Air purifiers use a fan to pull in dirty air at one end, remove pollutants, and output clean air at the other end. The exact pollutants that can be cleaned depend on the technology used. All of the air purifiers we’ve tested here use a HEPA filter, which will capture a minimum of 99.97% of airborne particles 0.3µm in size. Also known as particulate matter, these particles can penetrate your lungs causing respiratory problems, particularly in allergy sufferers. Particulate matter sources include dust, with cleaning likely to disturb dust and throw it into the air, pet hair and dander, smoke and pollens. HEPA filters are no good where dust has settled, since the filters can remove only airborne particles. In other words, you still need to clean effectively. HEPA filters also have a lifespan, after which they’ll need to be replaced. The exact timing will depend on the air purifier and how dirty your air is, but expect to buy a new filter between three months and 12 months of operation. Volatile Organic Compounds (VOCs) are harmful chemicals that are often found in cleaning products, paints, and some beauty products. VOCs are also found in some manufactured products, such as synthetic carpets and old furniture. Most air purifiers can’t touch these pollutants; those that can, can’t filter all VOCs. Instead, you’re better off with an air monitor, such as the excellent Foobot, to monitor your home to see what’s giving off VOCs. Where possible, switch to less harmful products. Some models of air purifier also have an ionisation option. These use negative ions, which causes particles to stick to surfaces – such as wall or floor. Ionisation has been shown to reduce the risk of some viruses. Ionisation on its own isn’t particularly effective, so always combine with a proper air purifier.