Wearing two masks is becoming a popular option to take precaution against Covid19, but a new study shows that there is a right — and a wrong — way to do it. The efficacy of double masking depends completely on how you wear the masks: the better the fit, the more filtration against harmful particles, and the safer you are.

The study, published in JAMA Internal Medicine, figuratively puts a finger on what strategy makes double masking an effective option against Covid19 spread. “Double masking is most effective because of the improved fit that it offers, not because of the additional layers,” lead author Emily Sickbert-Bennett, a researcher at the University of North Carolina at Chapel Hill, U.S., tells Insider. The study compared the effectiveness of commonly available masks worn singly, doubled, or in combinations — and found that fit, not material, is the protective factor at hand.

Surgical masks are made from nonwoven polypropylene layers, the material used to make N95 respirators that help with filtration. But wearing two surgical masks is not exactly as effective as one might think. For instance, commonly used surgical masks are baggy and loose-fitting, and can allow infectious particles to bypass their filtration — the study found it made little difference if someone wore one surgical mask or two. Also, wearing a surgical mask over a fabric mask might not do the trick too because of the clumsy fitting.

Double masking works best when both, or one of the two masks, offers a different, improved fit than the other. Wearing a cloth mask on top of a surgical mask was found to be the optimal solution: it combined the filtration quality of surgical masks with an improved fit. Wearing a surgical mask under the cloth face-covering, instead of the other way round, improved the overall protection from 66% to 81%. Focus on the fit, instead of the layers, researchers say. The wrong way to double mask is when you wear two layers of masks, neither of which hugs the face along its edges.

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Previous studies have confirmed the benefit of wearing two masks: the U.S. government’s Centers for Disease Control and Prevention found double masking can improve protection by about 50%; immunologist and health expert Dr. Anthony Fauci has also previously endorsed double masking as a protective measure.

But there has been ambivalence around its proper usage among the public, because of a lack of convenience and awareness; double masking can be uncomfortable, particularly in the summer. The effective barrier that the double layer creates for outgoing and incoming particles, also makes it harder to breathe in the air we need. The present study’s findings are then helpful in tweaking our mask-wearing habits to improve the fit such that it offers protection. There is talk of mask braces that can help stick the surgical mask better to the face, or knotting the ends of a mask and tucking it on the chin.

This aligns with what we already know about masks and their utility during the pandemic. A Lancet study published last week, strengthened this argument by confirming that “there is consistent evidence to prove that the SARS-CoV-2 virus, behind the Covid19 pandemic, is predominantly transmitted through the air.” The findings spotlight mask-wearing behavior as an important factor: wearing masks whenever indoors (even if not within 6 feet, or 2 meters, of others) and paying more attention to mask quality and fit thus remain relevant and pertinent in addressing the Covid19 spread.

The researchers echo the CDC’s endorsement of this double masking approach — it can reduce the chances of spreading infectious particles by 95%, only when everyone follows it. “The best form of double masking is when you and the person you are interacting with both have a mask on,” Sickbert-Bennett said.