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.

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