The air around Delhi-NCR is already toxic, and soon, most of North India will be wrapped in its annual winter blanket of smoke. What frustrates me more than the haze itself is the misplaced faith people have in cheap air purifiers. Somehow, we’ve convinced ourselves that a single Rs. 7,000 unit sitting in one corner of a flat will protect an entire family from Delhi’s smog. It won’t. Just look outside– the air quality is beyond what any consumer-grade purifier can realistically handle.
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Yes, HEPA-based purifiers do work, but only within their limits. If you truly want cleaner air indoors, you’d need one purifier per room, doors and windows sealed most of the time, and the machines running almost 24/7. Even then, you must occasionally ventilate to balance gases like oxygen, nitrogen, and carbon dioxide.
Studies show that air purifiers can cut indoor PM2.5 levels by half in a closed bedroom and even deliver measurable cardiovascular benefits for people with existing health risks. But these gains are narrow, temporary, and heavily dependent on room size, placement, and continuous operation. In Delhi’s reality– where outdoor PM levels are off the charts and seep through every gap– a single, intermittently used purifier in a 3 BHK is little more than wishful thinking.
What HEPA really means
HEPA stands for High Efficiency Particulate Air. It’s a filtration standard, not a brand or technology type. A true HEPA filter must remove at least 99.97% of airborne particles that are 0.3 microns in size that’s about 300 times smaller than the width of a human hair.
This matters because PM2.5, the fine particulate matter common in Delhi’s smog, is roughly 0.1–2.5 microns. A genuine HEPA filter captures most of it, including dust, smoke, mould spores, pet dander, and bacteria.
However, HEPA filters only trap particles, not gases or odours. Pollutants like nitrogen dioxide, ozone, or volatile organic compounds (VOCs) pass right through unless the purifier also includes activated carbon filters or other gas-absorbing media.
HEPA: The catch
Not every purifier labelled “HEPA” is truly HEPA. Some cheaper ones use marketing terms like “HEPA-type” or “HEPA-like.” These copies don’t meet the strict 99.97% efficiency standard. In a city like Delhi, where most of the threat comes from fine particulates, buying a “HEPA-type” purifier is nearly useless.
HEPA filters are effective, proven, and essential for particle pollution. But they’re only one piece of the clean-air puzzle. Without proper sizing, airtight rooms, and continuous use, even the best HEPA purifier is fighting a losing battle.
Why one purifier feels like a band aid for a broken arm
- Localised effect. Portable purifiers clean the air that passes through them. Their reach is limited to the room they are placed in and declines with distance and doorway airflows. You cannot expect a living-room purifier to keep bedrooms, kitchens and bathrooms at the same low PM level.
- Wrong size, wrong job. Many people buy a single cheap unit without checking CADR or the AHAM sizing rules. A filter needs an appropriate CADR for the room to give multiple air changes per hour. Otherwise it barely moves the needle.
- Intermittent use defeats the purpose. Pollutants re-accumulate quickly. If you run a unit only at night or only occasionally, exposure during the day still matters for long-term health. Some trials that show benefits kept purifiers running continuously for weeks.
- Not all pollutants are particles. HEPA filters are excellent for particles such as PM2.5. They do not remove gases like nitrogen dioxide, many volatile organic compounds, or odours. Some consumer devices that claim to remove gases emit ozone or use ionisers, which can create other health risks. So a single HEPA unit is blind to some important urban pollutants.
- Outdoor source overwhelms local fixes. Delhi’s poor ambient air is not something a single indoor device can fix. When outdoor PM rises massively during a smog episode, infiltration and door openings quickly push indoor concentrations up. This is why city-scale measures matter more than individual band aids. Public health data make that point starkly.
Practical advice, if you have an air purifier
- Use the right size. Match CADR to each room. Use the AHAM rule of thumb to pick a unit that delivers adequate air changes for the room.
What CADR actually means? CADR, or Clean Air Delivery Rate, measures how quickly an air purifier can clean the air in a room. It’s expressed in cubic feet per minute (CFM) and tells you how much filtered air the device delivers every minute. There are usually three CADR numbers- one each for smoke, dust, and pollen- since purifiers remove these particles at slightly different rates. In most cases, the smoke CADR is used as a benchmark because smoke particles are the smallest and closest in size to PM2.5 found in Delhi’s air.
In short:
Higher CADR = faster cleaning.
Lower CADR = slower cleaning and less protection.
If the CADR is too low for your room size, the purifier simply won’t cycle and clean the air often enough to make a real difference.
The AHAM rule (how to size a purifier)
The AHAM rule comes from the Association of Home Appliance Manufacturers (AHAM), the US body that certifies purifier performance. It gives you a simple formula to choose the right purifier for your room:
Recommended room size (in square feet) = CADR × 1.55
Or flipped around: CADR = room area ÷ 1.55
So, if your bedroom is 150 sq. ft, you need a purifier with at least CADR 100 (150 ÷ 1.55 ≈ 97).
If your living room is 300 sq. ft, look for CADR around 200 or more. This ensures the purifier can clean the room’s air about 4 to 5 times per hour, which is what you need to keep PM2.5 levels under control in a polluted environment like Delhi.
HEPA filters are effective, proven, and essential for particle pollution. But they’re only one piece of the clean-air puzzle. Without proper sizing, airtight rooms, and continuous use, even the best HEPA purifier is fighting a losing battle.
Why one purifier feels like a band aid for a broken arm
- Localised effect. Portable purifiers clean the air that passes through them. Their reach is limited to the room they are placed in and declines with distance and doorway airflows. You cannot expect a living-room purifier to keep bedrooms, kitchens and bathrooms at the same low PM level.
- Wrong size, wrong job. Many people buy a single cheap unit without checking CADR or the AHAM sizing rules. A filter needs an appropriate CADR for the room to give multiple air changes per hour. Otherwise it barely moves the needle.
- Intermittent use defeats the purpose. Pollutants re-accumulate quickly. If you run a unit only at night or only occasionally, exposure during the day still matters for long-term health. Some trials that show benefits kept purifiers running continuously for weeks.
- Not all pollutants are particles. HEPA filters are excellent for particles such as PM2.5. They do not remove gases like nitrogen dioxide, many volatile organic compounds, or odours. Some consumer devices that claim to remove gases emit ozone or use ionisers, which can create other health risks. So a single HEPA unit is blind to some important urban pollutants.
- Outdoor source overwhelms local fixes. Delhi’s poor ambient air is not something a single indoor device can fix. When outdoor PM rises massively during a smog episode, infiltration and door openings quickly push indoor concentrations up. This is why city-scale measures matter more than individual band aids. Public health data make that point starkly.
Practical advice, if you have an air purifier
- Use the right size. Match CADR to each room. Use the AHAM rule of thumb to pick a unit that delivers adequate air changes for the room.
What CADR actually means? CADR, or Clean Air Delivery Rate, measures how quickly an air purifier can clean the air in a room. It’s expressed in cubic feet per minute (CFM) and tells you how much filtered air the device delivers every minute. There are usually three CADR numbers- one each for smoke, dust, and pollen- since purifiers remove these particles at slightly different rates. In most cases, the smoke CADR is used as a benchmark because smoke particles are the smallest and closest in size to PM2.5 found in Delhi’s air.
In short:
Higher CADR = faster cleaning.
Lower CADR = slower cleaning and less protection.
If the CADR is too low for your room size, the purifier simply won’t cycle and clean the air often enough to make a real difference.
The AHAM rule (how to size a purifier)
The AHAM rule comes from the Association of Home Appliance Manufacturers (AHAM), the US body that certifies purifier performance. It gives you a simple formula to choose the right purifier for your room:
Recommended room size (in square feet) = CADR × 1.55
Or flipped around: CADR = room area ÷ 1.55
So, if your bedroom is 150 sq. ft, you need a purifier with at least CADR 100 (150 ÷ 1.55 ≈ 97).
If your living room is 300 sq. ft, look for CADR around 200 or more. This ensures the purifier can clean the room’s air about 4 to 5 times per hour, which is what you need to keep PM2.5 levels under control in a polluted environment like Delhi.
2. Run it continuously when you occupy the room, especially at night in the bedroom. Short bursts do not reliably lower cumulative exposure.
3. Place it well. Keep it away from walls and corners so airflow is not blocked and put it in the main occupied room (bedroom while sleeping).
4. Control sources. Avoid frying, incense and indoor smoking. Use the kitchen exhaust hood while cooking and close doors to isolate cooking smoke. Get a air purifier for your kitchen as well!
5. Seal and ventilate smartly. During heavy outdoor pollution keep windows closed and use mechanical ventilation with recirculation and good filters if available. When outdoor air is relatively clean, ventilate to remove indoor gases.
A single midrange purifier can reduce PM2.5 in one room for a relatively modest running cost. That is not worthless. It is a worthwhile protective step if you or someone at home has asthma, COPD, cardiovascular risk or is pregnant. But if your objective is to protect everyone in a typical 2 or 3 BHK and you use the unit sporadically, the health returns are limited and you are mostly paying for comfort in one corner of the flat. Think of a purifier as targeted personal protection, not a household cure.
Owning a HEPA purifier is sensible if you use it correctly and understand its limits. Expecting one intermittently used unit to protect an entire Delhi apartment from winter smog is unrealistic. If you want meaningful protection, either invest in a properly sized solution for all occupied rooms, improve sealing and ventilation strategy, or accept the purifier as partial and situational protection while you focus on source control and safer behaviours.