Air Quality Index (AQI) has become as familiar as temperature on modern weather dashboards — yet far fewer people understand what those numbers actually mean, which pollutants they measure, and when the reading actually warrants changing your plans. Here is a clear breakdown.

What Is AQI?

Air Quality Index is a standardised scale for communicating how clean or polluted the outdoor air is and what health effects might be expected at different levels. The index was developed to transform complex air quality data — measurements of multiple pollutants at different concentrations — into a single, simple number that anyone can understand.

Different countries use slightly different AQI scales. The US EPA scale runs from 0–500. China uses its own scale. The World Health Organisation publishes separate air quality guidelines. However, the colour-coded tiered concept is internationally consistent.

What Pollutants Does AQI Measure?

AQI is typically calculated based on the most harmful pollutant present at any given time. The main pollutants measured include:

  • PM2.5 (Fine Particulate Matter): Particles smaller than 2.5 micrometres — small enough to penetrate deep into the lungs and enter the bloodstream. This is the most health-significant pollutant for most urban air quality measurements. Sources include traffic, industrial activity, and wildfire smoke.
  • PM10 (Coarse Particulate Matter): Larger particles (2.5–10 micrometres). Still irritating to the respiratory system but filtered more effectively by the body's defences. Sources include dust roads, construction, and quarrying.
  • Ozone (O3): Ground-level ozone (not to be confused with the protective stratospheric ozone layer) forms when sunlight reacts with vehicle emissions. Peaks on hot, sunny days in traffic-heavy cities. A respiratory irritant.
  • Nitrogen Dioxide (NO2): Primarily from vehicle exhaust and power plants. A respiratory irritant that worsens asthma.
  • Sulphur Dioxide (SO2): Primarily from coal burning and industrial processes. Causes acid rain and respiratory irritation.
  • Carbon Monoxide (CO): From incomplete combustion of fuels. Dangerous at high concentrations.

AQI Categories and Their Health Implications

  • 0–50 — Good (Green): Air quality is satisfactory. No health risk for the general population. Ideal for all outdoor activities.
  • 51–100 — Moderate (Yellow): Air quality is acceptable for most people. However, unusually sensitive individuals — particularly those with severe asthma or other respiratory conditions — may experience minor irritation during prolonged outdoor exertion.
  • 101–150 — Unhealthy for Sensitive Groups (Orange): Children, elderly, and those with respiratory or cardiovascular conditions should reduce prolonged outdoor exertion. Healthy adults generally not affected.
  • 151–200 — Unhealthy (Red): Everyone may begin to experience health effects with prolonged outdoor activity. Sensitive groups may experience more serious effects. Consider wearing a quality mask (N95/FFP2) outdoors.
  • 201–300 — Very Unhealthy (Purple): Serious health effects for everyone with prolonged exposure. Outdoor activities should be minimised. Stay indoors with windows closed if possible.
  • 301–500 — Hazardous (Maroon): Emergency conditions. Entire population is likely to be affected. Avoid all outdoor activity. Use air purifiers indoors if available.

Cities with Chronic Air Quality Challenges

Several major world cities regularly experience poor air quality. Beijing, Delhi, Lahore, Jakarta, and Dhaka consistently rank among the world's most polluted cities, largely due to traffic, industrial emissions, and — in South Asian cities — agricultural burning. In contrast, cities like Sydney, Reykjavik, and Wellington typically have excellent baseline air quality.

SunorSnow's weather dashboard displays real-time AQI for every city you track. The reading updates alongside temperature, humidity, and UV index, giving you a complete picture of outdoor conditions before you step outside.