When and how to replace air purifier filters

Air purifier filter replacement should follow the interval in the manual for your specific unit, shortened if the air is dirtier than the maker assumed or the unit runs more than the maker assumed. Any single number you see for all purifiers is a generalization. EPA’s Guide to Air Cleaners in the Home gives the principle in one line: “All filters need regular replacement. If a filter is dirty and overloaded, it won’t work well.” The rest of the job is knowing which filter you have, what happens as it fills, and how to take a dirty one out without putting its contents back in the room.

Why there is no single interval

A filter’s life depends on how much air passes through it and how dirty that air is. Two identical purifiers can need new filters at very different times if one runs all day in a house with smokers and the other runs in the evenings in a clean apartment. Things that shorten the interval include:

• longer run time and higher fan speed
• pets, cooking, smoking and renovation dust
• high outdoor pollution or smoke drifting indoors
• a filter stage that is small for the room

Many pages quote a month count for HEPA or carbon filters. The EPA documents cited here do not. The only numeric interval in them is for a different product: furnace and HVAC filters, for which it says “Manufacturers typically recommend replacement every 60 to 90 days.” It is a statement about what makers typically recommend for those filters, and it should not be applied to the cartridge in a portable unit.

The manual is the best available source because it was written for that filter’s size and media. EPA’s Technical Summary says the same: “Regular filter media replacement and/or cleaning are essential for ensuring performance. Follow manufacturer’s instructions for filter replacement and/or cleaning.”

The interval and the availability of replacement filters are both worth checking when choosing an air purifier, before the first filter is due.

What happens as a particle filter loads

A filter does not simply get worse from the first day. The Technical Summary says that, in general, fibrous media filters without an electrostatic charge “tend to increase in efficiency with dust loading over time”, while those with a charge “initially tend to decrease in efficiency with dust loading as the charge is diminished over time.” Check whether the maker says which kind you have.

Either way, there is an end point. The Technical Summary continues: “filters that become excessively loaded will tend to decrease the effectiveness of a furnace filter or portable air cleaner because of reduced airflow through the filter and/or increased bypass airflow around a clogged filter.” That applies to a HEPA filter as much as any other. A purifier’s job is the combination of efficiency and airflow, so a highly efficient filter that the fan can barely push air through is doing less work.

Signs that a particle filter is overdue:

1. A clearly reduced stream of air at the outlet on the usual fan setting.
2. A filter that is dark and heavily coated when you open the unit.
3. A fan that sounds strained or louder than it did.
4. Dust visibly building up around the intake or at the seams.

None of these is a precise measurement. EPA makes a useful point about furnace filters that applies by analogy: “If you notice that filters appear heavily soiled when you replace them, you should consider changing them more frequently.”

Carbon filters fill up where you cannot see

A carbon stage has a different failure. It does not clog in any way you can observe. EPA says adsorbent filters “have a finite capacity for adsorption and therefore must contain sufficient sorbent media for the application and must be replaced regularly”, and warns that thin layers “can become quickly saturated, and the filter can become a source of previously adsorbed pollutants.”

A smell returning with no change in the source is one clue the carbon is spent. The absence of smell proves nothing, since plenty of gases have none. The guide to activated carbon filters has more on saturation and why thin carbon sheets are limited.

What the replacement light really means

Many purifiers have a replacement indicator, and a light can work in different ways: it may only count running hours, or it may measure something. One that only counts hours can be early in a clean home and late in a smoky one. Check the manual for which kind a unit has and how to reset it after a change. The EPA documents cited here do not evaluate the accuracy of these indicators, so treat one as a reminder and not a reading.

Washable and reusable filters

Washable filters are where the evidence runs out. A purifier can have several stages, and they differ:

• A coarse pre-filter catches hair and lint. Some manuals allow vacuuming or rinsing it, and doing so as directed keeps it from choking the airflow.
• A HEPA or other fine particle filter. The Department of Energy’s HEPA definition, written for nuclear facilities, calls such a filter “throwaway.” Some consumer products are sold as washable, and the documents cited here do not test how they perform after washing.
• A carbon filter. The EPA documents cited here describe replacement, not washing, for sorbent media.

The EPA documents consulted here do not report tests of how well a home HEPA filter holds its efficiency after washing or vacuuming, and this page does not fill that gap with a guess. The cautious reading is to wash only what the maker explicitly says can be washed, using the maker’s method, and to let it dry completely before refitting.

How to change a filter

EPA’s Technical Summary says that during cleaning or replacement “an effort should be made to ensure that pollutants are not re-emitted into the air and do not come into contact with skin.” It offers specific precautions, which shape the steps below.

1. Turn the unit off and unplug it.
2. Keep air still. EPA says “excessive movement or air drafts should be avoided when filters are removed.” Close nearby windows and turn off fans.
3. Consider protection. EPA says “using an N-95 respirator (such as those sold for home improvement projects) and gloves can help provide additional protection during cleaning or filter replacement.” People with a health condition should ask a clinician about respirator use.
4. Lift the old filter out gently without tapping it or knocking it against a bin.
5. Bag it straight away. EPA says “used filters should be placed in sealed plastic bags or containers for disposal.”
6. Wipe the housing with a damp cloth, as the manual allows.
7. Remove all packaging from the new filter, including any plastic bag around it.
8. Fit it the right way round. For furnace filters, EPA says the arrow on the side should point in the direction of airflow. Portable cartridges vary, so check the markings and the manual.
9. Seat it fully. Bypass around a poorly seated filter cuts the air that gets cleaned, and EPA notes, of furnace filter frames, that bad ones “can cause bypass airflow, which significantly decreases filter effectiveness.”
10. Reset the indicator, and write the date somewhere you will see it.

Filters during and after wildfire smoke

Smoke events call for more frequent checks. EPA’s wildfire smoke fact sheet says to “check the filter often and change it when it gets dirty or starts to smell.” That line is written for indoor air filtration in smoke events, and it is better guidance during a fire than a calendar is.

After a heavy episode, look at the filter even if the indicator has not come on, and keep a spare on hand before fire season if your area sees it. Take the same care with a smoke-loaded filter as with any dirty one.

What a new filter does and does not do

A fresh filter restores the airflow and capacity the unit was designed around. It does not make a home free of pollutants, and EPA says “no air cleaner or filter will eliminate all of the air pollutants in your home.” A filter change is maintenance, and no filter cures, treats or prevents a health condition. Anyone whose symptoms seem linked to indoor air should see a clinician.