Air Purifier vs Air Cooler: One Cleans Air, the Other Can Worsen It

Evaporative cooling works by pulling outdoor air through a wetted honeycomb or wood-wool pad. Some coarse dust gets caught on the wet surface, and this is occasionally marketed as a purification benefit. It is not one — the pad has openings measured in millimetres and no gas-phase media at all, and it is saturated with standing water.

In practice, on a bad-air day a cooler is a mechanism for importing outdoor PM2.5 into your bedroom at a high flow rate. That is fine in April in Jodhpur. It is not fine in November in Delhi.

Evaporative cooling exchanges heat for moisture: the water that cools the air ends up in the air. A cooler running in a closed-ish room will typically take relative humidity from 35% to somewhere between 55% and 75%.

Dust mites thrive above about 50% RH, and mould germinates readily above 60%. If someone at home is allergic to dust mites — extremely common and frequently undiagnosed in India — a cooler running nightly in the bedroom is working directly against the purifier in the corner.

• Do not run them in the same room at the same time. The purifier will spend its energy treating a continuous stream of new outdoor air and never reach steady state.
• Run the cooler while you are out, and the purifier in a closed room while you sleep — the reverse of what most households do.
• Change the cooler pads at the start of every season. An old pad is a biological filter in the wrong direction.
• Track humidity with a ₹500 hygrometer. Keeping the bedroom between 40% and 55% RH is the single most useful thing an allergy household can do.
• In a genuinely dry climate, consider a fan plus purifier instead. You lose some cooling and gain a room you can actually keep clean.