We're told to wear masks, to keep our distance from others, and when indoors, to opt for spaces that are well ventilated. Now, with the onset of dry, wintry weather and heaters running full blast, add another strategy to reduce the risk of transmission: humidification.
One option is to use a humidifier, but be careful not to overdo it, said Dr. Robert T. Sataloff, chair of the department of otolaryngology at the Drexel University College of Medicine. Too much humidity, along with a failure to keep the units clean, can lead to a different health problem: the growth of mold.
"It's not as easy as going out and buying a humidifier, turning it on and forgetting about it," he said.
Other ways to keep the nasal passages moist include drinking plenty of water, or boiling a pot of water on the stove several times a day and inhaling the steam for a few minutes, he said. (But don't get too close.)
The reason moisture helps the nose filter viruses has to do with what lines its insides: tiny hair cells (called cilia) and mucus. These are not the kind of nose hairs you can see, but their microscopic cousins, which "beat" in a pulse-like fashion, carrying mucus toward the back of the nose to the throat.
It's the first line of defense in the immune system. A human swallows close to a quart of mucus every day, carrying countless viruses to the stomach where they are safely dissolved in acid.
Yet in a dry environment, mucus becomes thicker and the hairs are less able to move. Viruses can sail past this protective lining and penetrate cells deep in the airways.
Scientists have not directly studied how dry air hampers the nose's ability to filter the coronavirus, as it would be unethical to test a deadly virus in humans. But the evidence from studies of other airborne viruses and foreign particles is clear.
In 2019, the lab of Yale University immunologist Akiko Iwasaki captured this phenomenon on video. Scientists filmed the inside of the trachea (windpipe) in two groups of mice: one that had spent a week in air with 50% relative humidity, the other at 10% humidity.
Like the nose, the trachea is lined with mucus and cilia, and the videos revealed that in dry air, this filtration system cleared particle-laden mucus much more slowly, Iwasaki and her colleagues reported in Proceedings of the National Academy of Sciences. The animals at 10% humidity also were less able to repair damaged cells in their airways, and they were more susceptible to the flu.
In an article she co-wrote for the Washington Post, Iwasaki recommended that people try to maintain relative humidity in the 40-60% range.
Sataloff is keenly aware of the risks of dryness.
A trained vocalist and choir director, he and his fellow singers know that poor hydration not only hampers the immune system, but also reduces lubrication for the vocal cords.
"You can spot singers from the bottles of water coming out of their backpacks," he said.
As for how much water to drink, don't worry too much about that oft-repeated advice to consume eight glasses per day, he said. The right amount varies depending on body size, activity level and air temperature, among other factors. Most people can tell if they are getting enough fluids simply by whether they feel thirsty.
In older people, however, the sensation of thirst may be impaired. A better indicator is to watch what happens in the bathroom, Sataloff said.
"Your kidneys are a good computer," he said. "If you're too dry, your urine turns dark yellow or orange. If you're adequately hydrated, it's almost water color. Singers always say 'Sing wet, pee pale.' "
In addition to drying out the nose, cold weather can lead to more cases of COVID-19 in other ways. More activities take place indoors, where there is less fresh air and people cluster together, potentially spreading infection. And evidence suggests the actual virus particles fare better in cooler, drier temperatures.
As with so much else in the pandemic, various factors are at work. No one layer of protection is foolproof. But increasing hydration and humidity can help.
Your nose, Sataloff says, will thank you.