Air purifiers are a filter, a fan and a room you have to measure

The options around air cleaning equipment are set out side by side below, with the conditions that genuinely favour one over the other.

How a mechanical filter actually captures particles

Filter media do not work like sieves, since the fibres are spaced far wider apart than the particles they capture. Large particles are caught by impaction, because their momentum prevents them following the airstream around a fibre.

Mid-sized particles are caught by interception when they follow the airstream but pass close enough to touch a fibre. The smallest particles move erratically through diffusion, which increases the chance they contact a fibre despite their size. These mechanisms combine to produce a most-penetrating particle size where capture is least effective, which is why filter tests specify a particle size.

Filter grades and what the letters describe

High-efficiency particulate filter classifications describe a minimum capture percentage measured at a specified particle size under a defined test. Terms resembling those classifications, but modified with words like type or style, are not the classification and carry no test behind them. Testing regimes and class names differ between regions, so an equivalent-sounding grade may have been measured differently.

On the bench, activated carbon addresses gases and odours by adsorption rather than filtration, and its capacity is finite and exhausts silently. Pre-filters exist to catch large debris and protect the expensive stage behind them, which makes cleaning them a performance decision.

The fan, the airflow and the noise

A filter that captures more also resists airflow more, so a better filter with the same fan moves less air through the machine. Clean air delivery combines airflow and capture efficiency into a single figure, which is why either number alone is misleading. Pushing more air through more resistance requires more fan energy and produces more noise, and there is no way around that relationship.

Published performance figures are usually taken at the highest fan setting, which is often the setting nobody can tolerate in a bedroom. Checking whether a machine can meet the requirement at a liveable noise level is more useful than reading its maximum figure.

Room size and air changes

Manufacturers state a room size, but that statement assumes an air change rate that varies between makers and between regions. More air changes per hour reduce particle concentration faster, and the appropriate number depends on how quickly particles are being generated. Ceiling height is frequently ignored in room size claims, and a tall room contains substantially more air than the floor area suggests.

Open-plan spaces defeat the calculation entirely, since the machine cannot treat air it never draws in.

Placement matters as much as capacity, because a unit against a wall or behind furniture recirculates its own clean output.

Technologies that produce something as well as removing something

Ionisers charge particles so they stick to surfaces, which removes them from the air while depositing them on walls and furniture. Some electrical air treatments generate ozone as a by-product, and ozone is a respiratory irritant that regulators in several regions restrict. Photocatalytic and plasma systems can produce intermediate compounds, and independent evidence on their real-world output is limited and contested.

Where a device makes a claim about viruses or chemicals, the relevant question is what was tested, at what concentration and in what volume. Because health claims fall under different rules in different countries, a claim permitted in one market may be prohibited in another.

Model numbers change quietly, and a review from last year may describe a different machine.

What to check before choosing one

Confirm the filter is a standard part that will still be sold, since a discontinued filter ends the machine. Check the stated performance figure and the noise figure at the same fan speed, rather than accepting each at its most flattering setting. Look for an indicator that measures filter condition rather than counting hours, since a timer knows nothing about your air.

Match the room size claim to your actual room including ceiling height, and treat open-plan spaces as larger than they appear. For anyone managing a diagnosed respiratory condition, this is a matter for a qualified clinician rather than for a specification sheet.

Side by side

The takeaway

Judge an air cleaner by how much air it moves through how good a filter at a noise level you can live with.

Buy for the failure you can live with, not the feature you will use twice.