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Higher humidity may thwart flu virus, study shows

If all the coughing and sneezing associated with this year’s severe flu season have you worried you’ll catch the bug, consider boosting the humidity in the rooms around you.

Moderate to high humidity can actually reduce the ability of the flu virus to infect folks, according to a new study published in the journal PLOS One. Maintaining relative indoor humidity at 43 percent or higher could cut the bug’s threat to about 15 percent, researchers found.

The flu is often spread through the air, said John D. Noti, team leader of the infectious disease transmission program at the National Institute for Occupational Safety and Health. When we cough or sneeze, bits of virus hitch a ride on droplets of mucous or water, explained Noti, the report’s lead author.

While the large droplets hit the floor and other surfaces fairly quickly, the smallest ones can remain in the air for hours -- just waiting for you to breathe them in.

“Typically they stay in the air for an hour or so, but we have caught them as long as five hours out,” Noti says. “If you’re a health care worker in a room full of coughing patients there could be a lot of them.”

Earlier studies had suggested that the flu virus might be sensitive to humidity. So Noti and his colleagues set up an ingenious experiment, one that simulated a real-life situation, but exposed no humans to the nasty virus.

The researchers put two manikins about six feet apart in a closed-off room. One of the manikins was designed to “cough” flu particles into the air, while the one on the other side of the room “breathed” them in.

Noti and his colleagues collected samples from the inhaling manikin every few minutes and then deposited them in a cell culture to see if the virus particles would be able to infect the tissue cells.

The researchers ran their experiment at varying levels of humidity.

They found that when the humidity was less than 23 percent, the virus retained between about 71 percent of its ability to infect. When the humidity was boosted to 43 percent or higher, the infectivity dropped to about 15 percent.

And that drop-off in infectivity happened fast -- within 15 minutes of the initial cough.

The experiment may explain why we see so much more flu in the fall and winter months, Noti says. “In the winter months when the heat is on, the air is really dry,” he says. “It’s often down around 10 percent or less. So that may be what is driving all that February activity.”

The new study might offer interesting insight into the biology of the virus itself, says E. John Wherry, director of the Institute for Immunology at the University of Pennsylvania School of Medicine.

“There are lots of theories as to why the flu seasonally cycles: More people are inside, the weather is colder, mucous membranes might be more sensitive because of the dry conditions inside,” says Wherry. “But this suggests that humidity influences the virus itself rather than making our mucous membranes more susceptible.”

The researchers don’t address what happens to the virus, Wherry says. “But this virus is not very hardy. Changes in humidity, concentrations of salt or protein could have a major impact on it. The next step might be to look at what features of the virus change when the humidity is high. Those might offer targets for intervention. If you can show the weak point of the virus, that might be more broadly useful.”

In the meantime, it might make sense for folks to invest in a humidifier for their homes.

“Given that there’s very little downside to humidifying houses in the winter -- that looks like a good idea based on their data.”

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