How to Choose an Air Purifier: CADR, HEPA and Room Size Explained

Every winter, as the air quality index climbs across north India, air purifiers fly off the shelves. The listings make it sound simple: a big coverage number in square feet, a HEPA badge and a photo of a family breathing easy.

What decides whether a purifier actually cleans your room is less visible: how much air it moves, what its filter really catches, what it costs to keep running and whether you can stand it running at night. Here's how to choose an air purifier without paying for the wrong one.

Start with CADR and your room

CADR stands for clean air delivery rate. It tells you how much filtered air a purifier puts out, usually in cubic metres per hour (m³/h). It's the most useful number on the box, and more honest than the "coverage area" headline, which doesn't tell you how many times an hour your room's air actually gets cleaned.

For bad-air days, that number of air changes is what matters. A practical way to size it:

1. Work out your room volume. Length × width × ceiling height, in metres. A 12 × 12 ft bedroom with a 10 ft ceiling is about 3.7 × 3.7 × 3 metres, or roughly 41 m³.
2. Aim for around five air changes an hour. That's a sensible target when pollution is heavy.
3. Multiply. 41 m³ × 5 = about 205 m³/h of CADR.

Then go a step above that. CADR is measured at the highest fan speed, which is also the loudest. A purifier with spare capacity can do the job on a quieter setting.

If CADR is listed in CFM (cubic feet per minute), multiply by about 1.7 to get m³/h.

True HEPA vs "HEPA-type"

A true HEPA filter is tested to capture at least 99.97% of particles at 0.3 microns, the size that's hardest to catch. That covers PM2.5, the fine particles that make winter smog so harmful, along with dust and pollen.

• H13 and H14 come from a European grading for HEPA filters. Both are high grades; H14 is finer.
• "HEPA-type", "HEPA-like" or "99% HEPA" are marketing phrases, not standards. They usually mean a filter that hasn't been tested to the HEPA benchmark.
• The seal matters. A good filter in a leaky frame lets air slip around it. This is where cheap units and poorly fitting replacement filters fall short.

Most purifiers also have an activated carbon layer for smells and some gases, like cooking odours or smoke. A thin sheet of carbon does little, so if odours matter, look for a noticeably thicker carbon stage. A washable pre-filter catches hair and large dust and helps the main filter last longer.

Filter costs over a year

The purifier's price is only the entry fee. Filters are the running cost.

• Find the replacement filter price before you buy, and check that genuine filters are easy to get.
• Check the stated filter life, usually in hours or months. In heavy pollution seasons, filters clog faster than the estimate.
• Work out the yearly cost: filter price × replacements a year, plus electricity if it runs all day.

A cheaper purifier with expensive or hard-to-find filters can cost more over three years than a pricier one with affordable filters. Our guide to finding the best products within your budget shows how to think in total cost.

Noise at night

You'll probably run it in the bedroom while you sleep, so check:

• Noise on the lowest and medium settings, not just a "whisper quiet" claim.
• A sleep mode that slows the fan and dims the lights.
• Display brightness. A glowing PM2.5 number all night is surprisingly annoying.

This is another reason to buy more CADR than you strictly need: a bigger unit on a low setting is often quieter than a small one working flat out.

Sensors and auto mode

Many purifiers have a PM2.5 sensor and an auto mode that speeds up the fan when the air gets worse. It's useful, especially when you cook or someone opens a door.

Treat the sensor as a guide, not a lab instrument. Readings vary between devices and can drift over time. Use it to see trends, like whether the air is better than an hour ago, rather than to compare your number with the city's official reading. If auto mode keeps the fan too low, set it manually on bad-air days.

Ionisers and ozone: be careful

Some purifiers add an ioniser, which charges particles so they stick to surfaces or the filter. Some ionisers, along with certain "plasma" or UV features, can produce ozone as a by-product, and ozone irritates the lungs. That's the opposite of what you're buying a purifier for.

• Prefer purifiers that rely on mechanical filtration: HEPA plus carbon.
• If there's an ioniser, make sure you can switch it off.
• Avoid devices sold as "ozone generators" for home use.

Placement: where it goes matters

• One room at a time. A purifier cleans the room it's in, not the whole flat. Put it where you spend the most hours, usually the bedroom.
• Close doors and windows while it runs, or it'll be trying to clean the outdoors.
• Give it space. Keep it clear of walls, furniture and curtains so air can flow in and out freely.
• Keep it at the right level. Floor or table height near where you sit or sleep works well; tucked behind a sofa does not.

A purifier doesn't bring in fresh air or reduce carbon dioxide. Ventilate when the outdoor air is cleaner, and lean on the purifier when it isn't.

When you're down to two or three models, line up CADR, filter cost and noise side by side. Our guide to comparing electronics online shows how. Or let Nudge do the narrowing: answer a few questions about your room and budget, and get three picks, each with a reason it fits.

Frequently asked questions

What CADR do I need for my room?

Multiply your room volume in cubic metres by about five air changes an hour, then choose a purifier a step above that number so it can run quietly.

Is "HEPA-type" the same as true HEPA?

No. True HEPA is tested to capture at least 99.97% of particles at 0.3 microns. "HEPA-type" is a marketing term without that test behind it.

How often should I change the filter?

Follow the brand's schedule, but expect to change it sooner in heavy pollution seasons. Check the filter's price and availability before you buy.

Are ionisers in air purifiers safe?

Some can produce ozone, which irritates the lungs. Choose a purifier that relies on mechanical filtration and lets you switch any ioniser off.

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Cleaner air starts with the right size. Try Nudge.