A recent study by Duke University researchers tested a variety of masks to determine their effectiveness to block droplets that carry the coronavirus. They found that gaiter masks, or neck fleeces, were the least effective. But there is another kind of mask the CDC wants you to ditch: those with vents or valves. The reason for wearing a mask is not as much to protect yourself as it is to protect others, the CDC says. “The purpose of masks is to keep respiratory droplets from reaching others to aid with source control. Masks with one-way valves or vents allow exhaled air to be expelled out through holes in the material. This can allow exhaled respiratory droplets to reach others and potentially spread the COVID-19 virus. Therefore, CDC does not recommend using masks if they have an exhalation valve or vent,” the agency wrote on its site.