To demonstrate the quality of cleaning, the majority of air purifiers come with Clean Air Delivery Rate (CADR) ratings for smoke, pollen and dust. The higher the score, the better the performance. And, the individual ratings help you choose the right purifier for your needs. However, not all manufacturers quote CADR ratings and, instead, quote the percentage of particles below a certain size: the higher the percentage and the lower the quoted particle size, the better. Some – but not many – air purifiers are capable of removing Volatile Organic Compounds (VOCs) from the air, too. These are harsh chemicals that can be found in everything from old furniture to cleaning sprays. To test an air purifier’s performance, we use a three-second burst spray from a can of insecticide – a source of particulate matter and VOCs – directed into the middle of the room. With the air purifier placed in a corner running at maximum, we time how long the air takes to return to normal, using a separate air-quality monitor. We also light a smoke pellet and time how long it takes until our Nest Protect tells us that the air is safe again. We also test the sound levels of purifiers, running at minimum and maximum speeds from a distance of 1 metre.