Humidity cause for influenza peak?

Influenza peaks in winter in temperate zones and U.S. researchers suggest it may not be the heat but the humidity that is the cause. Linsey Marr, associate professor of civil and environmental engineering, at Virginia Tech in Blacksburg, Va., and colleagues, Wan Yang, a doctoral student, and Elankumaran Subbiah of the Virginia-Maryland Regional College of Veterinary Medicine measured the influenza A virus survival rate at various levels of humidity. The study, published in the journal PLoS One, said a relationship between the influenza A virus viability in human mucus and humidity over a range of relative humidity from 17 percent to 100 percent. The study found the viability of the flu A virus was highest when relative humidity was either close to 100 percent or less than 50 percent. The results in human mucus might help explain influenza's seasonality in different regions. "We added flu viruses to droplets of simulated respiratory fluid and to actual human mucus and then measured what fraction survived after exposure to low, medium, and high relative humidities," Marr said in a statement. At low humidity, respiratory droplets evaporate completely and the virus survives well under dry conditions but at moderate humidity, the droplets evaporate some, but not completely, leaving the virus exposed to higher levels of chemicals in the fluid and compromising the virus' ability to infect cells. The researchers said humidity could explain the seasonality of influenza by controlling the ability of viruses to remain infectious while they are in droplets or aerosols. The viruses survived best at low humidity, such as that found indoors in the winter, and at extremely high humidity such as 100 percent such as the rainy season in tropical regions, Marr said.