Keeping indoor air cleaner during haze
Published 19 October 2026
Most people spend most of a haze day indoors, so the air inside matters more than any mask. A few habits and one filter can make a large difference.
Indoors is not automatically clean
Smoke gets in. Fine particles pass through open windows, gaps around doors and the vents of an ordinary house, and on a heavy haze day the air in a room with the windows open can be close to the air outside. Shutting the house and filtering what is inside is what turns "indoors" into protection.
This is why the class advice moves indoors as the air gets worse: shut the windows during the worst hours at Unhealthy, run an air purifier at Very unhealthy, and at Hazardous stay indoors with the windows shut and make one room the clean room if you have no purifier.
Close up during the worst hours
- Shut windows and doors when the reading is poor. A rolled towel along the bottom of a door that leads outside helps.
- Set the air conditioner to recirculate, not to bring in outside air. Split units, the usual kind in Indonesian homes, mostly recirculate already; clean the filter, because a clogged one does nothing.
- In a car, use recirculation and keep the windows up.
Shutting up a house for days makes it stuffy, and in hot weather heat is a risk of its own. Ventilating briefly when the air outside is better is part of the plan, not a failure of it — see the last section.
Air purifiers
A purifier with a true HEPA filter catches the large majority of fine particles that pass through it. What matters is how much air it moves.
- Look at the CADR (clean air delivery rate) or the room size the maker recommends. A small unit in a large living room filters too little air to change much. Choose one rated for a room at least as large as the one you will use it in.
- Run it on a high setting in a room with the door and windows shut. On the lowest setting most units move a fraction of their rated air.
- Replace the filter as the maker says, and sooner in a long haze season; a filter loaded with smoke slows down.
- Avoid ozone generators and purifiers that work by producing ozone. Ozone irritates the lungs, which is the opposite of the point.
A home-made fan filter
If a purifier is out of reach, a fan with a filter attached does a useful job. The best-known design, often called a Corsi–Rosenthal box, tapes four or five furnace filters into a cube with a box fan on top, pulling air through them. A simpler version straps one filter to the back of a fan.
Use pleated filters with a fine rating (MERV 13 or similar, if you can find them), mark which way the air flows, and seal the edges with tape so air goes through the filter rather than around it.
A few cautions. Use a fan in good condition with its own guard, never leave a home-made unit running unattended for long stretches, keep it where it will not be knocked over, and stop using it if the motor gets hot or smells.
Do not add smoke of your own
Shutting a house also traps whatever is made inside it. During haze, avoid:
- mosquito coils and incense;
- smoking indoors;
- frying and grilling without the kitchen hood on, or with the door to the rest of the house open;
- burning rubbish, which also adds to the haze outside.
Make one clean room
If you cannot filter the whole house, filter one room well. Choose a room where people sleep, ideally one with few windows, shut it, and put the purifier or fan filter in it. Spending the night in clean air gives the lungs hours of relief every day, and this is the room for children, older relatives and anyone with asthma or a heart condition.
When to air out
Every city page shows an hourly forecast for five days and, under Going outside, the least-bad stretch of the day. Use it to pick when to open up: air the house during the better hours, then shut it again before the evening rise. In many cities the air is worst overnight and in the early morning, when cooler air holds smoke close to the ground, but the forecast for your own city is a better guide than a rule of thumb.
The forecast is a model, and models smooth the peaks of a dense haze event (methodology explains why). If you can smell smoke, trust that over the number.