Have you ever found yourself with only one mask option – wearing a disposable you’ve already worn once before? Maybe you had it in your jacket pocket or in your car’s cup holder. But should you really be wearing a disposable mask more than once?

There is no “true, hard and fast rule,” for wearing disposable masks, Chris Sulmonte, a project administrator for the Johns Hopkins Bio-Containment Unit, tells Nexstar. Instead, you should take into account two main factors.

First, Sulmonte says to evaluate the fit of the mask. Any mask, whether it’s a KN95, a light blue disposable, or a reusable mask, should fit well. If the ear loops are starting to expand or stretch, or there are gaps developing on the side or around the mask, it’s time to replace the mask.

Second is the material of the mask itself and the wear-and-tear it has experienced. If the mask appears worn out or discolored, Sulmonte recommends changing it out. How the mask was previously worn matters, he explained – there’s a big difference between a mask worn to an uncrowded store for light shopping and one used to work out.

“That [lightly-used] mask doesn’t have a lot of residue inside of it, a lot of sweat, things that break down the material or thin it out,” Sulmonte says. But if you take that same mask, wear it at the gym for a 60-minute workout and sweat in it, it’s best to not re-wear that mask. “A lot of times you will see some discoloration, you’ll see a lot of moisture inside of there, you’ll start to see the material and it’ll start to wear out a little bit. That’s probably a good time to change that out.”

If you’re wearing a reusable mask, Sulmonte says you can treat them like clothing.

“It’s the same thing that you have with a T-shirt,” he explains. “You can tell the difference when the shirt has been used and when you should probably have it cleaned.”

If your reusable mask doesn’t feel fresh or appears worn out, it’s time to wash it.

If you’re leaving a mask in your car, gym bag, or anywhere else for easy access, Sulmonte recommends storing it in a paper or plastic bag. Because there will always be moisture on a mask after you wear it, Sulmonte notes the bag should be able to stay open, allowing the mask to air out.

Overall, Sulmonte says the most important features of a mask are that it fits well and it has multiple layers.

“Those are really the most important pieces. So if you’re able to do that for most of the situations you’re going to run into, that’s going to be providing protection that you’re looking for.”