Low humidity, drier air may contribute to increased COVID-19 transmission risk, researchers find

A study published by researchers from the University of Sydney found that dry air and the prevalence of low humidity contributed to an increased risk of transmission of the novel coronavirus.

The study, published on Aug. 4 in Transboundary and Emerging Diseases, was led by Professor Michael Ward, an epidemiologist in the Sydney School of Veterinary Science at the University of Sydney.

Ward’s team of researchers analyzed daily coronavirus cases reported by health officials in New South Wales. They compiled daily reported case data from February to May and matched that information with recorded weather data. 

“It is important to highlight that SARS-CoV-2 cases used in this study occurred predominantly during the autumn season in southern hemisphere. In contrast, most SARS-CoV-2 cases in northern hemisphere have been reported during the winter and spring seasons,” the researchers noted. 

“When the humidity is lower, the air is drier and it makes the aerosols smaller,” Ward said, adding that aerosols are smaller than droplets. “When you sneeze and cough those smaller infectious aerosols can stay suspended in the air for longer. That increases the exposure for other people. When the air is humid and the aerosols are larger and heavier, they fall and hit surfaces quicker.”

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The study’s authors estimated that for every 1 percent decrease in relative humidity, COVID-19 cases could increase by 7 to 8 percent.

"Dry air appears to favour the spread of COVID-19, meaning time and place become important," Ward said. "Accumulating evidence shows that climate is a factor in COVID-19 spread, raising the prospect of seasonal disease outbreaks."

A separate June 8 study by the U.S. Centers for Disease Control and Prevention (CDC) found that the coronavirus was more stable in low-temperature and low humidity conditions.

The CDC researchers added that “warmer temperature and higher humidity shortened half-life. Although infectious virus was undetectable after 48 hours, viral RNA remained detectable for seven days.”

The Australian study is the latest in research aimed at determining how climate impacts a respiratory virus like COVID-19. 

The new virus is genetically related to SARS and MERS. Severe Acute Respiratory Syndrome first broke out in China in late 2002 and ultimately sickened about 8,000 people worldwide before it was declared contained in July 2003.

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But the arrival of summer wasn’t what stopped SARS. Extraordinary measures that included shutting down travel from epicenters in Asia and Canada and a mass culling of palm civets that spread the disease to humans were largely credited for curbing the disease.

Although the transmission of Middle Eastern Respiratory Syndrome has never been entirely interrupted, its spread to humans from camels is mostly sporadic, sparking limited outbreaks since being identified in 2012.

“I don’t think there’s anything we can say about seasonality and the coronavirus based on what we’ve seen with SARS and MERS,” said Michael Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota. “I’ve been in the Arabian peninsula when MERS is spreading in 110-degree heat just fine,” he said.

The Associated Press contributed to this story. This story was reported from Los Angeles.