Air quality is one of the most searched topics in India every winter, and wildfire smoke now sends air quality alerts across North America and Europe. The air quality index, or AQI, is everywhere, but few people know what the number actually measures, or whether the air purifier they bought is doing anything.
What the AQI measures
The AQI combines several pollutants into one simple number. The most important for health is PM2.5: tiny particles 2.5 micrometres or smaller, about thirty times thinner than a human hair. They travel deep into the lungs and can enter the bloodstream. The AQI also accounts for PM10, ozone, nitrogen dioxide, sulphur dioxide and carbon monoxide.
India's AQI scale
| AQI | Category | What it means |
|---|---|---|
| 0 to 50 | Good | Minimal impact |
| 51 to 100 | Satisfactory | Minor breathing discomfort for sensitive people |
| 101 to 200 | Moderately polluted | Discomfort for people with lung or heart conditions, children and older adults |
| 201 to 300 | Poor | Breathing discomfort for most people on prolonged exposure |
| 301 to 400 | Very poor | Respiratory illness on prolonged exposure |
| 401 to 500 | Severe | Affects healthy people; serious impact on those with existing disease |
The US AQI uses different bands. An AQI of 150 in an American app is not the same as 150 on an Indian app, so always check which scale you are reading.
How an air purifier cleans air
Most home purifiers pull room air through a HEPA filter, a dense mat of fine fibres. Under the widely used US standard, a true HEPA filter captures at least 99.97% of particles 0.3 micrometres in size, which is the hardest size to catch. Larger and smaller particles are captured even more efficiently. Many purifiers add an activated carbon layer for odours and some gases.
Do they actually work?
Yes, for particles, if the purifier is the right size for the room and the room is kept closed. The key number is CADR, clean air delivery rate, which tells you how much filtered air the machine delivers. A common guideline is a CADR (in cubic feet per minute) of at least two-thirds of the room's floor area in square feet.
Buying and using a purifier well
- Check the CADR, not just the marketing claims about room size.
- Keep doors and windows closed while it runs, especially at night in the bedroom.
- Replace filters on schedule. A clogged filter moves far less air.
- Avoid purifiers that produce ozone. Ozone is itself a lung irritant.
- Run it continuously on high pollution days, not just for an hour.
Other ways to reduce indoor pollution
- Use the kitchen exhaust fan while cooking, especially when frying.
- Avoid incense and mosquito coils indoors on bad air days.
- Wear a well-fitted N95 mask outdoors when AQI is very poor. Cloth masks do little against PM2.5.
Frequently asked questions
Do indoor plants clean the air?
Plants are pleasant, but you would need an impractically large number to match even a small purifier for particle removal.
Is outdoor AQI the same as indoor?
Indoor PM2.5 usually follows outdoor levels closely in leaky homes, and can be higher when cooking or burning things indoors.