Best air purifiers UK 2026: the CADR your room needs for allergies, mould and sleep, not the m² printed on the box

The sum fits on one line: clean air needed = floor area × ceiling height × air changes an hour. You know the first two. The third comes from AHAM, the US association that writes the standard CADR is measured against, which sizes its seal at 4.8 air changes an hour. Most room sizes you'll read, on boxes and in reviews, leave the ceiling out, and in the UK two official figures are worth having to hand: the government's nationally described space standard, which local plans in England apply to new homes, sets a double bedroom at 11.5 m² or more and a single at 7.5 m², with 2.3 m from floor to ceiling over at least 75% of the floor; London's housing design guidance raises that ceiling to 2.5 m. The calculator below does the sum with your figures, sets it next to the floor area the same flow would give depending on who divides it, and adds the other sum that decides: the filter over three years.

CADR stands for clean air delivery rate: the volume of filtered air a purifier delivers, in cubic metres an hour in Europe and in cubic feet a minute in the US. The standard that defines it is ANSI/AHAM AC-1-2020, which rates it separately for three kinds of particle, 'dust, pollen and tobacco' smoke in AHAM's words. That is what makes it useful before you buy: two machines from two brands, compared on one number.

Square metres can't do that job, and the reason is arithmetic. A purifier doesn't clean a floor, it moves a volume of air. The area on the box is the flow divided by a coefficient each maker picks, and the coefficient is almost never printed. Our count of the Spanish Amazon catalogue shows how rarely even the flow is.

Mould spores in the air are particles, so a HEPA purifier catches them the way it catches dust, and the same CADR measures it. What a purifier doesn't touch is the reason the mould is there. The UK government's guidance on the health risks of damp and mould, written by the Office for Health Improvement and Disparities with the housing ministry and the UK Health Security Agency, sets out the causes: moisture from 'showering, drying laundry and cooking' that builds up and condenses on the colder parts of a home, water that gets in through defects in walls, roofs, windows or floors, and leaks. It also puts a size on the problem: in 2019, damp and mould in English homes was estimated to be associated with about 5,000 cases of asthma and 8,500 lower respiratory infections. And its line on the visible patch: 'Simply removing surface mould will not prevent the damp and mould from reappearing'.

The same guidance lists who is most at risk, and it is the reader of this section: 'people with a pre-existing health condition (for example allergies, asthma, COPD, cystic fibrosis, other lung diseases and cardiovascular disease)', children and older adults. So the order is moisture and ventilation first (an extractor fan close to the hob, windows opened wide and closed again once the air has changed, laundry dried outdoors where possible), and a purifier in the bedroom as a supplement for the spores already in the air. For the moisture itself the appliance is a dehumidifier, not a purifier, and never a humidifier, which puts water back into the air.

For allergies and asthma the reference figure is the 4.8 air changes an hour AHAM uses to set the room size on its seal: 'Look for the AHAM Verifide seal to find the suggested room size based on 4.8 air changes per hour', in its own words. Below three, which AHAM cites as indoor-air researchers' minimum ventilation recommendation, the machine doesn't keep up with the dirty air coming in. For hay fever, the CADR to read is the pollen one, because AHAM rates dust, pollen and smoke separately.

In the UK you'll also see the Allergy UK Seal of Approval on some purifiers, and it is a different kind of evidence, worth reading for what it is. Allergy UK says approved products are 'independently scientifically proven to be effective at reducing/removing allergens', under a scheme 'Established in 1995' and run through Allergy Research Limited. What the seal doesn't give you is a room size: for that you still need the CADR and the sum at the top of this page.

Running it overnight is where a purifier earns its keep: eight hours in the room where you spend the most time. The catch isn't the noise, it's that the flow and the noise are never measured at the same moment. The EPA says it in one line: the CADR 'is typically measured at the air cleaner's highest speed'. The decibels on a box are the opposite end, the lowest speed.

Philips' leaflet for the 600i prints both ends: 19 dB in sleep mode and 49 dB in turbo, to IEC 60704, the standard its 2200 series also cites. Of the documents we read for this guide, it is the only one that sets the loud figure next to the quiet one. If you're going to sleep with it on, our advice is to size it with headroom: none of the 36 listings in our count publishes the flow in night mode, so assume it delivers well below its CADR.

The sum is watts × hours ÷ 1,000 × the price of a kilowatt-hour, and for the last quarter of 2026 the UK has a reference price: Ofgem's cap for 1 October to 31 December 2026 puts electricity at 26.32p per kWh on average by Direct Debit, with no VAT on electricity bills until 31 March 2027 after the government removed it. Philips publishes the two figures you need for the 600i: under 12 W at full speed and 2 W in sleep mode. Round the clock at full speed that is 105 kWh a year, about £27.67; eight hours a night in sleep mode, the way a bedroom purifier usually runs, it is 5.8 kWh, about £1.54.

Which? gives a range for the purifiers it assesses of 'as little as £6 and as much as £45 a year'. The figure that decides is the wattage the maker declares, and in our Amazon Spain count 32 of the 36 listings don't give it: it is worth asking for before you buy, because it's the only way to do this sum.

The European standard for these filters, published in the UK as BS EN 1822-1:2019, 'High efficiency air filters (EPA, HEPA and ULPA)', names its three groups in its own title. The H in H13 is the HEPA group. What the class doesn't tell you is how much air passes through the filter, and that decides everything: an excellent filter in a machine that moves little air cleans little.

In our Amazon Spain count, 30 of the 36 listings say HEPA, eight say H13 and two write 'true HEPA', which is none of the standard's three groups. The maker that does cite the standard is Dyson, in the small print of its Spanish website: it states its filtration efficiency at 0.1 microns '(EN1822, ISO29463)'. That is the reference to look for, and almost nobody prints it.

The other half of the money isn't in the machine. None of the 36 listings in our Amazon Spain count says how long its filter lasts, and the lives the makers do publish run from six months to three years. Over the same three years that is five replacements or none: six filters through the machine instead of one.

The three podium machines cover the whole range. Levoit's UK page says to replace the Core 300S filter 'every 6-8 months'; Philips' leaflet says 12 months for the 600i; Philips rates the 2200's FY2200/30 filter for up to three years, with a reminder from the machine when it's due. This is the only sum in this guide that can reorder a whole podium, and the calculator above does it with the price you pay for a replacement.

There are two problems here and only one has a number. Smoke is particles and gases at once: the particles are caught by the filter and rated by the CADR; the gases are caught by activated carbon and rated by nobody. The EPA puts it without hedging: 'there are no widely used performance rating systems for portable air cleaners or filters designed to remove gases. The CADR rating system is for particles only.'

For the part that is rated, AHAM raises the bar: for smoke it asks for a smoke CADR equal to the room's square feet, 1.55 times its usual rule, which works out at about 7.4 air changes an hour. That is the third setting in the calculator at the top of the page.