AQI Calculator

Convert PM2.5, PM10, Ozone, NO₂, SO₂, or CO readings into AQI instantly.

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Pollutant Concentration

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How AQI is Calculated

The Air Quality Index (AQI) uses a piecewise linear function to convert raw pollutant concentrations into a 0–500 scale. The EPA defines six breakpoints for each pollutant — Good (0–50), Moderate (51–100), Unhealthy for Sensitive Groups (101–150), Unhealthy (151–200), Very Unhealthy (201–300), and Hazardous (301–500).

When multiple pollutants are measured simultaneously, the overall AQI equals the highest sub-index across all pollutants. This means a single severely polluted pollutant drives the entire reading.

PM2.5 AQI Breakpoints (EPA 2024)

PM2.5 is the most commonly reported pollutant because it poses the greatest health risk. The 2024 EPA standard tightened the annual standard from 12 µg/m³ to 9 µg/m³, reflecting growing evidence of harm at lower concentrations.

Readings below 12 µg/m³ fall in the Good category. Between 12.1 and 35.4 µg/m³ is Moderate. Above 55.5 µg/m³ is considered Unhealthy for everyone — not just sensitive groups.

PM2.5 vs PM10 — Which Matters More?

PM2.5 particles are 2.5 micrometers or smaller — roughly 30 times thinner than a human hair. They penetrate deep into lung tissue and can enter the bloodstream. PM10 particles are coarser and get trapped in the upper respiratory tract. Consequently, PM2.5 is the stronger predictor of cardiovascular and respiratory disease.

Sensitive Groups

Children, elderly adults, pregnant women, and people with asthma or heart disease experience symptoms at lower AQI values. The Unhealthy for Sensitive Groups category (AQI 101–150) is specifically designed to warn these populations before the air becomes harmful for the general public.

What is a good AQI number?

AQI 0–50 is Good — air quality is satisfactory and poses little or no health risk. AQI 51–100 is Moderate — acceptable quality, though some unusually sensitive people may have symptoms.

Is AQI 100 safe to exercise outside?

For most healthy adults, AQI 100 is acceptable for outdoor exercise. Sensitive groups — children, elderly, asthma sufferers — should reduce prolonged or heavy exertion at this level.

How often does AQI update?

Official monitoring stations typically report hourly averages. Some networks report every 10 minutes. Satellite-derived estimates update 1–4 times per day.

Does rain improve AQI?

Yes. Rainfall washes particulate matter out of the air and suppresses dust, which typically drops PM2.5 and PM10 readings significantly within an hour of sustained rain.