Throughout the Covid-19 pandemic, the use of masks has been hotly debated. The vast majority of public health authorities agree that masks can help curb the spread of Covid-19 when used along with other measures like social distancing and good hygiene. Many scientists argue they’re a cheap, life-saving intervention.
A vast majority does not mean everyone is in agreement. The World Health Organization still says that if you are healthy, you only need to wear a mask if you are taking care of a sick person. This lack of universal support as opened the door to controversy, inviting in the opinion that mask science is dubious.
What masks really are, experts tell Inverse, is imperfect. That imperfection needs to be considered within the context that our understanding of the disease has also changed. That's why the CDC now endorses the use of cloth face masks, which saves up PPE for medical professionals, when it originally did not.
Jeremy Howard is the co-founder of the #Masks4All movement and a researcher at the University of California San Francisco. He describes masks as one of the most effective and easiest to use weapons we have.
"This is the way we can stop this thing dead, even before we have a vaccine," Howard tells Inverse.
As authorities ease stay-at-home restrictions and people begin to move about more freely, people may wonder when and how often to use a mask. To help answer those questions, here's the science behind why masks can make a crucial difference.
How do masks work? — Covid-19 mainly spreads through respiratory droplets, the Centers for Disease Control explains. Every time people sneeze, speak, or cough, they're also spewing hundreds — even thousands — of droplets into their immediate vicinity.
These potential coronavirus-containing droplets can land on the noses or mouths of others, and inhaled into the lungs. Outdoors, these droplets seem to quickly dissolve in the air. But indoors, especially in areas with stagnant air, droplets can linger in the air between eight to 14 minutes.
People are commonly contagious before showing symptoms. These folks can operate as invisible carriers, inadvertently spreading viral droplets around, if these droplets are not contained in a mask. It was the realization that asymptomatic individuals could transmit the virus that motivated the CDC to support the wearing of cloth face masks coverings in public.
What's the latest data on masks and Covid-19? — In video and letter published in April in the New England Journal of Medicine, researchers used laser light scattering techniques to track just how far spittle can spray when people speak at various volumes. In the video below, a speaker utters the phrase "stay healthy" repeatedly, spraying droplets around the immediate area.
As the speaker raises their voice, you can watch the droplets increasing in size and spread.
If you can physically block potentially infection-causing droplets from spraying into the air or reaching others, you can disrupt this crucial transmission vector — effectively stopping the virus' spread in its tracks. That's where masks and cloth face coverings come in.
The team replicated the experiment when the speakers' voice was covered with a damp washcloth, a condition designed to mimic mask-wearing. With this face covering, spittle spray was contained significantly.
Importantly, there haven't been any randomized controlled trials — the "gold standard" of clinical research — conducted on how masks influence transmission of Covid-19. The missing large-scale evidence, as well as dire supply shortages, were two driving factors causing public health authorities to hesitate before initially recommending masks or face coverings to the public, Howard says.
In a recent pre-print report (which has not completed peer-review), Howard and 18 other scientists synthesized the current data on cloth masks' role in curbing the spread of Covid-19. After combing the available research on the Covid-19 virus and other coronaviruses, the team found evidence in favor of widespread mask use to reduce community transmission.
In the studies analyzed, masks made with household materials filtered 49 and 86 percent of small droplets. They also helped people keep their own droplets to themselves, blocking droplets and particles from being transmitted from the wearer.
The available evidence suggests that "near-universal adoption of non-medical masks when out in public," along with complementary public health measures like social distancing and large-event bans, could successfully stop community spread, the report argues.
"Masks are incredibly effective in simple physical experiments of blocking the primary transmission vector of Covid-19," Howard says. "So what happens if you use them? Well, obviously, the virus stops transmitting."
If populations reach an 80 percent compliance with mask-wearing, then we could expect the virus to "stop [spreading] entirely," Howard says.
"That would be the end of the pandemic."
If used in conjunction with other measures like social distancing and banning large gatherings, we may only need 70 percent compliance, Howard explains.
Meanwhile, a not peer-reviewed study released this week by the University of Hong Kong found that surgical masks could reduce the transmission rate of droplets by as much as 75 percent. Crucially, this research was conducted on hamsters — not people — but the team argues their results are applicable to humans.
People in masks — Currently, over 50 countries have made some level of mask-wearing or cloth face coverings compulsory when people leave home. In the United States, 40 states require some level of mask-wearing, although who is required to wear a mask varies by state.
A Gallup poll conducted in April shows 68 percent of Americans say they always or sometimes wear a mask in public. The vast majority of Americans support mask-wearing, despite a vocal minority of opponents, Howard says.
"You can physically create the barrier that literally stops the transmission vector from an infected person to somebody else," Howard says.
"Anybody can do it and it costs like five cents but we don't...," Howard questions. "I'm just like, what the hell?"
Other public health experts and authorities don't go so far as to say masks are the absolute best and cheapest intervention.
Elaine Shuo Feng, an epidemiologist at the University of Oxford's Vaccine Group, previously told Inverse that, for the general public, masks "would be helpful in flattening the curve."
But if the world moves towards universal mask use, Feng notes that limited supplies of medical masks need to be closely monitored.
Feng also cautions that mask usage — medical, cloth, or homemade — shouldn't replace public health measures like social distancing and hand-washing.