Humidity > Influenza Virus

Photo, James Gathany Photo credits: CDC

Photo, James Gathany
Photo credits: CDC

Each year in the winter influenza or “flu” season rolls around and this sickness runs rampant throughout the country. This virus, and many in general, are quite scary because of how easily they can be spread through the air ie. via coughing. Researchers of the National Institute for Occupational Safety and Health (NIOSH of the CDC) have recently linked higher humidity and transmission particle size to more inactivation of influenza (and less spread).

Influenza is an RNA virus that affects mammals and birds causing a small scale pandemic every year in the United States. Viruses tend to mutate very quickly, therefore, it is hard to find a single band-aid to blanket cure this bug. Most people can spread the virus between -1 and 6 days after showing symptoms. Airborne transmission is most likely when this virus appears since many people are in closed quarters attempting to stay warm at home with others during winter.

To test the rate at which influenza stays infectious in different humidities, researchers set up coughing and breathing test dummies in an isolated room. The sneezing dummy released about 4.5×103 virus/liter of air into the room, where 4.6% of that virus was seen to be infectious upon discharge. Of that, upwards of 77.2% of the virus retained infectivity at lower relative humidity (7-23%), while only 14.6% retained infectivity at higher relative humidity of 43%. As a grounding point, average in home humidity during winter is just above 20%, while summer humidity indoors is just below 60%.

Next, the rate of loss of infectivity was measured comparing low and moderate humidity (20% and 45%). Within 15 minutes, 52% of the virus’ infective ability was lost at 45% humidity compared to 20% humidity.

Together these two showed more virus lost its ability to infect new people/etc. at higher relative humidities.

Finally, transmission particle size was measured to see if it had bearing on influenza’s spread and infection at differing humidities. Three particle sizes were used, <1 µm, 1-4 µm, and >4 µm. At the two extremes, <1 & >4 µm, 94% infectivity was seen as lost in the first 15 minutes after coughing virus into the air. However, the 1-4 µm range of particles only showed 29% decrease in infectivity of the virus after 15 minutes. All sizes did, however, show increased loss of infectious ability with more time passed.

It seems as though humidity has great bearing on the inactivation/spreading ability of the influenza virus. At higher humidities, 45% +, the influenza virus showed very little strength in infecting new hosts. This was doubly true when the particle size they rode on was really small or really big.

The best way to minimize your chance to get influenza each year without getting a flu shot?…move to a state with really high humidity in the winter.

John D. Noti, Francoise M. Blachere, Cynthia M. McMillen, William G. Lindsley, Michael L. Kashon, Denzil R. Slaughter, Donald H. Beezhold. High Humidity Leads to Loss of Infectious Influenza Virus from Simulated Coughs. PLoS ONE, 2013.