As far as all of the pieces that make up a bedding set go, pillowcases are the most important to keep clean. Your pillow makes prolonged contact with your hair and skin oils—and, if we’re being honest, sometimes drool.

It’s common practice to wash your pillowcases at the same frequency as the rest of your sheets, but should you actually be changing and washing them more often? We went straight to an expert source to find out.

The Bare-Minimum Frequency for Washing Pillowcases

The longest you should go without washing your pillowcase, at a maximum, is one week.

Danielle Hurhula, head of home merchandising at Quince, notes you should really only go for seven days under certain conditions, which brings us to our next point.

Factors that Call for More Frequent Washing

Some situations call for changing a pillowcase twice a week. Here’s when and why you may need to make a little extra time for this important hygiene practice.

When You’ve Been Sweating More

Sweat is a big reason to make it a priority to keep your sheet sets clean, especially your pillowcases. Of course, there are some days and weeks when you sweat more than others.

“The frequency with which you wash your pillowcases can vary throughout the year depending on your lifestyle, with summer or other particularly active periods requiring more frequent washes,” Hurhula says.

According to her, the same rule applies if you’re naturally a hot sleeper.

You Have Allergies or Asthma

Any outside influences, like pollen and air pollutants, can wind up on your pillowcase through your skin and hair. If this transfers to your pillowcase (which it inevitably will), it can build up quite a bit over the course of an entire week.

Hurhula notes that you should bump up the washing frequency to twice a week if you suffer from allergies or asthma. The same goes for your pillow protectors, if you use those.

You Go to Sleep With Hair and Skincare Products

Going to bed after your skincare routine with unwashed hair isn’t really a bad habit. However, your pillowcase can be a magnet for the excess products as you toss and turn.

To keep the build-up at bay, Hurhula says to increase your pillowcase washing frequency to twice a week.

You’ve Recently Been Sick

If you’re in the thick of dealing with a contagious illness, you’ll want to up your sheet-washing regime. Not only because you’ve probably spent more time in bed recently, but also because you don’t want to run the risk of prolonging your illness or getting someone else in your household sick.

You Co-Sleep With Kids or Pets

The more sleeping companions you’ve got, the more sweat, dandruff, hair, and skin cells will build up in your sheets. If you sleep with pets and kids in your bed, your pillowcases will start to show it after just a few days.

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Will Frequent Washing Harm Your Pillowcases?

Hurhula notes that washing quality pillowcases frequently won’t harm them, but rather it could actually extend their life and help prevent stains and maintain their color. In case you’re worried about their longevity, Hurhula has another tip.

“To prolong the life of your pillowcases and ease your laundry burden, we always recommend purchasing a second set of pillowcases and swapping them out between trips to the washing machine,” she says.