Indoor air in smog season: what actually lowers PM2.5 inside a home

Delhi recorded an average AQI of 166 on 1 October 2026, the highest reading for that date since 2022, and the usual pattern says it will get worse as winter arrives. Most of us cannot change the air outside. We can change a surprising amount about the air in the rooms where we sleep and work.

The pollutant that matters most here is PM2.5: particles small enough to reach deep into the lungs. They come in through gaps, open windows and doors, and they are also produced indoors by cooking, incense and smoking.

1. Stop making it indoors

This is the cheapest step and often the most overlooked. Frying and tadka on a gas flame, agarbatti, dhoop, mosquito coils and candles all release fine particles. On bad-air days, use the kitchen exhaust every time you cook, keep incense for special occasions, and switch to electric mosquito repellents or nets.

2. Seal the rooms you use most

On high-pollution days, keep windows shut during the worst hours, usually late night and early morning when the air is still. Weather strips on doors and windows reduce the amount of outdoor air that leaks in. You still need some ventilation, so air the room briefly in the afternoon when levels are often lower.

3. Use a purifier properly

A purifier with a true HEPA filter is the most effective tool for reducing PM2.5 in a closed room. The two numbers that matter are the clean air delivery rate (CADR) and the room size it is rated for. A unit that is too small for the room will run constantly and achieve little.

• Run it in the bedroom with the door closed
• Keep it on a medium setting continuously rather than on high occasionally
• Change filters on schedule; a clogged filter moves very little air

4. Build a box-fan filter as a budget option

A furnace filter strapped to a box or table fan is a well-studied low-cost alternative. Its performance depends on the filter grade, but it can reduce particles noticeably in a small room. It is louder and less tidy than a commercial purifier.

5. Clean without stirring it up

Dry sweeping and dusting push settled particles back into the air. Wet mopping and a damp cloth trap them. If you use a vacuum, one with a HEPA filter is better than one that blows fine dust out of the exhaust.

What does not help much

Houseplants are lovely, but a room would need an impractical number of them to make a measurable difference to particles. Ionisers and ozone generators can add harmful gases. Scented sprays only mask smells and add their own chemicals.

Measure if you can

A basic indoor air monitor shows whether your changes work. Readings often fall sharply within an hour of closing a room and starting a purifier, and climb within minutes of frying something. That feedback is what makes the habits stick.

A note on health

Children, older people and anyone with asthma or heart conditions are most affected by poor air. If symptoms worsen during smog season, talk to a doctor; indoor measures help, but they are not a substitute for medical advice.