What the AQI Number on Your Phone Actually Measures

On the same winter morning, the weather app on your phone might show an AQI of 160 in red for one city and 40 in green for another. We all understand that a bigger number is worse. But what is the number actually counting?

The answer is less obvious than it looks. The AQI isn’t a direct measurement of anything. It’s a score, built from several measurements, designed to tell you quickly how the air might affect your health. And depending on where you live, the same air can get a different score.

The short answer

Monitoring stations measure the amounts of several pollutants in the air, usually in micrograms per cubic metre (µg/m³). An air quality index converts each of those measurements into a score on a common scale, called a sub-index.

On the US scale, and on most others, the AQI you see is simply the highest of those sub-indices. In other words, it tells you about the worst pollutant at that moment, not an average of all of them.

The pollutants that usually count

Some countries add others. India’s index, for example, also includes ammonia and lead. In most cities, PM2.5 is the pollutant that sets the number most often, because it can travel deep into the lungs and is linked to heart and lung disease.

The AQI tells you about the worst pollutant at that moment, not an average of all of them.

Why every country’s scale is different

Each country sets its own index, based on its own air quality standards. Here are the ones you are most likely to see:

Even scales that look alike can disagree. A 24-hour PM2.5 level of 60 µg/m³ falls in the Unhealthy band on the US scale, but at the top of Satisfactory on India’s scale. Neither is wrong: they simply draw the lines in different places.

The World Health Organization’s guidelines are stricter than all of them. Its 2021 guideline for PM2.5 is no more than 15 µg/m³ as a 24-hour average, and 5 µg/m³ as a yearly average. By that measure, most of the world’s population breathes air that is worse than recommended.

Why your apps disagree

If you’ve ever seen one app say 90 and another say 160 for the same city, there are three common reasons.

1. Different scales

Many international weather apps use the US AQI wherever you are. Your government’s official figure may use a national scale, which gives a different number for the same air.

2. Different time periods

Official indices usually use averages, often over 24 hours for particles and 8 hours for ozone. Some apps show the latest hourly reading or a shorter average, which can jump up and down much more.

3. Different stations

Pollution can vary a lot between neighbourhoods. An app may be using the nearest government station, a low-cost private sensor, or a computer model that estimates air quality between stations.

For official figures, check your national environment agency, such as AirNow in the US or the European Environment Agency’s air quality index in Europe.

Using the AQI in daily life

The AQI is most useful as a quick guide for planning your day:

• When the air is in an “unhealthy” band on your local scale, consider moving hard outdoor exercise indoors or to a cleaner time of day, especially for children, older people and anyone with asthma or a heart condition.
• On the worst days, limit time outdoors where you can, and keep windows closed during the most polluted hours.
• Check the trend, not just the number. In winter, air is often worst early in the morning and late at night, when cold air traps pollution near the ground. In summer, ozone usually peaks in the afternoon.
• Ask your doctor if you have a lung or heart condition and want specific advice, such as on masks or medicines.