 ug/m3 gives an index of 848 and the category stays Hazardous. Treat those numbers as a continuation of the scale rather than as a separately defined category.
How to use
- Get a PM2.5 concentration in micrograms per cubic metre from a monitor, a sensor or an air quality service.
- Enter the figure, using the 24-hour average when you are checking a daily report.
- Read the index and the category, then the breakpoint band so you can see which part of the table was used.
- Check the health guidance line, which is the EPA advice for that category.
- If your reading is a short term peak rather than a daily average, compare it with the daily figure before making plans.
- For wildfire smoke, recheck through the day, since concentrations can swing by a factor of several within hours.
Formula
- C
- PM2.5 concentration in micrograms per cubic metre, truncated to one decimal place
- C low
- Lower concentration breakpoint of the band
- C high
- Upper concentration breakpoint of the band
- I low
- AQI value at the lower breakpoint
- I high
- AQI value at the upper breakpoint
The truncation to one decimal place before interpolating is part of the official procedure, and it keeps two people with the same reading from getting different indices. Rounding happens only at the end, to the nearest whole number. Above 325.4 ug/m3 the top segment is extended for indices over 500, which is what the hazardous category footnote in Appendix G directs, so a reading of 500 ug/m3 gives 848.
Worked example
Example: 12 ug/m3 of PM2.5
A daily PM2.5 reading of 12 ug/m3, which sits in the Moderate band.
- Find the band: 12 falls between 9.1 and 35.4, which maps to an index of 51 to 100.
- Apply the formula: (100 - 51) / (35.4 - 9.1) x (12 - 9.1) + 51.
- The slope is 49 / 26.3 = 1.863, and 12 - 9.1 = 2.9, so the product is 5.40.
- Add the lower index: 51 + 5.40 = 56.4, which rounds to 56.
- The category is Moderate, and the guidance is that unusually sensitive people may feel effects.
- For comparison, 5 ug/m3 gives 28 and 100 ug/m3 gives 182.
Reference table
| PM2.5 (ug/m3) | AQI | Category |
|---|---|---|
| 0.0 to 9.0 | 0 to 50 | Good |
| 9.1 to 35.4 | 51 to 100 | Moderate |
| 35.5 to 55.4 | 101 to 150 | Unhealthy for sensitive groups |
| 55.5 to 125.4 | 151 to 200 | Unhealthy |
| 125.5 to 225.4 | 201 to 300 | Very unhealthy |
| 225.5 to 325.4 | 301 to 500 | Hazardous |
Limits
- This is the PM2.5 sub-index only, while the AQI a city reports is the highest of all its pollutant sub-indices.
- The index is defined for 24-hour averages, so a short peak can read differently from the daily number.
- It does not account for your personal health, medication or exertion level, and the guidance is general rather than medical.
- Low cost sensors drift and need correction, so a single sensor reading can be off by several micrograms per cubic metre.
Common mistakes
- Comparing a reading with an old AQI chart made before the 2024 PM2.5 revision.
- Treating the index as the concentration, when 50 index points do not mean 50 ug/m3.
- Using a one hour peak as if it were the 24-hour average that the index is built on.
- Assuming the reported city AQI is the PM2.5 sub-index, when it may come from ozone or another pollutant.
- Reading a low cost sensor at face value without checking it against a nearby regulatory monitor.
Try it yourself
Quick checks. Answers sit tucked under each question.
What index does 5 ug/m3 of PM2.5 give?
28, in the Good category. Inside the 0 to 9.0 band the index is 50 / 9 x 5 = 27.8, which rounds to 28.
What index does 100 ug/m3 give?
182, in the Unhealthy category. The band is 55.5 to 125.4, so 151 + 49 x (100 - 55.5) / 69.9 = 182.2, which rounds to 182.
What index does 300 ug/m3 give?
449, in the Hazardous category. Inside 225.5 to 325.4 the index is 301 + 199 x (300 - 225.5) / 99.9 = 449.4, which rounds to 449.
Questions
What is PM2.5?
Airborne particles with a diameter of 2.5 micrometres or less, including smoke, soot and secondary sulfate and nitrate. They are small enough to reach the deepest parts of the lungs.
Why did the PM2.5 breakpoints change in 2024?
Because the EPA tightened the annual PM2.5 standard and redrew the index bands to match the current health evidence. The Good band now tops out at 9.0 ug/m3, and the upper bands were revised too.
Is the AQI the same everywhere in the world?
No. The US scale runs to 500 and uses these breakpoints, while other countries use different bands and different reference periods, so the same concentration can map to a different number abroad.
Should I exercise outside at AQI 56?
For most people, moderate air quality is acceptable, though unusually sensitive people may notice effects. Anyone with heart or lung disease should use their own medical advice rather than the general band.
Does the index include wildfire smoke?
Wildfire smoke is largely PM2.5, so the PM2.5 sub-index captures it well. During a smoke event, check the PM2.5 figure and the daily average rather than a single hourly reading.
What does the AQI not tell me?
It does not measure indoor air, it does not cover every pollutant, and it does not personalise risk. It is a public communication tool, not a medical test.
Sources
- AirNow: how the Air Quality Index works
- US EPA: final updates to the AQI for particulate matter, fact sheet
- 40 CFR Part 58 Appendix G: AQI breakpoints and calculation