Heat and relative humidity affect how coronavirus spreads, new study finds

The weather may play some role in the behavior of SARS-CoV-2, the virus that causes COVID-19, but, according to the new findings of two physics professors in Cyprus, perhaps not in the way that many in the scientific community have believed up until this point.

The weather plays some role in the behavior of SARS-CoV-2, the virus that causes COVID-19 -- that much is certain, according to the new findings of two physics professors in Cyprus. However, what if some previously held beliefs about how weather conditions affect the spread of the virus are based on faulty logic?

In a recent study published last month by the American Institute of Physics, researchers from the University of Nicosia in Cyprus looked at the roles that temperature, wind and humidity play in influencing the transmission and survival of respiratory droplets containing the coronavirus. However, their findings may say more about what we don't yet know rather than what we can better understand.

"Our research was motivated by the fact that we know very little about the combined effects of environmental factors on COVID-19," Prof. Dimitris Drikakis, one of the study's authors, told AccuWeather in an email.

While previous studies have tried to help better frame the coronavirus with our base of knowledge regarding viruses such as the influenza, Drikakis and co-author, Dr. Talib Dbouk, recognized that the shape structure of a coronavirus capsid, the protein shell surrounding the virus, influenced the virus's behavior in the air uniquely.

Previously, experts have argued that high rates of humidity would likely decrease the transmission of the virus because the respiratory droplets would be heavier, thus helping them fall to the ground quicker rather than stay in the air.

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A waiter wearing a face mask and behind a plastic curtain to prevent the spread of coronavirus serves a coffee at a bar in the southern neighborhood of Vallecas in Madrid, Spain, Tuesday, Sept. 22, 2020. (AP Photo/Bernat Armangue)

However, the results of this study suggest otherwise.

"The results of the study revealed, to our surprise too, that the contaminated droplet cloud's traveled distance and concentration remain significant at any temperature if the relative humidity is high," Drikakis told AccuWeather. "We discovered that the viability of coronavirus depends on the combination of the air temperature, the wind speed, and the relative humidity."

Experts have previously speculated that certain environmental factors could arrive to assist the vulnerable public. From high temperatures to strong sunlight to low humidity, numerous weather factors gave the world hope for when seasonal changes may aid in stamping out the virus, which has killed more than 1 million worldwide.

Transmission electron micrograph of SARS-CoV-2 virus particles, isolated from a patient. Image captured and color-enhanced at the NIAID Integrated Research Facility (IRF) in Fort Detrick, Maryland. (National Institute of Allergy and Infectious Diseases, NIH)

For those banking on weather conditions to do that work, the researchers' findings may suggest that, at the very least, far more needs to be understood before any one factor can be credited with curbing the spread of the coronavirus.

"The results reveal that a significant reduction of virus viability occurs when both high temperature and low relative humidity occur," the authors wrote in the paper's abstract. "The droplet cloud's traveled distance and concentration remain significant at any temperature if the relative humidity is high, which is in contradiction with what was previously believed by many epidemiologists."

Migrant workers wait to get their COVID-19 tests done as they return to look for jobs in New Delhi, India, Tuesday, Aug. 18, 2020. (AP Photo/Manish Swarup)

That reversal on the understanding of humidity's role in the spread of the virus may help explain why infection rates continued and, in some places, escalated, through the warm, humid months of summer in the Northern Hemisphere.

"The droplet cloud's traveled distance and concentration remain significant at any temperature if the relative humidity is high, which is in contradiction with what was previously believed by many people," the authors said, adding that humidity levels could explain the increase of cases in crowded cities like New Delhi, India, where both high temperature and high relative humidity values were recorded. As of this writing, New Delhi has recorded more than 282,000 COVID-19 cases and more than 5,400 fatalities.

It could also greatly change the expectations that some countries have for the coming winter season.

"Moreover, it creates a crucial alert for the possibility of a second wave of the pandemic in the coming autumn and winter seasons where low temperatures and high wind speeds will increase airborne virus survival and transmission," Drikakis cautioned.

A Palestinian girl wears a face mask during a lockdown imposed following the discovery of coronavirus cases in the Gaza Strip, Thursday, Aug. 27, 2020. (AP Photo/Hatem Moussa)

For indoor spaces, a controlled microclimate will keep temperatures between 73 F and 75 F (23 and 25 C) with a relative humidity of around 65%, Drikakis said. In summer months, that relative humidity can decrease below 50% in some places, he said.

"Our research shows that the viability of coronavirus is higher at low air temperature values below or equal to 75 F (25°C) and at high relative humidity values greater than or equal to 65%," Drikakis told AccuWeather. "Therefore, in countries with the above weather conditions or indoor places with the above environmental conditions (at those conditions) are more at risk."

Moreover, the researchers believe, the weather conditions inside crowded indoor spaces could be crucial to the spread of the virus. According to the author, high relative humidity should be avoided indoors whenever possible. Using ventilation systems can help reduce humidity, Drikakis said.

A sign indicating mask use is mandatory is seen during an NFL football game between the Kansas City Chiefs and the Baltimore Ravens, Monday, Sept. 28, 2020, in Baltimore. (AP Photo/Nick Wass)

Penn State University professor Dr. William Bahnfleth previously told AccuWeather that proper ventilation also helps indoor spaces reduce the concentration levels of any contagions present in the air, thus lowering the risk of infections.

"We should also avoid places of low temperature. If this is not possible, then strictly respect the social distance, at least 2 meters if no air circulation is present," Drikakis said. "And in case of air or wind speeds, this social distance should be up to 6 meters depending on the wind speed," he added.

Outside, however, may be a different subject

Regarding wind speed, the authors previously published another piece, titled "On coughing and airborne droplet transmission to humans" that sought to better understand the mechanisms of airborne transmission of viruses.

Foreigner tourists wearing face masks as a precaution against coronavirus as they walk along a beach in Bali, Indonesia, on Wednesday, Sept. 30, 2020. (AP Photo/Firdia Lisnawati)

In that study, the pair found that saliva droplets can travel nearly 20 feet (6 meters) at wind speeds varying from 2.5 mph to just over 9 mph (4 km/h to 15 km/h) with a decrease in the concentration and liquid droplet size in the wind direction.

Drikakis and Dbouk said they hope their findings better inform the public about not just why social distancing is necessary, but what factors influence the effectiveness of social distancing.

"Our findings reinforce the importance of social distancing and the use of face masks to prevent full virus spread. The results reveal the extent of weather conditions in the virus's viability," the professors said. "They could guide the design of measures in both indoor and outdoor environments to reduce airborne virus transmission in private and public places."

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