About six months into quarantine and there is another reason to wear your mask. A recent study published in Transboundary and Emerging Diseases confirms that drier air is associated with an increase in COVID-19 cases.

A 1% Drop in Humidity Linked to a 7-8% Increase in COVID-19 Cases

The study, entitled “Humidity is a Consistent Climatic Factor Contributing to SARS‐CoV‐2 Transmission,” is the second to confirm the impact of humidity on the spread of COVID-19.

Professor Michael Ward, an epidemiologist in the Sydney School of Veterinary Science at the University of Sydney, and researchers Shuang Xiao and Zhijie Zhang from the Fudan University School of Public Health in Shanghai, China, cite a growing body of evidence pointing to the role of humidity and COVID-19 transmission.

Their study assessed the link between climatic factors and COVID-19 during the southern hemisphere summer and autumn periods in New South Wales, Australia, when the pandemic grew exponentially and declined.

The authors connected relative humidity to COVID-19 transmission. Specifically, a 1% drop in relative humidity was associated with a 7-8% increase in COVID-19 cases. The study found no connection between cases and temperatures, wind speed, and rainfall.

Zip Codes and Weather Leave Clues for Researchers

To determine the connection between climate and COVID-19, the researchers accessed case reports in New South Wales, Australia, between January and May 2020.

They used locally acquired cases that included a residence postcode. After creating a daily time series of cases, the researchers used the closest weather observation station to reported postcodes to make daily observations about the weather; average values for each day were estimated and used to create a time series for the weather.

Comparing locally acquired cases with local weather allowed researchers to determine the effect of various climatic factors on COVID-19, including humidity.

Why Humidity Matters

Low humidity implies dry air. Dry air makes the infectious aerosols we exhale into the air smaller, helping them to remain suspended for an extended period, increasing the risk of exposure.

In contrast, humid air makes the infectious aerosols larger, and when they're larger, they fall to the ground, or other surfaces, more quickly.

Previous research has confirmed that the primary mode of COVID-19 transmission is person-to-person contact (i.e. as standing near an infected person), not touching surfaces contaminated with COVID-19

The Importance of Wearing a Mask and a Winter Warning

While mask-wearing has almost become routine, it's easy to grow complacent. However, this research makes it clear that we need to continue to wear masks.

People infected with COVID-19 need to wear masks to prevent their infectious aerosols from escaping into the air and infecting others. Those who are not infected need to wear masks to avoid exposure to infectious aerosols in the atmosphere.

Furthermore, winter air is dry air, and now that we know low humidity is linked to an increase in COVID-19 transmission, there's more reason to be concerned. As fall comes to an end, people need to remember that the pandemic is far from over, and masks need to be worn, especially as we start to spend more time together indoors — dry winter air means COVID-19 will have strong potential to spread more easily.