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Winter is when many Indian households start thinking about an air purifier. The short answer on how to choose one: match the purifier’s CADR (Clean Air Delivery Rate) to the size of your room, pick a HEPA filter for fine dust and smoke, add an activated carbon layer if odours or fumes bother you, and avoid any machine that deliberately produces ozone. Everything else, from app control to colour-changing lights, is secondary.
This guide is based on the US Environmental Protection Agency’s (EPA) guide to air cleaners in the home, last updated on 1 October 2026, and on consumer guidance from the California Air Resources Board (CARB). We have not tested any purifier ourselves; the aim is to help you read a spec sheet with confidence.
Step 1: Work out your room size
Measure the length and width of the room where you spend the most time, usually the bedroom, and multiply them. A 12 ft × 14 ft bedroom is 168 sq ft (about 15.6 m²). The EPA’s figures assume a ceiling of about 8 ft, so if your ceiling is higher, aim for a bigger purifier. If the room opens into a hall or kitchen with no door, the EPA suggests treating the connected space as one area or using more than one unit.
Step 2: Match CADR to the room
CADR tells you how much filtered air a purifier delivers, usually measured at its highest fan speed. Higher CADR means it cleans a bigger room, or the same room faster. The EPA lists these minimum CADRs. US brands quote CADR in cubic feet per minute (cfm), while most purifiers sold in India quote cubic metres per hour (m³/h), so we have converted them (1 cfm ≈ 1.7 m³/h).
| Room size | Approx. area | Minimum CADR (cfm) | Approx. in m³/h |
|---|---|---|---|
| 100 sq ft | 9.3 m² | 65 | 110 |
| 200 sq ft | 18.6 m² | 130 | 221 |
| 300 sq ft | 27.9 m² | 195 | 331 |
| 400 sq ft | 37.2 m² | 260 | 442 |
| 500 sq ft | 46.5 m² | 325 | 552 |
| 600 sq ft | 55.7 m² | 390 | 663 |
These are minimums. When outdoor air is very polluted and windows leak, a purifier with more CADR than the table suggests will bring the room down to a cleaner level faster. Running a bigger purifier at a lower, quieter speed is often more comfortable than running a small one flat out.
Which CADR number to look at
Some brands list separate CADRs for smoke, dust and pollen. The EPA says that for the smallest particles, such as PM2.5, you should look for a high CADR for tobacco smoke, because smoke is the reference for small particles. If a brand gives only one CADR figure, check which pollutant it refers to.
Step 3: Choose the right filter
| Filter or technology | What it does | What to know |
|---|---|---|
| HEPA (true HEPA) | Traps fine particles, including PM2.5 dust and smoke | The main reason most purifiers achieve a high CADR |
| Activated carbon | Absorbs some gases, odours and VOCs | Thicker, heavier carbon beds work better than thin coated sheets |
| Pre-filter | Catches hair and large dust | Protects the main filter; check the manual for how to clean it |
| Ionizer / plasma / UV / PCO | Marketed as killing germs or breaking down gases | EPA says these have not been shown to remove gases effectively in home portable units; some can emit ozone |
| Ozone generator | Deliberately produces ozone | Avoid; CARB strongly advises against using them at home |
Note that CADR measures particle removal only. There is no widely used rating for how well a purifier removes gases, so be sceptical of big claims about removing smells or chemicals.
Step 4: Avoid ozone
Ozone irritates the lungs. CARB says ozone can inflame the airways and worsen breathing problems such as asthma, and it “strongly advises against” ozone generators at home. The EPA adds that some ionizers, electrostatic precipitators, plasma units and UV units without proper lamp coatings may also release ozone. In the US, CARB keeps a list of certified air cleaners that meet its ozone limit, and AHAM runs a CADR verification programme. Indian listings rarely mention either, so if a model sold in India is also sold in the US with CARB certification or AHAM-verified CADR, that is a useful extra check. If a listing talks about “activated oxygen” or ozone as a feature, skip it.
Step 5: Check running costs and noise
- Filter price and life. Replacement filters are the real long-term cost. Check the price and availability of the replacement filter before you buy the machine. A clogged filter performs poorly, so replace it when the indicator or the manual says so, and sooner if it looks heavily soiled.
- Noise. The EPA notes that about 50 decibels is roughly as loud as a modern refrigerator. If a purifier is too loud to run at night, you will switch it off, which defeats the purpose. Look for the published noise level at the speed you will actually use.
- Power use. Longer running time and higher fan speeds clean more air but use more electricity. Check the wattage on the spec sheet.
- Service network. In India, easy access to genuine filters and service in your city matters as much as the specs.
How to choose: a quick checklist
- Measure the room and find the minimum CADR in the table above. Add a margin for high ceilings, leaky windows or heavy smog.
- Insist on a true HEPA filter; add activated carbon if odours, cooking smoke or fumes are a concern.
- Reject anything that produces ozone by design.
- Compare the cost and life of replacement filters across two or three shortlisted models.
- Check the noise level at the speed you plan to use overnight.
- Prefer brands with good service coverage in your city.
Once you have a shortlist, you can compare HEPA air purifiers with activated carbon on Amazon India and check the current price, filter price and stated CADR for each model.
Getting the most from your purifier
- Place it in the room you use most, with clear space around it, and not behind curtains or furniture.
- Keep doors and windows closed while it runs; a purifier cannot keep up with a wide-open window.
- Run it continuously on a moderate speed rather than in short bursts at maximum.
- Remember its limits. The EPA says no air cleaner removes all pollutants. Reducing sources such as incense, smoke and dusty surfaces matters too, and purifiers do not fix mould or damp.
On the health side, keep expectations realistic. The EPA says studies generally show small health benefits that are not always noticeable. If you have asthma or another condition, follow your doctor’s advice rather than relying on a purifier alone.
Related reads: India’s cleanest cities by AQI, smart home automation for busy households and how to secure your home Wi-Fi if you plan to use a purifier’s app.
FAQ
What CADR do I need for a 150 sq ft bedroom?
The EPA’s guide works out to about 100 cfm, or roughly 170 m³/h, for 150 sq ft with an 8 ft ceiling. Going higher gives you faster cleaning and a quieter low-speed option.
Are ionizer purifiers bad?
Not all of them, but the EPA warns that some ionizers can emit ozone, and it says such technologies have not been shown to remove gases effectively in home units. A purifier that relies mainly on a HEPA filter is the safer choice.
Does an air purifier remove smells?
Only if it has a decent amount of activated carbon or a similar absorbent filter. A HEPA filter alone traps particles, not gases.
Bottom line
Ignore the marketing and focus on four things: enough CADR for your room (more if your home is leaky), a true HEPA filter, a meaningful carbon layer if you need odour control, and no ozone. Then compare filter costs and night-time noise. A well-sized purifier running steadily in a closed bedroom does far more than an oversized feature list.
Sources
- US EPA: Guide to Air Cleaners in the Home (updated 1 October 2026)
- California Air Resources Board: Air cleaner information for consumers
