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An airborne virus is a threat worth taking seriously

The writer is a science commentator

How does coronavirus spread? Scientists are increasingly convinced that airborne transmission plays a role. Last week, more than 200 specialists penned an open letter to the World Health Organization urging it to officially accept that the Covid-19 virus can spread through the air further than social distancing recommendations.

The WHO says the possibility cannot be ruled out but its reluctance so far to embrace airborne transmission has put it at odds with aerosol chemists and engineers, who now believe the virus can spread through tiny particles that stay aloft for hours on air currents, in addition to the usual accepted route of larger droplets of saliva and mucus sprayed out from coughs and sneezes. The tension highlights the dilemma of using incomplete and potentially outdated scientific evidence to make critical judgment calls in a rapidly changing situation. 

The virus is worryingly contagious, with nearly 13m confirmed cases worldwide. Droplets (defined as particles at least 5-10 microns across) are widely regarded as the master key to respiratory infection. They can be expelled in coughs and sneezes, as well as by singing, shouting and talking. Due to gravity, droplets generally fall to the floor within several metres — hence the need for distancing. Infection can occur directly, when virus-laden droplets enter the eyes, mouth or nose of another person, or indirectly, when a person touches a surface on which droplets have fallen and then transfers the virus to their own eyes, mouth or nose, which explains handwashing guidance.

But the appearance of large infection clusters at indoor gatherings has led many experts, such as Kimberly Prather, an atmospheric chemist at the University of California, San Diego, to question whether droplets explain everything. For some cases, she told Nature, the only way to explain certain transmission clusters is “you put it in the air and everyone breathes that air”. 

Airborne spread involves particles that are smaller than droplets, called aerosols (also called droplet nuclei or microdroplets). These fine particles, smaller than 5 microns, can be emitted in the same way that droplets are, but also by breathing. They linger in the air, where they can be inhaled by others. Last month, Prof Prather argued for the aerosol transmission of viruses to be “acknowledged as a key factor in the spread of infectious respiratory diseases”, with implications for indoor mask-wearing, ventilation and crowding, as well as wider testing.

A patchwork of studies can indeed be pieced together into plausible scenarios of airborne infection. People produce clouds of aerosols when they breathe and talk; some individuals are “superemitters”, exhaling particularly large clouds; aerosols can remain buoyant for hours; research from the 2002-2003 Sars outbreak show aerosols can contain an infectious virus. Interestingly, airborne transmission may explain some perplexing superspreading events when distancing and hygiene rules were followed. One restaurant outbreak was traced back to an air conditioning unit. 

Other observations provide circumstantial support: asymptomatic and pre-symptomatic spreaders do not, by definition, sneeze or cough appreciably and yet are known to transmit the virus to others. Aerosols might be even more dangerous than droplets: smaller particles can potentially penetrate deeper into the airways, possibly bypassing the immune system and causing more severe disease.

For now, the WHO maintains that only “aerosol-generating procedures” in medical settings, such as the insertion of breathing tubes, are proven to spread Covid-19 through the air. But it agreed last week that more research is needed. Prof Prather points out that some of the science guiding today’s advice was gathered in the 1930s, and requires updating; others complain that the bar for admitting new evidence is set too high.

The current uncertainty over airborne spread mirrors the international debate about masks. An anxious pro-mask consensus was building for months before the WHO’s advice formally changed in June to support face coverings. Strong evidence always trumps weak, but, in an extraordinary situation, weak evidence — especially when it converges from multiple disciplines — is arguably better than none. The final answer on airborne coronavirus transmission might well be blowing in the wind. 

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