Scientists don’t know why influenza viruses (commonly referred to as the flu) are associated with cold weather, but a recent paper in the journal PNAS provides a new association: When absolute humidity is low, flu virus survival and transmission are high.

Humidity is the amount of water vapor in the air. Absolute humidity is the actual water vapor content of the air. Relative humidity, expressed as a percentage, is the ratio between absolute humidity and water vapor saturation – the measure used by most television meteorologists when they discuss humidity.

Relative humidity varies as a function of temperature. “Air with relative humidity of 50% at 20°C has much more water vapor than air with 50% relative humidity at 5°C,” writes Jeffrey Shaman, an assistant professor in the College of Oceanic and Atmospheric Sciences at Oregon State University in Cornwallis. (Humidity can be measured in many ways. Shaman uses vapor pressure.)

A 2007 study linked flu transmission and relative humidity. The problem is that relative humidity is affected by absolute humidity and by temperature. The previous study didn’t tease apart whether absolute humidity or temperature were associated with flu survival and transmission.

To make things even more confusing, during the winter, when the outdoor temperature is low, relative humidity is high outdoors but low indoors. Absolute humidity is low everywhere, indoors and outdoors, during the winter and high everywhere during the summer.

So which is it, relative or absolute humidity?

Nobody had ever seriously considered whether absolute humidity influences influenza survival. So Shaman calculated vapor pressure (absolute humidity) from the relative humidity and temperature data presented in the 2007 study. He found that when absolute humidity is low, flu virus survival and transmission is high – a condition that did not correlate as strongly with temperature or relative humidity.

Before you run out and buy a humidifier for the winter, you might want to consider disease-causing mold thrives at higher humidity, so there’s a potential trade-off in health benefits. Moreover, we don’t understand how absolute humidity affects flu virus survival. The consensus among experts is that the flu vaccine is the best way to avoid the flu. Another caveat: The experiments on flu survival and transmission were done using guinea pigs, not humans, and used only one species of influenza virus.

Shaman’s results also suggest a positive side to global warming. A warmer world should increase absolute humidity, which would decrease flu survival and transmission.

Source: “Absolute humidity modulates influenza survival, transmission, and seasonality,” by Jeffrey Shaman and Melvin Kohn, published in the March 3 issue of PNAS.