AQI Explained: Understanding the Air Quality Index

What Is AQI?

The Air Quality Index (AQI) is a scale that tells you how polluted the air is right now and what that means for your health. The higher the number, the worse the air quality and the greater the health risk.

Two widely encountered systems are the US EPA AQI, used in the United States and referenced internationally, and the European AQI, published by the European Environment Agency for locations across Europe. Many countries also publish national or local indices, so check which scale an app or authority uses before comparing numbers.

What Pollutants Does AQI Measure?

Both systems calculate AQI based on a set of key pollutants. Each one is measured independently, and the overall AQI is determined by whichever pollutant scores the worst; that pollutant is reported as the “dominant” pollutant. So an AQI of 120 doesn’t mean every pollutant is at 120; it means at least one has reached that level.

The EPA index covers the six rows below, treating PM2.5 and PM10 as two forms of particle pollution. The European AQI covers all except CO.

Pollutant What It Is PM2.5 (Fine Particulate Matter) Microscopic particles that penetrate deep into the lungs and bloodstream. Sources: vehicle exhaust, industrial emissions, wildfires. Often the dominant pollutant in urban areas. PM10 (Coarse Particulate Matter) Larger particles from dust, pollen, construction, and road debris. O₃ (Ground-Level Ozone) Formed when pollutants such as nitrogen oxides and volatile organic compounds react in sunlight. Levels often build on hot, sunny days, although weather and transported pollution also matter. Harmful to breathe, unlike the protective ozone layer high in the atmosphere. NO₂ (Nitrogen Dioxide) From vehicle engines and power plants. Contributes to smog and irritates airways. CO (Carbon Monoxide) Colourless, odourless gas from incomplete combustion. Reduces oxygen delivery to organs. SO₂ (Sulphur Dioxide) From burning fossil fuels. Causes respiratory problems and contributes to acid rain.

US EPA AQI

The EPA converts pollutant concentrations into a number from 0 to 500, divided into six colour-coded levels. For each pollutant, an AQI of 100 generally corresponds to the short-term national air quality standard. Above 100, the air is considered unhealthy first for pollutant-specific sensitive groups, then for more people as the value rises.

AQI Level What It Means 0–50 Good Air quality is satisfactory and pollution poses little or no risk. 51–100 Moderate Air quality is acceptable, but unusually sensitive people may be at risk. 101–150 Unhealthy for Sensitive Groups Members of sensitive groups may experience health effects. The general public is less likely to be affected. 151–200 Unhealthy Some people may experience health effects, while sensitive groups may experience more serious effects. 201–300 Very Unhealthy Health alert: the risk of health effects is increased for everyone. 301–500 Hazardous Health warning of emergency conditions: everyone is more likely to be affected.

These categories describe risk. The best action depends on the dominant pollutant, your health and how long or intensely you plan to be outdoors. Use the pollutant-specific advice in your local alert or the AirNow activity guides.

EPA Concentration Breakpoints

EPA pollutant indices use different averaging periods. The table below shows the breakpoints used for AQI values up to 500. Ozone uses an 8-hour value through AQI 300; the hazardous range is based on a 1-hour value. A 1-hour ozone index may also be reported from AQI 101 upward.

Level PM2.5, 24 h (µg/m³) PM10, 24 h (µg/m³) O₃, 8 h (ppb) O₃, 1 h (ppb) NO₂, 1 h (ppb) CO, 8 h (ppm) SO₂, 1 h (ppb) Good 0–9.0 0–54 0–54 Not used 0–53 0–4.4 0–35 Moderate 9.1–35.4 55–154 55–70 Not used 54–100 4.5–9.4 36–75 Unhealthy for Sensitive Groups 35.5–55.4 155–254 71–85 125–164 101–360 9.5–12.4 76–185 Unhealthy 55.5–125.4 255–354 86–105 165–204 361–649 12.5–15.4 186–304 Very Unhealthy 125.5–225.4 355–424 106–200 205–404 650–1249 15.5–30.4 305–604 Hazardous 225.5–325.4 425–604 Not used 405–604 1250–2049 30.5–50.4 605–1004

Source: EPA AQI Breakpoints.

European AQI (EAQI)

The European Environment Agency takes a different approach. Its hourly index maps pollutant concentrations directly to six named levels, and the overall level is determined by the pollutant with the poorest result. It describes short-term air quality, not annual exposure or legal compliance.

Level General Population Sensitive Population Good Enjoy your usual outdoor activities. Enjoy your usual outdoor activities. Fair Enjoy your usual outdoor activities. Enjoy your usual outdoor activities. Moderate Enjoy your usual outdoor activities. Consider reducing intense outdoor activity if you experience symptoms. Poor Consider reducing intense outdoor activity if you develop sore eyes, a cough or a sore throat. Consider reducing physical activity, particularly outdoors, especially if you experience symptoms. Very Poor Consider reducing intense outdoor activity if you develop sore eyes, a cough or a sore throat. Reduce physical activity, particularly outdoors, especially if you experience symptoms. Extremely Poor Reduce physical activity outdoors. Avoid physical activity outdoors.

EAQI Concentration Ranges

Each level is defined by pollutant concentration thresholds (all values in µg/m³):

Level PM2.5 PM10 O₃ NO₂ SO₂ Good 0–5 0–15 0–60 0–10 0–20 Fair 6–15 16–45 61–100 11–25 21–40 Moderate 16–50 46–120 101–120 26–60 41–125 Poor 51–90 121–195 121–160 61–100 126–190 Very Poor 91–140 196–270 161–180 101–150 191–275 Extremely Poor >140 >270 >180 >150 >275

Source: European Air Quality Index.

How the Two Systems Compare

Both systems follow the same core principle: low numbers mean clean air, high numbers mean take precautions. The main differences:

• The EPA uses a single numerical scale (0–500) that works across all pollutants. The EAQI maps concentrations directly to named levels.
• The EAQI is hourly. EPA pollutant indices use different averaging periods, most often 8 or 24 hours, with 1-hour values for some gases.
• The EAQI excludes CO, while the EPA includes it.

Airpult uses the US EPA scale as it’s the most widely recognised internationally and provides more granularity at the higher end.

Who counts as a “sensitive group”?

The answer depends on the dominant pollutant. EPA guidance names different groups for ozone, particle pollution, CO, NO₂ and SO₂. These include people with heart or lung disease, people with asthma, children, older adults and people who are active or work outdoors. The World Health Organization also identifies pregnant people and outdoor workers as more vulnerable to air-pollution exposure.

Tips for High Pollution Days

• Check the dominant pollutant, AQI category and local health advice before outdoor activity. Sensitive groups should reduce prolonged or heavy exertion when AQI rises above 100. At higher levels, everyone may need to shorten or move strenuous activity.
• Pay attention to symptoms such as coughing, wheezing, chest tightness, sore eyes or shortness of breath. Reduce exposure and seek medical advice if symptoms are severe or do not settle.
• During smoke or particle-pollution events, close windows and doors when it is safe to do so and use a correctly sized portable air cleaner that does not produce ozone. Do not let the home become dangerously hot.
• If you have a heart or lung condition, keep prescribed medication available and follow the action plan agreed with your healthcare professional.

How Airpult Shows AQI

On Airpult, you can see the current AQI for any location alongside the weather forecast. The AQI is displayed on a colour-coded gradient scale: green through yellow, orange, red, purple, and dark red, matching the EPA levels above. You can also see which pollutant is driving the current reading and check pollen levels where available.

Open Airpult’s air quality map to check current conditions, then Explore to browse locations near you.