Face masks with valves or vents do not prevent spread of the corona virus : CDC

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The CDC recommends wearing cloth masks instead of masks with exhalation valves or vents.

Wearing a mask is one among the simplest and handiest ways to curb the spread of the corona virus and save lives. But, as a burgeoning number of advisories makes clear, not every mask is useful.

In guidance updated late last week, The Centers for Disease Control and Prevention warned against wearing masks with exhalation valves or vents, a kind of face-covering made for hot and dusty construction work that has become a popular pandemic accessory thanks to its seemingly high-tech design.

“The purpose of masks is to keep respiratory droplets from reaching others to help with source control,” the agency’s guidance reads. “However, masks with one-way valves or vents allow air to be exhaled through a hole within the material, which may end in expelled respiratory droplets which will reach others. This sort of mask doesn’t prevent the person wearing the mask from transmitting COVID-19 to others.

“Therefore, the CDC does not recommend using masks for source control if they need an exhalation valve or vent.”

3M, which makes valve masks for construction work, illustrates on its website how they work: inhaled air is filtered through the material a part of the mask, and hot, humid exhaled air goes out through the valve. The system could also be what you would like when tearing out a kitchen for transforming, but the valve defeats the purpose when you’re trying to slow the spread of a virus.

Public health experts recommend mask-wearing to forestall respiratory droplets from spreading into the air once you exhale, speak, cough or sneeze, and therefore the valves allow those droplets through. Medical masks, you’ll notice, do not have valves.

The CDC recommends simple cloth masks instead. A couple of layers of cotton prevent most of the potentially infectious respiratory droplets from escaping into the air around you, and that they also are much cooler than the form-fitting N95 masks.

Masks with valves are banned by the main U.S. airlines, with American Airlines on Wednesday becoming the most recent to announce a policy change, citing the CDC guidance.

Researchers at Duke University found those coverings could also be worse than not wearing a mask at all because they break up larger airborne particles into a spray of little ones more likely to linger longer in the air.

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