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"Double masking" with a cloth mask over a medical mask could substantially reduce exposure to aerosols even from unmasked sources, a CDC lab study suggested.

Moreover, improving the fit appeared at least as important as adding fabric layers for blocking aerosol particles.

Compared to an entirely maskless arrangement, fitting a dummy head with a three-ply cotton mask over a three-ply medical mask reduced aerosol exposure 83% when a similar dummy with no mask was the source; the reduction was nearly the same when the source dummy was double-masked and the "receiver" was not, reported John Brooks, MD, of the CDC, and colleagues, writing in an early edition of the .

And when both source and receiver dummies were double masked, the reduction reached 96% compared with no masks on either, the researchers found.

The CDC recommends universal masking to slow the spread of COVID-19. Fourteen states currently have universal mask mandates, and mask wearing is now mandated on federal property via a recent executive order from President Biden.

CDC Director Rochelle Walensky, MD, emphasized the study results at a White House COVID-19 briefing Wednesday morning. "The bottom line is this: masks work, and they work best when they have a good fit and are worn correctly," she said. "Masks should be used in combination with other prevention measures to offer you and your community the most protection from COVID-19. Stay at least 6 feet apart from people you don't live with, avoid crowds and travel, and wash your hands often."

Both Walensky and the CDC researchers also discussed a newer option in mask-wearing: mask-fitters. "At least two recent studies examined use of mask fitters to improve the fit of cloth and medical procedure masks," the researchers noted. "Fitters can be solid or elastic and are worn over the mask, secured with head ties or ear loops. The results indicated that when fitters are secured over a medical procedure mask, they can potentially increase the wearer's protection by ≥90% for aerosols in the size range considered to be the most important for transmitting SARS-CoV-2 (generally <10 μm)."

Walensky added that "with cases, hospitalizations, and deaths still very high, now is not time to roll back mask requirements." She cited a survey from Porter Novelli in which half the respondents who reported wearing masks in the past week said they had worn their mask incorrectly in public. "The new CDC data underscore the importance of wearing a mask correctly and making sure it fits closely and snugly over your nose and mouth," Walensky said.

For the MMWR report, Brooks and colleagues conducted laboratory experiments to simulate a variety of masked and unmasked situations during exposure to aerosols. These involved double-masking, where the dummies wore a medical mask and a cloth mask over it, and "knotting and tucking the medical procedure mask" (where the ear loops are knotted where they attach to the mask, and the extra mask material is flattened to minimize side gaps). Potassium chloride particles of 0.1-7 μm were used as the aerosol, emitted from a mouthpiece on the source dummy.

First, the researchers tested a simulated cough only, and found that an unmodified medical mask blocked 42% of aerosol particles, while a cloth mask blocked 43% of those particles. Double masking blocked about 93% of cough particles.

The team then examined exposure to aerosols from breathing with a masked source and an unmasked receiver, where double masking reduced cumulative exposure of the unmasked receiver by 82%, and "knotting and tucking" a medical mask reduced exposure by about 63%.

The researchers also examined when the source was breathing unmasked, and found that if the receiver was double masked, cumulative exposure was reduced 83%, and "knotting and tucking" a medical mask reduced exposure by about 65%.

When both source and receiver were double masked or both wore knotted and tucked medical masks, cumulative exposure to the receiver was reduced about 96% in each scenario.

The researchers noted obvious limitations to the study, such as that it was a laboratory experiment, and that the finding about effectiveness may not be representative of real-world settings. The findings may also not be generalizable to children or men with beards or other facial hair, due to the fit of masks.

Ultimately, the team concluded, "the data in this report underscore the finding that good fit can increase overall mask efficiency."

Washington Editor Joyce Frieden contributed to this story.

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