What doesn’t kill us makes us stronger, or so the saying goes. If you believe that, you might assume that living in a city packed with people, pests, and the countless microorganisms they lug around would strengthen a person’s immune function. But is that true? The short answer: Maybe. It’s complicated.
“There’s some biological plausibility that specific microbes in [a person’s] environment can affect immune function,” says Elizabeth Matsui, a professor of pediatrics, epidemiology, and environmental sciences at Johns Hopkins University.
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Matsui brings up something called the “hygiene hypothesis,” which is the science-backed idea that early life exposure to microbes can teach a person’s immune system to stay cool in the presence of allergens and other invaders. But when it comes to city environments—as opposed to rural or suburban settings—the evidence on immune benefits is mixed.
For example, there’s a growing body of research that suggests growing up on a farm and around animals can lower a person’s risk for allergies, asthma, and hay fever. Especially if you spend the first year of your life around stables, you’re likely to enjoy a drop in allergy and asthma risk, a European study shows.
Some related research from China shows kids who grow up in rural areas experience lower rates of asthma and allergy than city kids. More research shows that kids born in developing countries who move to the US tend to start out with lower rates of allergies and asthma, but that rates of both tick up after they’ve lived in ultra-sterilized, germophobic America for a decade or longer.
But before you write off cities, Matsui says some recent research has linked early life exposure to cats, mice, and cockroaches in urban environments to immune system benefits. “What we suspect is there are specific microbes associated with mice and cockroaches that may be conferring some protection,” she says.
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You may be picking up on a trend here, which is that young kids exposed to critters and their germs tend to be better off. Soil, too, seems to harbor microorganisms that can ramp up a person’s immune system if he or she spends time digging in the dirt at a young age. But whether one environment—city or country—is superior for a person’s immune function is not a simple question to answer, says Melody Carter, a staff clinician at the National Institute of Allergy and Infectious Diseases.
Carter says your genetic predisposition—coupled with your specific microbial environment and your age of exposure—all come together to change how your immune system reacts to threats. “It’s not one thing but a combination of things,” she says.
Pollution is also a factor. “Pollution particles increase sensitivity to allergens,” Carter says. If you live on a street with heavy vehicle or bus traffic—something that’s a lot more likely in a city or suburb than in a rural setting—that exposure isn’t doing your immune function any favors. On the other hand, exposure to agricultural pesticides may be a serious health risk for people in rural communities.
Finally, when it comes to your built-in defenses against colds and flu, catching other people’s sick germs can gradually build up your resistance to those pathogens, Carter says.
“Once you catch a viral illness, your body builds up immunity to that virus so you can’t get it again,” she says. If you’re riding the subway every day in a city, you’re probably more likely to be exposed to illness-causing pathogens than someone living in a more isolated locale. But there are more than 200 common cold viruses, Carter adds, so trying to catch them all to make yourself impervious isn’t really a workable option. Still, it’s nice to know the colds you’ve had to deal with can’t strike you down a second time.
Put all this together and what do you get? A lot of unanswered questions. “I hear parents saying they’re going to move to a farm to protect their kids from allergies, and that makes me worry,” Matsui says. “I think the data supports the idea that living in a sterile environment is bad, but when you try to transfer findings from a population level to an individual, I think that is fraught.”
“There are some things in rural environments that seem to be protective—especially early in life—but the same is true of some things in urban environments,” she adds. To underscore just how much a person’s risk for immune issues can vary even within a single urban environment, consider a recent New York City report that found rates of asthma among school-aged kids ranges from 3 percent to 19 percent depending on the youngster’s neighborhood.
Staying away from pollution and pesticides is a good idea. And exposing young kids to pets, gardens, and other safe sources of immune-boosting microbes also seems to be beneficial. But if you’re wondering whether your city, suburban, or rural ’hood is doing your immune system any favors, experts today have few cut-and-dry answers.
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