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The World Health Organization acknowledged Thursday that the coronavirus could possibly spread through small airborne particles that linger in the air, suggesting the virus could spread indoors in poorly ventilated rooms and buildings.
The distinction, which critics say doesn't go far enough, would be important for advocates of using face masks in almost all indoor spaces outside the home.
The primary spread of the coronavirus, which has led to more than 552,000 deaths worldwide, is still believed to be through "person to person" contact, but the WHO's new scientific brief has strengthened speculation among some scientists that the virus's reach via air could be playing a larger role in its spread.
The WHO has already said the virus can be spread through droplets that infected people emit through coughing, sneezing, talking, or singing. Someone in close contact or within a meter can be infected.
But on Thursday, the organization said it was looking at the possible spread of the virus through aerosols, smaller particles that linger in the air for longer periods of time.
It could mean the virus can also spread through the air in settings like "choir practices, in restaurants, or in fitness classes," the WHO said.
Earlier this month, a group of 239 scientists from 32 countries published a paper outlining evidence that smaller particles in the air could allow people to get infected and called on the WHO to revise its recommendations, the New York Times reported.
The WHO had already stated that airborne transmission in aerosols was possible during specialized medical procedures, but if airborne transmission in smaller particles were confirmed, the finding could mean a need to address ventilation systems in places like schools or nursing homes.
Earlier this month, Benedetta Allegranzi, the WHO's technical lead on infection control, told the New York Times that the evidence of the virus going airborne was "not supported by solid or even clear evidence."
But on Thursday, the WHO said infections in crowded, indoor locations with inadequate ventilation "cannot be ruled out."
"WHO's updated scientific brief of transmission of COVID-19 is a step in the right direction because they now acknowledge that airborne transmission may be occurring outside of specialized medical procedures," Dr. Linsey Marr, professor of civil and environmental engineering at Virginia Tech, said in a statement.
Under the new recommendations to slow the spread of the virus, the WHO is recommending avoiding large crowds and ensuring good ventilation in buildings. It has also recommended the use of face masks when physical distancing is not possible.
States and local health departments across the US have already recommended people avoid large gatherings to slow the spread of the virus. Marr said she believed the WHO's recommendations don't go far enough considering the possibility the virus could be airborne.
"I would like to see WHO make a stronger recommendation for use of face coverings at all times indoors, except in one's own residence or when in a room by oneself, because there is increasing evidence that face coverings are very effective at slowing transmission, and they are a relatively inexpensive intervention," she said.
Part of the scientific debate regarding the virus's path through the air has centered over the definition of droplets versus aerosols. The WHO, for example, defines droplets as being larger than 5 microns (a micron is 1 millionth of a meter) and up to 10 microns. It considers aerosols to be 5 microns or less.
Marr argued it'd be better to acknowledge that path of transmission, rather than size, for the definition.
"Droplets land on the body, while aerosols are inhaled into the respiratory system," she said. "With this definition, a size cutoff of somewhere in the range of 50–100 microns is more appropriate."
More than just a scientific debate, the definition could have implications for recommendations issued to the public and a consideration for people possibly being infected by breathing in the virus.
"At close contact, transmission by aerosols might be as important or more important than droplet spray transmission because an infected person's respiratory plume is most concentrated right in front of their face," Marr told BuzzFeed News. "In poorly ventilated rooms, respiratory aerosols can build up in the air and pose a risk to others in the same place."
Correction: Benedetta Allegranzi's name was misspelled in an earlier version of this post.