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 minimise recirculating air and add powerful new filters. Ultraviolet lights may be needed to kill viral particles floating in tiny droplets indoors.
The World Health Organisation (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 metre.)
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 handwashing as a primary prevention strategy, even though there is limited evidence for transmission of the virus from surfaces. (The Centres for Disease Control and Prevention now says surfaces are likely to play only a minor role.)
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 organisation 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.
"We've known since 1946 that coughing and talking generate aerosols," said Dr Linsey Marr, an expert in airborne transmission of viruses at Virginia Tech. Scientists have not been able to grow the coronavirus from aerosols in the lab. But that doesn't mean aerosols are not infective, Dr Marr said: Most of the samples in those experiments have come from hospital rooms with good air flow that would dilute viral levels.
In most buildings, she said, "the air-exchange rate is usually much lower, allowing virus to accumulate in the air and pose a greater risk."
Many experts said the WHO should embrace what some called a "precautionary principle" and others called "needs and values" - the idea that even without definitive evidence, the agency should assume the worst of the virus, apply common sense and recommend the best protection possible.
"There is no incontrovertible proof that Sars-CoV-2 travels or is transmitted significantly by aerosols, but there is absolutely no evidence that it's not," said Dr Trish Greenhalgh, a primary care doctor at the University of Oxford in Britain.
"So at the moment we have to make a decision in the face of uncertainty, and my goodness, it's going to be a disastrous decision if we get it wrong," she said. "So why not just mask up for a few weeks, just in case?" Even cloth masks, if worn by everyone, can significantly reduce transmission, and the WHO should say so clearly, Dr Don Milton, an aerosol expert at the University of Maryland.