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Cold, Flu, & Cough Health Center

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Higher Indoor Humidity Levels Might Slow Flu


The study authors cautioned that it remains to be seen whether humidity adjustments can undermine infection risk as effectively in a real-world setting.

If confirmed, however, the protective impact of humidity levels of 40 percent and above probably would be of the most practical benefit in hospital settings, where the ability to protect medical staff by strictly regulating humidity levels would be most feasible.

"I totally buy this," said Dr. Marc Siegel, a clinical associate professor of medicine at NYU Langone Medical Center in New York City. "It's very hard to prove that lower humidity increases the risk of transmissibility, but it's not surprising because the reasoning makes sense, which is that droplets fall to the ground in high humidity because water travels on dry air, not on water. If you combine airborne viral droplets with water droplets, they fall."

Dr. Philip Tierno a clinical professor of microbiology and pathology at NYU Langone, agreed.

"It's a well-understood phenomenon that moisture prevents the movement of germs, as it does, for that matter, bio-terror weapons like sarin gas and other chemicals," he said. "They combine with moisture and become heavy, and even drop."

"This is not surprising from a scientific standpoint," Tierno added. "And 45 percent relative humidity is not a lot. It's at the 70 percent level where you get discomfort. But 45 [percent humidity] does not make you feel uncomfortable.

"The problem," he said, "is that in the wintertime, when the virus is most likely to spread, it's very difficult to maintain 45 percent humidity because you have to compete with all the dry heat that's being pumped in, which means that typically you're talking about rooms that have a relative humidity of 20 percent, if you're lucky."

"That's why it would be difficult to use this approach in a residential space, unless you seclude the patient to one room and use room humidifiers that are large enough to handle the space in question and keep the doors closed," Tierno added.

More information

For more on influenza, visit the U.S. National Institutes of Health.

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