AQI explained simply (and what's actually safe)

What the number actually is

AQI combines five pollutants into one number. The most important one for everyday health is PM2.5, the tiny particles your lungs can't filter out.

The scale

• 0 to 50: Good. Air is clean. Do whatever you want outside.
• 51 to 100: Moderate. Fine for most people. Asthmatics should pay attention.
• 101 to 150: Unhealthy for sensitive groups. Kids, older adults, people with lung conditions should limit time outside.
• 151 to 200: Unhealthy. Everyone starts to feel it. Shut the windows. Run a purifier if you have one.
• 201 and up: Very unhealthy or hazardous. Stay indoors. Wear an N95 if you have to go out.

What PM2.5 is, and why it's the headline

PM2.5 means particles smaller than 2.5 microns. To compare: a human hair is around 70 microns thick. These particles slip past your nose hairs, slip past your throat, and end up deep in your lungs. From there some of them get into your bloodstream.

Long-term exposure damages your heart and lungs. Short-term spikes can trigger asthma, headaches, and brain fog.

Where it comes from

• Vehicle exhaust, especially diesel
• Wood-burning fireplaces and bushfires
• Industrial emissions
• Coal and gas power plants
• Gas cooking, especially in a kitchen with no extractor

What to do at each level

AQI over 100: Shut the windows. Run a HEPA purifier in the room you spend most time in. Postpone outdoor exercise.

AQI over 150: Wear an N95 outside. Check in on elderly relatives. Asthmatic kids should not be doing PE.

AQI under 50: Open everything. Ventilate. This is a good day for the dog walk and the hanging out of washing.

How to know yours

Open Window Today shows your live AQI for any city and tells you exactly what to do about it.

What the AQI is really measuring

The air quality index condenses several pollutants, chiefly fine particulate matter known as PM2.5, plus ozone, nitrogen dioxide, and others, into a single number on a colour-coded scale. The index reports whichever pollutant is worst at the time, so a high reading does not tell you which pollutant is the problem, only that one of them has reached a concerning level. PM2.5 is usually the one that matters most for health because the particles are small enough to lodge deep in the lungs and enter the bloodstream.

Reading the bands without overthinking them

From 0 to 50 is good and the air poses little risk. From 51 to 100 is moderate; fine for most people, though unusually sensitive individuals may notice it. From 101 to 150 is unhealthy for sensitive groups, meaning people with asthma, heart or lung conditions, children, and the elderly should ease back on strenuous outdoor activity. From 151 to 200 is unhealthy for everyone, and above that the effects become serious for the whole population. The practical takeaway: under 100 most people can carry on normally, between 100 and 150 sensitive groups should take care, and above 150 everyone should cut back outdoor exertion.

What to actually do at each level

The index is only useful if it changes your behaviour. On good and moderate days, live normally. When it climbs past 100, sensitive people should move hard exercise indoors, keep reliever medication handy, and avoid the busiest roads. Past 150, everyone should shorten time outside, skip intense outdoor workouts, and keep windows shut while running a purifier if they have one. The numbers can change through the day, often worse in still evening air or during a smoke event, so checking the current reading rather than the daily average gives you the picture that matters for what you are about to do.

Frequently asked questions

What AQI is safe to breathe?

Anything under 50 is good for everyone. Up to 100 is acceptable for most people. Above 100, sensitive groups should take care, and above 150 everyone should reduce outdoor exertion.

What does AQI actually measure?

It combines several pollutants, most importantly fine particulate matter (PM2.5), into one number and reports whichever is worst at the time. PM2.5 is usually the most health-relevant because it penetrates deep into the lungs.

Why is the AQI worse at some times of day?

Pollution often builds in still morning and evening air, during rush hour, or when smoke is present. Because it shifts through the day, the current reading is more useful than the daily average for deciding what to do right now.

Related reads