Humidity Kills The Flu Virus

February 13, 2009

A new study conducted by Oregon researchers has found that the cold and dry air is more likely to give a better chance of getting the flu. A new analysis of previous data shows a significant correlation between a complete humidity and influenza virus survival and transmission. When absolute humidity is low - as in peak flu months of January and February - the virus turns out to survive longer and spreads faster from one person to another.

"It seems that the influenza virus' ability to survive and be transmitted person-to-person is greatly affected by how dry or wet the air is," says Jeffrey Shaman, Ph.D., an atmospheric scientist at Oregon State University in Corvallis, who specializes in ties between climate and disease transmission, and a co-author of the new study.

According to Shaman, it is strongly recommended to use humidifiers in places where the spread of influenza poses a serious threat, such as intensive care units or even at home with a sick kid- as long as sensitivity to moisture-loving mold and spores does not cause any problem.

A link between humidity and flu prevalence and its transmission has been suspected by scientists for a long time, but the focus has been on relative humidity, not an absolute humidity, according to background information in the OSU news release. The scientists have always been wondering about why some parts of the world have such a pronounced winter flu season with almost no flu activity in warmer months. Potential explanations are that people spend more time indoors in winter and, therefore spread the infection to each other more easily; less sunlight may have a chemical effect on the virus and people's immune system response, or there might be some not known environmental control. Shaman believes that the secret is in humidity. Absolute humidity, which is particularly low in cold weather.

Shaman and his colleague Melvin Kohn, an epidemiologist with the Oregon Department of Health Services, Portland, re-analyzed information from a 2007 PLoS Pathogens study by researchers at Mount Sinai School of Medicine in New York.

The 2007 PLoS Pathogens study examined and analyzed the effects of temperature and relative humidity on transmission of influenza using influenza-infected guinea pigs in climate-controlled chambers. Twenty different combinations of temperature and relative humidity have been used in order to identify a primary reason for changes in transmission of the virus between infected guinea pigs and adjacent control animals. At around 40 degrees Fahrenheit, flu virus infected guinea pigs at humidity that stopped infections at room temperature. Infected animals shed virus two days longer at the colder temperature, probably because noses cannot shed virus as effectively when mucus is colder and thicker.

In general, that study found that there were more infections when it was colder and drier. However, the Oregon researchers concluded that relative humidity explained only about 36 per cent of flu virus survival and 12 per cent of transmission in the 2007 study, while in their new research absolute humidity explained 90 per cent of flu virus survival and 50 per cent of influenza transmission.

The researchers also analyzed the studies dating back to the 40s of airborne flu virus survival. Some of them included information on relative humidity, which the researchers converted to absolute humidity. Again, the relationship between survival and air moisture got much stronger.

"The correlations were surprisingly strong. When absolute humidity is low, influenza virus survival is prolonged, and transmission rates go up," explained Jeffrey Shaman. The finding "is very important for the scientific community and the medical community to know to develop better prediction models of influenza," he added. It will give a better opportunity understand and predict the spread of the disease. The scientist advised that in some cases it would be beneficial to add humidity to the air, but at the same time gave his warning to people to be careful of overdoing it - too much humidity, he said, can lead to other problems, such as mold.

The correlation with flu and low humidity is important because in cold winter months, when flu is most common, even a high relative humidity reading may indicate little actual moisture in the air, and the less moisture there is, the better environment for the flu virus to spread.

Absolute humidity is expressed in weight of water, grams or pounds, in a volume of air, such as a cubic meter or yard. The higher the reading - the wetter it gets. The wetter it gets - the worse for flu virus to survive.

"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," Shaman said. "Consequently, outbreaks of influenza typically occur in winter when low absolute humidity conditions strongly favor influenza survival and transmission."

Their findings were published in this week's issue of the Proceedings of the National Academy of Sciences.

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Tags: Health

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