The first study, by the Leibniz Institute for Tropospheric Research in Leipzig and the National Physical Laboratory in New Delhi, looked at airborne transmission of the virus in indoor environments, and the role played by humidity. And it found that in places where relative humidity was low (less than 40%) the virus transmitted more readily.
At the same time, in environments where humidity was over 90%, there was less transmission of the virus. However, the optimum level of indoor humidity recommended for human health lies between 40% and 60%.
The reasoning is that in a dry environment, the tiny droplets of an aerosol, such as produced by normal speaking, remain separate from each other and partially evaporate, allowing them to become smaller and remain in the air longer and travel further, in some cases further than the recommended social distance of 1.5m or six feet.
On the other hand, in a humid environment, aerosol particles tend to join together, become heavier and thus more likely to drop out of the air.
The study points out that most government have standards covering air quality in buildings relating to temperature and pollutants, but not generally humidity.
“Based on research findings, for future scenarios, setting a minimum [relative humidity] standard of 40% for public buildings will not only reduce the impact of COVID-19, but it will also reduce the impact of further viral outbreaks, both seasonal and novel,” the authors conclude.
At the same time, a study by the University of Sydney and Fudan University in Shanghai, published this week, has confirmed the conclusions of a previous study which suggested that lower humidity is associated with a higher rate of community transmission.
“This second study adds to a growing body of evidence that humidity is a key factor in the spread of COVID-19,” Professor Michael Ward leading the study said.
Concretely, a reduction of 10% in humidity correlates to a 100% increase in the number of infections. At the same time, a cut of 1% leads to an increase of 7-8% in confirmed cases.
“Dry air appears to favour the spread of COVID-19, meaning time and place become important,” he said. “Accumulating evidence shows that climate is a factor in COVID-19 spread, raising the prospect of seasonal disease outbreaks.”
For the northern hemisphere, winter generally means colder and more humid air outside, while inside the air is warmer and drier because of central heating – and inside is where we spend most of our time.