The coronavirus is finding new victims worldwide, in bars and restaurants, offices, markets and casinos, giving rise to frightening clusters of infection that increasingly confirm what many scientists have been saying for months: The virus lingers in the air indoors, infecting those nearby.
If airborne transmission is a significant factor in the pandemic, especially in crowded spaces with poor ventilation, the consequences for containment will be significant. Masks may be needed indoors, even in socially distant settings. Healthcare workers may need N95 masks that filter out even the smallest respiratory droplets as they care for coronavirus patients.
Ventilation systems in schools, nursing homes, residences and businesses may need to minimize re-circulating air and add powerful new filters. Ultraviolet lights may be needed to kill viral particles floating in tiny droplets indoors.
The World Health Organization (WHO) has long held that the coronavirus is spread primarily by large respiratory droplets that, once expelled by infected people in coughs and sneezes, fall quickly to the floor.
But in an open letter to the WHO, 239 scientists in 32 countries have outlined the evidence showing that smaller particles can infect people and are calling for the agency to revise its recommendations. The researchers plan to publish their letter in a scientific journal.
Even in its latest update on the coronavirus, released June 29, the WHO said airborne transmission of the virus is possible only after medical procedures that produce aerosols, or droplets smaller than 5 microns. (A micron is equal to 1 millionth of a meter.)
Proper ventilation and N95 masks are of concern only in those circumstances, according to the WHO.
Instead, its infection control guidance, before and during this pandemic, has heavily promoted the importance of hand washing as a primary prevention strategy, even though there is limited evidence for transmission of the virus from surfaces. (The Centers for Disease Control and Prevention now says surfaces are likely to play only a minor role.)
Dr Benedetta Allegranzi, the WHO's technical lead on infection control, said the evidence for the virus spreading by air was unconvincing.
Especially in the last couple of months, we have been stating several times that we consider airborne transmission as possible but certainly not supported by solid or even clear evidence, she said. There is a strong debate on this.
But interviews with nearly 20 scientists - including a dozen WHO consultants and several members of the committee that crafted the guidance - and internal e-mails paint a picture of an organization that, despite good intentions, is out of step with science.
Whether carried aloft by large droplets that zoom through the air after a sneeze, or by much smaller exhaled droplets that may glide the length of a room, these experts said, the coronavirus is borne through air and can infect people when inhaled.
In early April, a group of 36 experts on air quality and aerosols urged the WHO to consider the growing evidence on airborne transmission of the coronavirus. The agency responded promptly, calling Dr Lidia Morawska, the group's leader and a long-time WHO consultant, to arrange a meeting.
But the discussion was dominated by a few experts who were staunch supporters of hand washing and felt it must be emphasized over aerosols, according to some participants, and the committee's advice remained unchanged.