Air Purifiers & HEPA Filters: Do You Need One, and How to Choose

In India's polluted cities, the air purifier has gone from luxury to near-necessity — and for good reason: it's the most direct way to cut the PM2.5 that dominates indoor air. But purifiers are also over-marketed, with confusing claims and gimmick features. This guide cuts through it honestly: how a purifier actually works, what its filters do and don't remove, how to size one properly (the one number that matters), and where a purifier genuinely helps versus where fresh air or fixing the source matters more.

It builds on the Indoor Air Quality Guide, part of the HVAC Knowledge Hub.

The safety line & scope. Air purifiers are plug-in consumer devices — homeowner-level. This guide helps you decide and choose; for whole-home filtration built into HVAC/fresh-air systems, a qualified professional designs it.

How an air purifier works

A purifier is simple: a fan draws room air through filters and returns it cleaner, cycling the room's air repeatedly. Its effectiveness comes down to the filters and how fast it moves air:

• The pre-filter catches large dust and hair (and protects the main filter).
• The HEPA filter — the heart — captures fine particles, including PM2.5.
• The activated-carbon filter — adsorbs odours, smoke and some gases/VOCs (a different job from HEPA).
• The fan — moves the air; faster airflow means more of the room cleaned per hour.

The key: a purifier only cleans the air that passes through it, so it must move enough air for your room size — which is where CADR comes in.

What the filters remove

Matching the filter to the pollutant:

• HEPA — a High-Efficiency Particulate Air filter captures at least 99.9%+ of fine particles at the standard test size; this is what removes PM2.5, dust and smoke particles. Look for a genuine HEPA filter (beware "HEPA-type" imitations).
• Activated carbon — adsorbs gases, odours, smoke smell and some VOCs — a different function; HEPA alone doesn't remove smells or gases.
• What purifiers don't fix — they don't add fresh air or remove CO₂ (only ventilation does that), and standalone units clean one room, not the whole home.
• Be wary of — "ioniser" and ozone-generating features (ozone is itself a pollutant), and unsubstantiated claims. HEPA + carbon + adequate airflow is what genuinely works.

Sizing it: CADR is the number that matters

The single most important spec — and the one buyers ignore:

• CADR (Clean Air Delivery Rate) — how much clean air the purifier actually delivers per hour (in m³/hr or CFM). A purifier's CADR must match your room's size to clean it effectively.
• Too small a CADR for the room = the purifier runs forever and never really cleans it. This is the commonest mistake.
• Rule of thumb — pick a CADR that gives the room several air changes per hour (roughly 4–5×), so buy for your room's floor area, and size up for a large room.
• Manufacturers state a "room size" rating — sanity-check it against CADR and your actual room, and don't put a small bedroom unit in a large living room.

Where a purifier genuinely helps — and where it doesn't

An honest placement:

• It genuinely helps — in polluted cities with high outdoor PM2.5, in the bedroom (where you spend 8 hours), during cooking or high-pollution days, and for allergy/asthma sufferers. Real, measurable benefit.
• It's less the answer when — the problem is CO₂/stuffiness (you need ventilation, not a purifier), or a pollution source you could fix (a proper kitchen exhaust, low-VOC materials, managing damp).
• The right combination — ventilate and cut sources first, then purify the PM2.5 that remains. A purifier in an unventilated, high-source room is fighting uphill.

For a bedroom in a polluted city, a well-sized HEPA purifier is one of the best health buys there is; as a substitute for ventilation or source control, it isn't.

Choosing and running one well

• Genuine HEPA + activated carbon, from a reputable brand.
• CADR matched to the room — the decisive spec.
• Noise — check the noise at the speed you'll actually run it (especially in a bedroom).
• Filter cost and life — filters are a recurring cost; factor it, and replace filters on schedule (a clogged filter barely works).
• Running it — run it continuously on high-pollution days and overnight in the bedroom; a monitor (IAQ guide) shows it working.
• Skip the gimmicks — ozone/ioniser marketing; buy filtration and airflow.

The one-line answer

An air purifier draws room air through a pre-filter, a genuine HEPA filter (which captures PM2.5) and activated carbon (which adsorbs odours and gases) and returns it cleaner — so in India's polluted cities it's the most direct way to cut the PM2.5 that dominates indoor air, with real benefit in the bedroom, while cooking, on bad-air days and for allergy sufferers. The one spec that matters is CADR (clean air delivery rate): it must match your room size to actually clean it — under-sizing is the commonest mistake, so buy for your room's area with several air changes per hour. But be honest about its limits: a purifier doesn't add fresh air or remove CO₂ (only ventilation does), doesn't fix a pollution source you could eliminate, and cleans one room. So ventilate and cut sources first, then purify — a well-sized HEPA-plus-carbon unit is one of the best health buys in a polluted city, but not a substitute for fresh air. Skip ozone/ioniser gimmicks and replace filters on time.

Where to go next

• The full picture: Indoor Air Quality Guide.
• Fresh air (what a purifier can't give): Home & Building Ventilation Guide.
• Whole-home fresh air with filtration: HRV & ERV Guide.
• Size the airflow: Air Changes Per Hour Calculator.

References

• HEPA filter classification (EN 1822 / ISO 29463); CADR (Clean Air Delivery Rate) — AHAM test method.
• Central Pollution Control Board (CPCB) — indoor air quality guidance; World Health Organization PM2.5 guidelines: https://cpcb.nic.in/
• ISHRAE — indoor air quality guidance; ASHRAE Standard 62.1 (filtration & ventilation).

Filter and CADR standards are set by international bodies; verify a product's genuine HEPA rating and CADR. Whole-home filtration built into HVAC is professional work.