Last updated: Feb 09, 2009
The finding also suggests that using a humidifier may be a good idea in places where the spread of influenza poses a serious threat, like intensive care units or even a home with a sick childas long as sensitivity to moisture-loving mold and spores isnt a problem, according to Jeffrey Shaman, PhD, of Oregon State University in Corvallis, a coauthor of the new study.
“It seems that [the influenza viruss] ability to survive and be transmitted person-to-person is greatly affected by how dry or wet the air is,” says Shaman, whose study is published in this weeks Proceedings of the National Academy of Sciences.
The name for this potentially deadly respiratory infection comes from the Italian word for influence; centuries ago, people believed that the influence of the planets made people sick with the disease. Our science is a little more solid today, but researchers still arent 100% sure how and why the virus spreads, and they remain stumped about why some parts of the world have such a pronounced winter flu season with almost no flu activity in warmer months.
Shaman believes hes found the answer: Its all about humidity. Absolute humidity, that is, which is particularly low in cold weather.
Shaman and his colleague Melvin Kohn of the Oregon Department of Health Services in Portland revisited a 2007 study that found higher humidity slowed the spread of the flu among guinea pigs. The researchers had measured air dampness using relative humidity, or how saturated the air is with water vapor.
For example, 75% relative humidity would mean the air is holding 75% of its total capacity of water vapor. Relative humidity is strongly influenced by temperature; the warmer the air, the more water vapor it can hold, while colder air cant hold as much water vapor.
Absolute humidity, on the other hand, refers to the actual amount of water vapor in the air regardless of saturation. Relative humidity is like a cars gas gauge, Shaman notes. It tells you how full your tank is. Absolute humidity represents how many gallons you have in your tank, regardless of the tanks size.
Shaman converted the guinea pig data from relative humidity to absolute humidity and found the link between air moisture and flu spread got much stronger. “Absolute humidity, for reasons that remain undetermined, is affecting how long the virus remains viable,” he says. “It really explains why you have this pronounced seasonality in temperate regions.”
The researcher also looked at studies dating back to the 1940s of airborne flu virus survival. Some included information on relative humidity, which he converted to absolute humidity. Again, the relationship between survival and air moisture got stronger.
For example, in the wettest possible absolute humidity, less than 20% of the virus was still viable after an hour, while at the driest conditions 80% of the virus was still capable of making someone sick. After 23 hours, the viruses in the dampest conditions were all dead, while 60% of those under the most arid conditions were still alive.
“The idea that humidity affects the life of virus has been known for decades,” says Michael Gardam, MD, PhD, an infectious disease specialist with the University Health Network in Toronto and Ontarios public health agency. While Shamans findings are “important,” according to Dr. Gardam, the challenge will now be to see if air humidity affects the spread of the disease among humans in the real world. “Based on the papers that have been done thus far, I wouldnt run out and buy humidifiers, but I would definitely buy humidifiers to study humidifiers.”
“This is sort of nibbling around the edges,” he added. “Its useful, but its not sufficient to say, ‘We now have the answer to change the world.”
Its “frustrating,” Dr. Gardam says, that we still dont know if the virus is spread mainly by physical contact or through the air or both. For this reason, he adds, there remains no scientific evidence that widely promoted protective measures such as hand washing and mask wearing will actually stop people from getting sick. “Weve just sort of taken for granted we know how it transmits, and we really dont know,” he says.
“I think theyve done a good job. I agree with their work,” says Anice Lowen, PhD, a researcher at the Mt. Sinai School of Medicine in New York and one of the authors of the guinea pig study. “Im not an expert in the methods that they used, but it seems to make sense.”
The reasons why the flu virus might survive and spread more easily in dry conditionsand why absolute humidity might be a better gauge of these conditions than relative humidityare still not clear, Lowen notes. It could have something to do with moisture evaporating from surfaces more speedily when humidity is low, she says.
Lowen agrees that keeping indoor air more humid might help slow down the spread of the flu. But, she adds, “the best way to protect yourself against influenza is to get vaccinated.”