It’s arguably the most divisive personal choice of the pandemic period: Should you wear a face mask while outdoors to help stop Coronavirus from spreading?

Although the main way that SARS-CoV-2 coronavirus spreads from person to person wasn’t initially certain, scientists now say the world should face the reality that the virus is transmitted through the air. Many believe that airborne transmission is actually the dominant route for the spread of Covid-19.

Coronavirus is carried by droplets or aerosols that leave the body when a person coughs, sneezes, breathes or talks. As a consequence, the chances of catching or spreading the virus will obviously increase when you’re close to other people.

The odds should be higher in a confined area, where virus-laden particles will circulate in the air. Following this reasoning, if there’s no significant risk of transmitting the virus in an unconfined space, you might not need a mask.

But is it really true that you’re relatively safe from worrying about Covid-19 while out in the open? Almost all studies (that I’m aware of) have taken place inside buildings, [detecting viruses in the air within hospital rooms][3], for instance, and little research has compared indoor transmission to what happens outdoors. Some work is currently being peer-reviewed, however.

An unpublished study of 110 Covid-19 cases in Japan tested individuals then used contact-tracing to follow-up on secondary cases. The results showed that people are much more likely to catch Coronavirus indoors: the odds that a primary case transmitted the disease in a closed environment was 18.7 times greater compared to the open air.

In an unpublished review article on how the virus spreads in different settings, epidemiologists at the London School of Hygiene & Tropical Medicine summarized what we know: “The difference in transmission risk between households and larger communal settings is unclear, as is the difference between indoor and outdoor transmission.”

One factor that affects the relative risk of catching a virus is how far it could potentially travel through the air. Some scientists distinguish between ‘droplet’ and ‘aerosol’ (using a cut-off size such as 0.005mm) partly because bigger particles should fly a greater distance and hang around for longer. One laboratory experiment found that airborne SARS-CoV-2 survived for three hours.

The artificial conditions of an indoor lab might not apply to outdoor locations. But it’s difficult to test how far the virus travels outside because experiments are ideally carried out in a closed system (effectively a box) so scientists can control for potentially confounding factors that might affect their results — and there are plenty of those factors in an open environment.

Wind is one environmental influence that could make a virus travel further. If you believe wearing a mask is a good idea — even when nobody is nearby — it may be because you worry about being ‘downwind’ of someone who’s infected, yet out of sight, as air currents enable the virus to be spread over a wider range.

The possibility that Coronavirus travels further than your field of view isn’t supported by evidence, but one mathematical model does suggest that wind might drive cough droplets over longer distances. The model, which used fluid dynamics (accounting for effects like condensation and evaporation) and consisted of two virtual humans, predicted that the influence of wind might allow airborne transmission over at least three metres (10 feet) — beyond the ‘six-foot-rule’ currently used for physical distancing.

By contrast, some observations suggest that wind might help stop the spread. A comparison of weather in Singapore found relationships between the number of Covid-19 cases and deaths with meteorological conditions: greater SARS-CoV-2 transmission was associated with high temperatures and humidity, whereas wind speed and ventilation were associated with fewer cases and deaths.

Although those associations seem to suggest that windy weather helps limit the spread of disease, note that they are correlations and don’t show causation. The negative correlation could also be caused by more people staying in when it’s windy, for instance, raising population density and helping the virus spread.

The main reason why you’re probably safer outside is ventilation. Inside closed environments like a building or vehicle, contaminated air will simply circulate. This is what happens in poorly-ventilated spaces and might explain why Covid-19 cases are so common on public transport and in nursing homes.

Researchers in the Netherlands used lasers to track aerosols and measured their distribution, travel distance and velocity. By analysing the droplets produced by coughs or speech from healthy volunteers, the scientists found that better ventilation substantially reduced the airborne time of respiratory droplets that can carry SARS-CoV-2, and concluded that “improving ventilation of public spaces will dilute and clear out potentially infectious aerosols.”

Ventilation in open environments allows virus particles to be diluted as wind continuously bring fresh air into a space. As Julian Tang, consultant virologist at the Leicester Royal Infirmary in the UK has explained, “If the wind is blowing the virus towards you, there may be an increased risk of infection. But there will also be a massive dilution factor which will generally act to reduce the exposure even if the wind is blowing it in the right direction.”

Without a host to infect, airborne viruses are gradually destroyed by sunshine through a double-whammy of heat and light. Heat from solar radiation will cause Coronavirus to dry-out so that it loses its outer shield and the spike proteins that enable it to invade cells.

A virus is like a burglar who uses a lock-pick to break into a building through a window: removing its shield and spikes is akin to stripping the burglar naked and taking away their tools. Light kills viruses by damaging genetic material, which would be like leaving the burglar trapped inside a factory that can manufacture clones that could then escape, but without instructions for how to operate the machinery.

Taken together, the science of Coronavirus suggests that you probably don’t need a face mask when going outdoors.

That statement comes with an important caveat: your decision should also depend on whether you’re likely to be in close proximity to other people. If you’re walking through a rural area, a mask isn’t necessary, but you might consider wearing one if you’re in an urban park full of picnics and barbecues, where it’s harder to avoid contact with potential asymptomatic cases of Covid-19.

With knowledge of your local environment, you can then calculate the risk and make an informed decision on whether to wear a mask outdoors.

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