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Does Being Cold Make You More Susceptible to Getting a Cold?

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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.

Today we're answering 20 big questions like this one. We'll plan more days like this, so if you have something you're dying to know, leave us a comment or tweet @mental_floss with the hashtag #bigquestions.

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Big Questions
Why Does Turkey Make You Tired?
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Why do people have such a hard time staying awake after Thanksgiving dinner? Most people blame tryptophan, but that's not really the main culprit. And what is tryptophan, anyway?

Tryptophan is an amino acid that the body uses in the processes of making vitamin B3 and serotonin, a neurotransmitter that helps regulate sleep. It can't be produced by our bodies, so we need to get it through our diet. From which foods, exactly? Turkey, of course, but also other meats, chocolate, bananas, mangoes, dairy products, eggs, chickpeas, peanuts, and a slew of other foods. Some of these foods, like cheddar cheese, have more tryptophan per gram than turkey. Tryptophan doesn't have much of an impact unless it's taken on an empty stomach and in an amount larger than what we're getting from our drumstick. So why does turkey get the rap as a one-way ticket to a nap?

The urge to snooze is more the fault of the average Thanksgiving meal and all the food and booze that go with it. Here are a few things that play into the nap factor:

Fats: That turkey skin is delicious, but fats take a lot of energy to digest, so the body redirects blood to the digestive system. Reduced blood flow in the rest of the body means reduced energy.

Alcohol: What Homer Simpson called the cause of—and solution to—all of life's problems is also a central nervous system depressant.

Overeating: Same deal as fats. It takes a lot of energy to digest a big feast (the average Thanksgiving meal contains 3000 calories and 229 grams of fat), so blood is sent to the digestive process system, leaving the brain a little tired.

Have you got a Big Question you'd like us to answer? If so, let us know by emailing us at bigquestions@mentalfloss.com.

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Big Questions
How Are Balloons Chosen for the Macy's Thanksgiving Day Parade?
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The balloons for this year's Macy’s Thanksgiving Day Parade range from the classics like Charlie Brown to more modern characters who have debuted in the past few years, including The Elf On The Shelf. New to the parade this year are Olaf from Disney's Frozen and Chase from Paw Patrol. But how does the retail giant choose which characters will appear in the lineup?

Balloon characters are chosen in different ways. For example, in 2011, Macy’s requested B. Boy after parade organizers saw the Tim Burton retrospective at the Museum of Modern Art. (The company had been adding a series of art balloons to the parade lineup since 2005, which it called the Blue Sky Gallery.) When it comes to commercial balloons, though, it appears to be all about the Benjamins.

First-time balloons cost at least $190,000—this covers admission into the parade and the cost of balloon construction. After the initial year, companies can expect to pay Macy’s about $90,000 to get a character into the parade lineup. If you consider that the balloons are out for only an hour or so, that’s about $1500 a minute.

Have you got a Big Question you'd like us to answer? If so, let us know by emailing us at bigquestions@mentalfloss.com.

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