Key points
- The Air Quality Index (AQI) turns measurements of five common pollutants into one number on a scale of 0 to 500, in six colour-coded bands.
- Almost the whole world population — 99% — breathes air above WHO guideline limits, and air pollution is linked to about 7 million premature deaths a year (WHO).
- Scales differ by country: the US AQI is not the same as the scales used in China, India or the EU, so numbers from different systems do not compare directly.
- Nigeria has no public AQI portal. This guide notes the absence and the nearest published source.
Direct answer: The Air Quality Index (AQI) turns measurements of five common pollutants into one number on a scale of 0 to 500, in six colour-coded bands from Good to Hazardous. Almost the whole world population — 99% — breathes air above WHO guideline limits, and air pollution is linked to about 7 million premature deaths each year. This article explains what the number means, how it is calculated, and why the same number can mean different things in different countries.
What the Air Quality Index is
The AQI is a communication tool, not a measurement itself. Monitors measure pollutants in the air — ground-level ozone, particle pollution (PM2.5 and PM10), carbon monoxide, sulfur dioxide and nitrogen dioxide — and the index translates those measurements into one number that a general reader can follow. The US system, run by the EPA’s AirNow programme, is the most widely quoted version, and it is the scale this site’s lookup tool uses.
Particle pollution deserves a word of its own. PM2.5 means particles smaller than 2.5 micrometres across; PM10 means particles smaller than 10 micrometres. The smaller the particle, the deeper it can travel into the lungs, which is why PM2.5 is watched most closely.
The six bands
The US AQI bands, as published by EPA AirNow, are:
| Band | Range | What it means (EPA AirNow) |
|---|---|---|
| Good | 0–50 | Air quality is satisfactory. |
| Moderate | 51–100 | Air quality is acceptable. |
| Unhealthy for Sensitive Groups | 101–150 | Sensitive groups may experience health effects. |
| Unhealthy | 151–200 | Everyone may begin to experience health effects. |
| Very Unhealthy | 201–300 | Health alert: the risk of health effects is increased for everyone. |
| Hazardous | 301–500 | Health warning of emergency conditions. |
WHO sets guideline values for each pollutant — the levels below which health effects are minimised — but WHO does not run an index of its own. The 99% figure is the share of the world’s population living where air quality does not meet those guideline limits.
How the AQI is calculated
Each pollutant gets its own sub-index. The measured concentration is matched against a breakpoint table — a set of concentration ranges, each mapped to an index range — and the sub-index is worked out from where the measurement falls inside its range. The highest sub-index of the five pollutants becomes the AQI for that place and time, and the pollutant behind it is named as the main pollutant.
That is why the AQI can stay flat while one pollutant rises: only the worst pollutant drives the number. It is also why two places can show the same AQI for different reasons — one driven by ozone on a hot afternoon, the other by particle pollution from traffic or smoke.
Why scales differ by country
The US AQI is not the only system. China, India, Vietnam and the EU all run their own indices, with different bands, breakpoints and labels. A reading of 150 means “Unhealthy” on the US scale but sits in a different band on some other national scales, so numbers from different systems do not compare directly. Always check which scale a number comes from before comparing two places.
National portals publish the official readings for their own countries: AirNow for the United States, and the national environment agencies elsewhere. Global platforms such as the World Air Quality Index project and OpenAQ gather readings from many sources into one place, but a reading from a global feed is not a regulatory monitor reading — the distinction matters when a number is quoted.
Nigeria: no public AQI portal
Not every country publishes an index. Nigeria has no public AQI portal: there is no official Nigerian site where a general reader can look up a published AQI reading for Lagos or Abuja. The nearest published source is the Nigerian Meteorological Agency (NiMet), whose quarterly bulletins report on air quality in general terms rather than as a live index. This guide notes the absence rather than filling it with a figure from an unverified source — a missing portal is a fact about the data, not a gap to paper over.
How to check the air near you
Start with the official route for the country: AirNow in the United States, the national environment agency elsewhere. Where no official portal exists, a global feed such as the World Air Quality Index project or OpenAQ is the next best source — and its reading should be quoted with its source and time, as the lookup tool on this site does. Be wary of any page that shows an AQI with no source, no time and no named scale: without those three, the number cannot be checked.
What the number does not say
The AQI does not say whether the air is safe for you personally. It is a published reading with an official band label, for a place and a time. It gives no personal verdict, no personal advice, and no guidance for any individual’s health decisions.
Frequently asked questions
Is the AQI the same everywhere?
No. The US AQI, China's AQI, India's AQI and the EU's common air quality index use different bands and breakpoints, so a number from one system does not mean the same as the same number from another.
How is the AQI calculated?
Each pollutant gets a sub-index from its measured concentration and a breakpoint table. The highest sub-index becomes the day's AQI, and the pollutant behind it is named as the main pollutant.
Does the AQI say if the air is safe for me?
No. It is a published reading with an official band label. It gives no personal verdict and no personal advice.