- There is a higher chance of contracting SARS-CoV-2 through the air we breathe than from the surfaces we touch
- Experts are, therefore, urging people to improve ventilation in order to reduce the spread of the virus
- Still, fomite transmission, i.e. through contaminated surfaces or objects, cannot be entirely ruled out
In the early days of the Covid-19 pandemic, public health guidance was largely based on what we knew about past disease outbreaks. To reduce our risk of infection, we were advised to disinfect potentially contaminated surfaces or objects, known as fomites, such as doorknobs and tabletops, as these were thought to be one of the main ways through which one could contract the virus.
Over more than a year, however, scientists have learned a lot more about SARS-CoV-2, the virus that causes Covid disease, and increasing evidence suggests that our focus should instead shift to preventing airborne transmission.
Updated guidance by the US Centers for Disease Control and Prevention (CDC) notes that “the relative risk of fomite transmission of SARS-CoV-2 is considered low” when compared with direct contact, droplet transmission, or airborne transmission.
In an editorial published in the British Medical Journal (BMJ) this month, four authors explain how improving indoor ventilation and air quality can help us to stay safe from infection.
“Wearing masks, keeping your distance, and reducing indoor occupancy all impede the usual routes of transmission, whether through direct contact with surfaces or droplets, or from inhaling aerosols,” they wrote.
“One crucial difference, however, is the need for added emphasis on ventilation because the tiniest suspended particles can remain airborne for hours, and these constitute an important route of transmission.”
According to the authors, the Covid pandemic has helped to redefine airborne transmission of viruses. People infected with the new coronavirus produce many small respiratory particles – laden with the virus – as they breathe, talk, sneeze, cough, or sing.
Some of these, they explain, will be inhaled almost immediately by those within a typical conversational short-range distance of under one metre, while the remainder would disperse over longer distances of more than two metres, where they may be inhaled by people further away.
They add that there’s been some confusion over the precise definition of air transmission of infection in the past. Traditionally, the larger, short-range particles are known as “droplets”, whereas the smaller long-range particles are termed “droplet nuclei”, but since they can be inhaled directly from the air, the authors claim that they are all “aerosols”.
The important factor to bear in mind is that if a person can inhale particles, regardless of their size or name, they are breathing in aerosols, they further explain.
Since more and more evidence points to the virus being able to linger in the air, our attention should centre on ventilation, the four experts say.
“If we accept that someone in an indoor environment can inhale enough virus to cause infection when more than two metres away from the original source – even after the original source has left – then air replacement or air cleaning mechanisms become much more important.”
Essentially, this means that we should keep windows open, or install or upgrade heating, ventilation, and air conditioning systems, as outlined in a recent World Health Organization (WHO) guidance document, they wrote, adding: “People are much more likely to become infected in a room with windows that can’t be opened or lacking any ventilation system.”
"Super-spreader events", where one highly contagious individual infects an "unusually high" number of other people at once (usually in a crowded indoor space with poor ventilation) also strongly point to airborne transmission of the virus, engineer Linsey Marr at Virginia Tech in Blacksburg wrote in an op-ed for The Washington Post.
The power of the face mask
Of course, we shouldn’t forget about face masks, which have been one of the most powerful tools in controlling the pandemic, and, according to the paper, a second crucial implication of airborne spread is the quality of the face mask.
The higher the quality of the mask, the more effective it can protect against inhaled aerosols. Although masks generally impede large droplets from landing on covered areas of the face, and the majority are at least partially effective against inhalation of aerosols, two key components can enhance protection, they say.
These are high filtration efficiency, and a good fit, as “tiny airborne particles can find their way around any gaps between mask and face”, they explain.
Covid may become seasonal
Like influenza (the flu), experts believe Covid-19 may become seasonal. If this is the case, governments and health leaders should heed the science and focus their efforts on airborne transmission, the authors advise.
“Safer indoor environments are required, not only to protect unvaccinated people and those for whom vaccines fail, but also to deter vaccine-resistant variants or novel airborne threats that may appear at any time. Improving indoor ventilation and air quality, particularly in healthcare, work, and educational environments, will help all of us to stay safe, now and in the future,” they conclude.
Other research also indicates that fomite transmission of the virus, while it cannot be completely ruled out, is relatively low.
In July 2020, Emanuel Goldman, a microbiologist at Rutgers New Jersey Medical School in Newark, analysed the evidence around fomites and found that surfaces presented relatively little risk in terms of virus transmission, Health24 reported.
"You have to make up some really convoluted scenarios in order to explain super-spreading events with contaminated surfaces," she said. Goldman added that handwashing is still crucial, but that, like the experts who wrote the editorial state, it is more important to improve ventilation systems or to install air purifiers than to sterilise surfaces.
"If we've already paid attention to the air and we have some extra time and resources, then yes, wiping down those high-touch surfaces could be helpful," she explained.
Reducing airborne spread
This Health24 article lists some helpful preventive measures to reduce airborne transmission of the virus where possible. These include avoiding large gatherings and crowded spaces; opening windows or doors in public buildings; and avoiding coughing/sneezing openly (rather, do so into a handkerchief or into an elbow).