Scientists have tested 14 different types of face covering and found that one of them actually increased the risk of coronavirus infection.
Researchers at Duke University in North Carolina tested covers that range from healthcare professionals to throat fleece and knitted masks.
The study's authors compared the distribution of droplets from a participant's breath while wearing one of the covers to the results of a control experiment in which his mouth was completely exposed.
The least effective face covering in the study was a Neck fleece what has been found to actually increase the risk of infection by a "droplet transfer rate" of 110%.
Duke University researcher Martin Fischer, who compiled the test, told CNN: "We were extremely surprised that the number of particles measured with the fleece actually exceeded the number of particles measured without a mask.
"We want to emphasize that we really encourage people to wear masks, but we want them to wear masks that actually work."
A bandana was the second worst face coverage but did not increase the risk of infection, while a Knitted cover was third worst.
The most effective face covering in the study was a customized one N95 mask without valves with a droplet transfer rate of 0.1%.
The hospital grade drapes are used by frontline healthcare workers.
A surgical mask came second best, while a polypropylene mask came in third.
Handcrafted cotton face covers They have also been shown to work well, eliminating a significant amount of the spray from normal speech.
The study was published in the journal Science.
Mr. Fischer said: "We have confirmed that when you speak, tiny droplets are emitted, so that you can spread disease by speaking without coughing or sneezing.
"We also found that some face coverings performed much better than others at blocking ejected particles."
Eric Westman, one of the study's authors, said, "Wearing a mask is a simple and easy way to reduce the spread of COVID-19.
"About half of the infections come from people who show no symptoms and often don't know they are infected. They can unwittingly spread the virus by coughing, sneezing, and just talking.
"If everyone was wearing a mask, we could stop up to 99% of these droplets before they hit anyone else.
"In the absence of a vaccine or antiviral drug, this is the tried and true way to protect others and yourself."
Study participants wore one of the covers before speaking in a black box in the direction of a laser beam.
Droplets from the person's breath would then scatter light as they move through the laser beam. This process was recorded with a cell phone camera.
The study's authors said a "simple computer algorithm" was used to count the drops in the video.
The ranking list of the 14 tested masks (picture number):
1. N95 mask, no exhalation valve, mounted (14th)
2. Surgical mask (1)
3. Cotton-polypropylene-cotton mask (5)
4th Two-layer polypropylene apron mask (4)
5. Two-layer, pleated cotton mask (13)
6. Two-layer, pleated cotton mask (7)
7th N95 mask with exhalation valve (2)
8th. Two-layer, Olston style mask made of cotton (8th)
9. Single-layer Maxima AT mask (6)
10. Single-layer, pleated cotton mask (10)
11. Two-layer, pleated cotton mask (9)
12th Knitted mask (3)
13th bandana (12)
14th Gaiter neck fleece (11)