Air Quality Index Scale
The Air Quality Index (AQI) is a standardized indicator of air quality that translates complex pollutant data into a simple scale. Understanding it helps you make informed decisions to protect your health.
Good
Air quality is satisfactory, and air pollution poses little or no risk.
Moderate
Air quality is acceptable. However, there may be a risk for some people who are unusually sensitive to air pollution.
Unhealthy for Sensitive Groups
Members of sensitive groups may experience health effects. The general public is less likely to be affected.
Unhealthy
Some members of the general public may experience health effects; sensitive groups may experience more serious effects.
Very Unhealthy
Health alert: The risk of health effects is increased for everyone.
Hazardous
Health warning of emergency conditions: everyone is more likely to be affected.
Each pollutant has its own concentration ranges that correspond to AQI values. The dominant pollutant determines the overall AQI.
Fine Particulate Matter
Tiny particles less than 2.5 micrometers in diameter that can penetrate deep into the lungs and enter the bloodstream.
Coarse Particulate Matter
Inhalable particles with diameters 10 micrometers or smaller, including dust, pollen, and mold spores.
Ozone
A reactive gas formed by sunlight acting on pollutants. Ground-level ozone can trigger respiratory problems.
Nitrogen Dioxide
A reddish-brown gas produced by vehicle emissions and power plants. Irritates airways and worsens respiratory diseases.
Sulfur Dioxide
A gas produced from burning fossil fuels containing sulfur. Can harm the respiratory system and aggravate asthma.
Carbon Monoxide
A colorless, odorless gas produced by incomplete combustion. Reduces the blood's ability to carry oxygen.
At Weather World AI, we recommend that people with asthma or seasonal allergies take extra care when AQI exceeds 51 (Moderate).
Asthma
- • Check AQI before 10:00 AM — ozone builds through the day; morning air is often cleaner.
- • Keep your rescue inhaler accessible when AQI exceeds 51.
- • If ozone is dominant, avoid afternoon exertion. If PM2.5 is high, wait until mid-morning after rush hour.
- • During wildfire smoke, stay indoors with HEPA filtration. Use N95 if you must go out.
Seasonal Allergies
- • High PM2.5 can worsen allergic inflammation — check both AQI and pollen forecasts.
- • Best time for a walk: right after a light rain (clears pollen and particles).
- • Start allergy medications 1–2 weeks before your typical season.
- • Shower and change clothes after outdoor exposure on high-pollen or high-AQI days.
For more detailed guidance, see our Air Quality & Health guide.
Stay Indoors
When AQI is high, stay inside with windows and doors closed. Use air purifiers with HEPA filters if available.
Wear Protection
Use N95 or KN95 masks when outdoors during unhealthy air quality. Standard cloth masks offer limited protection against fine particles.
Time Activities Wisely
Ozone levels peak in afternoon heat. Schedule outdoor exercise for early morning or evening when pollutant levels tend to be lower.
Monitor Symptoms
Watch for coughing, throat irritation, shortness of breath, or chest tightness. Seek medical attention if symptoms persist.
Reduce Exposure
Avoid exercising near busy roads. Reduce physical exertion outdoors when air quality is poor — breathing harder draws more pollutants into your lungs.
Stay Informed
Check air quality forecasts daily. Use real-time AQI monitoring to plan outdoor activities around the best air quality windows.
How AQI Is Calculated
The AQI is calculated by measuring concentrations of major pollutants (PM2.5, PM10, O₃, NO₂, SO₂, and CO) and converting each to a standardized index using EPA-defined breakpoint tables. Each pollutant's concentration maps to a value on the 0–500 scale through linear interpolation between its breakpoints.
The overall AQI is the highest individual pollutant sub-index. That pollutant is reported as the "dominant pollutant" — the primary driver of air quality concern at that moment.