Mask wearing and social distancing

Social distancing and mask wearing are important ways we can help limit the spread of COVID-19. Can physics tell us what distance is a safe social distance, and which masks are really effective?

The main way the SARS-CoV-2 virus, which causes COVID-19, spreads is through large droplets laden with virus being shared among people who are in close contact with each other. This is why social distancing and mask wearing are crucial; preventing people from spreading the virus to the people around them. But the social distancing rules imposed by different countries are almost as diverse as the types of face protection people wear around the world. Why?

A safe social distance?

A country’s choice of a safe social distance is as much based on politics as it is on science. Too short a distance risks runaway infections and deaths. Too long a distance risks huge harm to society and the economy. This is half of the reason why countries have opted for a reasonable 1–2 metre rule.

The other half is based on scientific evidence; though not evidence from COVID-19, nor even the 21st Century. A 1942 study ‘Atomizing of mouth and nose secretions into the air as revealed by high-speed photography’ by Marshall W Jennison is thought to be the origin of the 1–2 metre rule. In it, he found that most droplets in his experiment travelled 2–3 feet (≈1 metre), though he provided no details about how he reached this conclusion.

Based on an idea from even earlier, the 1930s, that droplets come in only two forms—‘large’ and ’small’—later studies supported Jennison’s conclusion. Eventually, 1–2 metre social distances became general recommendations by the likes of the World Health Organization and US Centers for Disease Control and Prevention (CDC) to avoid catching viruses.

Recent work makes the picture less clear. In a series of papers before the pandemic, MIT’s Lydia Bourouiba and colleagues looked at the turbulent gas cloud we make when we exhale, sneeze and cough. They found that this cloud can contain virus-bearing droplets of all sizes, not just large and small. What’s more, these droplets can travel well beyond two metres. In fact, 7–8 metres, suggesting current 1–2 metre social distancing rules might not be enough.

In contrast, the first study to bring together all direct information on COVID-19 concluded that physical distancing dramatically lowers risk of infection; by 13% within one metre, and just 3% beyond that distance. The authors’ key takeaway was that in general social distancing works, and the more distance the better.

Yet there are a number of other risk factors beyond distance that come into play. Chief among these are time spent with people with the virus (exposure time) and how well-ventilated the location is.

For instance, one recent study gives an insight into ventilation. Japan’s National Institute of Infectious Diseases found that the chances of transmitting the virus were 19 times higher indoors than outdoors after looking at 110 cases of COVID-19 and tracing the contacts of the people with the virus.

The results of this and many other studies provide new understanding of the virus, but make a safe social distance hard to pin down. In fact, they suggest social distancing is not enough to deal with people spreading the virus from close contact.

An effective mask?

Luckily, social distancing isn’t the only protection we have. Many studies have shown that medical-grade masks slash the chances of both transmitting and catching coronavirus in healthcare facilities. In fact, some studies even hint that masks might reduce the severity of infection if people do contract the disease.

Combining face-masks and social-distancing is the most effective way to prevent the transmission of coronavirus.

In an experiment led by Kwok-Yung Yuen at the University of Hong Kong, healthy and coronavirus-infected hamsters were placed in neighbouring cages. Without a barrier, 2/3 of the uninfected animals caught the virus. With surgical mask material separating them, only ~1/4 were infected. And surprisingly, the hamsters that were infected had milder symptoms. Scientists are now exploring whether these findings translate to humans.

Unfortunately, there is less evidence supporting the use of the various different face masks the public wear on a daily basis. With some people wearing valved designer models or homemade multi-layer face-coverings, and others buying simple cloth masks or pulling bandanas over their mouths and noses, it can be easy to think that all that matters is that your face is covered. And in one sense this is true, as any mask covering your mouth and nose has been shown to be of benefit. But there are differences.

What to avoid is clear. A mask with an exhalation valve means the air you breathe out is unfiltered. So if you have COVID-19, these masks offer no protection to anyone you come into contact with. Also, loosely-woven fabrics like scarves, stretchable coverings like neck gaiters, and any poorly fitting masks reduce protection for both the wearer and others.

At the other end of the scale, properly fitted N95 respirators without exhalation valves are the gold standard. They are called N95s because they filter out approximately 95% of airborne particles that measure 0.3 µm (for comparison, a human hair is about 75 µm across) and larger, and block a similar proportion of outgoing particles.

However, N95 masks have been in short supply throughout the pandemic, rightly being prioritised for healthcare workers. This makes choosing the best mask that will provide you and others around you with the most protection far from obvious.

One study highlighted that surgical and cloth masks, even homemade ones, are 67% effective at protecting the wearer, with other research suggesting multi-layer and multi-material masks offer superior protection, perhaps as good as N95 masks. To be most effective, people need to combine proper social distancing with mask wearing to stop the spread of the virus in their communities.