Why more flu in winter? U.S. scientists test new idea

William Atkins
Wednesday, 11 February 2009 20:28

Science - Health

Their paper “Absolute humidity modulates influenza survival, transmission, and seasonality” was published in print on February 9, 2009 in the journal Proceedings of the National Academy of Sciences (PNAS).

In its abstract, its authors, Jeffrey Shaman (College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, Oregon) and Melvin Kohn (Public Health Division, Oregon Department of Health Services, Portland, Oregon), state that relative humidity (RH) has been thought by scientists to more likely bring on the flu.

Specifically, previous studies have indicated that RH “affects both influenza virus transmission (IVT) and influenza virus survival (IVS).”

However, these scientists don’t really have a strong case as to why flu (influenza A) is more frequent during the cold temperatures as opposed to warm temperatures.

Consequently, the two researchers decided to analyze absolute humidity (AH) on IVT and IVS, rather than relative humidity (RH).

Relative humidity is defined as the amount of water vapor in a gaseous mixture of air and water. When we talk about “humidity” (like, 'It is really humid today!'), then we are talking about relative humidity.

More specifically, RH is the ratio of the partial pressure of water vapor (P-partial, vapor) in the gaseous mixture to the saturated (maximum) pressure of water vapor (P-saturated, vapor) at a prescribed temperature of the mixture.

RH equals P-partial (vapor) divided by P-saturated (vapor) times 100%, and thus expressed as a percentage.

Thus, if 20 grams of water vapor were present in each kilogram of dry air, and the air becomes saturated (completely filled) with 40 grams of water vapor per kilogram of dry air, then the relative humidity would be 20/40=50%.

However, the higher the temperature, then the more water vapor in the air. Hotter air will take more water out of liquid form and put it into gaseous form. RH is affected by temperature.

So, a specific relative humidity level at a warmer temperature means there is more water vapor in the air than at a colder temperature (which is caused by the interplay of evaporation and condensation).

Page three defines absolute humidity, along with the conclusion of their study.