During winter, as the air gets colder, it also becomes more dry. But turning on the heating indoors also reduces the amount of moisture in the air – as well as inside our sinuses. As mucus gets drier, it does not trap infections in our airways as effectively.
“We’ve found that low humidity, which you can find indoors in winter, has a very negative impact on the ability of the respiratory tract to remove inhaled particles,” Akiko Iwasaki, professor of immunobiology at Yale University and a vocal supporter of global guidelines, told the Telegraph.
In laboratory trials, Prof Iwasaki’s team found that mice infected with influenza and housed at a 10 per cent relative humidity had more severe symptoms and were more likely to die than those at 50 per cent.
“So in dry air, host resistance to viruses is weaker,” Prof Iwasaki added. “But the stability of the virus is also increased when relative humidity is below 40 per cent, and third the particles that carry the virus also remain in the air for longer.
“All of these things combined mean low humidity is the perfect storm for successful coronavirus transmission,” she said.
In a preprint published last month, researchers in America found that Sars-Cov-2 survives best at low temperatures and low relative humidities, but even when the temperature is warmer a lack of moisture in the air facilitates Covid transmission.
Prof Iwasaki said that opening a window to allow air to circulate, or using a humidifier or air purifier could help prevent dry air. This is particularly important in our homes, offices, hospitals and care homes, she added.
But outdoors, face masks can help our noises remain moist.