The coronavirus that causes COVID-19 is believed to pass from person to person primarily via respiratory droplets and contaminated surfaces, so properly used face masks can act as a barrier to help prevent its spread.

One major problem: Masks, along with other protective gear, are in short supply. That includes cloth and thin surgical masks as well as N95 respirators, which are used by construction workers and in hospital wards because they block at least 95% of airborne particles. Recognizing this, the U.S. Centers for Disease Control and Prevention recently issued guidance allowing health care workers to reuse N95 masks under certain conditions.

But what if you’re not a health care worker and you have a limited supply of masks at home?

Before you worry about reusing a mask, consider which ones you should use. If you have an N95 mask and you’re a healthy person not taking care of an infected individual, you could donate that unused mask to health care providers in dire need of them.

If you are outside running errands, use a cloth mask. When social distancing outside is not possible, the CDC recommends the use of cloth masks in public settings.

Reusing surgical masks is not ideal. “The goal is not to reuse masks. It’s really a stopgap measure for the mask shortage that we’re having, but it’s not the best practice,” said Lucy Wilson, chair of the department of emergency health services at the University of Maryland Baltimore County. “The best practice would be single-use.”

The reality, though, is that supply is dwindling. You can reuse and rewash different mask materials under some conditions.

CDC.GOV There are different kinds of face masks.

To prevent self-contamination, put on and take off your mask very carefully.

If you’re using any kind of mask, you need to first make sure it doesn’t become one more contamination zone. Wilson emphasized that you need to wash your hands before you put a mask on, before you take it off and after you take it off.

The CDC has illustrated how to properly don and take off masks. The organization warns that after you use a mask, the front of the mask is contaminated, and so you need to “grasp bottom ties or elastics of the mask/respirator, then the ones at the top, and remove without touching the front.”

“Do not touch the inside of that mask, because then you are contaminating the inside of the mask,” Wilson said. “And if you put it back on, then you’re just putting that directly on your mucous membranes. You could self-contaminate.“

If your mask is cloth, wash it.

One 2015 study published in the medical journal BMJ Open cautioned against the medical use of cloth masks, compared with disposable ones. “Moisture retention, reuse of cloth masks and poor filtration may result in increased risk of infection” among health care workers, the study concluded.

But individuals who are using their own cloth masks should ideally wash them after each use or frequently machine-wash them, Wilson said. “Say you’re caring for somebody and you’re going face-to-face with them. You could wash it between uses or you could wash it at regular intervals,” she said.

If you’re using a homemade cloth mask, know that your protection is only going to be marginally effective at preventing the spread of an infectious disease, compared with a surgical mask or an N95 mask. One 2013 study on mask efficacy found that surgical masks were three times more effective at preventing the spread of influenza than cloth masks made from T-shirt material.

“A homemade mask should only be considered as a last resort to prevent droplet transmission from infected individuals, but it would be better than no protection,” the study concluded.

Allow all masks to dry out between uses.

When you safely take a mask off, put it somewhere it can dry out from whatever moisture it collected. The CDC recommends a clean, breathable container like a paper bag for storing N95 masks. Remember to regularly dispose of these containers, so they don’t contaminate your mask later.

Hilary Lin, an internal medicine physician at PlushCare, said paper containers are recommended because you don’t want the N95 mask to become compressed by something heavier and not fit as well on your face.

If your mask is not made of flammable material, Lin said, you can try putting it in an oven for 30 minutes at 158 degrees Fahrenheit, because the coronavirus is fragile. Doing so can kill off the virus, and “it doesn’t destroy the mask very much,” she said, adding that hanging the mask above a pot of boiling water for about 10 minutes could also help with killing off the virus, but it would need to be air-dried afterward.

You can also put another layer on top of your reused mask.

If you have a limited number of masks, you can create layers with a scarf to protect each one as you wear it, Lin said.

“If you’re protecting others from yourself, or yourself from others, you can maintain the cleanliness of the mask by putting something over it,” Lin said. “Scarves and other materials are terrible as filtration barriers, but if you put it over the mask, then you create more of a barrier and also you protect your mask from getting contaminated by droplets.”

Know when you need to stop reusing a mask.

If you are reusing a mask, you need to know when it’s time to let it go and safely dispose of it. “Visible dirt or damage to it ― that’s a good time to throw it away,” Lin said.

The World Health Organizations advises throwing away a mask as soon as it is damp, discarding it in a closed bin. When you are removing such a mask, don’t touch the front, which could be contaminated. Instead, remove it from behind, the organization advises. A paper mask should be discarded when it is visibly soiled or wet, Wilson noted.

If your masks are in limited supply, know that reusing them is not your only option to stop the spread of the coronavirus. Washing your hands, staying home, not touching your face and regularly cleaning surfaces in your home that other hands touch are other critical preventative measures, Wilson said.

“Masks are just one part of the prevention,” Wilson said. “The most important part is handwashing. Unless you are within six feet of someone who is actively coughing, your risk of infection is really from inadvertently touching something that’s contaminated in the environment.”