The influenza virus survives and travels best when "absolute" humidity is low, researchers at the Oregon State University said in findings published Monday.
The research provides a scientific answer to the age-old question of why more people tend to come down with the flu particularly in the cold winter months, said the study authors.
"When absolute humidity is low, as in peak flu months of January and February, the virus appears to survive longer and transmission rates increase," said the study in the Proceedings of the National Academy of Sciences.
"Absolute" humidity is a measure of the actual amount of water vapor in the air, regardless of temperature.
Previous studies found higher flu infection rates when temperatures are colder and drier, therefore some correlation between flu transmission and "relative" humidity, which measures the ratio of water vapor in the air to the saturating level, which fluctuates according to the temperature.
But the research team led by Jeffrey Shaman, an Oregon State University atmospheric scientist, re-examined previous studies and found that "relative" humidity could only account for 12 percent of transmission cases and 36 percent of flu virus survival, or how long the virus remains viable when airborne.
"The Oregon researchers then retested the various data using absolute humidity and found a dramatic rise in accounting for both transmission (50 percent, up from 12 percent) and survival (90 percent, up from 36 percent)," said the study.
"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," said Shaman, the lead author.
"Consequently, outbreaks of influenza typically occur in winter when low absolute humidity conditions strongly favor influenza survival and transmission."