Outbreaks of the flu during winter may be due to dry air, according to a US study which contradicts the notion that the virus needs a moist environment to thrive.
The researchers found influenza A - a major cause of flu in humans - appears to live longer and transmission rates increase when absolute humidity is low.
Assistant Professor Jeffrey Shaman, an atmospheric scientist at Oregon State University, and epidemiologist Dr Melvin Kohn of Oregon Health Department report their findings in the Proceedings of the National Academy of Science.
Water in the air is believed to affect transmission and survival of viral particles in airborne droplets.
Previous studies have indicated relative humidity affects virus transmission and survival, but Shaman and Kohn found absolute humidity constrains transmission and survival more than relative humidity.
Absolute humidity quantifies the actual amount of moisture in the air, while relative humidity is the ratio of air water vapour content compared to the saturating level, which varies with temperature.
The researchers looked at the data in a 2007 study that had previously found a tenuous relationship between low relative humidity and transmission.
That study used different temperatures and levels of relative humidity to find a trigger point for changes in transmission between infected and uninfected guinea pigs.
It found that more infections when the environment was colder and drier.
Reanalysis of the data by Shaman and Kohn found a dramatic rise in virus survival and transmission after they substituted absolute for relative humidity.
Warmer globe, less flu
The researchers say previous studies have shown that low humidity favours increased influenza virus survival.
They also suggest that global warming may decrease the survival and transmission of the influenza virus.
"Absolute humidity is predicted to increase in a warming world," they said. "Our findings indicate such changes would decrease [flu] virus survival and transmission rates."
Melbourne virologist and influenza specialist Emeritus Professor Greg Tannock says it is an interesting hypothesis which turns preconceived ideas on their head.
"If increased humidity could stop transmission of the flu it could have more lasting benefits than vaccination or viral drugs," says Professor Tannock, who is also a visiting fellow at the Macfarlane Burnet Institute for Medical Research and Public Health.
Professor Tannock says the influenza virus is enclosed within droplets and a suggestion is that the smaller the droplets, the further the infective unit can penetrate to lower parts of the lung, causing pneumonia.
Larger droplets might deposit in the upper respiratory tract, causing 'just a sniffle'.
"Maybe cold dry air and low absolute humidity favour smaller droplets," he says.
Expert not convinced
Another Melbourne virologist and flu expert Doctor Alan Hampson, says the data is based on a small study and appears to contradict research indicating flu often starts in tropical regions.
He says humidity could be one of several factors aiding transmission, along with reduced solar radiation and vitamin D in winter, indoor air, reduced immune systems and stress levels.
Dr Hampson - a World Health Organisation consultant - is chairman of the industry-funded Australian Influenza Specialist Group comprising medical and scientific specialists.
The researchers say humidification of indoor air in nursing homes and emergency departments might reduce spread, but Dr Hampson says it is premature to implement such measures.
By Helen Carter for ABC Science