How often you need to change the filter on your air purifier depends on several factors, not the least of which is how polluted your air is to begin with, and how often and at what speed you are running your air purifier. Most air purifiers have a multistep filtration system, including a prefilter in addition to the HEPA filter, which in some cases may be washable, and is used to capture large particles such as pet hair. Taking good care of the prefilter will help extend the life of the HEPA filter. Most of the brands on this list recommend changing the filter every six months. A few suggest every year, largely depending on the size of the HEPA filter and the prefiltration system. New filters and prefilters are often available from the same retailers that sell the air purifiers, or you can buy them through the brands' websites, some of which offer subscription programs for replacement filters at a discount.