Using an air purifier well
Placement, fan speed, filter changes and the features to switch off, so the purifier you own delivers what it promises.
A portable air purifier is a fan that pushes room air through a filter. For fine particles (PM2.5), the filter that matters is a mechanical HEPA filter. How much clean air it delivers is its CADR (clean air delivery rate). What to look for when buying is covered in the purifier spec guide. This page is about getting the most from one.
Placement
- In the room it is sized for, with the door closed. A purifier cleans its own room, not the whole house.
- Give it space. Keep intake and outlet clear: about 30 cm from walls and furniture, not tucked in a corner or behind a curtain.
- Near where people breathe. For a bedroom, place it where clean air flows towards the bed but not as a draught on the sleeper’s face.
- Away from the door and window if possible, so it isn’t mainly cleaning air that is about to leak out.
Fan speed: the part most people get wrong
The CADR on the box is measured at the highest fan speed. Sleep or quiet modes often deliver a fraction of that. If the purifier only runs on its lowest setting all night, it may be far too weak for the room.
- Run the highest speed the occupants can tolerate, especially overnight in a sensitive person’s room.
- Many people find a steady medium speed easier to sleep through than a unit that ramps up and down on “auto”.
- If the speed you can live with is not enough, a larger purifier on a lower speed is often quieter than a small one on full.
- Use a PM2.5 monitor to check the room at the speed you actually use at night.
Run it continuously during bad spells
Run the purifier all day and all night during the pollution season, not in short bursts. Rooms get dirty again within an hour or two of switching off. Running costs are covered in the buying guide. A typical 30–50 W purifier uses about as much electricity as a ceiling fan.
Filters
- Replace on schedule, or sooner in Delhi winters. Heavy pollution loads filters faster than the manufacturer’s “normal use” assumption. Signs: airflow drops, a musty smell, or the monitor shows the room cleaning more slowly than before.
- Buy genuine or certified-equivalent filters that match the exact model number. A cheap filter that doesn’t seal in its slot lets air bypass it.
- Keep one spare before the season starts: stock runs out in November.
- Some filters have a washable pre-filter: vacuum or rinse it as instructed, and let it dry fully. Never wash a HEPA filter.
Turn off ozone-making features
Some purifiers have an “ioniser”, “plasma”, “anion” or UV mode. These can produce ozone or other by-products, add little or nothing against PM2.5, and ozone irritates the lungs. Switch them off. Do not buy a device that only works by generating ozone. See ionisers and ozone generators.
Carbon filters and gases
A thin carbon layer can reduce some odours, but it has limited capacity for pollution gases such as nitrogen dioxide or ozone, and it saturates quickly. For PM2.5, the HEPA layer is what counts.