Air Quality Explained: How to Read PM2.5 and AQI

Last updated: 11 October 2026

Air quality is one of the few things about a city that can directly affect your health over the long term, and it varies a lot between cities and even between streets. This guide explains the numbers you will see on air quality maps and reports, and how to use them when choosing where to live.

What PM2.5 and PM10 mean

PM stands for particulate matter: tiny particles of dust, soot, smoke and other material in the air. The number is the maximum particle size in micrometres (millionths of a metre).

PM10: particles up to 10 micrometres across, such as road and construction dust and pollen.
PM2.5: fine particles up to 2.5 micrometres, mostly from burning fuel (traffic, heating with wood or coal, industry). Because they are so small, they can travel deep into the lungs, which is why health agencies pay most attention to PM2.5.

Both are measured in micrograms per cubic metre of air (µg/m³).

What levels are considered safe

The World Health Organization’s 2021 Air Quality Guidelines recommend that long-term (annual average) exposure should not exceed:

PM2.5: 5 µg/m³
PM10: 15 µg/m³

Very few large cities meet these levels every year. Legal limits set by governments are usually higher than the WHO guidelines, so a city can be “within legal limits” and still above the WHO recommendation. When comparing cities for a long stay, the annual average is more useful than any single day’s reading.

Air quality index (AQI) numbers

Many apps and websites turn pollution measurements into an index, often on a scale where higher numbers are worse and colours run from green to red or purple. The problem is that there are several different indexes: the US AQI, the European Air Quality Index and various national ones. They use different scales and thresholds, so a “50” in one index does not mean the same as a “50” in another. Always check which index a figure uses, or look at the PM2.5 value in µg/m³, which is the same everywhere.

Why one reading can mislead you

It changes by the hour. Rush hour, weather and wind direction can double or halve readings in a day.
It changes by season. In many European cities, PM2.5 is higher in winter when homes burn wood or other fuels for heating and still, cold air traps pollution.
It changes by street. Living on a busy road is very different from living by a park, even in the same district.

This is why Placeivo does not rank cities on live air quality readings. A snapshot from one hour says little about what living in a city will be like.

How to check air quality before you move

Look for annual averages of PM2.5 for the city, published by the national environment agency or, in Europe, the European Environment Agency.
Check the neighbourhood on an official monitoring map. Compare stations near busy roads with those in residential areas.
Look at the winter months if you are sensitive to pollution, since that is often when levels are highest.
Think about your daily routine. A long commute on foot or by bike along a main road matters more than the city average.

Who should pay most attention

Air quality matters most for children, older people, pregnant women and anyone with asthma, heart or lung conditions. If that applies to you or your family, ask a doctor how local air quality might affect you and consider it alongside cost when choosing a city and a neighbourhood.

General information only, not medical advice. For local air quality, use your destination’s official environment or health agency.