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“Put a jacket on if you’re going out there, or you’ll catch a cold.”
It’s a common refrain of grandmothers all over the world. Are they right, though? Do low temperatures have anything to do with catching the common cold? Most of the scientific evidence is strongly against her, but Granny just might be on to something.
Sure, people tend to get sick during the late fall and winter. An estimated 5 to 20 percent of Americans come down with colds or flu every year around that time, and the temperature’s supposed influence is easily seen in both the names cold and influenza (traced to the Italian influenza di freddo, or “influence of the cold”). The folk wisdom naturally goes that the two must be connected.
But, as any doctor will tell you, colds and the flu are caused by viruses that happen to surge seasonally. Scientists used to think that viruses from the temperate regions went into a dormant state during the summer months, but now they think that the viruses are actually quite busy during the “off season” and are transmitted throughout populations all over the world. A 2007 study by researchers at Pennsylvania State University found that the influenza A virus, for example, exchanges genetic information with viral strains from below the equator – theoretically in a geographic area that would act as an influenza melting pot and viral reservoir – during its globetrotting, and is reintroduced to its home turf with enough genetic differences to fool our immune systems. It’s kind of like the swallows’ annual return to San Juan Capistrano, only the swallows come back to give everyone runny noses and coughs.
Scientists still struggle, though, with what exactly triggers people getting infected with the reintroduced viruses in fall and winter. Researchers have proposed several explanations, which might work alone, simultaneously but separately, or in combination with each other. They include:
Weather and climate - The flu and colds appear to do very well in cold winter temperatures and the dry air that goes with them. They can survive longer in dry air than moist air and hold out longer on exposed surfaces (counters, doorknobs, keyboards, etc.) when they’re cold. Dry air means dehydrated mucus and drier nostrils and airways, which could make it easier for the viruses to make themselves at home once they're passed to us. A study on guinea pigs showed that the transmission of influenza is enhanced in dry (20 percent humidity), cold (41 degrees) air and declines as the temperature and humidity rise (at 86 degrees or 80 percent humidity, it wasn’t transmitted at all).
Human behavior - With school in session and people generally spending more time indoors and in close contact with each other, the viruses have an easy time being transmitted among sizable groups of hosts. Even in tropical and equatorial regions that don’t have a winter and where flu occurs throughout the year, there are spikes during the rainy season when people spend time together indoors.
Human physiology - Humans and many other mammals experience seasonal physiological changes, often tied to the light/dark cycle. In the winter, any number of tweaks to our immunity - such as a decrease in vitamin D production - could make us more susceptible to the virus for a few months out of the year.
Paging Dr. Grandma
Babcias, bubbies and nanas, it turns out, also have at least a little evidence on their side. A few years ago, Ron Eccles, director of the Common Cold Centre in Cardiff, UK, tested the idea that getting cold or damp might activate the viruses, and found that there might be a grain of truth to folk wisdom, and that being cold could indirectly contribute to getting sick.
Eccles chilled the feet of half of his subjects in the lab, and let the other half keep their feet warm in socks and shoes. After returning to their lives, people tracked their cold symptoms in journals. After four or five days, the subjects who got cold feet in Eccles’ experiment had more than twice as many sick people among their ranks (14.4% of the group) as the control group (5.6%).
One possible explanation for Eccles' results is that cold causes vasoconstriction, or a tightening of the blood vessels. This happens especially to the vessels close to the outside world, like in your nose, throat and mouth. This slows the flow of infection-fighting white blood cells to these areas, and also leads to dryness and hinders the nose's ability to filter the air.
(An important caveat to keep in mind with Eccles’ study is that people who had been cold in the lab only reported that they had cold symptoms afterwards. No medical tests were done to confirm that they had an infection.)
All this being said, we think you should feel free to go outside without your mittens on or with wet hair, unless Grandma is actually around. The bulk of the medical research isn’t on her side, but a stern look from Nana can trump all the empirical evidence in the world.
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