Flu viruses survive longer and are more easily transmitted when humidity levels are low, Oregon researchers say.
Humidity is at its lowest in the middle of winter-January and February-and this also happens to be peak time for flu.
A link between humidity and flu prevalence and transmission has long been suspected, but the focus has been on relative humidity, not absolute humidity, according to the Oregon State University (OSD) study. Relative humidity is the ratio of air water vapor content to the saturating level, which varies with temperature. Absolute humidity refers to the actual amount of water in the air, irrespective of temperature.
In this new study, the Oregon team reanalyzed data from a 2007 Mount Sinai School of Medicine study that identified a weak relationship between flu transmission and relative humidity. The reanalysis revealed a strong link between absolute humidity and flu virus survival and transmission.
"The correlations were surprisingly strong. When absolute humidity is low, influenza virus survival is prolonged, and transmission rates go up," said study author Jeffrey Shaman, PhD, an atmospheric scientist at OSU who specializes in ties between climate and disease transmission.
Dr. Shaman and colleague Melvin Kohn, MD, MPH, an epidemiologist with the Oregon Department of Health Services, concluded that relative humidity explains only about 36% of flu virus survival and 12% of transmission, while absolute humidity explains 90% of flu virus survival and 50% of transmission.
Their findings were published in the Proceedings of the National Academy of Sciences.
"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," Dr. Shaman said. "Consequently, outbreaks of influenza typically occur in winter when low absolute humidity conditions strongly favor influenza survival and transmission."