Air purifier specs that matter
CADR, true HEPA, noise at sleep speed, filter supply and running cost. A checklist for comparing any purifier without trusting the marketing.
The checklist
| Spec | What to look for | Why |
|---|---|---|
| Particle (smoke) CADR | At least what the size calculator gives for your room, in m³/h (or CFM × 1.7). US EPA and AHAM’s rule of thumb, smoke CADR in CFM ≥ ⅔ of floor area in ft², works out to the same 5 air changes an hour | The single most important number: how much clean air it delivers |
| Filter | Mechanical H13 HEPA (EN 1822 / ISO 29463 “ISO 35 H”) or US-standard True HEPA | Captures the fine particles. “HEPA-type” is not the same |
| Noise at a usable speed | Decibels at medium/sleep speed, not just at minimum; under about 35–40 dB(A) is comfortable for sleep | A purifier that is too loud gets turned down or off |
| CADR at that speed | Some brands list CADR per speed. Ask if they don’t | Sleep modes often deliver a fraction of the headline CADR |
| Power use | Watts at the speed you’ll use | Runs 24 h a day for months |
| No ozone | No ioniser, or one that can be switched off; certified low-ozone where stated | Ozone irritates lungs. See ionisers |
| Filter supply and cost | Genuine filters easy to buy locally, price known before you buy | Filters are the real long-term cost |
| Seal | Filter sits in a gasketed slot, no visible gaps | Air that bypasses the filter isn’t cleaned |
| Warranty and service | Local service network | It will run for years |
Nice to have, not essential: a built-in PM2.5 display (handy, but often optimistic; a separate monitor is more trustworthy), app control, a filter-life indicator (often just a timer).
Ignore: “room coverage” claims without CADR, UV lights, “plasma”, “anion”, “99.97%” claims for the whole device, and decorative carbon claims.
Compare a model
Check a purifier before you buy
Copy the numbers from the spec sheet of any purifier you are considering.
The purifier
Your room and use
Purchase price not included. Use the CADR and wattage at the speed you'll actually run, if the spec sheet lists them.
One big or two small?
- For one closed room, one purifier with enough CADR is simplest.
- For a large living room or open plan, two units at opposite ends mix the air better, and their CADRs add up.
- A bigger unit on a lower speed is usually quieter than a small unit on full speed.
DIY box-fan filters
A box fan with MERV 13 filters attached can deliver a lot of clean air for little money. In US EPA lab tests, a fan with one filter delivered about 110 CFM (≈ 190 m³/h). Adding a cardboard shroud raised that by about 40%, and a “Corsi–Rosenthal” cube with four filters reached about 400 CFM (≈ 680 m³/h). It’s a useful stopgap for living areas.
EPA’s safety advice, adapted for India:
- EPA recommends fans made 2012 or later with UL or ETL certification, which have thermal cut-offs. Most fans sold in India carry an ISI mark instead, which is not the same safety test. Treat any DIY unit as supervised use only.
- Never leave one running unattended or while sleeping, and never in a child’s or older adult’s bedroom. Use a commercial purifier with known performance and electrical safety there.
- No extension cords. Don’t use a damaged fan. Keep working smoke detectors.
- Match the filters’ airflow arrows to the fan’s airflow, seal the joins, and replace filters when they turn dark grey. Dirty filters make the unit almost useless.