The 2007 PLoS Pathogens study, by researchers at Mt. Sinai School of Medicine in New York, looked at the effects of temperature and relative humidity on transmission of influenza using influenza-infected guinea pigs in climate-controlled chambers. The researchers used 20 different combinations of temperature and relative humidity in an effort to identify a trigger point for changes in transmission of the virus between infected guinea pigs and adjacent control animals.
In general, the study found that there were more infections when it was colder and drier. However, Shaman and Kohn demonstrated that relative humidity could only explain about 12 percent of the variability of influenza virus transmission from these data. In addition, numerous other experiments, dating back to the 1940s, have shown that low relative humidity favors increased influenza virus survival.
However, in their PNAS analysis, Shaman and Kohn demonstrated that relative humidity only explains about 36 percent of influenza virus survival. 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).
The cold air holds less water. Absolute humidity drops. That increases flu virus survival.
Keep this news in mind the next time we get a big killer flu pandemic. But individuals can't do much to up the absolute humidity in our environment. We can do something in our homes with humidifiers. But most of our exposure to infectious people comes outside the home. Few of us have any control over the temperature or humidity where we work or shop or go for services such as in dental and medical offices.