Guidance is a faithful summary of the EPA/AirNow AQI guidance. It is informational, not medical advice; follow official local alerts and advice from a qualified clinician.
((100 - 51) / (100 - 51)) × (35.4 - 9.1) + 9.1These are inverse-interpolation estimates for the U.S. PM2.5 AQI. PM2.5 uses µg/m³; the common plain-text spelling is ug/m3.
| AQI | Estimated PM2.5 | EPA category |
|---|---|---|
| 25 | 4.5 µg/m³ | Good |
| 50 | 9.0 µg/m³ | Good |
| 75 | 22.0 µg/m³ | Moderate |
| 100 | 35.4 µg/m³ | Moderate |
| 150 | 55.4 µg/m³ | Unhealthy for Sensitive Groups |
| 200 | 125.4 µg/m³ | Unhealthy |
| 300 | 225.4 µg/m³ | Very Unhealthy |
| 400 | 275.2 µg/m³ | Hazardous |
| 500 | 325.4 µg/m³ | Hazardous |
The U.S. AQI uses a separate sub-index for each pollutant. For PM2.5, the method truncates concentration to one decimal place, selects its breakpoint interval, applies linear interpolation, and rounds the resulting AQI to a whole number.
AQI = ((Ihi - Ilo) / (Chi - Clo)) × (C - Clo) + Ilo
Inverse estimate: PM2.5 = ((AQI - Ilo) / (Ihi - Ilo)) × (Chi - Clo) + Clo
C is PM2.5 in µg/m³; I is AQI; lo and hi are the matched breakpoint bounds.
Reverse conversion is an estimate, not a recovery of an exact measurement. Because the forward method truncates concentration and rounds AQI, one reported whole-number AQI may correspond to several one-decimal PM2.5 values. The result panel enumerates those values and reports their interval.
This converter is informational, not medical advice or an official monitoring result. During smoke or pollution events, follow local public-health alerts, AirNow, and advice from a qualified clinician.
Breakpoint: AQI 51-100 and PM2.5 9.1-35.4 µg/m³.
((100 - 51) / (100 - 51)) × (35.4 - 9.1) + 9.1
= 35.4000 → 35.4 µg/m³
So AQI 100 has an inverse estimate of 35.4 µg/m³. In the forward method, truncated concentrations from 35.2 through 35.4 µg/m³ all round to AQI 100.
35.5 is already at one decimal. Breakpoint: PM2.5 35.5-55.4 and AQI 101-150.
((150 - 101) / (55.4 - 35.5)) × (35.5 - 35.5) + 101
= 101.0000 → round to AQI 101
This is the first value in Unhealthy for Sensitive Groups.
AQI 75 is inside the Moderate interval rather than on an edge.
((75 - 51) / (100 - 51)) × (35.4 - 9.1) + 9.1
= 21.9816 → 22.0 µg/m³
The possible one-decimal PM2.5 interval that forward-rounds to AQI 75 is 21.8-22.2 µg/m³.
First truncate, without rounding: 10.99 → 10.9 µg/m³.
((100 - 51) / (35.4 - 9.1)) × (10.9 - 9.1) + 51
= 54.3536 → round to AQI 54
Truncation matters: using 11.0 instead would produce a different unrounded result.
Boundary example: 9.0 µg/m³ is the last Good value and calculates to AQI 50; 9.1 µg/m³ is the first Moderate value and calculates to AQI 51. Likewise, 35.4 is Moderate/AQI 100, while 35.5 begins Unhealthy for Sensitive Groups/AQI 101.
| Category | AQI | PM2.5 concentration | Official color |
|---|---|---|---|
| Good | 0-50 | 0.0-9.0 µg/m³ | Green |
| Moderate | 51-100 | 9.1-35.4 µg/m³ | Yellow |
| Unhealthy for Sensitive Groups | 101-150 | 35.5-55.4 µg/m³ | Orange |
| Unhealthy | 151-200 | 55.5-125.4 µg/m³ | Red |
| Very Unhealthy | 201-300 | 125.5-225.4 µg/m³ | Purple |
| Hazardous | 301-500 | 225.5-325.4 µg/m³ | Maroon |
An app's overall AQI is the highest pollutant sub-index. It may be driven by ozone or PM10 rather than PM2.5.
The U.S., India, China, and other jurisdictions use different breakpoints and calculation rules.
A raw hourly or minute reading can change quickly. The PM2.5 breakpoints here apply to a 24-hour average or an EPA NowCast concentration.
AirNow's NowCast weights recent hourly readings to respond to changing conditions before converting the weighted concentration to AQI.
Low-cost and indoor monitors may use different calibration, humidity correction, averaging, or proprietary conversion methods.
Services may choose different stations, update times, spatial estimates, or forecasts. PM2.5 truncation and AQI rounding can also create small differences.
Methodology review: Starlight Tools checked the formulas, data handling, breakpoint table, and particle-pollution guidance against the sources below on July 18, 2026. No individual medical or scientific reviewer is claimed.
Revision note: This page uses the revised PM2.5 breakpoints that took effect May 6, 2024. Older U.S. tables placed the top of Good at 12.0 µg/m³; the current upper breakpoint is 9.0 µg/m³.
PM2.5 is a pollutant concentration measured in micrograms per cubic meter. AQI is a unitless public-health index calculated from a pollutant concentration. One overall AQI may represent whichever pollutant has the highest sub-index.
Only when the reported number is specifically the U.S. AQI sub-index for PM2.5. An overall AQI driven by ozone, PM10, carbon monoxide, sulfur dioxide, or nitrogen dioxide cannot be converted to PM2.5.
It may use a different pollutant, AQI system, time window, monitor, correction method, or station. AirNow may use NowCast weighting, while a low-cost sensor may show a raw short-term reading.
For an official-style U.S. PM2.5 AQI comparison, enter a 24-hour average or an EPA NowCast concentration. A single raw hourly or minute sensor reading is not directly equivalent to either one.
They use the older U.S. PM2.5 AQI breakpoints. EPA lowered the upper end of Good from 12.0 to 9.0 µg/m³, effective May 6, 2024. This converter uses the revised table.
This converter uses only the U.S. EPA AQI scale for PM2.5. India, China, and other countries use different index rules or breakpoints, so their AQI values are not interchangeable.
It means micrograms of particulate matter per cubic meter of air. It is the concentration unit used for PM2.5.
It uses the U.S. EPA PM2.5 AQI breakpoints effective May 6, 2024: 0.0-9.0, 9.1-35.4, 35.5-55.4, 55.5-125.4, 125.5-225.4, and 225.5-325.4 µg/m³ for AQI 0-500.
EPA AQI calculations truncate PM2.5 to one decimal place and round the calculated index to a whole number. Those two operations discard precision, so several concentrations can produce the same reported AQI.
The standard displayed AQI scale ends at 500. PM2.5 above 325.4 µg/m³ is still Hazardous, but this converter reports it as above 500 instead of extrapolating a precise standard AQI.
No. The converter runs locally in your browser and does not send the AQI or PM2.5 values you enter to a server.