Dry air a breeding ground for Covid-19 virus, researchers find

• Humidity seems to play a key role in the spread of the new coronavirus
• A second study, concentrated on the Greater Sydney area, found that drier air was linked to a higher number of cases
• This evidence suggests that wearing a face mask is more important than ever

The relationship between weather conditions and SARS-CoV-2, the virus that causes Covid-19, has been studied since the early stages of the pandemic.

The first study, published in Rapid Communication, focused on the Greater Sydney area and found a link between lower humidity, or "drier air", and an increase in community transmission of the virus.

Now, a second study, published in Transboundary and Emerging Diseases this week, and carried out by the same team, confirms this risk. 

Decrease in humidity leads to increase in transmission

The research was led by Professor Michael Ward, an epidemiologist in the Sydney School of Veterinary Science at the University of Sydney, and two researchers from their partner institution Fudan University School of Public Health in Shanghai, China.

The published, peer-reviewed study is the second to investigate weather conditions and transmission of the virus in Australia, and found reduced humidity in several different regions of Sydney to be consistently linked to increased case numbers. However, the same link was not found for other weather factors, including rainfall, temperature and wind.

"This second study adds to a growing body of evidence that humidity is a key factor in the spread of [the] Covid-19 [virus]," Ward said in a news release.

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

The team estimated that as a result of a 1% decrease in relative humidity, SARS-CoV-2 cases might increase by 7–8%.

Lower humidity, higher transmission explained 

According to Ward, biological factors are responsible for the transmission of airborne viruses.

"When the humidity is lower, the air is drier and it makes the aerosols smaller," he said.

The virus has been confirmed to spread via respiratory droplets, while aerosol transmission is still debated by scientists. This Health24 article explains that droplets are bigger than aerosols and travel relatively short distances, whereas aerosols can accumulate in poorly ventilated areas and are carried by air currents.

"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,” Ward explained.

Don’t let go of that face mask

Wearing a face mask is one of the non-pharmaceutical interventions (NPIs) that have been implemented by countries globally in order to lower the spread of infection. In the case of the latest evidence, this is yet another reason in favour of this NPI.

"This suggests the need for people to wear a mask, both to prevent infectious aerosols escaping into the air in the case of an infectious individual, and exposure to infectious aerosols in the case of an uninfected individual," Ward said.

The authors are encouraging further studies on humidity for the remainder of the year, as these are required to determine how the humidity relationship works and the extent to which it drives infection rates.