Dry air blamed for flu outbreaks

http://www.msnbc.msn.com/id/35706376/ns/health-cold_and_flu/

By Emily Sohn
updated 9:46 a.m. ET, Thurs., March. 4, 2010

Extremely low humidity levels in winter, according to new research, fuel influenza outbreaks. Particularly dry spells make the problem worse. The discovery might help scientists prepare for epidemics and for the rash of secondary illnesses, like pneumonia, that often slam people once they're already down.

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But relative humidity isn't the best measurement for studying flu outbreaks, Shaman said, because relative humidity varies with temperature. So, there is actually less moisture in the air on a rainy winter day in Seattle than there is on a sunny summer day in the same city.

He thought it would be more useful to look at absolute humidity, which measures exactly how much moisture is in the air, regardless of temperature.

On that scale, Shaman said, winters are usually twice as dry as summers in a place like San Diego and Arizona, four times drier in New York, and up to five times drier in a particularly cold state like Minnesota.

Along with colleagues, he analyzed 31 years of data from around the United States and used a computer model to show that influenza outbreaks were more likely to occur when absolute humidity levels were low. Like a sliding scale, progressively drier air led to progressively higher likelihood that an outbreak would occur, the researchers reported in the journal PLoS Biology. Temperature didn't play much of a role.

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