What do you actually run in Delhi when outdoor AQI sits above 300 for weeks? Purifiers

Budget for filters as a consumable, not an accessory. At sustained outdoor levels like that a pre-filter is doing most of the work and it wants vacuuming weekly, which will roughly double how long the expensive filter behind it lasts. Two units sized for the two rooms people sleep in, running continuously through the season, beats one large unit in the living room by a wide margin, because the exposure that matters most is the eight hours somebody spends breathing in one room.

Sealing matters more than most people expect and it does not have to be permanent. Weatherstripping tape on the window sashes, a draught excluder under the bedroom door, and a rolled towel at the balcony door took my bedroom from tracking outdoor levels within an hour of switching the unit off to holding for most of a night. The cooking problem is real, so I run the kitchen extractor while cooking and accept that the bedroom is the sealed room rather than the whole flat.

A box fan with four MERV 13 filters in a cube gets you into the same ballpark as a lot of retail units. Estimates from a university test put that design somewhere around 165 to 240 CFM of clean air delivery, against about 111 for a single filter taped to the same fan. It is loud and it is ugly and it is a fraction of the price, and in a season where you may want a second one in a hurry that is worth knowing.

Check the replacement filter price and availability in your city before you commit to a brand, not after. I bought a unit on a good discount and then found the filters were import-only, three weeks of shipping, and cost close to a third of the machine each time. The boring locally-serviced brand you have never heard of on a review site is often the correct answer for exactly this reason.

One thing nobody mentions until you live with it: put the unit somewhere with clear space around the intake and outlet, not tucked behind the sofa because it is ugly. I lost most of a winter's performance to a purifier that was breathing its own exhaust in a corner.

Two units in the two occupied bedrooms, continuous running, pre-filter vacuumed weekly, main filters bought in advance before the season spikes the price. Everything else is optimisation.