When do I need to change the filters in an air purifier?

Three filter stages with three different intervals, why the indicator light on most machines is only a timer, and how to tell a spent carbon filter from a spent HEPA one when neither looks any different.

Difficulty: beginner
Time: 15 min
Read: 6 min

Short answer

Rinse or vacuum the pre-filter monthly. Replace activated carbon every three to six months, and sooner if smells stop being removed. Replace the HEPA filter every six to twelve months, or when airflow at the outlet has clearly dropped. The indicator light is a running-hours timer on most models, so treat it as a prompt to inspect rather than as a measurement.

An air purifier is a consumable-driven appliance, and its filters fail in ways you cannot see. A HEPA filter loading up gets better at capturing and worse at passing air, so the machine quietly delivers less clean air per hour while looking and sounding much the same. A carbon filter simply becomes saturated — every adsorption site occupied — and stops removing odours with no change in appearance at all. Neither announces itself, which is why intervals and a couple of simple tests matter more here than for most appliances.

What you'll need

The correct replacement filters for your exact model
A vacuum with a brush attachment, for the pre-filter
A marker pen, to date the filter (optional)

Step by step

Identify which stages your machine has.Most have a pre-filter, a HEPA filter and some carbon. On many models the HEPA and carbon are bonded into one cartridge that is replaced together, which simplifies the schedule but means you replace the HEPA on the carbon's shorter cycle or vice versa. Check before buying replacements.
Clean the pre-filter monthly.It is usually a washable mesh or a fine screen on the intake face. Vacuum it with a brush attachment, or rinse and dry it fully if the manual allows. This is the highest-value ten seconds of maintenance on the machine, because it is what stops the expensive filter behind it clogging early.
Replace carbon every three to six months, and judge it by smell.Carbon works by adsorption and has a finite capacity. When it is full it stops working, silently. The test is functional: if cooking smells that used to disappear now linger, the carbon is spent regardless of what the indicator says. Heavy cooking, smoking or a nearby road shortens the interval.
Replace HEPA every six to twelve months, and check the airflow.Hold a hand at the outlet on the highest setting and compare it against your memory of the machine when new, or against the same setting after fitting a new filter. A noticeable drop in airflow, or a machine that has to run at a higher speed than it used to for the same result, means the HEPA is loaded.
Look at the filter as well as the calendar.Pull the HEPA out and look at the intake face. Even grey discolouration is normal loading. Visible fluff, hair or a felted mat means the pre-filter is not doing its job. Any tear, crushed pleat or gap at the gasket means replace it now, whatever the age, because unfiltered air is bypassing it.
Treat the indicator light as a timer, not a sensor.Most machines count running hours. A machine run on low in a clean room and one run on maximum in a smoky one will hit the indicator at the same time and be in completely different condition. Some premium models measure pressure drop, which is a real measurement — check which yours does.
Write the date on the filter frame as you fit it.In marker, on the plastic frame. It removes all guesswork at the next inspection and it survives the machine being moved, unplugged or reset.
Never wash a HEPA filter to extend it.Water mats and collapses the fibre structure that does the capturing. The filter will look fine, filter poorly, and often never dry properly inside the pleats — which then grows mould in the machine.
Shorten every interval for the conditions that shorten them.Pets, smoking, a wood burner, a busy road, building work nearby, or running the machine constantly on high. Halve the intervals in any of those cases and inspect rather than assume.
Buy genuine or properly specified filters.A cheap replacement that does not seal correctly in the housing lets air bypass it entirely, which makes the machine a fan. If buying a third-party filter, check it is stated as true HEPA and that the frame and gasket match.

Tips

Keeping the pre-filter clean is what most extends the life of the expensive filter behind it. It is the cheapest lever available and almost nobody pulls it.
Buy the next set of filters when you fit the current ones. Machines routinely run for months past due because the replacement had not been ordered.
If the machine has started to smell when running, that is usually a saturated carbon stage or a damp filter, not a fault.
Vacuum the outlet grille and the machine's exterior when you change a filter. Dust drawn in through gaps bypasses the filtration entirely.

Common mistakes

Washing a HEPA filter to save money — It destroys the fibre structure. The filter still fits, so the machine seems fine while delivering a fraction of the filtration, and a damp filter can grow mould inside the housing.
Relying on the indicator light alone — On most machines it is a running-hours timer with no knowledge of how dirty the air was. It can be months out in either direction.
Ignoring the pre-filter — It exists to protect the expensive filter behind it. Neglected, it lets hair and fluff felt onto the HEPA and can halve its useful life.

If it doesn't work

Machine noticeably quieter or weaker at the outlet

Cause: Loaded HEPA filter restricting airflow — Fix: Replace the HEPA. Airflow should return immediately; if it does not, check for a blocked intake or a fan fault.

Cooking smells linger where they used to clear

Cause: Saturated carbon stage — Fix: Replace the carbon filter. Carbon gives no visual warning at all, so this functional test is the only reliable one.

A musty smell from the machine itself

Cause: A damp filter, or one washed and refitted before drying — Fix: Replace the filter rather than drying it, and check the machine is not standing somewhere damp or in the path of a humidifier.

Questions people ask

How often should air purifier filters be changed?

Pre-filter: cleaned monthly. Carbon: replaced every three to six months, sooner if odours stop being removed. HEPA: replaced every six to twelve months, sooner with pets, smoking or heavy use. Halve all of them if the machine runs constantly on a high setting.

Can I clean an air purifier filter instead of replacing it?

The pre-filter, yes — vacuum or rinse it monthly. The HEPA and carbon, no. Vacuuming a HEPA filter damages the pleats and removes only surface fluff, and washing it collapses the fibres. Carbon cannot be regenerated at home at all.

What happens if I do not change the filters?

The machine keeps running and progressively stops doing its job. A loaded HEPA delivers less clean air per hour, a saturated carbon stops removing smells entirely, and a filter that has become damp or mouldy can make the room's air worse than no machine at all.