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Humidity key to minimize COVID-19 transmission

Ian P. Johnson
August 20, 2020

Dry rooms and air-conditioned indoor spaces hike Covid-viral infection, conclude Indian and German researchers in their meta-study. They're urging optimum humidity standards for building interiors and public transport.

A humidifier in a room
Image: picture-alliance/dpa/R. Guenther

Relative humidity "strongly influences" the spread of viruses among people indoors, especially in dry rooms. That's the conclusion reached by an Indian-German research team which evaluated 10 mostly recent international studies. 

"The role of humidity seems to be extremely important to the airborne spread of COVID-19 in indoor environments," according to the report, which was also based on findings derived from past tests with similar viruses, H1N1 for influenza and MERS-CoV.

Public buildings should have at least 40% humidity indoors, capped at 60%, to reduce viral-spread risks for occupants, said analysts led by Sumit Kumar Mishra  of India's CSIR National Physical Laboratory and Alfred Wiedensohler and Ajit Ahlawat of Germany's Leibniz Institute for Tropospheric Research (TROPOS).

Read more: Dangers of COVID-19 aerosols are underestimated

How humidity affects transmission

Their report says humidity affects viral spread in three ways: droplet size, how viral-loaded aerosols float for "hours," and stay viral on landing surfaces.

In humid places, the viral droplet — a solution of salts, water, organics and attached viruses — grows and it falls faster, "providing less chances for other people to breathe in infectious viral droplets."

But in dry indoor air, micro-droplets shrunk by evaporation become lighter and stay adrift — an "optimal route" for viruses to be "inhaled by other residents, or finally settle on surfaces where they can survive for many days," warns the report.

Maintaining relative room humidity at between 40% and 60%, "like opening of windows," they say, can also reduce absorption of viruses though a person's nasal passages.

"Dry air also makes the mucous membranes in our noses dry and more permeable to viruses, said Ahlawat.

Read more: Coronavirus pandemic: Is the second wave already here? 

Threat ahead

The Northern Hemisphere's approaching winter meant higher risks for "millions of people" in heated rooms, warned Wiedensohler because colder outdoor air was typically sucked indoors via air-conditioning systems.

Heating that inflowing air to a comfortable temperature would "significantly lower the indoor RH (relative humidity), which creates an extremely dangerous situation for indoor residents, particularly during the COVID-19 pandemic," says the paper.

Citing Singapore and Malaysian studies, they also warned tropical residents to avoid "extreme cooling" systems because resulting dried indoor airflows would also "promote more COVID viability."

Read more: Coronavirus: How well do face masks protect against viruses, droplets and dust?

In conclusion, the team said building supervisors and governments would play an "extremely important role" in updating standards.

"Authorities should include the humidity factor in future indoor guidelines," said CSIR's Mishra.

"Based on research findings, for future scenarios, setting a minimum RH standard in public buildings" said the report, "will not only reduce the impact of COVID-19, but it will also reduce the impact of further viral outbreaks."

For the moment, aside from wearing masks, the team urged: "Keep social distancing, having as few people per room volume as possible."