In temperate regions, such as North America and Europe, flu peaks during the winter season, when humidity is low. But in some tropical regions, influenza thrives during the rainy season, noted the researchers from Virginia Polytechnic Institute and State University, in Blacksburg.
After measuring the influenza A virus survival rate at different humidity levels, the researchers concluded that flu is more common in winter months because the virus's viability in mucus increases when the relative humidity is below 50%. The virus, however, also thrives when the humidity is close to 100%, they found.
How the study was done
"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," Linsey Marr, an associate professor of civil and environmental engineering at Virginia Tech, said in a university news release.
When the humidity is low indoors in the winter, the researchers explained, respiratory droplets evaporate completely. Under these dry conditions, the virus is better able to survive.
When the humidity rises to moderate levels, however, droplets do not evaporate completely so the virus remains exposed to higher levels of chemicals and is less able to infect cells.
The researchers concluded that humidity affects the concentrations of salts and proteins in respiratory droplets, which affects the survival of the flu virus. That's why flu is seasonal, they reasoned.
In the United States, flu season can start as early as October and continue until late May, according to the US. Centers for Disease Control and Prevention.
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