How to choose an air purifier in India: CADR, filters and running cost

1. Start with the room you will actually clean

A portable purifier is normally intended for one room or connected area, not an entire flat with closed doors. Measure the room's length and width, multiply them to get floor area, and note the ceiling height. Most published room-size tables assume an eight-foot ceiling. A taller room contains more air, while an open kitchen or passage lets pollutants continually mix back in, so both need more capacity than the same floor area in a closed bedroom.

Choose for the room where you spend the most time, usually a bedroom or work room. Buying one powerful unit and expecting it to clean several separated rooms is rarely realistic. If two occupied rooms need filtration at the same time, two correctly sized purifiers may be more useful than moving one device after the pollution has already built up.

2. Use smoke CADR, not a giant coverage claim

Clean Air Delivery Rate, or CADR, describes how much contaminant-free air a purifier delivers. AHAM reports separate CADR values for tobacco smoke, dust and pollen because particle sizes differ. For India's PM2.5-heavy pollution episodes, smoke CADR is the most relevant of the three. A bare fan airflow number is not the same thing: CADR measures the complete purifier's combination of airflow and particle removal.

AHAM's normal sizing rule is a smoke CADR in cubic feet per minute equal to at least two-thirds of the room area in square feet. A 150 sq ft room therefore needs roughly 100 cfm, or about 170 m³/h. That is a sensible minimum for an eight-foot ceiling. Higher ceilings, leaky rooms or a severe outdoor smoke load justify extra margin. Compare products using the same CADR unit and test basis instead of trusting a brand's unexplained maximum-area headline.

• 100 sq ft room: look for at least about 65 cfm smoke CADR.
• 200 sq ft room: look for at least about 130 cfm smoke CADR.
• 300 sq ft room: look for at least about 195 cfm smoke CADR.
• If a listing gives only m³/h, divide by roughly 1.7 to estimate cfm before comparison.

3. Treat HEPA as a filter description, not a performance score

A genuine high-efficiency particle filter is useful, but the word HEPA does not tell you how much clean air the finished machine delivers. A small filter behind a weak fan can have excellent media efficiency and still clean a room slowly. Air bypass around a poorly sealed filter can also reduce real performance. This is why measured smoke CADR and room sizing should come before marketing terms such as H13, medical grade or multi-stage.

Check that the replacement filter is made for the exact model and seals into the housing properly. Washable pre-filters can capture hair and larger dust before it reaches the main filter, but a pre-filter does not replace the particle filter. Never wash a HEPA filter unless the manufacturer explicitly says that exact filter is washable.

4. Know what the purifier cannot remove

Particle filters target airborne particles such as PM2.5, smoke, pollen and some dust. They do not automatically remove gases or odours. EPA guidance says an activated-carbon or other sorbent filter is needed for gases, and effectiveness depends partly on having a meaningful amount of material. A thin black sheet may reduce some odour briefly but should not be treated like a deep carbon bed.

CADR ratings apply to particles, not gas removal. A purifier also cannot fix a continuing moisture leak, remove settled dust from surfaces, replace a kitchen exhaust, or make indoor smoking harmless. Source control and ventilation with cleaner outdoor air remain the first steps; filtration is a supplement when those measures cannot remove enough pollution.

5. Avoid products that intentionally generate ozone

Ozone is a lung irritant, not a feature you need in an occupied room. EPA advises avoiding portable cleaners that intentionally produce ozone and notes that some ionizers, electrostatic devices, plasma systems and poorly designed UV systems may emit it. A mechanical fan-and-filter purifier is the simpler default when your goal is particle removal.

If a purifier includes ionisation, plasma or another electronic cleaning mode, verify whether it can be disabled without reducing normal fan-and-filter operation. Do not assume that words such as active oxygen, sterilisation or freshness describe a health benefit. Look for a recognized low-ozone test or certification and a clearly published emissions result.

6. Compare noise at the speed you need

CADR is usually measured at the highest fan speed, which may also be the loudest. A purifier that reaches the required CADR only in an intolerable turbo mode will spend most nights on a lower setting and clean much more slowly. Check noise measurements at high and medium speeds, not only the quietest sleep mode. Independent tests are particularly useful when they measure both particle-cleaning speed and sound at repeatable settings.

For a bedroom, ask whether you can sleep with the machine running continuously at a useful airflow. For a work room, check whether fan tone interferes with calls. A slightly oversized purifier can often deliver the required clean air at a lower, quieter speed, but excessive oversizing adds purchase cost, bulk and energy use.

7. Calculate filter cost before checkout

The purifier is a recurring-cost product. Find the exact replacement-filter model, current availability and expected replacement interval before buying the machine. Then calculate at least two years of ownership: purchase price plus replacement filters plus electricity. A cheap purifier with a scarce proprietary filter can become unusable long before its fan fails.

Replacement timing depends on pollution load, operating hours and pre-filter maintenance, so a fixed app timer is only an estimate. A filter that is visibly damaged, smells persistent or causes severely reduced airflow needs attention even if the timer says otherwise. Vacuum or clean only the parts the manual permits; aggressive cleaning can damage filter media and seals.

8. Read electricity efficiency and smart features correctly

Purifiers work best when they run for long periods, so watts matter. ENERGY STAR's current criteria compare smoke CADR with operating power through an integrated efficiency measure. When two models deliver similar verified CADR, the lower-watt option can reduce long-term cost. Compare power at useful cleaning speeds rather than standby consumption alone.

A built-in PM sensor and auto mode are convenient, but they do not prove purification performance. Sensor placement, calibration and software thresholds vary. India's CPCB AQI also combines the worst sub-index among several outdoor pollutants; a purifier display based on one indoor PM sensor is not a complete CPCB AQI station. Use the display for trends in that room, not as a certified public-health measurement.

9. Place and operate it so air can move

Keep the intake and outlet clear of curtains, furniture and walls according to the manual. Place the unit in the occupied room rather than a corridor and close unnecessary openings when outdoor air is heavily polluted, while still following ventilation advice appropriate to cooking, moisture and carbon-dioxide buildup. A purifier hidden behind a sofa cannot deliver its laboratory airflow into the room.

Run the purifier before the room becomes badly polluted, use a higher speed for an initial clean-up, then reduce speed only as far as comfort allows. Clean the pre-filter on schedule and check door seals after every filter change. The best purifier is not the one with the most modes; it is the one that delivers enough verified clean air every day.

10. Use this India buying checklist

Reject a listing that hides the numbers needed to complete this checklist. Once the essentials are verified, compare convenience features and price among the remaining options.

For India-specific claims, BIS lists IS 17531:2021 as the product specification for portable electric indoor air purifiers and shows that it was reviewed in 2025. The same BIS record currently lists certification as N/A. That means the existence of an Indian Standard should not be presented as proof that every purifier must carry a compulsory BIS product-certification mark. If a seller advertises BIS certification, ask for the exact standard, licence or registration details and verify the claim against the official record rather than relying on a marketplace badge.

• Room area, ceiling height and intended room are known.
• Verified smoke CADR meets the room-size requirement with sensible margin.
• Filter type, seal, exact replacement code, availability and two-year cost are clear.
• Noise and power are acceptable at a speed that provides useful CADR.
• Gas or odour claims are backed by substantial sorbent media, not particle CADR.
• The product does not intentionally generate ozone, and electronic modes can be disabled.
• Warranty, invoice, service route and replacement-filter seller are traceable.

The simple recommendation

For a typical bedroom, shortlist mechanical fan-and-filter purifiers by verified smoke CADR first. Choose enough capacity for the measured room, then eliminate models whose high-speed noise or replacement-filter cost will stop you from running them. Add a substantial carbon stage only when gases or odours are a real priority. Search Gaandfaad with the room and constraint—for example, ‘air purifier for a 200 sq ft Delhi bedroom with low filter cost’—instead of asking for one universal best purifier.