For decades, scientists struggled to explain why flu season arrives so predictably each winter. While many assumed that cold weather itself was to blame, recent research has revealed that the real culprit is the dry air we breathe indoors.
When temperatures drop, the air becomes drier because cold air holds less moisture. This dry environment allows flu viruses to survive longer and travel further in the droplets from coughs and sneezes. Laboratory studies on guinea pigs have shown that flu spreads much more easily in low humidity, and analysis of 30 years of climate and health data confirms that flu epidemics almost always follow a drop in air humidity.
Another key factor is that people spend more time indoors during winter, in closer contact with others who may be carrying germs. Public transport, for instance, becomes a hotspot for transmission as commuters are pressed together in poorly ventilated spaces.
While our behaviour and physiology also play a role – such as reduced vitamin D levels and constricted nasal blood vessels – the indoor environment appears to be the missing link. Understanding this could lead to simple measures, like improving ventilation or using humidifiers, to help stem the spread of infection.
