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Hot, wet weather raises risk of airborne coronavirus transmission, shows study

September 24, 2020 06:05 AM

COURTESY HT SEPT 24

Hot, wet weather raises risk of airborne coronavirus transmission, shows study


Sanchita Sharma

letters@hindustantimes.com

NEW DELHI : Rapid evaporation of saliva droplets contaminated by the Sars-CoV-2 virus in hot, dry weather lowers the spread of the coronavirus disease (Covid-19), but high humidity can boost it, which may explain why cases surged during the monsoon in Delhi and Mumbai, and many other crowded cities around the world, according to new research that reinforces the importance of social distancing and the use of face masks to stop the pandemic from spreading.

Quick evaporation emerged as a critical factor in the transmission of the infectious particles, which are immersed in respiratory clouds of saliva drops, found the study, which analysed the effects of relative humidity, temperature, and wind speed on the survival of Sars-CoV-2, the virus responsible for the disease.

“High temperature and low relative humidity lead to high evaporation rates of saliva-contaminated droplets, thus significantly reducing the virus viability,” according to the study on weather effects that fuel the spread of Covid-19. The study was published in the journal Physics of Fluids of the American Institute of Physics.

It found that travel distance and concentration of the droplet cloud were significant even at high temperatures if the relative humidity is high. Wind speed is another critical factor that can negate the benefits of social distancing.

“The study examines three factors that affect transport of the virus, temperature, relative humidity and wind speed, of which relative humidity is the most significant. It also explains why infections rose in India during the monsoon and why cases are low in the Middle East and Australia, which experienced its summer during the northern hemisphere winter,” said Dr NK Mehra, former dean, All India Institute of Medical Sciences, New Delhi.

The new report reinforces the findings of past studies, including one from Princeton University published in the journalSciencein May, that said without effective control measures, strong outbreaks are likely in more humid climates and high summer temperature will not substantially limit the spread of the pandemic.

“Our work focuses on the potential risk of being infected — rather than disease transmission itself — from exposure to an airborne cloud of contaminated saliva droplets... The results reveal the importance of the weather conditions in the virus’s viability,” write the researchers who conducted the new study.

Because research on heat and mass transfer around and within respiratory droplets containing the virus is scarce, the researchers developed theoretical correlations for the unsteady evaporation of coronavirus-contaminated saliva droplets.

They implemented the theory in an advanced computational fluid dynamics platform and studied the effects of weather conditions on airborne virus transmission.

The study adds to the understanding of airborne droplet transmission but does not link the evaporation rate change to disease transmission as the viral load required for someone to be infected depends on several factors such as age, gender, underlying medical conditions, genetic factors and cross-infections, among others.

“It highlights the importance of weather conditions in the virus’s viability, but what we need to factor in India is pollution levels, particularly suspended particulate matter, which can transport the virus. In India, air quality index, along with rising wind speed, are the other important factors that will affect droplet transmission over the winter months,” said Dr Mehra.

Air pollution increases the risk of viral transmission. A Harvard University study on air pollution and Covid-19 deaths in the United States found that an increase of only 1 μg/m3 in PM2.5 levels is associated with an 8% increase in the Covd-19 death rate, which underlined the need for continued enforcement of existing air pollution laws. PM2.5, or fine particulate matter, is an air pollutant that is a concern for people’s health when its levels in the air are high.

The new study provides a warning on the possibility of a second wave in the northern hemisphere autumn and winter, when temperatures fall and wind speeds rise.

Public health experts said it will help guide the design of measures to reduce airborne virus transmission in both indoor and outdoor environments. “Such studies reinforce the need for wearing masks and maintaining social distancing in both indoor and outdoor environments to reduce airborne virus transmission, whether you’re at home or in a public place,” said a health ministry official, requesting anonymity

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