The coronavirus may linger in the air in crowded indoor spaces, spreading from one person to the next, the World Health Organization formally conceded Thursday.

The WHO had described this form of transmission as rare and possibly insignificant. But growing scientific and anecdotal evidence suggests this route may be important in spreading the virus, and this week more than 200 scientists urged the agency to revisit the research and revise its position.

In updated guidance documents, the agency also acknowledged unequivocally for the first time that the virus can be spread by people who do not have symptoms. "Infected people can transmit the virus both when they have symptoms and when they don't have symptoms," the agency said.

The WHO previously said asymptomatic transmission, while it may occur, was probably "very rare."

Some experts said both revisions were long overdue and not as extensive as they had hoped.

"It is refreshing to see that WHO is now acknowledging that airborne transmission may occur, although it is clear that the evidence must clear a higher bar for this route compared to others," Linsey Marr, an aerosol expert at Virginia Tech, said in an e-mail.

An aerosol is a respiratory droplet so small it may linger in the air. In its latest description of how the virus is spread, the agency said transmission of the virus by aerosols may have been responsible for "outbreaks of COVID-19 reported in some closed settings, such as restaurants, nightclubs, places of worship or places of work where people may be shouting, talking or singing."

The WHO had maintained that airborne spread is only a concern when health care workers are engaged in certain medical procedures that produce aerosols. But mounting evidence has suggested that in crowded indoor spaces, the virus can stay aloft for hours and infect others, and may even seed so-called superspreader events.

U studying spread

Engineering researchers at the University of Minnesota have already been studying the conditions under which such virus-carrying aerosols could spread in different indoor environments. The researchers on Thursday published their findings showing the hot spots that would be created amid different levels of ventilation if infected people were spreading the virus in a hypothetical elevator, classroom and supermarket.

Results will hopefully give school leaders and building owners information they can use to increase or optimize ventilation and reduce the buildup of aerosols in any one area that could present infection risks, said Jiarong Hong, a U assistant professor of mechanical engineering and one of the report authors.

The agency still largely emphasizes the role played by larger droplets that are coughed or inhaled, or by contact with a contaminated surface, also called a fomite. And in a longer document detailing scientific evidence, the WHO still maintained that "detailed investigations of these clusters suggest that droplet and fomite transmission could also explain human-to-human transmission."

In addition to avoiding close contact with infected people and washing hands, people should "avoid crowded places, close-contact settings, and confined and enclosed spaces with poor ventilation," the agency said; and homes and offices should ensure good ventilation.

These recommendations are "what is needed to help slow transmission in communities," Marr said.

There is debate about the relative contribution of airborne spread compared with transmission by droplets and surfaces. The new guidance still skirts that question.

"I interpret this as saying while it is reasonable to think it can happen, there's not consistent evidence that it is happening often," Bill Hanage, an epidemiologist at the Harvard T.H. Chan School of Public Health, said in an e-mail.

Outdoors, any virus in small or large droplets may be diluted too quickly in the air to pose a risk. But even a small possibility of airborne spread indoors has enormous implications for how people should protect themselves.

People may need to minimize time indoors with others from outside the household in addition to maintaining a safe distance and wearing cloth face coverings. Businesses, schools and nursing homes may need to invest in new ventilation systems or ultraviolet lights that destroy the virus.

Some experts have criticized the WHO for being slow to acknowledge the possibility of airborne spread while emphasizing hand-washing as the primary preventive strategy. Even in the new guidance, it's clear that members of the committee interpreted the evidence differently, said Dr. Trish Greenhalgh, a professor of primary health care at the University of Oxford.

"The push-pull of that committee is palpable," she said. "As everyone knows, if you ask a committee to design a horse, you get a camel."

Airborne transmission is the most likely explanation for several clusters of infection, according to some scientists.

But WHO staff members have yet to accept the importance of these case studies and instead have "dreamed up an alternative story" in which an infected person spat on his hands, wiped it on something and "magically" infected numerous other people, Greenhalgh said.

The WHO easily accepts that droplet and fomite transmission occur but seems to want more definitive proof of spread by aerosols, some experts said.

WHO staff members are reluctant to make statements when they do not have irrefutable proof of certain phenomena, and they are slow to update their hypotheses, scientists have charged. "They are still challenged by the absence of evidence and the difficulty of proving a negative," Hanage said.

"The WHO is being overly cautious and shortsighted unnecessarily," Dr. Julian Tang, honorary professor of respiratory sciences at the University of Leicester in the United Kingdom, said in an e-mail.

"By recognizing aerosol transmission of SARS-CoV-2 and recommending improved ventilation facilities to be upgraded or installed, you can improve the health of people" by eliminating a variety of hazards, including indoor pollutants and allergens, he added. "Isn't that what the WHO stands for — the improvement of human health from all angles?"