How often should you clean or change an air purifier filter?

Short answer

Clean the pre-filter every month, main filter every 6 to 12 months. A room air purifier has a coarse pre-filter that catches hair and lint, and behind it a main filter, usually a dense pleated one, that catches the fine dust, pollen and smoke you bought the purifier for. The pre-filter vacuums clean in a couple of minutes. The main filter is replaced, not cleaned, and a new one costs $20 to $100 depending on the purifier. The whole job takes 10 to 20 minutes.

Why it matters

A purifier works by pulling room air through its filters and pushing clean air back out. As the main filter loads up with everything it has caught, air moves through it more slowly. The fan spins just as fast but less air passes through, so the purifier cleans less of the room each hour. Past a certain point it's mostly a fan with a filter-shaped weight in it, drawing power and making noise while the air stays as it was.

The pre-filter is there to protect the expensive main filter from the big stuff. Keep it clean and the main filter lasts its full life. Let it mat over with pet hair and the main filter is starved of air before it's half full. The fan motor also runs hotter working against a blocked intake, and motors that run hot don't last.

When to do it more often

Wildfire smoke or heavy pollen: check the main filter every month. A week of smoke can load a filter that would otherwise go all year.
Pets that shed: vacuum the pre-filter whenever you see hair on it, which may be every week or so in shedding season.
A purifier that runs on high all day: expect the short end of the 6 to 12 month range for the main filter.
A sealed bedroom with the door closed: the air gets cleaner faster, and the filter loads slower, so lean toward the long end.

What you need

A vacuum with a brush attachment, for the pre-filter
A replacement main filter made for the purifier's model

Steps at a glance

Unplug the purifier.
Open the front panel or back cover and lift out the pre-filter.
Vacuum the pre-filter on both sides, or rinse it under the tap if the manual says it's washable, and let it dry.
Pull out the main filter and look at it against the light.
If it's gray through or past its date, unwrap a new one and seat it in the same orientation.
Put the pre-filter and cover back, plug in, and reset the filter indicator if the purifier has one.

You're done when the cover latches, the purifier starts without a filter warning, and air blows from the outlet with the same force it had when it was new.

Signs it's overdue

The filter indicator light, which on most purifiers counts hours of use rather than measuring the filter.
Visible dust on the main filter when you hold it to the light, with the pleats gray instead of white.
Lower airflow from the outlet than you remember.
Odors the purifier used to remove, such as cooking or smoke, that now linger.

DIY or pro

This is an easy job that the purifier is built for. The covers open by hand and the filters lift out. There's no professional service for room purifiers, and no reason there should be.

The only decision is which replacement filter to buy. Filters are made for a model, and a filter that's close but not exact leaves gaps that air slips around. Use the model number on the purifier's label and match it. Off-brand filters that list your model number work fine as long as they fit snugly.

FAQ

Can you wash an air purifier filter?

The pre-filter, often yes. The main pleated filter, no. Water collapses the fibers, and a damp filter grows mold. Check the manual for which parts are washable.

How do I know when to replace the air purifier filter?

Hold it up to a light. A new filter is white and lets light through the pleats. A filter that looks gray throughout, or that you can't see light through, is done, whatever the indicator says.

Does the filter indicator light mean the filter is full?

Usually not. Most purifiers count run time and light up on a schedule. Treat it as a reminder to look, not a measurement.

Should an air purifier run all the time?

It cleans the air only while it runs, so continuous use on a low setting works best. Expect the main filter to need changing at the short end of its range if it runs all day.