Blog | Monday, March 4, 2013

QD: News Every Day--Higher room humidity could reduce flu virus transmission

Maintaining indoor relative humidity of more than 40% may significantly reduce the ability of the flu virus to travel across the room, researchers found.

In winter, people spend more time indoors and flu virus remain suspended in dry air longer than in humid air, which forces the particles to fall faster and presumably, not spread as far.

To address whether humidity contributes to how coughing can spread the flu, researchers set up a simulated examination room that contained a coughing manikin and breathing manikin to simulate a health care worker's exposure.

Nebulized influenza was coughed into the examination room and samplers collected size-fractionated aerosols (less than 1 micrometer, from 1 to 4 micrometers and more than 4 micrometers aerodynamic diameters) adjacent to the breathing manikin's mouth and throughout the room. Humidity was varied from 7% to 73%.

Results appeared online Feb. 27 at PloS ONE.

At humidity, there was little loss in infectivity of virus from any particle fraction within the first hour. Moderate humidity inactivated 60% to 80% of the virus, depending on the viral particle size. The fastest rate of inactivation was seen in the particle sizes more than 4 micometers, where 78% of infectivity was reduced within 16 to 30 minutes of the cough.

When room humidity was held to less than 23%, the flu virus retained 70.6% to 77.3% infectivity for 60 minutes. But when humidity was held at 43% or more, the flu virus retained 14.6% to 22.2% infectivity.

Researchers wrote, "At low relative humidity, influenza retains maximal infectivity and inactivation of the virus at higher relative humidity occurs rapidly after coughing. Although virus carried on aerosol particles less than 4 micrometers have the potential for remaining suspended in air currents longer and traveling further distances than those on larger particles, their rapid inactivation at high humidity tempers this concern. Maintaining indoor relative humidity greater than 40% will significantly reduce the infectivity of aerosolized virus."

Marc K. Seigel, MD, FACP, a clinical associate professor of medicine at NYU Langone Medical Center in New York City, who was not part of the study, told HealthDay, "I totally buy this. 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."