So much air pollution! Do we need an air purifier?

Every winter the same photograph comes back: Delhi under a grey blanket.

Then someone at home asks whether we should finally buy an air purifier.

How is air quality measured?

India uses the National Air Quality Index, or AQI.

It is one number from 0 to 500, and lower is better.

The scale runs Good, Satisfactory, Moderate, Poor, Very Poor and Severe.

Anything above 200 is Poor.

That is usually when you start feeling it in your throat.

Most of that number comes from particulate matter, written as PM.

PM is simply the solid and liquid particles floating in the air around you.

PM 10 and PM 2.5 refer to how big those particles are, measured in microns.

1 mm is 1000 microns, so a 2.5 micron particle is small enough to travel straight into your lungs.

The official readings come from the Central Pollution Control Board.

They publish them on their website and in their free SAMEER app.

aqicn.org shows the same numbers on a world map.

Easiest of all, your phone's weather app already shows air quality for your area.

If your area sits in Poor or worse for weeks together, yes.

If someone at home has asthma or allergies, definitely.

A purifier cleans one room, not your city.

Shut the windows and run it in the bedroom while you sleep.

That is where it earns its keep.

Outdoors, a well-fitted N95 mask does more than any gadget.

A true HEPA filter, ideally H13, because that is the part that actually traps PM 2.5.
A CADR, or clean air delivery rate, that matches the size of your room.
Filters you can actually buy here, so look for the replacement on Amazon.in first.
The noise rating in decibels, because you will be running it at night.
Wi-Fi and an app, so you can check the live PM 2.5 reading and the filter life.
Skip anything whose main selling point is an ioniser or ozone.

Plan for a filter change every six to twelve months, depending on how hard you run it.

The app will usually tell you when.

Be safe.