New Delhi, Dec. 26 -- Most air purifiers disappear from your attention bubble long before they stop doing their job. The first few days, you notice everything - the numbers changing, the fan ramping up, and the faint hum at night. A month later, it's just another appliance in the corner, quietly running while life carries on around it. The only sign something's off usually comes much later. Suddenly, the air purifier sounds louder than it used to and airflow feels weaker if you hold your hand near the outlet. Owing to this, the room doesn't feel quite as fresh, though you can't say exactly why. At that point, many people assume the machine is ageing. In reality, it's almost always the filter asking for attention.

Open most home air purif...