Hong Life CalculatorsUnderstanding the air quality index (AQI) calculator — concept and principle
The US Air Quality Index (AQI) turns a pollutant concentration into a number from 0 to 500 with six colour-coded categories, so that the public can understand health risk at a glance. This calculator handles 24-hour PM2.5, 24-hour PM10 and 8-hour ozone using the US EPA breakpoints, including the PM2.5 revision of 2024.
How it is calculated
- AQI = (I_hi − I_lo) ÷ (BP_hi − BP_lo) × (C − BP_lo) + I_lo, rounded to a whole number.
- C is the concentration, first truncated: PM2.5 to one decimal (µg/m³), PM10 and ozone to whole numbers (µg/m³, ppb).
- BP_lo and BP_hi are the breakpoints that bracket C; I_lo and I_hi are the AQI values for those breakpoints.
- PM2.5 breakpoints: 0–9.0, 9.1–35.4, 35.5–55.4, 55.5–125.4, 125.5–225.4, 225.5+ µg/m³.
- PM10: 0–54, 55–154, 155–254, 255–354, 355–424, 425–604 µg/m³. Ozone (8-h): 0–54, 55–70, 71–85, 86–105, 106–200 ppb.
The principle
Each breakpoint is tied to a health-based level, such as the national ambient air quality standard, which is set at AQI 100. Between breakpoints the index rises linearly, so equal steps in concentration give equal steps in AQI within a band, while bands of different width give different slopes. This piecewise scale lets very different pollutants share one easy-to-read number. When several pollutants are measured, the reported AQI is the highest of the individual values.
Worked example
- Inputs: PM2.5, C = 35 µg/m³ (already one decimal).
- 35 lies in the 9.1–35.4 band, which maps to AQI 51–100.
- AQI = (100 − 51) ÷ (35.4 − 9.1) × (35 − 9.1) + 51 = 49 ÷ 26.3 × 25.9 + 51 = 99.25.
- Rounded: AQI 99, category Moderate.
| Concentration | AQI |
|---|---|
| 8.75 | 48 |
| 17.5 | 67 |
| 26.2 | 83 |
| 35 | 99 |
| 43.8 | 121 |
| 52.5 | 143 |
| 70 | 161 |
How to read the result
- 0–50 Good; 51–100 Moderate; 101–150 Unhealthy for sensitive groups; 151–200 Unhealthy; 201–300 Very unhealthy; 301–500 Hazardous.
Above 100, people with heart or lung disease, older adults and children should reduce prolonged outdoor exertion; above 150 everyone should. Other countries use different scales: Korea's PM2.5 forecast, for example, uses 0–15 good, 16–35 moderate, 36–75 bad and 76+ very bad (µg/m³), so the same air can carry different labels.
Common mistakes
- Entering a 1-hour reading where the scale expects a 24-hour or 8-hour average.
- Entering ozone in ppm instead of ppb (0.070 ppm = 70 ppb).
- Comparing a US AQI with a national index from another country as if they were the same scale.
Frequently asked questions
Why did my AQI change in 2024 for the same PM2.5 level?
The US EPA lowered the annual PM2.5 standard to 9.0 µg/m³ and moved the top of the Good band from 12.0 to 9.0, so the same concentration now gives a higher AQI.
Can ozone 8-hour AQI exceed 300?
The 8-hour ozone scale stops at 200 ppb; higher levels are reported with the 1-hour ozone scale, which this calculator does not include.
Sources: US EPA, Technical Assistance Document for the Reporting of Daily Air Quality – the Air Quality Index (AQI), 2024 · US EPA, Reconsideration of the National Ambient Air Quality Standards for Particulate Matter, final rule, 2024