Hot, Dry Conditions May Decrease Survival Time of SARS-CoV-2 on Surfaces

By Linda Carroll

June 10, 2020

(Reuters Health) - Temperature and humidity may play a role in how long the SARS-CoV-2 virus survives on surfaces, a new study suggests.

Virus packed droplets produced when an infected person coughs or sneezes tend to dry faster under hotter, dryer conditions, researchers found using a mathematical model. The importance of this is that once a droplet dries, the virus particles contained in it would desiccate and die, they explained in an article published in Physics of Fluids.

"The take home message is that a correlation between the outdoor weather and growth rate of COVID-19 seems to exist," said Rajneesh Bhardwaj, an associate professor at the Indian Institute of Technology Bombay, in Mumbai. "The outdoor weather is important since it determines the duration of drying of respiratory droplets deposited on surfaces. The drying time is linked to the survival of the coronavirus inside the droplets."

The model may explain some of the variation in infection rates seen across the globe, Bhardwaj said in an email.

"This could explain a slow or fast growth of the infection in a particular city," Bhardwaj said. "This may not be the sole factor but definitely the outdoor weather matters in the growth rate of the inflection and our study provides some evidence for this fact."

Lower temperatures coupled with higher humidity would allow the virus to last longer on surfaces, Bhardwaj and his coauthor, Amit Agrawal, a professor of mechanical engineering at IIT Bombay, noted.

Using a mathematical model Bhardwaj and Agrawal examined the impact of surface materials and weather conditions on drying time of respiratory droplets. The researchers specifically looked at how long it would take for droplets to dry out in six cities around the world: New York, Chicago, Los Angeles, Miami, Sydney and Singapore. They plotted the growth rates of COVID-19 infections in these cities against the estimated drying time for a droplet and found that cities with the largest infection growth rate had the longest drying times.

The new findings can be used by government authorities to plan for the future, Bhardwaj said.

"Thus, in the coming weeks/months, if outdoor weather would be more conducive for the survival of the coronavirus, this scientific evidence/data can be used for implementing policies for avoiding the spread by policy makers," he explained. "This is especially important for cities where the peak of the infections has not been reached, say in Mumbai, India and Sao Paulo, Brazil, and cities where a second wave of infections could occur. For example, Singapore data shows that there was a surge of daily infections in late May (second wave), perhaps due to the advent of Southwest Monsoon."

The researchers also found that certain surfaces fostered longer survival times.

"Our study suggests that surfaces such as smartphone screens and wood need to be cleaned more often than glass and steel surfaces, because droplets form blob-like shapes on the former surfaces and the droplets evaporate slowly on such surfaces, thereby increasing the survival of the coronavirus," Bhardwaj said.

A model like the one described by Bhardwaj and Agrawal could help with plans to combat the virus, said Jared Evans, a molecular virologist who specializes in emerging infectious diseases at the Applied Physics Laboratory at Johns Hopkins University.

"The authors present a thoughtfully created algorithm to predict drying times for virus-containing droplets," Evans said in an email. "They reference a number of previous studies to define the parameters and assumptions. From this approach, they developed a tunable simulation that can be very useful."

"A model to identify the variance in drying time is an important tool that could be used to inform infection prevention and public health experts for surveillance efforts, Evans said. "It should be noted that most models and simulations require empirical testing for validation and acquiring data or proof that specific actions should be taken. This simulation can provide guidance to that testing."

SOURCE: Physics of Fluids, online June 8, 2020.