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

The W.H.O. had described this form of transmission as doubtful and a problem mostly in medical procedures. But growing scientific and anecdotal evidence suggest 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 an updated scientific brief, the agency also asserted more directly than it had in the past that the virus may 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 W.H.O. 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 W.H.O. 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 email.

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 W.H.O. 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.