magine you think there are mice in your house. You can see the evidence – mouse droppings; gnawed or damaged skirting boards; holes left in food packaging. You call a local pest control team who confirm that you have mice and advise you on what is needed to remove them. Neither of you have actually had to see a mouse to reach this conclusion. The same kind of thinking can be applied to the transmission of coronavirus. We don’t need to see the virus to understand how it spreads. Recent studies from China show that patients infected with Covid-19 in clinical settings exhale large amounts of virus, which remain present in the air and can be sampled and detected. Because of this, scientists can reasonably infer that the virus contaminates its surrounding environment. People nearby may inhale it, and as the virus floats through the air, spreading further in poorly ventilated environments, those who are further away could also become infected. Importantly, scientists haven’t yet demonstrated that someone walking through a cloud of exhaled virus would develop Covid-19 from that particular exposure, and research in this area is ongoing. But there is growing evidence that the virus which causes Covid can remain in the air, and therefore pose a risk to people in that airspace. The evidence that Covid can spread via aerosol transmission takes one of two different forms. First, many scientists now think that aerosol transmission explained some early outbreaks, such as those in an air-conditioned restaurant in Guangzhou, China, and at a choir practice in the US, where the virus was exhaled by people and may have remained in the surrounding environment before infecting others who inhaled it. Second, aerosol transmission partly explains why countries that were early adopters of policies which targeted this kind of transmission have been more successful at controlling the virus. For example, south-east Asian countries were far quicker to adopt face masks, which are shown to reduce the spread of aerosols, than many western countries including the UK. I’m not saying that aerosols are the only transmission route. Research suggests there are in fact two main ways the virus can transmit. The first is via contaminated surfaces, also known as “fomites”. The UK government emphasised this transmission route at the beginning of the pandemic, issuing public health messages that focused on hand-washing. Yet as the World Health Organization conceded in a recent Covid-19 brief, “despite consistent evidence as to … the survival of the virus on certain surfaces, there are no specific reports which have directly demonstrated fomite transmission”. This doesn’t mean that surfaces don’t play a role in the spread of Covid-19. The WHO still considers them a “likely mode of transmission”, given consistent findings about how the virus contaminates its environment, and the fact that other coronaviruses can transmit this way. But contact with contaminated surfaces may not play as big a role as we initially thought. Indeed, a review of studies by theScientific Advisory Group for Emergencies suggested hand-hygiene policies only led to a 16% reduction in acute respiratory infections. The other way that coronavirus spreads is likely through a combination of droplets and aerosols. Droplets are larger than aerosols; too big to remain suspended in the air, they fall more quickly on to surfaces. Aerosols are tiny by comparison and can be spread at far greater distance. Both droplets and aerosols are produced while talking, coughing, sneezing and singing, but where aerosols can remain in the air for minutes or even hours, droplets fall and contaminate surfaces from where they can contribute to transmission. In an open letter that I co-authored with many other scientists, we outlined these ideas and cited evidence from various studies suggesting that aerosol transmission is a potentially significant route by which Covid-19 is spread. Surfaces are still important, and people should continue washing their hands, but a growing scientific consensus suggests this may not be the main way that Covid is transmitted. Why does this matter? First, because understanding how the virus spreads is key to controlling it. On the basis of what we know about how Covid transmits, a combined approach that targets aerosols, droplets and surfaces, and gives priority to aerosols, is likely the best way to control its spread. This means a combination of policies, such as social distancing, face masks, improving indoor ventilation, reducing contact between households and enforcing the rule of six will all reduce the level of transmission. Many countries including Britain have already adopted variations of each of these policies. So why does emphasising aerosols as one of the main routes by which the disease spreads make any difference? The reason for recognising the virus as potentially airborne (as we do with chickenpox, measles and tuberculosis) is that this will release additional funding around the world to combat airborne infections. If Covid-19 is globally recognised as caused by an airborne virus, there will be more of an imperative to fund research and initiatives, such as improving indoor ventilation, that will be crucial to preventing the spread of this virus and others like it. Emphasising that Covid-19 can be spread through the air would allow us to add more weapons in our arsenal to fight this virus – which may be with us for some time to come. Julian Tang is a clinical virologist and honorary associate professor in the respiratory sciences department at the University of Leicester