Hot, dry air increases the risk of Covid-19 contagion, compared to humid air. Since the virus spreads in droplets when infected people cough and sneeze, in dry air, droplets contract and turn into aerosol, which can penetrate deeper into the respiratory tract. This is what two studies conducted in Sydney indicate, where the epidemic started in February, in the height of summer, and began to grow at an exponential rate. As the air became wetter, the trend of the epidemic declined, while taking into account the effect of lockdown periods.
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The two studies
The studies were coordinated by Professor Michael Ward, epidemiologist at the Sydney School of Veterinary Science at the University of Sydney, in collaboration with Fudan University in Shanghai and published in the journal Transboundary and Emerging Diseases. “These two studies add to a growing body of evidence that humidity is a key factor in the spread of COVID-19,” writes Ward. “This confirms the need to wear a mask, to prevent both infectious aerosols from leaking into the air by infectious individuals, and exposure to infectious aerosols from non-infected individuals,” he adds.
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With humid air, heavier aerosols fall to the ground
Studies have calculated that for a 1% decrease in relative humidity, Covid-19 cases could increase by 7-8%. A similar trend was found in the parallel study on the outbreak of Covid-19 in China and also during the related coronavirus outbreaks, which caused Sars and MERS. The estimate is about 2 times the increase in infections for a 10% decrease in relative humidity. “The growing body of evidence confirms that climate is a factor in the spread of Covid-19, raising the prospect of seasonal disease outbreaks,” adds Ward, who identifies biological reasons why humidity is important in the transmission of airborne viruses. “When the humidity is lower, the air is drier and makes aerosols smaller than droplets, ”he writes. “When the air is humid the aerosols are bigger and heavier, and they fall on surfaces faster».
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18 August 2020 (change August 18, 2020 | 10:51)
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