It’s still flu season. The work of two researchers from Oregon State University shows that humidity levels have a big impact on the survival, transmission, and seasonality of influenza A - the type that peaks during winter in temperate regions.

If your customers are getting the flu a lot, maybe they need to have their home’s absolute humidity examined.

“The basis for this pronounced seasonality is not understood, nor is it well documented how influenza A transmission principally occurs,” said researchers Jeffrey Shamana and Melvin Kohnb, of the College of Oceanic and Atmospheric Sciences, Oregon State University, and Public Health Division, Oregon Department of Health Services.

“Previous studies indicate that relative humidity (rh) affects both influenza virus transmission (IVT) and influenza virus survival (IVS).” The researchers took a closer look at the effects of absolute humidity on the virus’s survival and transmissibility.

The researchers found that absolute humidity (ah, the actual amount of humidity in the air) constrains or restricts both the transmission efficiency and IVS “much more significantly than rh,” they reported. “In temperate regions, both outdoor and indoor ah possess a strong seasonal cycle that minimizes in winter.” The cycle, they said, “is consistent with a wintertime increase in IVS and IVT and may explain the seasonality of influenza.”

They tested influenza A transmission on guinea pigs, and the results suggested that “airborne transmission may indeed be an important mode of IVT” - no surprise. Transmission rates increase at lower rh levels - again, no surprise. But they set out to prove why.

Were virus-laden droplet nuclei more efficiently produced at lower rh and did expelled droplet particles keep the virus airborne longer? Or did the virus survive longer as rh decreased because the airborne virus remained viable longer at lower rh?

“Rh is a meaningful physical quantity and for certain organisms may affect biological response; however, measures of ah, i.e., the actual water vapor content of air irrespective of temperature, can be of greater biological significance for many organisms,” they wrote.

Using data from the guinea pig study, they found that low wintertime ah levels, both indoor and outdoor, “increase influenza IVS and thus increase IVT efficiency.

“These findings also suggest that humidification of indoor air, particularly in places where transmission to those at high risk for complications, such as nursing homes and emergency rooms, may help decrease the spread and the toll of influenza during influenza season.”

Exactly how ah affects the virus’s survivability isn’t really understood. Naturally, this article is skipping over a lot of the really intense scientific data and cutting to the meat for our readers.

“The findings presented here indicate that IVS responds to the amount of water vapor in the surrounding air (i.e., ah), and not how close that air is to saturation (i.e., rh).”

The bottom line is, dry air is proven to make the flu virus live longer and be easier to transmit. You might want to let your customers know.

For more information, visit

Publication date: 03/16/2009