New Delhi, Jan. 8 -- Air purifiers have a strange talent - they look busy even when they're losing the fight. The lights blink, the fan whirs, the display behaves like a stock ticker and yet the AQI refuses to budge. It's the kind of stall that makes you question the entire purchase. But the issue usually isn't the purifier's motor or its filter. It's the room behaving in ways you don't expect.

Most people assume that closing the door is enough. It isn't. Air leaks behave like cracks in a boat. A tiny gap under the door, a window that doesn't fully lock, or an AC vent with a loose flap is enough to let outdoor air slip in faster than the purifier can clean it. You see this most dramatically on high-pollution days - PM2.5 outside is 300+,...