Dry Winters Linked to Flu Outbreaks.

• Position: INFLUENZA

The seasonal increase of influenza long has baffled scientists, but a study published in PLoS Biology has found that seasonal changes of absolute humidity are the apparent underlying cause of these wintertime peaks. The study also found that the onset of outbreaks might be encouraged by anomalously dry weather conditions, at least in temperate regions.

For some time, scientists have suspected a link between humidity and seasonal (epidemic) flu outbreaks, but most of the research has focused on relative humidity--the ratio of water vapor content in the air to the saturating level, which varies with temperature. Absolute humidity quantifies the actual amount of water in the air, irrespective of temperature.

Absolute humidity is much higher in the summer. "In some areas of the country, a typical summer day can have four times as much water vapor as a typical winter day--a difference that exists both indoors and outdoors," says lead author Jeffrey Shaman, atmospheric scientist at Oregon State University, Corvallis.

The researchers used 31 years of observed absolute humidity conditions to drive a mathematical model of influenza and found that the model simulations reproduced the observed seasonal cycle of influenza throughout the U.S. They also discovered that the start of many influenza...