Coronavirus: How dehumidifiers and air conditioning could be helping COVID-19 spread

A common household appliance could be putting you at greater risk of contracting COVID-19 - the dehumidifier. 

And air conditioning at work could also be making it easier for particles of the SARS-CoV-2 virus, which causes the disease, to get around, rendering physical distancing near useless.

Researchers in Germany and India have been looking at the effects of humidity and temperature on the virus' ability to spread. It's become increasingly clear in recent months the virus is airborne, meaning it can linger in the air and find new hosts, even after the infected person has left.

 "For a long time, the main transmission route of viral droplets was considered to be direct human-to-human contact, because of infected people sneezing or coughing and secreting the virus," the team from the Leibniz Institute for Tropospheric Research (TROPOS) in Leipzig and the CSIR National Physical Laboratory in New Delhi said in a statement.

"Because these drops are relatively large and heavy, they fall very quickly to the ground and can only cover very short distances in the air. The recommendation to keep a minimum distance of 1.5m to 2m - social distancing - is based on this assumption."

But they've found a flaw in the plan. When it's humid, droplets containing the virus gather water molecules, gaining weight and eventually falling to the ground. 

"If the relative humidity of indoor air is below 40 percent, the particles emitted by infected people absorb less water, remain lighter, fly further through the room and are more likely to be inhaled by healthy people," said Ajit Ahlawat, study co-author.

"In addition, dry air also makes the mucous membranes in our noses dry and more permeable to viruses."

The team looked at 10 previous studies on coronaviruses, including SARS-CoV-2, SARS-CoV-1 (better known simply as SARS), and the pathogen behind MERS (Middle East Respiratory Syndrome). 

While air conditioning isn't common in New Zealand, almost all homes in the US have it - and with winter coming up, there are fears it might give COVID-19 another boost. 

"Heating the fresh air also ensures that it dries," said Alfred Wiedensohler of TROPOS. "In cold and temperate climate zones, therefore, the indoor climate is usually very dry during the heating season. This could encourage the spread of coronaviruses."

All of this would be moot if people wore masks, the scientists say.

"Keep social distancing, having as few people per room volume as possible, and wearing masks. The lowest risk of infection [is] still where there are no viruses in the air."