Glossary
HEPA filter
A filter standard capturing at least 99.97 percent of very fine particles, used in higher-end air purifiers.
HEPA stands for High Efficiency Particulate Air, and a true HEPA filter is certified to capture at least 99.97 percent of particles at a specific, very small size, which covers most dust, pet dander, pollen and smoke particles that pass straight through a basic filter.
Not every filter marketed with HEPA-style language meets that certified standard. Terms like HEPA-type or HEPA-like are used on filters that approximate the performance without being independently tested and certified to the actual specification, so the exact wording on a product listing matters if fine-particle filtering is the reason you're buying a purifier in the first place.
In a small apartment, a true HEPA filter paired with a correctly sized CADR rating does most of the practical work against everyday indoor air complaints, from pet dander to dust kicked up in a room with a lot of foot traffic. Filters also need periodic replacement to keep performing at that level, since a clogged filter loses efficiency even if the initial rating was accurate. Replacement frequency varies by household, but a visibly gray or dusty filter is a reliable sign it's overdue regardless of the manufacturer's suggested schedule.