A couple of weeks ago, the temperature in New York dropped something like 30 degrees in the span of a day. A month or so earlier, the city had seen the reverse— a 40-something-degree day was followed soon after by an unseasonable 70-degree one. Weather is my favorite boring thing to talk about, so whenever I saw someone after these rapid temperature changes I said something like, “Crazy weather, huh?” Usually they replied “Yep!” or “I know,” bringing our conversation to a welcome early close. But a few friends I spoke to instead took this opportunity to introduce my second-favorite boring discussion topic: mild seasonal illness. “I’m sick,” they’d say. “It’s because of the weather.” Always, I’d nod. This is a line of thinking I have heretofore taken as self-evident truth—a sizable change in temperature, in either direction, can give you a cold, or a cough, or a sore throat. The reason is ... well, I didn’t know the reason, actually. (Something about barometric pressure, maybe?) Many places experience a number of wacky weather reversals at every changing season, but not everyone falls sick every time. So who is susceptible, and why?

As Ray Casciari, a pulmonologist at St. Joseph Hospital in Orange, California, told me, changing weather can make you sick, but it has less to do with the actual temperature change, and more to do with environmental effects associated with those changes. Research shows that, despite what your mom has told you, simply existing in cold weather isn’t, itself, likely to make you get sick. A 2002 meta-analysis examined related studies, and concluded that exposing one’s skin to cold temperatures does not put one automatically at risk for contracting the common cold. As such, there is no direct link between avoiding the cold and staying healthy. As Casciari puts it, “Staying indoors, bundling up, and not exercising is supposed to make you get better faster. Not so.”

If you’re exposed to a sudden drop in temperature, your risk of infection does increase, but not simply because of the temperature itself. It’s because of the drop in humidity that likely accompanies it. For that reason, “real” temperature changes in the weather and artificial ones (like walking into brisk air-conditioning from outside on a hot day) can both affect your health in different ways, depending on the types of environments these temperatures help create. In a low-humidity environment, “your eyes tend to dry out, the mucous membranes in your nose dry out, and your lungs dry out, and you’re therefore much more susceptible to bacteria and viruses,” Casciari says. It’s more likely that someone would get sick after a rapid drop in temperature than after a rapid increase in temperature, because viruses themselves can survive longer in the cold. “Many viruses live longer and can replicate faster in colder temperatures. As a result, a highly contagious virus such as influenza can stay active and linger for up to 24 hours on a hard surface,” says Katharine Miao, the medical director at CityMD. It’s not so much that the cold creates the infection; rather, colder temperatures allow it to survive and spread.

There is a social component to this phenomenon, too—as Casciari explains it, where people congregate, illness spreads. This could mean a crowded day at the park or the zoo on an unseasonably warm day, but more often, people gather indoors, in more enclosed spaces, when the weather gets cold. “When you’re [gathered indoors], where you’re all really close together, and it’s hot and humid because you’ve got a lot of people in there, that’s when viruses like the common cold and influenza and certain bacteria actually spread,” says Casciari. While you can pick up a virus anywhere (hot or cold, indoors or out), they’re more likely to spread in crowded, humid environments. Like many human mouths in close proximity. So if you were looking for an excuse to skip your office holiday party, there you go.

There is good news here: Because getting sick after a rapid temperature change is largely due to correlational factors like humidity and crowdedness, there is something you can do to prevent it. Casciari suggests liberal use of alcohol wipes in public, and particularly on airplanes. Miao suggests combating winter’s dry air by using a humidifier. And finally, don’t let the placebo effect win: Just because the temperature dropped or rose 20 degrees in a day, it doesn’t mean you’ll definitely get sick. “If you took a person and you stuck them in a completely sterile room, and you raised and lowered the temperature, they wouldn’t be any more likely to get infected,” says Casciari. Of course, rapidly changing and unseasonably warm weather are concerning for many other reasons, but let’s deal with things as they come.

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