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

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Sledding image via Shutterstock

“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
Should You Keep Your Pets Indoors During the Solar Eclipse?
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By now, you probably know what you’ll be doing on August 21, when a total solar eclipse makes its way across the continental United States. You’ve had your safety glasses ready since January (and have confirmed that they’ll actually protect your retinas), you’ve picked out the perfect vantage point in your area for the best view, and you’ve memorized Nikon’s tips for how to take pictures of this rare celestial phenomenon. Still, it feels like you’re forgetting something … and it’s probably the thing that's been right under your nose, and sitting on your lap, the whole time: your pets.

Even if you’ve never witnessed a solar eclipse, you undoubtedly know that you’re never supposed to look directly at the sun during one. But what about your four-legged family members? Shouldn’t Fido be fitted with a pair of eclipse glasses before he heads out for his daily walk? Could Princess Kitty be in danger of having her peepers singed if she’s lounging on her favorite windowsill? While, like humans, looking directly at the sun during a solar eclipse does pose the potential of doing harm to a pet’s eyes, it’s unlikely that the thought would even occur to the little ball of fluff.

“It’s no different than any other day,” Angela Speck, co-chair of the AAS National Solar Eclipse Task Force, explained during a NASA briefing in June. “On a normal day, your pets don’t try to look at the sun and therefore don’t damage their eyes, so on this day they’re not going to do it either. It is not a concern, letting them outside. All that’s happened is we’ve blocked out the sun, it’s not more dangerous. So I think that people who have pets want to think about that. I’m not going to worry about my cat.”

Dr. Jessica Vogelsang, a veterinarian, author, and founder of pawcurious, echoed Speck’s statement, but allowed that there’s no such thing as being too cautious. “It’s hard for me to criticize such a well-meaning warning, because there’s really no harm in following the advice to keep pets inside during the eclipse,” Vogelsang told Snopes. “It’s better to be too cautious than not cautious enough. But in the interest of offering a realistic risk assessment, the likelihood of a pet ruining their eyes the same way a human would during an eclipse is much lower—not because the damage would be any less were they to stare at the sun, but because, from a behavior standpoint, dogs and cats just don’t have any interest in doing so. We tend to extrapolate a lot of things from people to pets that just doesn’t bear out, and this is one of them.

“I’ve seen lots of warnings from the astronomy community and the human medical community about the theoretical dangers of pets and eclipses, but I’m not sure if any of them really know animal behavior all that well," Vogelsang continued. "It’s not like there’s a big outcry from the wildlife community to go chase down coyotes and hawks and bears and give them goggles either. While we in the veterinary community absolutely appreciate people being concerned about their pets’ wellbeing, this is a non-issue for us.”

The bigger issue, according to several experts, would be with pets who are already sensitive to Mother Nature. "If you have the sort of pet that's normally sensitive to shifts in the weather, they might be disturbed by just the whole vibe because the temperature will drop and the sky will get dark,” Melanie Monteiro, a pet safety expert and author of The Safe-Dog Handbook: A Complete Guide to Protecting Your Pooch, Indoors and Out, told TODAY.

“If [your pets] have learned some association with it getting darker, they will show that behavior or at a minimum they get confused because the timeframe does not correspond,” Dr. Carlo Siracusa of Penn Vet Hospital told CBS Philly. “You might put the blinds down, but not exactly when the dark is coming but when it is still light.” 

While Monteiro again reasserts that, "Dogs and cats don't normally look up into the sun, so you don't need to get any special eye protection for your pets,” she says that it’s never a bad idea to take some extra precautions. So if you’re headed out to an eclipse viewing party, why not do your pets a favor and leave them at home. They won’t even know what they’re missing.

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Big Questions
Why Can't Dogs Eat Chocolate?
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Even if you don’t have a dog, you probably know that they can’t eat chocolate; it’s one of the most well-known toxic substances for canines (and felines, for that matter). But just what is it about chocolate that is so toxic to dogs? Why can't dogs eat chocolate when we eat it all the time without incident?

It comes down to theobromine, a chemical in chocolate that humans can metabolize easily, but dogs cannot. “They just can’t break it down as fast as humans and so therefore, when they consume it, it can cause illness,” Mike Topper, president of the American Veterinary Medical Association, tells Mental Floss.

The toxic effects of this slow metabolization can range from a mild upset stomach to seizures, heart failure, and even death. If your dog does eat chocolate, they may get thirsty, have diarrhea, and become hyperactive and shaky. If things get really bad, that hyperactivity could turn into seizures, and they could develop an arrhythmia and have a heart attack.

While cats are even more sensitive to theobromine, they’re less likely to eat chocolate in the first place. They’re much more picky eaters, and some research has found that they can’t taste sweetness. Dogs, on the other hand, are much more likely to sit at your feet with those big, mournful eyes begging for a taste of whatever you're eating, including chocolate. (They've also been known to just swipe it off the counter when you’re not looking.)

If your dog gets a hold of your favorite candy bar, it’s best to get them to the vet within two hours. The theobromine is metabolized slowly, “therefore, if we can get it out of the stomach there will be less there to metabolize,” Topper says. Your vet might be able to induce vomiting and give your dog activated charcoal to block the absorption of the theobromine. Intravenous fluids can also help flush it out of your dog’s system before it becomes lethal.

The toxicity varies based on what kind of chocolate it is (milk chocolate has a lower dose of theobromine than dark chocolate, and baking chocolate has an especially concentrated dose), the size of your dog, and whether or not the dog has preexisting health problems, like kidney or heart issues. While any dog is going to get sick, a small, old, or unhealthy dog won't be able to handle the toxic effects as well as a large, young, healthy dog could. “A Great Dane who eats two Hershey’s kisses may not have the same [reaction] that a miniature Chihuahua that eats four Hershey’s kisses has,” Topper explains. The former might only get diarrhea, while the latter probably needs veterinary attention.

Even if you have a big dog, you shouldn’t just play it by ear, though. PetMD has a handy calculator to see just what risk levels your dog faces if he or she eats chocolate, based on the dog’s size and the amount eaten. But if your dog has already ingested chocolate, petMD shouldn’t be your go-to source. Call your vet's office, where they are already familiar with your dog’s size, age, and condition. They can give you the best advice on how toxic the dose might be and how urgent the situation is.

So if your dog eats chocolate, you’re better off paying a few hundred dollars at the vet to make your dog puke than waiting until it’s too late.

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