South Korean officials noticed a distinct pattern during a superspreader event in March: Nearly all of the infected employees at a call center in Seoul sat in the same section on the 11th floor. This suggested that employees who had longer interactions with an infected patient were at higher risk of contracting the virus.

But a person's viral load the amount of viral particles they release into the environment also plays a role in transmission.

A new study from Swiss researchers found that people with mild or asymptomatic cases can still be "high emitters," meaning they release large viral loads into the air around them. In small spaces with poor ventilation like offices or restaurants these individuals can expel hundreds of millions of virus copies per cubic meter, the study found.

"Individuals are at serious risk of infection if they spend more than a few minutes in a room with a person who is infected and has a high viral load, and this is true even when keeping distance from that person," the researchers wrote. "Sharing a workplace in a small room with a person with asymptomatic COVID-19 is not advised."

High emitters can release 5,000 times as many virus copies as average patients

Research indicates that there's little difference in the viral loads between coronavirus patients who show symptoms and those who don't. Both can be high emitters.

A typical coronavirus patient releases about 0.005 virus copies per cubic meter in a single breath, according to the Swiss study. But coughing can increase the amount of particles that get expelled into the surrounding air that same typical patient might expel 277 virus copies per cubic meter in a cough.

High emitters release around 5,000 times as many virus copies through these same activities as typical patients do, according to the new study. The researchers also found that high emitters release nearly 25 virus copies per cubic meter in one breath and nearly 1.4 million copies per cubic meter in one cough.

These infectious particles don't disappear right away: The longer an infected person spends in a confined space, the more virus particles accumulate in the air.

In rooms with poor ventilation, a high emitter who is coughing could release up to 286 million virus copies per cubic meter over the course of nearly three hours, the researchers found. Other studies have shown that just a few hundred copies of a respiratory virus are enough to infect another person.

Eventually, however, the concentration of particles in a room will start to plateau. This happens sooner in rooms with better ventilation. In hospital rooms, for instance, the virus concentration plateaus after about 30 minutes, but it takes more than an hour for the concentration to plateau in a typical office.

coronavirus office worker
coronavirus office worker
Laurie Dieffembacq/Belga/AFP via Getty Images

In a well-ventilated room, an average coronavirus patient who isn't coughing may not release enough particles to infect another person.

"A person spending time in a room with an individual emitting at a typical rate and breathing normally has the chance of inhaling only a few copies of the virus when keeping distance from that person," the researchers wrote. "However, the situation is worse in the presence of an individual with a high emitting rate and worse still if the individual is coughing."

High emitters aren't common, but they could become super-spreaders

The good news is that high emitters are rare, according to the researchers. But prolonged exposure to a high emitter could result in superspreader events like the one in the South Korean call center, in which large groups of people get infected.

Researchers in Hong Kong found that superspreader events may account for the majority of coronavirus transmission. They estimated that more than half of Hong Kong's community-spread cases between January 23 and April 28 were connected to just six superspreader events including a wedding, religious activities at a local temple, and people frequenting local bars inthe city's Lan Kwai Fong district.

Superspreader events become even more probable when high emitters are engaged in loud speaking or singing. The Swiss researchers found that both activities can increase viral emissions by one to two orders of magnitude.

That's consistent with research from the National Institutes of Health, which found that speech droplets from asymptomatic carriers are a likely mode of coronavirus transmission. The researchers found that just one minute of loud speech can produce over 1,000 coronavirus-containing droplets that linger in the airfor eight minutes or longer.

For this reason, the Swiss researchers recommend caution for individuals returning to cramped workplaces even if they're social distancing or wearing a face mask.

"While wearing a surgical face mask can be an effective source control, the protective factors may still be insufficient if an extended amount of time is spent in the same room with a coughing individual who has a high viral load," the researchers wrote. "Workplaces should not be shared as long as there are no rapid tests to differentiate between individuals without COVID-19 and individuals with asymptomatic COVID-19."

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