What actually helps at home?

At home, the evidence points to a few things that work when done properly: a well-fitted respirator outdoors, a purifier sized for the room you're actually in, and a closed room for it to work in. None of them fixes the air outside.

This chapter summarises studies. It recommends no products and contains no affiliate links. For personal health questions, ask a doctor.

Do masks protect against Delhi's air?

They can, but the face seal matters more than the filter. The clearest test comes from Beijing. Cherrie and colleagues took nine masks sold there as PM2.5 protection and measured how much of a diesel-exhaust aerosol got through the material: from 0.26% to 29%, depending on the mask and airflow.¹ They then fitted four of those masks on volunteers and measured what reached the wearer, including air leaking around the edges. Average total inward leakage ranged from 3% to 68% while sitting, and 7% to 66% while active. Only one mask type kept average leakage under 10% in both conditions.¹ The authors concluded that many masks may not give adequate protection, primarily due to poor facial fit.¹

The practical reading: a filter rating printed on a mask describes the material, not the gap between the mask and your face. A mask that fits snugly, with no air escaping at the nose or cheeks, delivers much more of the material's promise than one that doesn't.

Do air purifiers work in Delhi homes?

In a closed room, yes, by a lot; across a whole leaky house, much less.

The best real-home evidence comes from controlled trials that compare a working purifier with a disguised “sham” one. In one such trial in New Jersey and New York homes, a portable HEPA cleaner cut PM2.5 by 78.8% in the room where it ran and by 57.9% in another room of the same home.² The unit's measured cleaning rate on the setting used was below its advertised maximum, a reminder that box figures describe the top setting under test conditions.²

Delhi's outdoor air is a different starting point. In winter 2015–16, Vyas, Srivastav and Spears ran 72 timed tests of two low-cost HEPA purifiers in a 150 sq ft bedroom in Hauz Khas, with outdoor PM2.5 averaging about 260 µg/m³ over the period.³ The purifiers produced substantial reductions in indoor particle counts, but indoor levels during purifier use were often still above those measured in cities such as Pittsburgh and Edinburgh. Indoor counts rose with outdoor counts: each extra 10,000 particles outside was associated with roughly 3,000 more inside.³

What do CADR and room size mean?

A purifier's clean air delivery rate (CADR) is the volume of particle-free air it supplies per unit of time. What matters is that rate relative to the room's volume: the same machine that clears a small bedroom quickly will barely touch an open-plan living room. Two habits follow from the studies above. Size the purifier for the room's volume, not its floor area alone. And close the door; leaking air from the rest of the house, or the outdoors, is continuously replacing the air the machine just cleaned.³²

Should windows stay closed on bad days?

On the worst days, keeping outdoor air out of the room you're cleaning is what lets a purifier work, and the Delhi experiment shows indoor particle counts tracking outdoor ones.³ There is a trade-off: a closed room also keeps in whatever is produced indoors, such as smoke from cooking. Ventilating briefly when outdoor air is at its least bad in the day is a reasonable compromise, and the live data in How do you read Delhi's live air data yourself? shows when that is.

Is it worth measuring indoor air?

An indoor monitor is the only way to know whether any of this is working in your room. Treat an inexpensive one as a way to compare, not as an official measurement: check the reading before and after you close a window, run a purifier or cook, and watch the change rather than the absolute number.

What does not work as advertised is covered in Which Delhi air myths don't hold up?