New York: Low humidity is a key reason behind people falling sick and even dying from flu during the winter months, says a study.

Decreased humidity lowers the immune system's defences against flu infection, says a study published in the journal Proceedings of the National Academy of Sciences (PNAS). It offers an insight into why the flu is more prevalent when the air is dry.

For the study, researchers used mice genetically modified to resist viral infection as humans do. The mice were housed in chambers at the same temperature, but with either low or normal humidity. They were then exposed to the influenza A virus.

They found that low humidity hindered the immune response of animals in three ways. It prevented cilia, which are hair-like structures in airway cells, from removing viral particles and mucus. It also reduced the ability of airway cells to repair damage caused by the virus in lungs.

The third mechanism involved interferons, or signalling proteins, released by virus-infected cells to alert neighbouring cells to the threat. In the low-humidity environment, this innate immune defence system failed.

"It's well known that when humidity drops, a spike in flu incidence and mortality occurs. If our findings in mice hold up in humans, our study provides a possible mechanism underlying this seasonal nature of flu disease," said Akiko Iwasaki, Professor at Yale University in the US.

While humidity is not the only reason in flu outbreaks, it's an important factor that should be considered during the winter season.

Increasing water vapour in the air with humidifiers at home, school, work and even hospital environments is a potential strategy to reduce flu symptoms and speed up recovery, researchers said.