What Is AQI & How
Is It Calculated?
The Air Quality Index (AQI) is the universal standard for communicating how clean or polluted the air is. Here's everything you need to understand it — from how the number is calculated to what each pollutant actually does to your body.
How the AQI Number Is Calculated
1
Measure
Government sensors measure concentrations of 6 pollutants every hour at monitoring stations.
2
Convert
Each pollutant's concentration is converted to a 0–500 sub-index using EPA breakpoint tables.
3
Take the max
The highest single-pollutant sub-index becomes the AQI. The worst pollutant sets the score.
4
Report
The AQI and the 'dominant pollutant' responsible for the score are published hourly.
The AQI Scale
0–50
Good
The air is clean and healthy.
✓ Safe
51–100
Moderate
Air quality is acceptable.
~ Mostly safe
101–150
Unhealthy for Sensitive Groups
Sensitive groups are at risk.
âš Use caution
151–200
Unhealthy
Air quality is unhealthy today.
✕ Not safe
201–300
Very Unhealthy
Health alert in effect.
✕ Not safe
301+
Hazardous
Hazardous — emergency conditions.
✕ Not safe
The 6 Pollutants That Make Up AQI
Each pollutant has different sources, health effects, and safe thresholds.
🔴
PM2.5
μg/m³
Fine Particulate Matter
Particles smaller than 2.5 microns bypass the nose and throat and embed deep in lung tissue. Linked to heart attacks, lung cancer, and premature death.
ðŸŸ
PM10
μg/m³
Coarse Particulate Matter
Larger than PM2.5 but still inhalable. Irritates the nose, throat, and airways. Particularly problematic for people with allergies or asthma.
🟡
O₃
ppb
Ground-Level Ozone
Irritates the respiratory system. Reduces lung function during exercise. Different from the protective ozone layer high in the atmosphere.
🟤
NOâ‚‚
ppb
Nitrogen Dioxide
Inflames airways and increases susceptibility to respiratory infections. Also contributes to the formation of ground-level ozone and PM2.5.
âš«
SOâ‚‚
ppb
Sulfur Dioxide
Irritates the eyes, nose, and throat. At high levels causes severe respiratory distress. Also forms particulate matter when it reacts in the air.
🔵
CO
ppm
Carbon Monoxide
Colorless and odorless. Reduces blood's ability to carry oxygen. At high levels causes confusion, loss of consciousness, and death.
Frequently Asked Questions
Why does the AQI change throughout the day?
Air quality follows predictable daily patterns. It's typically best in early morning (5–9 AM) when overnight cooling and reduced traffic keep pollution low. It worsens during morning and evening rush hours as vehicle emissions peak. Ozone specifically peaks in the afternoon because it requires sunlight to form. Weather also plays a huge role — wind disperses pollutants while temperature inversions trap them near the ground.
Is indoor air quality better than outdoor?
In most cases, yes — indoor air is typically 2–5x cleaner than outdoor air during pollution events. However, indoor sources like cooking, candles, cleaning products, and building materials can make indoor air worse than outdoor air on clean days. Running a HEPA air purifier and avoiding indoor combustion (candles, gas stoves without ventilation) keeps indoor air quality high.
What's the difference between US AQI and other AQI scales?
Different countries use different AQI scales. The US EPA AQI runs from 0–500 and is the most widely used internationally. China has its own AQI with different breakpoints. The EU uses an Air Quality Index that runs 1–100. This site uses the US EPA standard, which is the global benchmark for most air quality reporting services.
How is the AQI number actually calculated?
The EPA measures concentrations of six pollutants (PM2.5, PM10, ozone, NO₂, SO₂, and CO) at monitoring stations. Each pollutant's concentration is converted to a sub-index using EPA breakpoint tables. The highest single-pollutant sub-index becomes the overall AQI. So an AQI of 120 might mean ozone is at 120 while PM2.5 is only at 45 — the worst pollutant sets the score.
Should I trust the AQI if there's no monitoring station near me?
AQI values are interpolated between monitoring stations and can miss highly localised events like a nearby wildfire or industrial incident. For your immediate neighbourhood, services like PurpleAir (which use low-cost sensors) can supplement official monitoring. Official government stations are the most accurate but are spaced miles apart in most regions.
Our Data Sources
EPA AirNow
Official US government air quality monitoring network. Gold standard for US city data. Updated hourly.
airnow.gov
World Air Quality Index
Aggregates data from 11,000+ stations in 1,000+ cities worldwide. Backed by UNEP. Updated hourly.
aqicn.org
Open-Meteo
Open-source weather data from national meteorological services. Provides wind, humidity, and temperature context.
open-meteo.com