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 isn’t a problem, according to Jeffrey Shaman, PhD, of Oregon State University in Corvallis, a coauthor of the new study.
“It seems that [the influenza virus’s] 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 week’s 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 aren’t 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 he’s found the answer: It’s 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 can’t 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 car’s 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 tank’s 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.