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Of all the three-word phrases this pandemic has popularized — “flatten the curve,” “six feet apart” — perhaps none has resonated as deeply as “wear a mask.” (Or, as Marco Rubio, the Republican senator from Florida, put it back in late June: “Everyone should just wear a damn mask.”)

It’s one of the simplest and most effective ways to curb the spread of the coronavirus and save lives.

But, as a burgeoning number of advisories makes clear, not every mask is helpful.

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

“The purpose of masks is to keep respiratory droplets from reaching others to aid with source control,” the agency’s guidance reads. “However, masks with one-way valves or vents allow air to be exhaled through a hole in the material, which can result in expelled respiratory droplets that can reach others. This type of mask does not prevent the person wearing the mask from transmitting COVID-19 to others.

“Therefore, CDC does not recommend using masks for source control if they have 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 fabric part of the mask, and hot, humid exhaled air goes out through the valve. The system may be what you want when tearing out a kitchen for remodeling, but the valve defeats the purpose when you’re trying to slow the spread of a virus.

Public health experts recommend mask-wearing to prevent respiratory droplets from spreading into the air when you exhale, speak, cough or sneeze, and the valves allow those droplets through.

Medical masks, you’ll notice, do not have valves.

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

Masks with valves have been banned by the major U.S. airlines, with American Airlines on Wednesday becoming the latest to announce a policy change, citing the CDC guidance.

A recent study also suggested people should avoid the newly popular neck gaiters, which are made of thin, stretchy material.

Researchers at Duke University found those coverings may 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.