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Cold, Flu, & Cough Health Center

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Influenza Linked to Absolute Humidity

Flu Virus Thrives in Low Humidity, Study Says
WebMD Health News
Reviewed by Louise Chang, MD

Feb. 13, 2009 -- Absolute humidity more than relative humidity is linked to influenza and the spread of the virus, a new study suggests.

Absolute humidity is a measure of the actual amount of water in the air, regardless of temperature. Relative humidity is the ratio of air water vapor content to the saturating level, which varies with temperature.

Scientists have long suspected a link between humidity and flu transmission and prevalence, but the main suspect has been relative humidity, says lead study author Jeffrey Shaman, PhD, of Oregon State University, who specializes in studying ties between disease transmission and climate. But the real culprit, he contends, is absolute humidity. The researchers explain that when humidity is low, as in peak flu months of January and February, the virus appears to thrive longer, and transmission rates zoom.

Shaman took a fresh look at data from a 2007 study that had found a “tenuous relationship” between flu transmission and relative humidity. He reused that research data, but substituted absolute humidity for relative humidity to search for correlations.

That led to additional investigation of the relationship between absolute humidity and survival of flu virus, or the length of time the bug remains viable once airborne.

“The correlations were surprisingly strong,” Shaman says in a news release. “When absolute humidity is low, influenza virus survival is prolonged and transmission rates go up.”

That 2007 study, in general, found more infections when it was colder and drier.

But Shaman and co-author Melvin Kohn, an epidemiologist with the Oregon Department of Health Services, found that relative humidity could explain only about 12% of the variability of influenza virus transmission.

They showed that relative humidity explains only about 3% of flu virus survival. After looking at the data focusing on absolute humidity, they found “dramatic” rises in both variability of transmission (from the 12% found earlier, to 50%) and survival (from 36% to 90%).

So conventional wisdom, backed by much research pointing at relative humidity, may be all wet, the researchers suggest, adding that absolute humidity is the more likely villain.

The conclusion, they say, is clear: “Outbreaks of influenza typically occur in winter when low absolute humidity conditions strongly favor influenza survival and transmission.” They add that "these findings also suggest that humidification of indoor air, particularly in places where transmission to those at high risk for complications, such as nursing homes and emergency rooms, may help decrease the spread and toll of influenza during influenza season."

The study is published in the Proceedings of the National Academy of Sciences.

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