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HEPA air purifier standing in a living room

Air Purifiers: Room Size, Filters and Running Costs

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August 12, 2026 On July 8, 2026

Air purifiers are sold on features and decided by two numbers: the clean air delivery rate and the cost of the filters. Everything else is secondary.

CADR and room size

CADR measures how much filtered air a unit delivers, usually in cubic feet per minute, and it is quoted separately for smoke, dust and pollen.

A widely used rule of thumb is that the smoke CADR should be at least two-thirds of the room’s floor area in square feet. A 300 sq ft room therefore wants a smoke CADR of about 200.

A unit rated for a room “up to 1,000 sq ft” is usually rated at its highest and noisiest setting. Size up so that you can run it on a quieter setting, because a purifier you turn off is not filtering anything.

Filter types

True HEPA captures 99.97% of particles at 0.3 microns and is the standard worth having. Be wary of “HEPA-type” and “HEPA-like”, which are not the same claim.

Activated carbon handles odours and gases. Particle filters do nothing for smells, so if odour is your concern the carbon stage is what matters and its thickness varies enormously between models.

Pre-filters catch hair and large dust and are usually washable. Cleaning the pre-filter monthly substantially extends the life of the HEPA behind it.

The running cost nobody calculates

Filters are consumables. Work out the annual filter cost before buying, because a cheap unit with expensive proprietary filters costs more over three years than an expensive unit with reasonable ones.

HEPA filters typically last 6 to 12 months in normal use and carbon rather less. Where a manufacturer publishes an interval, we list it.

Check that replacements are actually available for the model. Discontinued filters turn a purifier into an ornament.

What they do and do not help with

Help: dust, pollen, pet dander, smoke particles, and — with a good carbon stage — cooking and pet odours.

Do not help: humidity, mould already growing in the fabric of the building, gases from a fault such as a leaking appliance. A purifier is not a substitute for ventilation or for fixing a damp problem.

Ozone generators and some ionisers produce ozone, which is a respiratory irritant. Avoid them.

Placement

A few feet from walls and furniture so air can circulate around it. In the room you actually spend time in, which for most people means the bedroom rather than the living room.

A purifier running quietly in a bedroom overnight does more for most people than a larger one running in an empty lounge.

Noise

Check the decibel rating at the setting you will actually use. Anything above about 50 dB is intrusive in a bedroom.

This is the specification most closely correlated with whether the unit stays switched on, which is the only thing that matters.

Choosing

Room size, CADR at a quiet setting, true HEPA, a real carbon stage, and filters that are available and affordable. See Air, Seal and Comfort or ask us for the filter cost on any model.