Air purifiers and allergies

What they do

An air purifier draws room air through a filter and pushes it back out. A good one measurably reduces the quantity of small particles floating in that room — dust, smoke, pet dander, pollen that has drifted in.

That much is straightforward and not in dispute. The harder question is whether reducing airborne particles makes an allergic person feel better, and the honest answer is more qualified than the packaging suggests.

The evidence, stated honestly

Trials of air filtration in allergic disease exist, and they are mostly small, short and mixed.

The pattern that emerges is of modest benefit. Several small studies have found improvements in quality of life or night-time symptoms. Lung function generally does not change. An earlier pooled analysis found only small gains in overall symptoms and sleep, with no clear effect on how much medication people needed.

The strongest results come from studies where filtration was one part of a wider package — better heating and ventilation filters, dehumidifiers, and other measures together — rather than from a purifier alone.

None of this means they are useless. It means the realistic expectation is a modest improvement in comfort, most noticeably at night in a bedroom, rather than the relief that a leaflet implies.

It depends what the allergy is

Lumping all allergens together is where the overclaiming happens, because they behave quite differently in a room.

Cat allergen is where the evidence is strongest, such as it is. It is light, stays airborne for a long time, and filtration does reduce it. Results on symptoms are still mixed, and the effect disappears quickly once the cat is back in the room.

Pollen has very little evidence either way. What exists suggests some benefit from a unit running over the sleeping area.

House dust mite is the awkward case, and it is awkward because it is also one of the commonest indoor allergies.

Mite allergen is carried on relatively heavy particles. They become airborne when a bed is disturbed or a carpet walked on, and then settle again within a matter of minutes to hours. For most of the day they are not in the air at all — they are in the mattress, the bedding and the carpet, where a filter cannot reach them. A review of the evidence found no demonstrated benefit from HEPA room cleaners in dust mite allergy.

Someone buying a purifier specifically for dust mite allergy is buying the wrong thing, and that is the single most useful sentence in this article.

Ozone: avoid these outright

Some devices are sold as air purifiers that work by generating ozone. Older writing on this subject — including the version of this article that stood here before — treats them as a normal option and reassures readers that household levels are too low to matter.

That reassurance is wrong, and the position of the US Environmental Protection Agency, which has assessed these devices, is worth stating plainly:

• no government agency has approved ozone generators for use in occupied spaces
• ozone concentrations can exceed health standards even when the manufacturer's instructions are followed
• at concentrations that do meet health standards, ozone is generally ineffective at controlling indoor air pollution
• ozone does not remove particles at all — not dust, not pollen, not the particles responsible for most allergies

Ozone damages the lungs. Even modest amounts can cause chest pain, coughing, shortness of breath and throat irritation, it worsens asthma, and it reduces the body's resistance to respiratory infection.

So an ozone generator bought to help with allergies does not do the thing it was bought for, and can make the breathing of the person who bought it worse. There is no version of this that is a good purchase.

Ionisers and charged plates

A related category works by giving particles an electrical charge so they stick to metal plates inside the device, or simply to the walls and furniture of the room.

The old version of this article recommended these. The current advice is to avoid them: they offer little benefit compared with a proper filter, and many produce ozone as a by-product.

There is also something faintly self-defeating about a device whose mechanism is to deposit allergens onto the surfaces of the room. They have not been removed; they have been moved, and they can be stirred up again.

If buying one anyway

A mechanical filter is the type worth having, and that means HEPA — high efficiency particulate air. These trap particles in a dense mat of fibres rather than charging them.

Points that matter more than price:

• Room size. A unit rated for a small room will not cope with a large one. The rating is a capacity, not a marketing figure.
• One room only. A portable unit cleans the air in the room it is standing in. The bedroom is usually the place that justifies it, because that is where the most consecutive hours are spent.
• Filter replacement. HEPA filters load up and stop working well; they are consumables, not permanent. Replacement cost and frequency are part of the price of the machine, and are worth checking before buying rather than after.
• Noise. A purifier that is too loud to sleep next to gets switched off at night, which is exactly when it would have done the most good.

What matters more than the purifier

Filtration is a supplement to removing the source of the problem, not a replacement for it, and every serious assessment of it says the same thing: no single measure is likely to be enough on its own.

For dust mite, the useful measures are in the bedding and the bedroom rather than in the air — washing bedding hot, reducing humidity, and keeping soft furnishings down where that is practical. For pet allergen, keeping the animal out of the bedroom does more than any machine. For pollen, keeping windows shut at the times of day when counts are highest does more than filtering what has already come in.

An air purifier can add something on top of that. It is a poor substitute for it.