Winter is flu season in the world's temperate regions, but scientists still aren't sure why. Now, it turns out that the answer may have been staring them in the face all along. A reanalysis of data from past studies suggests that low absolute humidity--not low relative humidity, the factor many scientists have studied--helps the virus survive and the flu spread.
Explanations abound for why flu is king in winter, only to disappear come summer. Researchers have suggested that the virus might survive better in colder temperatures, that people are more exposed when huddled indoors, and that lower melatonin and vitamin-D levels can weaken immune systems. Humidity has also been implicated. Indoor heating makes the air in many homes drier, and several studies have shown that the virus survives better, and is more easily transmitted, when there's less moisture in the air.
As a measure of air moisture, researchers have always taken relative humidity, which is the ratio of actual water content in the air to the maximum possible level. That maximum increases exponentially with temperature, however; a 75% relative humidity at 25°C means much more vapor in the air than at 5°. Climate physicist Jeffrey Shaman of Oregon State University in Corvallis and epidemiologist Melvin Kohn of the Oregon Health Department in Portland believed that what might matter more to the virus is absolute humidity--especially because the evidence for the impact of relative humidity was never all that strong.
The duo first reanalyzed data from a 2007 paper in PloS Pathogens by Peter Palese of Mount Sinai School of Medicine in New York City and his colleagues, who found that virus transmission between two guinea pigs housed in neighboring cages falls as relative humidity rises (ScienceNOW, 19 October 2007). The link was only marginally statistically significant.
When Shaman and Kohn performed a similar analysis using absolute humidity--which is easy to calculate if you know the temperature--they found a much stronger correlation. Then, the researchers reanalyzed the data from several other studies on virus survival and transmission, by Palese's group and others, including some over 40 years old. They found the same pattern, they report online today in the Proceedings of the National Academy of Sciences. Shaman says he has no idea why other researchers never looked at absolute humidity. "Maybe it's because they always see the local weatherman talking about relative humidity," he says.
Palese read the paper at Science's request but declined to comment on it. Raymond Tellier, a microbiologist at the Hospital for Sick Children in Toronto, Canada, says the work is promising but that new studies are needed to confirm it. "In order to make the point definitively, you'd really like to see more data points," he says.
Shaman and Kohn say the data bolster the idea, suggested previously by others, that increasing humidity in nursing homes and emergency rooms might help prevent flu among vulnerable patients. Tellier agrees, but he warns that this might boost the numbers of other pathogens, such as molds.