The Centers for Disease Control and Prevention recently recommended wearing two masks to ensure protection against COVID-19, especially as more contagious variants are sweeping the country.

But, according to Best Life, the agency does not advocate double masking with two disposable masks. The CDC said “disposable masks are not designed to fit tightly and wearing more than one will not improve fit. Masks offer the best protection when they sit over your nose, under your chin, and against your cheeks without any gaps.”

Do not layer two KN95 or N95 masks together, said the CDC. According to Paul Hickey, president of PuraVita Medical, a company that manufactures KN95 masks, this could be dangerous.

“KN95 masks are designed to be a respirator,” he told Best Life. “A respirator is designed to create an airtight seal around your face, so you only breathe air that comes in through the respirator material and the air you breathe out only goes through the respirator material. Because of this, if you double layer a KN95 respirator or an N95 respirator you will have a difficult time breathing. Respirators are designed to only be only one layer and not double stacked.”

Other experts point out that the well-designed N95 and KN95 masks already are 95% effective in blocking pathogens so there is no clinical need to double mask. Some people, however, use a cloth mask over their respirators to keep the pricey masks cleaner, longer.

According to Best Life, universal masking is one of the strategies the CDC recommends to slow the transmission of SARS-CoV-2, the virus that causes COVID-19. In a report published this month, the CDC said that simply wearing a single medical mask blocks 56% of potentially contagious aerosol particles but adding cloth mask over it blocks over 85% of pathogens.

Furthermore, when participants in the study double knotted their ear loops to provide an even tighter seal, the protection jumped to 96%.