Coronavirus outbreak. Pathogen affecting the respiratory tract. COVID-19 infection.

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RALEIGH — “How big is it?” It’s a common question to answer when describing something new. But it’s also a crucial and controversial part of understanding the spread of COVID-19.

The size of the coronavirus is well-known to scientists. At 100 nanometers, 200 virions — that’s the word for a single particle of virus — could fit in the width of the thinnest of human hairs. But viruses need to be carried by something.

The coronavirus commonly stows away on the droplets that are released when a person exhales. And the size of the droplets that can carry a virion may mean the difference between safely going back to schools and offices versus another round of telework or homeschooling.

Scientists don’t agree on how big those droplets have to be.

Last week, the debate went public when over 200 scientists published an open letter to health agencies around the world. The signatories argued that emerging evidence shows that COVID-19 may be an airborne disease carried by the smallest droplets. A few days later, the World Health Organization issued slightly revised guidance addressing this possibility.

“Calling something airborne changes dramatically the way you treat a patient and the way you treat that problem from a public health perspective,” said Glenn Morrison, a professor of environmental sciences and engineering at UNC-Chapel Hill, and one of the 239 scientists who signed the letter. “The assumption is that it can get anywhere.”

With the exception of certain medical settings, the current guidelines from most public health organizations are designed to protect people from large droplets that carry the coronavirus. They tend to quickly fall to the ground within a few feet from their source. This is why people are encouraged to stay 6 feet apart in public.

However, “microdroplets” or “droplet nuclei” that are smaller than 5 microns can stay in the air as aerosols, which can hang suspended for hours and travel long distances.

When people exhale, whether a cough or a normal breath, they release droplets of all sizes. Barbara Turpin, the chair of the environmental sciences and engineering department at UNC, who also signed the letter, said someone infected with COVID-19 can produce about 600 droplets containing the coronavirus in a minute.

“There’s reasonable scientific debate that is going on about how COVID-19 is spread,” Turpin said. The debate is about the relative importance of droplet transmission compared to airborne transmissions by aerosols, based on what sizes can hold the virus.

Morrison said scientists don’t know enough to prove that airborne transmission is so rare that it can be ignored. Research looking back at the SARS epidemic, which was caused by a closely related virus, shows that disease was airborne.

Jim Zhang, chair of the ecotoxicology and environmental health program at Duke’s Nicholas School of the Environment, points out that the scientists’ letter doesn’t argue that current precautions are unimportant. “We’re not disputing that those are the right things to do, like washing hands or wearing face masks to cover your face and nose.”

The concern is that the current recommendations don’t adequately protect the population if the concentration of virus-containing aerosols builds up in indoor environments with poor ventilation. This could be especially important as businesses and schools consider reopening.

Imagine a classroom or bar with infected people isolated to one side and healthy people on another, Turpin said. A droplet-spread virus can’t infect the healthy people if they keep their distance. But an airborne virus can waft farther, and the odds of infection rise the longer everyone stays in the room, exhaling more virus into the air.

The three experts agreed that the most important way to reduce spread of an airborne virus is to reduce its concentration in the air through good ventilation. One way to do this is move more interactions outside. “Any activity that you can do inside is always safer outside,” Turpin said.

Zhang likens it to smoking. “That’s why we asked people to smoke outside. Smoking one cigarette has the same amount of content,” but outdoors, the smoke and pollutants dilute quickly.

Buildings should also try to increase the amount of fresh air they can bring in through their HVAC system. However, this can be limited by infrastructure. Bringing in new air requires more energy to heat or cool compared to recirculating air, and older systems may not be able to increase intake.

Zhang argues that if a system is capable of drawing in more air, it’s worth the cost compared to the expenses of allowing the coronavirus to spread.

Unfortunately, it can be hard for the average person to tell if a building or room has good ventilation. Just because air is moving doesn’t mean that it’s fresh. In fact, a COVID-19 cluster attributed to airborne spread at a restaurant was believed to have been caused by the air conditioning blowing so hard it spread the coronavirus between tables.

One relatively accessible way to measure ventilation could be to get a carbon dioxide monitor, which Morrison said could provide an indirect measure. If the carbon dioxide concentration gets above 1,000 parts per million, the space probably does not have adequate ventilation for the number of people inside.

It may sound counterintuitive, but it may be even more important to keep wearing face masks if there’s aerosol spread. This is because many microdroplets first come out a person’s mouth or nose as large droplets. Large droplets become smaller as water evaporates and they can become so small they stay in the air.

The biggest takeaway from the scientists’ letter is that there is still a lot to learn about COVID-19.

“What people are seeing is science in action. It is normal for scientists to have a healthy disagreement about how things work,” Turpin said. “With time, we’ll figure it out.”

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