Testing by our counterpart consumer organisations around the world has found that HEPA filtration is the most important feature for an air purifier.

HEPA stands for high-efficiency particulate air. It's a very effective type of filter which traps very small particles, invisible to the naked eye. Generally, HEPA filters can trap at least 99.95% of dust, smoke, mould and other particles in the air, down to a tiny 0.3 microns in size.

HEPA filters are common in good-quality vacuum cleaners and you should look for HEPA filtration in an air purifier too.

Other filters:

Carbon or charcoal

Said to be good for trapping odours and VOCs such as formaldehyde.

Ozone 

Said to be good for removing odours. But at low levels it's been shown to be ineffective at removing air pollution, and at high levels it can cause breathing irritation. We recommend you avoid these models.

Ionic or ionisation

Said to be good for removing fine particles such as dust and smoke. However, ionisation of air can produce ozone (see above).

Ultraviolet (UV) sterilising

Said to be good for killing viruses, bacteria and mould spores. But UV light takes at least a few minutes to kill germs, and it's unlikely any germs passing through the air purifier will be exposed to the UV that long. Don't rely solely on a sterilising filter if this level of cleanliness is important for you.

Filter cleaning and replacement

An air purifier may have multiple filters (including the HEPA filter) and these will need regular cleaning or replacement. It's more economical to be able to wash and reuse a filter than to have to buy replacements each time, especially if you will be using the air purifier frequently. Most pre-filters and carbon filters can be washed and replaced; HEPA filters are not usually washable and you'll need to factor in buying replacements for these.

Some models have separate filters (HEPA, pre-filter, carbon etc) which can be replaced separately as needed. Others have an all-in-one filter cartridge which can be more convenient, but may mean you're replacing the whole lot when only one of the filter types actually needs replacing.

A good air purifier will have a warning indicator to let you know when it's time to clean or replace the filters. Alternatively, the instructions might simply specify to replace the filters after a certain period of time, such as every six months. But check how the replacement time is calculated; it might assume that the purifier is run all day every day, so if you use it less often, you shouldn't need to replace the filters as frequently.

Replacement filter costs can add up quickly – In some cases these can be well over $100 per year. If you are able to run the air purifier only occasionally (such as only on smoky or high-pollen days), the filters will of course last longer and your costs per year will be less. However, if you're very sensitive to air pollution, you might need to run the air purifier all year round.