Researchers find how temperature, humidity and properties of different surfaces influence the evaporation rates of respiratory droplets infected with COVID-19.
Ever since the COVID-19 pandemic struck the world, there is a frenzy of guidelines advocating us to go ‘contactless’. We are urged to use masks, maintain distance from others and avoid touching anything while stepping out. This respiratory illness, caused by a kind of coronavirus, is thought to spread through respiratory droplets of infected people, which are ejected when they sneeze, cough or talk. These droplets can land on surfaces and pose a risk to others, who may touch it and get the infection. Hence, wiping and disinfecting frequently-touched surfaces, like door handles and elevator buttons, and washing hands often, are suggested to keep the disease at bay.
In a new study, researchers at the Indian Institute of Technology Bombay (IIT Bombay) have explored how long it takes for such respiratory droplets to evaporate from different surfaces. They found that humidity, temperature and the properties of the surface are vital in determining when the droplets dry up. The study was published in the peer-reviewed journal Physics of Fluids.
Like many respiratory illnesses, COVID-19 spreads through respiratory droplets, whose size is around twice the thickness of human hair.
“The survival of the virus inside the droplet depends on how fast it dries,” says Prof Rajneesh Bhardwaj, who led the study. He is a professor at the Department of Mechanical Engineering, IIT Bombay. Studies in the past have shown that coronaviruses need a medium, like a saliva droplet, to survive. “Once there is no medium, because of the evaporation of the droplets, the chances of the survival of the virus are very less,” he adds.
The researchers built a mathematical model, validated by previous experiments, to estimate the time it takes for respiratory droplets to evaporate. This model considered the ambient temperature, type of surface, size of the droplet and the relative humidity in its calculations. On surfaces that repel water, like the touch screen of our phones, they found that the evaporation time is slower by 60% when compared to surfaces like glass or steel. On water-repelling surfaces, the droplets don’t spread out flatly and hence take a longer time to evaporate. Besides, the size of the droplet also influences the time it takes to dry up.
“Our study suggests that surfaces such as smartphone screens and wood need to be cleaned more often than glass and steel surfaces,” says Prof Amit Agrawal from IIT Bombay, who was also involved in the study.
He also suggests sun-drying surfaces to destroy the virus, in places with lower humidity, as pointed out by other studies.
The current study also found that temperature and humidity affect the evaporation time, which is reduced by half for every 15oC rise in temperature. When the researchers changed the relative humidity from 10% to 90%, the drying time increased almost sevenfold.
“A higher ambient temperature helps to dry out the droplet faster and drastically reduces the chances of the survival of the virus. With higher humidity, the droplet stays on the surface longer and hence the virus has a greater chance to survive,” explains Prof Agrawal.
The researchers further explored the relationship between the droplet drying time and the growth rate of COVID-19 infections in five cities, which have varying humidity and temperature values. They selected New York, Chicago, Los Angeles, Miami, Sydney and Singapore, and found that the cities with a more significant growth rate of the pandemic, like New York, had a higher drying time. Singapore, on the other hand, which had the highest ambient temperature, despite the high humidity, has the lowest number of infections.
Prof Bhardwaj points out a similar trend in India, where higher temperatures in Delhi may have slowed down the infection rate, as compared to Mumbai, where the humidity is high. While he acknowledges that the handling of the pandemic by the governments may have played a role, “ambient weather is an important factor to be considered,” he says.
With the arrival of the southwest monsoon in India, these findings have implications in managing COVID-19 in the country.
“There is a likely scenario that humidity may help the virus survive in droplets for a longer time,” cautions Dr Bhardwaj.
He urges authorities to enforce a stringent rule to use masks in areas with higher humidity. The proposed model of estimating the survival of a virus in a drying droplet, the researchers say, can also aid in understanding other diseases that are transmitted through respiratory droplets, like influenza A.
This article has been run past the researchers, whose work is covered, to ensure accuracy.