John Ogden-McKee, a self-described “extremely clean person,” has a penchant for filtration. In his home in Dallas, Ogden-McKee, 39, uses five air purifiers with built-in HEPA (short for high-efficiency particulate air) filters — one in every room but the kitchen — drinks water from a filtered container, and fills his drinking glasses with cubes created in an ice maker with a filtration component. He replaces the MERV 13 filter in the air conditioner on the first of every month; as soon as the “change filter” light on the other devices illuminates, he swaps out the used filter for a new one. “We just always have extra,” he says of his stash of assorted filters, which he keeps stored in a closet. “I can’t stand any chaos.”

Recently, Ogden-McKee discovered a crack in his highly filtered facade. While cleaning the water fountain for his kittens, Push Pin and Paperclip, Ogden-McKee accidentally discovered the device had a filter — one with four months’ worth of cat water grime. “It was almost like this horror that was in my house that I had no idea about,” Ogden-McKee says.

Over the course of a day, you’re likely to encounter any number of filters: furnace/air conditioning filters (they’re the same filter if your home has central AC), vacuum filters, water filters, car cabin air filters, humidifier filters, dishwasher filters, cat fountain filters. The pandemic helped raise the status of air filters as public health officials touted the benefits of ventilation and air purifiers in clearing SARS-CoV-2 in the air. Designed to remove contaminants like dust, pet hair, and chemicals from your air or water, filters need to be cleaned or replaced regularly or the devices they aid won’t function as necessary. Even more crucially, air filters help prevent all the gunk in the air in the home from getting into your body.

“Indoor air can be more polluted than air outside,” says Kelsey Hei, a 3M Filtrete Brand application engineer. “With our homes being built more airtight, there isn’t as much air circulation to capture and remove particles from your home’s air without the help of heating and cooling systems or air purifiers, aside from opening windows and doors.”

However, most people don’t regularly swap out or clean filters on their appliances, says Kelly Winslow, the director of engineering at EnviroHome, which specializes in environmental engineering and indoor air quality. “People abuse filters,” Winslow says, “because they just put it in, leave them in, and think the job is done. A filter is a temporary solution.”

As evidence of my own lack of filter replacement knowledge, during my last oil change, the mechanic took a picture of my car’s air filter and wrote a defiant “ICK!” alongside. “A lot of people especially forget to change out their car air filter,” says Dee Ray, the chief marketing officer at FiltersFast.com.

Filters can cost anywhere from upward of $50 for a refrigerator water filter to around $5 for a basic air conditioner or HVAC filter, but in the long run it’s about keeping your products, the water, and the air in your home — and the people you live with — healthy and in top shape. “Spending $10 on a filter quarterly, that sounds like a lot of money,” Winslow says. “But when you have to replace a whole unit for $4,500 after six years when it could have gone 14, that $40 looks pretty good for a year.” If you were looking for a reminder to replace any of your filters, this is it.

Air filters

There are several household appliances that filter dust, hair, debris, and other contaminants out of the air: central and window air conditioners, furnaces, dehumidifiers, range hoods or over-the-range microwaves, air purifiers, cars, vacuum cleaners, clothes dryers, even the refrigerator. “The key thing is your air conditioning system,” Winslow says. Air conditioner manufacturers typically recommend changing the filter every other month, but Winslow suggests refreshing the filter every quarter (four times a year). You’ll want to use a high-quality filter with a MERV rating of at least 8. “MERV ratings on a filter go from 1 to 20,” Winslow says. “A 20 is a filter that’s used in a hospital room and a MERV 4 is a rating for a filter that is a very basic, low-quality filter you see in many homes.”

Vacuum filters should be regularly cleaned or replaced, too. Some vacuums have foam filters that can be cleaned with all-purpose cleaner and water. However, if your vacuum has a HEPA filter (made of woven fibers), don’t wash it; just get a new one when your old one’s dirty. The issue with vacuums, Winslow says, is they “are notoriously bad pieces of equipment for indoor air quality unless they are higher-end vacuums. … Otherwise, the vacuum just pulls the stuff out of the carpet and literally throws it into the air, which is where you don’t want it to be.” Winslow’s rule of thumb is to avoid carpets if at all possible.

Every filter should be changed at various times — like monthly for grease and charcoal filters on over-the-range microwaves or after every use on a dryer lint filter — so this unfortunately means doing some digging on your specific appliance.

Water filters

Water pitchers, refrigerators with water dispensers and ice makers, sink filters, whole house filters, water bottles, dishwashers, coffee makers, fish tanks, and pet fountains filter water, removing metals and chemicals. “Most casual users of water filter pitchers just ignore the recommendations and replace the filters as soon as they notice the water flow slow down too much that it becomes unusable,” says Paul Lewin, founder of Home Water Research, who has reviewed hundreds of water filters, by his count. “Others go by taste, and if the water starts not to taste as good as before, it’s time for a change.” Standard Brita filters, for example, need to be replaced every two months; LG refrigerator water filters last six months.

Dishwasher filters help ensure the machine’s pump isn’t clogged and prevent bits of food from sloshing around and flinging back on your dishes. Generally, the filter will be on the bottom of the dishwasher and can be cleaned once or twice a year with dish soap and water.

Lewin says to read the filter manufacturer’s instructions for when to swap out your filter. These recommendations are often for optimal water conditions “and may be a bit generous,” Lewin says. Extra credit for the overachievers: If you’d like a more precise idea of how long your filter will last, Lewin suggests testing your water before you buy a filter. “High-quality tests are available cheaply on Amazon and other online stores,” he says. “You collect the water sample yourself and send it back to the lab, and often get results in less than a week. Once you know exactly what’s in your water and what it is you want to filter out, you can look for filters that target those specific contaminants.”

Maintenance

Because some appliances have disposable filters (like water purification systems) and some just need cleaning (clothes dryers) plus varying replacement schedules, keeping this all straight can be confusing. Winslow recommends marking a calendar with the dates you should replace specific filters. Ray suggests signing up for auto-delivery so as soon as your new filter arrives, you can swap it out. Filtrete also has an app that sends notifications to your phone when it’s time to change the filter, Hei notes, or you can set reminders in your phone. If it’s easier for you to lump filter maintenance in with other seasonal chores, try, for example, changing the filter in your window AC unit when you’re taking it out in the late summer or early fall so you’ll be ready to go come next spring.

As a rule of thumb, Ray says people should change an air filter every two to three months and a water filter every six months. Renters should clarify with their landlord whether they are responsible for replacing filters in appliances like the air conditioner or ice maker.

After discovering the filter in his cats’ water fountain, John Ogden-McKee, the filter-happy cat owner, ordered eight filters and immediately installed a fresh one once his order arrived. “I felt bad,” Ogden-McKee says. “I know they’re cats, but they were drinking this water that was running through this disgusting filter. I was like, I failed the cats somehow.”

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