As Covid-19 has spread, so has our fear of surfaces. There are now some familiar scenes in public places around the world – people trying to open doors with their elbows, commuters studiously surfing their way through train journeys to avoid grabbing a handle, office workers rubbing down their desks each morning.
In the areas worst hit by the new coronavirus, teams of workers in protective clothing have been dispatched to spray a fog of disinfectant in plazas, parks, and public streets. Cleaning regimes in offices, hospitals, shops, and restaurants have been increased. In some cities, well-meaning volunteers even venture out at night to scrub the keypads of cash machines.
Like many respiratory viruses, including flu, Covid-19 can be spread in tiny droplets released from the nose and mouth of an infected person as they cough. A single cough can produce up to 3,000 droplets. These particles can land on other people, clothing, and surfaces around them, but some of the smaller particles can remain in the air. There is also some evidence that the virus is also shed for longer in fecal matter, so anyone not washing their hands thoroughly after visiting the toilet could contaminate anything they touch.
It is worth noting that, according to the Centers for Disease Control and Prevention, touching a surface or object with the virus and then touching one's own face "is not thought to be the main way the virus spreads". Even so, the CDC, the World Health Organization and other health authorities, have emphasized that both washing one's hands and cleaning and disinfecting frequently touched surfaces daily are key in preventing Covid-19's spread. So although we still don't know exactly how many cases are being caused directly by contaminated surfaces, experts advise exercising caution.
One aspect that has been unclear is exactly how long SARS-CoV-2, the name of the virus that causes the disease Covid-19, can survive outside the human body. Some studies on other coronaviruses, including Sars and Mers, found they can survive on metal, glass, and plastic for as long as nine days unless they are properly disinfected. Some can even hang around for up to 28 days in low temperatures.
Coronaviruses are well known to be particularly resilient in terms of where they can survive. And researchers are now beginning to understand more about how this affects the spread of the new coronavirus.
Neeltje van Doremalen, a virologist at the US National Institutes of Health (NIH), and her colleagues at the Rocky Mountain Laboratories in Hamilton, Montana, have done some of the first tests of how long SARS-CoV-2 can last for on different surfaces. Their study, which has yet to be published in a scientific journal, shows that the virus could survive in droplets for up to three hours after being coughed out into the air. Fine droplets between 1-5 micrometers in size – about 30 times small than the width of a human hair – can remain airborne for several hours in still air.
It means that the virus circulating in unfiltered air conditioning systems will only persist for a couple of hours at the most, especially as aerosol droplets tend to settle on surfaces faster in disturbed air.
But the NIH study found that the SARS-CoV-2 virus survives for longer on cardboard – up to 24 hours – and up to 2-3 days on plastic and stainless-steel surfaces.
The findings suggest the virus might last this long on door handles, plastic-coated or laminated worktops and other hard surfaces. The researchers did find, however, that copper surfaces tended to kill the virus in about four hours.
But there is a speedier option: research has shown that coronaviruses can be inactivated within a minute by disinfecting surfaces with 62-71% alcohol, or 0.5% hydrogen peroxide bleach or household bleach containing 0.1% sodium hypochlorite. Higher temperatures and humidity also tend to result in other coronaviruses dying quicker, although research has shown that a related coronavirus that causes Sars could be killed by temperatures above 56°C or 132°F (hotter than even a bath scalding enough to cause injury) at a rate of about 10,000 viral particles every 15 minutes.
Although there is no data on how many virus particles will be in a single droplet coughed up by an infected person, research on the flu virus suggests smaller droplets can contain many tens of thousands of copies of the influenza virus. However, this can vary depending on the virus itself, wherein the respiratory tract it is found and at what stage in the infection the person is.
The researchers did find, however, that copper surfaces tended to kill the virus in about four hours
“We speculate due to the porous material, it desiccates rapidly and might be stuck to the fibers,” he says. Changes in temperature and humidity may also affect how long it can survive, and so may explain why it was less stable in suspended droplets in the air, as they are more exposed. “[We’re] currently running follow-up experiments to investigate the effect of temperature and humidity in more detail.”
The ability of the virus to linger for so long only underlines the importance of hand hygiene and cleaning of surfaces, according to Munster.
“There is a potential for this virus to be transmitted via a variety of routes,” he says.
