We’ve all gone through it – the temperature drops, your throat suddenly starts to feel itchy and next thing you know you’re holed up on the sofa with a cold or even the flu. In fact worldwide, up to five million people get the flu every year.
Does cold weather kill germs?
Strictly speaking, it would need to be extremely cold for germs to be killed.
For example, bacteria can be eliminated when heated to 100C – water’s boiling point.
When frozen at -18C, bacteria will stop growing and may even be killed.
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But while cold weather doesn’t necessarily kill germs, the flu and colds to tend to spread quicker when the mercury drops.
There are a number of theories around this mainly centred around human behaviour.
In winter we tend to spend more time indoors, which means we’re in closer contact with people who might be carrying germs.
Because it’s cold, we’re more likely to take public transport where any number of sneezes and coughs happen every minute, spreading germs all over the bus or train carriage.
Another popular theory is that cold weather wears down the body’s defences against infections.
Vitamin D helps power our immune systems and can be absorbed via exposure to the Sun.
But in winter, the days are short and the weather is often gloomy so we see less of our vitamin D giving star.
Furthermore, when we breathe in cold air, the blood vessels in our nose may constrict to stop us losing heat, and this may prevent white blood cells – which help figth germs – from reaching our mucus membranes and killing any viruses that we inhale.
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Analysis of these theories has not been completely able to explain why colds and the flu emerge yearly at the same time.
The answer might in fact be found in the air that we breathe in winter.
The laws of thermodynamics dictate that cold air can carry less water vapour before it falls as rain.
That means that while the weather may seem wetter, the air itself is drier as it loses the moisture.
Research has shown that drier conditions create environments better suited to the flu virus.
Lab experiments investigating how flu spreads among guinea pigs found that in moister air, the virus struggled to build momentum.
In drier conditions, however, it was able to spread much more effectively.
Jeffrey Shaman at Columbia University and colleagues compared 30 years of climate records with health records and found that flu epidemics almost always followed a drop in air humidity.
Any time we sneeze with a cold, a mist of particles from our nose and mouths expels into the air.
In moist air, these particles might remain relatively large and drop to the floor.
But in dry air, they break up into smaller pieces and eventually become so small they can stay aloft for hours or days.
The result is that in winter, you are breathing a cocktail of dead cells, mucus and viruses from anyone and everyone who has visited the room recently.
Viruses in drier air can float around and stay active for hours – until it is inhaled or ingested, and can lodge in the cells in your throat.