How to read a city's air quality index
An air quality index (AQI) condenses several pollutants into one easy number. Knowing what sits behind that number helps you decide when to run outdoors, open the windows or rethink a trip.
What the AQI actually measures
Every Live.City dashboard shows the US AQI, one of the most widely used scales. It runs from 0 upward and is calculated from concentrations of individual pollutants. Each pollutant gets its own sub-index, and the overall AQI is the highest of them — so a single bad pollutant is enough to raise the headline number.
The bands
| AQI | Category | What it means |
|---|---|---|
| 0–50 | Good | Little or no risk. |
| 51–100 | Moderate | Acceptable; a small number of unusually sensitive people may notice effects. |
| 101–150 | Unhealthy for sensitive groups | Children, older adults and people with heart or lung conditions should reduce prolonged exertion outside. |
| 151–200 | Unhealthy | Everyone may start to feel effects. |
| 201–300 | Very unhealthy | Health alert; avoid prolonged outdoor activity. |
| 301+ | Hazardous | Emergency conditions for everyone. |
The pollutants behind the number
PM2.5 are fine particles smaller than 2.5 micrometres — small enough to travel deep into the lungs. They come from vehicle exhaust, industry, wood and crop burning, and wildfire smoke. In many large cities, PM2.5 is the pollutant that drives the AQI most often.
PM10 are coarser particles such as dust, pollen and construction debris. They tend to spike on dry, windy days or near building sites.
Ozone (O₃) forms when sunlight reacts with other pollutants, so it usually peaks on hot, sunny afternoons and is lower in the morning. That's why a summer run is often better before breakfast.
Nitrogen dioxide (NO₂) is closely tied to traffic and tends to be highest near busy roads and during rush hour.
Why AQI changes so much through the day
Weather matters as much as emissions. Still air and temperature inversions — when a layer of warm air traps cooler air near the ground — can hold pollution over a city for days, which is common in winter in some regions. Rain and strong wind usually clean the air quickly. Comparing the AQI with the live wind speed and forecast on a city dashboard often explains a sudden change.
Practical steps on high-AQI days
- Move exercise indoors or to the cleanest time of day — often early morning for ozone, or after rain for particles.
- Keep windows closed during peaks and use a HEPA air purifier sized for the room.
- If you need a mask outdoors, a well-fitted N95/FFP2-type respirator filters fine particles far better than cloth or surgical masks.
- Choose routes away from heavy traffic when walking or cycling.
- People with asthma, heart disease or other conditions should follow their doctor's advice and local health alerts.
Reading AQI before a move
A single reading tells you about today, not about life in a city. Check air quality several times across different seasons and ask locals about the worst months. Our city comparison method includes air quality as a weighted factor for exactly this reason.
This guide is general information, not medical advice. Live AQI values on Live.City come from model data and can differ from official monitoring stations; follow your local authorities during pollution alerts.