Buzz
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'Put on a jacket if you're heading outside, or you'll catch a cold.'
This advice, often heard from grandmothers all around the world, begs the question: Is it accurate? Does cold weather truly have any bearing on the likelihood of catching the common cold? While the majority of scientific studies seem to dismiss this idea, Granny might not be entirely wrong.
It’s a fact that many people tend to get sick during the colder months of late fall and winter. Around 5 to 20 percent of Americans catch colds or the flu each year during this period, and the supposed connection to the weather is evident in the terms cold and influenza (the latter originating from the Italian phrase influenza di freddo, meaning 'influence of the cold'). It’s only natural that folk wisdom links these two things together.
However, as any doctor will tell you, both colds and the flu are caused by viruses that tend to peak during certain seasons. Researchers once believed that viruses in temperate regions went dormant during the summer months, but newer theories suggest that these viruses remain quite active during the 'off season,' spreading globally. A 2007 study from researchers at Pennsylvania State University revealed that the influenza A virus, for instance, shares genetic information with viral strains from the Southern Hemisphere, creating a viral melting pot that allows the virus to come back home with enough changes to trick our immune systems. It’s similar to the annual migration of the swallows to San Juan Capistrano, except these swallows bring along runny noses and coughs.
Despite this, scientists continue to struggle with understanding exactly why people fall ill when these reintroduced viruses circulate during the fall and winter. Several hypotheses have been proposed, which might operate independently, together, or in combination. These include:
Weather and climate - Colds and flu seem to thrive in colder winter conditions and the accompanying dry air. Viruses last longer in dry air than in moist air, and they can survive longer on surfaces like counters, doorknobs, and keyboards when it’s cold. Dry air causes our mucus to dehydrate, drying out nostrils and airways, which might make it easier for viruses to settle in once transferred to us. A study with guinea pigs showed that influenza transmission was more effective in dry (20 percent humidity), cold (41 degrees) air, and declined as the temperature and humidity increased (at 86 degrees or 80 percent humidity, it was not transmitted at all).
Human behavior - With schools in session and people spending more time indoors in closer proximity, viruses can easily spread within large groups. Even in tropical and equatorial regions, where flu occurs year-round, there are noticeable spikes in flu cases during the rainy season when people gather indoors.
Human physiology - Many mammals, including humans, experience physiological changes with the seasons, often linked to the light/dark cycle. During the winter, changes to our immunity—such as a reduction in vitamin D production—could make us more vulnerable to the virus for a few months of the year.
Dr. Grandma, Paging You!
It turns out that grandmothers, babcias, bubbies, and nanas might actually have some truth on their side. A few years ago, Ron Eccles, the director of the Common Cold Centre in Cardiff, UK, tested the idea that being cold or damp could trigger viruses, and he found that there could be a kernel of truth to this folk wisdom. It seems that cold weather could indirectly increase the chances of getting sick.
In his experiment, Eccles subjected half of his participants to cold feet while the other half kept their feet warm in socks and shoes. Afterward, the participants tracked their cold symptoms in journals. Within four or five days, the group who had cold feet showed more than twice the rate of sickness (14.4%) compared to the control group (5.6%).
One possible reason for Eccles' findings is that cold temperatures cause vasoconstriction, or the narrowing of blood vessels. This particularly affects the vessels near the surface, like those in your nose, throat, and mouth. As a result, the flow of infection-fighting white blood cells is slowed, and the nose’s ability to filter the air is compromised, leading to dryness.
However, it’s important to note that in Eccles' study, participants who had their feet chilled only reported experiencing cold symptoms afterward, with no medical tests to confirm actual infections were present.
That said, you’re free to step outside without your mittens or with wet hair—unless, of course, Grandma is present. While most medical research doesn’t support her advice, a single stern glance from Nana can easily outweigh all the scientific evidence in the world.
Today, we’re tackling 20 major questions just like this one. Stay tuned for more days like this, and if you have a burning question, feel free to leave a comment or tweet @mental_floss with the hashtag #bigquestions.
