THURSDAY, Feb. 28 (HealthDay News) -- As the flu season
continues to pack a punch for some Americans, new research suggests
there might be a simple way to reduce the risk for infection in an
indoor setting: hike up humidity levels.
By raising indoor relative humidity levels to 43 percent or
above, investigators reported that they were able to quickly render
86 percent of airborne virus particles powerless.
The finding is reported in the February issue of the journal
PLOS Oneby a team led by John Noti, a senior service fellow
with the U.S. National Institute for Occupational Safety and
Health's Health Effects Laboratory Division at the U.S. Centers for
Disease Control and Prevention in Morgantown, W.V.
To assess the role of humidity in flu transmission, Noti and his
colleagues relied on mechanized mannequins and tissue cultures
rather than actual humans.
Placed in a tightly sealed and disinfected model of a hospital
examination room, a coughing mannequin served as a flu patient and
was outfitted with aerosolized viral solution. The viral solution
was projected into the air via five mechanized "coughs," spread out
over one-minute intervals.
At the same time, a breathing mannequin (serving as a caregiver)
was set to face the coughing mannequin at a distance of a little
less than 7 feet. The breathing model was programmed to inhale in
sync with the coughing, and aerosol samples of inhaled air were
collected at various points around the mouth of the caregiver for
up to five hours post-coughing.
Throughout testing, humidity levels were adjusted from a low of
7 percent relative humidity to a high of 73 percent.
The result: the team found that when humidity levels were set to
43 percent, only 14 percent of the virus particles that were
released were able to transmit the influenza virus, compared with a
transmission rate of 70 percent to 77 percent in a relatively
low-humidity environment (23 percent).
What's more, the protective impact of higher humidity levels
appeared to be rapid, with the majority of viral inactivation
taking place within 15 minutes of when viral particles were first
"coughed" into a high-humidity environment.
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
"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.
For more on influenza, visit the
National Institutes of Health.
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