Do Air Purifiers Actually Work? We Tested It with Data
We ran particle counters for seven days with and without air purifiers in a real UK home. Here is what the numbers actually show.
If you are wondering whether air purifiers genuinely improve indoor air quality or whether they are just expensive fans with filters, you are not alone. It is one of the most common questions we receive, and the answer deserves more than a marketing claim. So we tested it.
Over seven consecutive days in January 2026, we measured PM2.5 and PM10 levels in a 28-square-metre living room in a Victorian terrace in South London. We ran three days without any purifier, then four days with a HEPA H13 air purifier. Same room, same occupants, same routine. Here is what we found.
The Science Behind Air Purification
Air purifiers work by drawing room air through one or more filters. The most effective type is the HEPA filter, which stands for High Efficiency Particulate Air. A true HEPA H13 filter is a dense mat of randomly arranged fibres, typically made from fibreglass. As air passes through, particles are trapped by three mechanisms.
Interception occurs when a particle following the airstream comes within one radius of a fibre and sticks to it. Impaction occurs when larger particles cannot follow the airstream around a fibre and collide directly with it. Diffusion occurs when the smallest particles move erratically due to collisions with gas molecules and eventually contact a fibre.
Together, these three mechanisms allow an H13 HEPA filter to capture 99.97% of particles at 0.3 microns, which is the hardest particle size to trap. Larger and smaller particles are actually captured more efficiently. This is not theoretical. It is the basis of the filtration standard used in hospital operating theatres and pharmaceutical cleanrooms.
"The evidence for portable HEPA air cleaners reducing indoor PM2.5 concentrations is now robust. Randomised controlled trials consistently show reductions of 25 to 80 per cent in particle concentrations."
— Professor Frank Kelly, Director of the Environmental Research Group, Imperial College London
Many purifiers also include an activated carbon layer. Carbon does not capture particles. Instead, it adsorbs gaseous pollutants like volatile organic compounds, formaldehyde, and odours through a chemical bonding process. Think of HEPA as the particle filter and carbon as the gas filter. Together, they cover the majority of indoor air pollutants.
What Air Purifiers Remove
Fine Particulate Matter (PM2.5 and PM10)
PM2.5 refers to particles smaller than 2.5 microns. These include soot, smoke, and combustion byproducts that penetrate deep into the lungs. PM10 includes larger particles like pollen, dust, and mould spores. HEPA filters capture both with very high efficiency. In our testing, PM2.5 levels dropped from an average of 35 micrograms per cubic metre to under 5 within 20 minutes of switching on the purifier.
Allergens
Pollen grains are typically 10 to 100 microns in diameter. Dust mite faecal particles, which are the primary allergen trigger, are 10 to 40 microns. Pet dander ranges from 2.5 to 10 microns. All of these are well within the capture range of an H13 HEPA filter. For hay fever and pet allergy sufferers, an air purifier makes a measurable difference to indoor allergen concentrations.
Volatile Organic Compounds
VOCs are gases released by paint, cleaning products, new furniture, air fresheners, and cooking. Common examples include formaldehyde, benzene, and toluene. HEPA filters do not capture gases. However, purifiers with an activated carbon filter can adsorb many VOCs, reducing their concentration in room air. The effectiveness depends on the weight and quality of the carbon used. Thin carbon layers in budget purifiers have limited capacity and need frequent replacement.
Bacteria and Mould Spores
Most bacteria range from 0.2 to 5 microns. Mould spores are typically 3 to 40 microns. Both fall within the HEPA capture range. An air purifier will reduce the concentration of airborne bacteria and mould spores in a room, though it will not address mould growing on surfaces. For homes prone to damp and mould, a purifier helps manage the airborne spores, but you still need to tackle the moisture source.
What Air Purifiers Cannot Do
Control Humidity
Air purifiers do not add or remove moisture from the air. If your home has a damp problem, you need a dehumidifier. If your air is too dry in winter from central heating, you need a humidifier. An air purifier addresses particles and gases, not humidity. We cover this in detail in our air purifier vs dehumidifier guide.
Reduce Carbon Dioxide
CO2 is a natural byproduct of breathing. In a closed room with several people, CO2 levels rise and can cause drowsiness and reduced concentration. Air purifiers do not filter CO2. The only effective solution is ventilation, either by opening a window or using a mechanical ventilation system with heat recovery.
Eliminate Odours at Their Source
An activated carbon filter can reduce cooking smells, pet odour, and cigarette smoke in the air. But it cannot stop the source. If your kitchen bin smells, the purifier will reduce the odour in the room but the bin still needs emptying. Carbon filters are reactive, not preventive. They also have a limited lifespan and lose effectiveness as the carbon becomes saturated.
Our PM2.5 Test Results
We placed a calibrated Temtop M2000 air quality monitor in the centre of our test room at table height. The room is a 28-square-metre living room with one double-glazed window and a gas cooker in the adjoining open-plan kitchen. Two adults live in the home and follow their normal routines during testing.
Without the purifier, the average PM2.5 over three days was 31 micrograms per cubic metre. Spikes above 80 micrograms per cubic metre occurred during cooking, particularly frying and grilling. Evening levels settled around 25 to 35 micrograms, likely influenced by road traffic outside and residual cooking particles.
With the purifier running continuously, the average PM2.5 over four days dropped to 6 micrograms per cubic metre. Cooking spikes still occurred but peaked lower, reaching 45 to 55 micrograms, and returned to under 10 within 15 minutes. Evening levels stayed between 3 and 8 micrograms.
That is an 80% reduction in average PM2.5. The World Health Organisation recommends annual average exposure below 5 micrograms per cubic metre and 24-hour average exposure below 15 micrograms. Without the purifier, our test room exceeded the 24-hour guideline on every day tested. With the purifier, it stayed within the guideline on all four days.
We also measured PM10 levels. Without the purifier, the average was 48 micrograms per cubic metre. With the purifier, it dropped to 9 micrograms. Dust, pollen, and pet dander were visibly reduced, and the surfaces in the room stayed cleaner over the testing period.
Who Benefits Most?
Allergy sufferers. If you have hay fever, dust mite allergies, or pet allergies, an air purifier with an H13 HEPA filter significantly reduces indoor allergen concentrations. During our pollen season testing in May 2025, indoor pollen counts dropped by over 95% within 30 minutes of activating the purifier with windows closed.
Asthma patients. The British Lung Foundation notes that reducing indoor particulate matter can reduce asthma triggers. Several purifiers we tested carry the Allergy UK Seal of Approval, which requires independent lab testing of allergen removal claims.
Pet owners. Homes with cats and dogs have elevated levels of pet dander, fur, and associated allergens. An air purifier reduces these particles continuously, which is especially helpful in bedrooms where pets sleep or visit.
Urban households. If you live near busy roads, construction sites, or industrial areas, outdoor pollution enters through gaps around windows and doors. Running a purifier with windows closed keeps indoor PM2.5 well below outdoor levels.
Homes with smokers. Cigarette and cooking smoke contain fine particulate matter and VOCs. A purifier with both HEPA and activated carbon filters addresses both the particle and gas components of smoke.
Are They Worth the Money?
Based on our testing, a quality HEPA air purifier costing between £150 and £300 delivers a genuine, measurable improvement in indoor air quality. The 80% reduction in PM2.5 we recorded is not marginal. It is the difference between exceeding and meeting World Health Organisation guidelines.
Running costs are modest. Most purifiers draw between 10 and 50 watts, costing 3 to 10 pence per day in electricity at current UK rates. Filter replacements add £25 to £60 per year. The total annual running cost for most models is under £80.
If you have allergies, asthma, pets, or live in an urban area with elevated outdoor pollution, an air purifier is worth the investment. If you live in a rural area with good ventilation and no specific health concerns, the benefit is smaller but still present, particularly during winter when windows are closed and indoor pollution accumulates.
For our recommended models, visit our best air purifiers guide where we rank the top five purifiers we have tested across all price ranges.
