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.

The two main systems you’ll encounter are the US EPA AQI (used in the United States and widely referenced internationally) and the European AQI (used across EU member states, published by the European Environment Agency). They cover largely the same pollutants but use different scales and categories.

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 tracks all six pollutants below, while 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 vehicle and industrial pollutants react with sunlight. Worst on hot, sunny days. 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 single number from 0 to 500, divided into six colour-coded levels. An AQI of 100 corresponds to the national air quality standard; anything above that starts to become a concern.

AQI	Level	Health Risk & What To Do
0–50	Good	Air quality is satisfactory. No precautions needed.
51–100	Moderate	Acceptable for most people. Sensitive individuals should consider reducing prolonged outdoor exertion.
101–150	Unhealthy for Sensitive Groups	Sensitive groups should reduce prolonged or heavy outdoor exertion. Everyone else can continue as normal but should watch for symptoms.
151–200	Unhealthy	Everyone should reduce prolonged outdoor exertion and move workouts indoors. Sensitive groups should avoid outdoor activity altogether.
201–300	Very Unhealthy	Everyone should avoid prolonged outdoor exertion and keep windows closed. Sensitive groups should stay indoors.
301–500	Hazardous	Emergency conditions. Stay indoors, avoid all outdoor physical activity, and use air purifiers if available.

EPA Concentration Breakpoints

Each AQI level corresponds to specific pollutant concentration ranges:

Level	PM2.5 (µg/m³)	PM10 (µg/m³)	O₃ (ppb)	NO₂ (ppb)	CO (ppm)	SO₂ (ppb)
Good	0–9.0	0–54	0–54	0–53	0–4.4	0–35
Moderate	9.1–35.4	55–154	55–70	54–100	4.5–9.4	36–75
Unhealthy for Sensitive Groups	35.5–55.4	155–254	71–85	101–360	9.5–12.4	76–185
Unhealthy	55.5–125.4	255–354	86–105	361–649	12.5–15.4	186–304
Very Unhealthy	125.5–225.4	355–424	106–200	650–1249	15.5–30.4	305–604
Hazardous	225.5–325.4	425–604	405–604	1250–2049	30.5–50.4	605–1004

Source: EPA AQI Breakpoints. Last verified February 2026.

European AQI (EAQI)

The European Environment Agency takes a different approach: instead of a single number, it maps pollutant concentrations directly to six named levels. The overall level is determined by whichever pollutant scores the worst.

Level	Health Risk & What To Do
Good	Air quality is satisfactory. No precautions needed.
Fair	Acceptable for most people. Sensitive individuals should consider reducing prolonged outdoor exertion.
Moderate	Sensitive groups should reduce prolonged or heavy outdoor exertion. Everyone else should watch for symptoms.
Poor	Everyone should reduce prolonged outdoor exertion. Sensitive groups should avoid outdoor activity.
Very Poor	Everyone should avoid prolonged outdoor exertion and keep windows closed. Sensitive groups should stay indoors.
Extremely Poor	Emergency conditions. Stay indoors and avoid all outdoor physical activity.

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. Last verified February 2026.

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 generally uses shorter averaging periods, so it reacts faster to sudden changes but can fluctuate more throughout the day.
• 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”?

People with lung conditions (including asthma), heart disease, older adults, children, people with diabetes, and anyone who spends extended time outdoors, whether for work or exercise.

Tips for High Pollution Days

• Check the AQI before outdoor activities, especially exercise; consider moving workouts indoors when levels exceed 100.
• Keep windows closed on high pollution days and use air purifiers with HEPA filters indoors.
• Children, older adults, and those with respiratory conditions should take extra precautions when AQI is elevated.
• If you have a respiratory condition, keep prescribed medications handy and consult your doctor about an action plan for high pollution days.

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.

For city-level comparisons, visit our Air Quality Rankings to see which cities have the best and worst air quality right now, or Explore to browse locations near you.