We all know that we're more likely to catch the flu in the winter. Now, researchers are closer to explaining exactly why.
It's long been known that the flu virus loves dry air. Air with little moisture allows the germ to survive longer and to transmit more easily from host to host.
Humidity levels are lowest during January and February - which just happen to be the peak flu months. Now, researchers have found that it is the "absolute humidity" of our air -- not the more commonly reported "relative humidity" - that is the most important factor in flu survival and transmission.
Absolute humidity measures the amount of water present in the air, regardless of temperature. Relative humidity is the ratio of air water vapour content to the saturating level and can vary depending on air temperature.
So a high relative humidity reading may not mean there is a lot of moisture in the air. For example, warm air at 30 per cent relative humidity may actually have the same amount of water in the air as cold air at 60 per cent relative humidity.
The researchers found that low absolute humidity levels provided the best conditions for the flu virus to survive longer and to easily transmit to others.
"The correlations were surprisingly strong," said Jeffrey Shaman, an Oregon State University atmospheric scientist who specializes in ties between climate and disease transmission.
"When absolute humidity is low, influenza virus survival is prolonged and transmission rates go up."
The study was published Tuesday in Proceedings of the National Academy of Science.
Previous research has linked flu rates and dry air, particularly a headline-grabbing study published in 2007 in the journal PLoS Pathogens by researchers at Mount Sinai School of Medicine in New York, which looked at flu-infected guinea pigs in climate-controlled chambers.
In this latest study, Shaman worked with co-author Melvin Kohn, an epidemiologist with the Oregon Department of Health Services, to reanalyze the data from the 2007 study.
They found that relative humidity could explain only about 12 per cent of the variability of flu virus transmission and 36 per cent of virus survival.
In their new analysis, Shaman and Kohn found that using absolute humidity explained 50 per cent of influenza transmission and 90 per cent of virus survival.
The finding offers the chance to better understand the spread of influenza and could help in developing better prediction models of influenza spread.
The findings suggest that it is likely a good idea to add humidity to the air. But beware of adding too much humidity, which can lead to other problems, such as mould.
