Respiratory viruses love the winter. These pathogens thrive in the cold and travel easier from host to host in dry air.

"When cold outdoor air with little moisture is heated indoors, the air's relative humidity drops to about 20 percent," Akiko Iwasaki, an immunobiologist at Yale University, said in a statement.

"This dry air provides a clear pathway for airborne viruses."

Relative humidity (RH) is a measure of how saturated the air is with water vapour. So in a room with 40 percent relative humidity, the air holds 40 percent of the total amount of moisture it could hold in total.

The drier the air, the lower the relative humidity, and the easier it is for viruses – including the coronavirus – to spread.

That's why Linsey Marr, an aerosol researcher from Virginia Tech University who studies coronavirus transmission, recommends using a humidifier in your home.

"You could invest in a humidifier and set it to keep the humidity above 40 percent but below 60 percent in the wintertime," she told Business Insider. "The virus doesn't survive as well under these conditions, and your immune response works better than when the air is dry."

Humidity and temperature affect how the coronavirus spreads
Research shows that the coronavirus spreads more easily when temperatures and humidity are low.

A July analysis by aerosol research scientist Ajit Ahlawat and his colleagues found that the chances of airborne transmission of the coronavirus in dry places are higher than in humid areas.

That's because coronavirus particles in drier, less humid air absorb less moisture and therefore remain aloft longer. That makes them more likely to be inhaled and infect someone new.

Plus, coronavirus particles become more stable as temperatures and humidity levels decrease, helping them remain stable enough to infect a new host when they arrive.

What's more, like the flu, the coronavirus is ensconced in a fatty layer called a lipid envelope that helps it survive the journey from one person to the next. This sheath dries out more quickly in higher temperatures.

Wetter air can also work against the protective layer by wreaking havoc on the structure of the lipid envelope, inactivating the virus.

"To control the coronavirus airborne transmission indoors, especially in poorly ventilated indoor places like certain hospitals, schools, and public buildings, we recommend the use of humidifiers," Ahlawat told Business Insider.

Like Marr, he recommends an indoor RH between 40 percent and 60 percent.

Higher temperatures can also hinder the virus' spread via surfaces, though that type of transmission is rare.

A study published in June revealed that warmer weather conditions can truncate how long the coronavirus survives on surfaces.

40 to 60 percent relative humidity benefits our immune systems, too
This fall, Iwasaki helped launch a petition calling on the World Health Organisation to set guidelines for indoor humidity levels. It calls 40-60 percent RH "a sweet spot," since indoor air in that range "allows our nose and throat to maintain robust immune responses" against many viruses.

Our immune systems' built-in protections – such as the mucus in our noses – work better when the air is wetter.

That's because mucus coats flexible hair-like appendages called cilia that jut out from cells in our airways (picture them like swaying seaweed underwater). Cilia are tasked with catching viral particles that try to float into our lungs.

According to a recent study by Iwasaki and her colleagues, low humidity dries out that mucus; as the lubricating mucus dries, those protective cilia fall flat, hampering their ability to snag viruses.

But experts warn against too much humidity
It's critical not to overdo the humidity, however.

"Be very careful to avoid getting above 65 percent, because that can promote mould growth," Marr said. The resulting mould can trigger asthma, and many people are allergic to mould spores.

Ahlawat also said a humidity level over 60 percent "would be too much uncomfortable for the indoor residents."

Numbers aside, some experts are against using humidifiers at all as a means of reducing viral transmission.

"This is an unproven approach and has potential for very bad side effects," Donald Milton, a professor of environmental health at the University of Maryland, told Elemental in November. "I don't recommend it."

Marr also cautioned that using a humidifier should not be seen as a panacea for stopping the virus' spread.

"The most important things to do are to wear a mask, maintain distance, ensure good ventilation and/or filtration of the air, and wash your hands," she said.

Business Insider

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