Here's how city life is actually affecting your healthS

There is a truism, often bolstered by pseudoscience, that people in the country are healthier than people in the city. Certainly there are many health problems associated with the metropolis. But it turns out that urbanites are often in better shape and live longer than their rural counterparts.

Most of our data on the health of people in cities comes from public health studies, like the one conducted by County Health Rankings in the United States, that compare the general health of people in various regions. Questions about regional health become more complicated when we look at it in a global perspective — partly due to vast differences in data-gathering practices — so in this article I'm going to focus mostly on studies that have explored U.S. health.

I began by saying that city dwellers are by and large healthier than their non-urban counterparts. But that's not true in every respect.

If you live in a city, you're more likely to develop asthma, allergies and dry eye.

People in cities, especially children, seem to be slightly more prone to allergies and asthma. One study suggests this is possibly because children in low-income urban areas are exposed to more toxins and stress at an early age, or suffer from more untreated respiratory illnesses. A similar pattern, with more asthma in cities, appears in a study of Scottish people. However, at least two U.S. studies suggest that asthma is equally prevalent in both the city and the country but isn't treated as aggressively among kids in the country. So it may appear that there are more asthma cases in cities simply because there are more health resources for parents who want to treat their kids — and thus, more parents report that their kids have asthma.

Also, city kids have more allergies, at least according to one study. This is a difficult claim to analyze, in part because there is so much disagreement over what causes allergies and even whether some conditions should be classified as allergies at all. Nevertheless, one theory behind this statistic is the "hygiene hypothesis," which suggests that people's immune systems don't grow as robust in artificially antiseptic and decontaminated environments. And such super-clean environments are more common in cities than in the country.

So city toxins may be causing asthma, and hyper-cleanliness may be causing the allergies. But dry eye is definitely caused by pollution.

Here's how city life is actually affecting your health

A higher percentage of urbanites will suffer anxiety disorders, mood disorders and maybe even schizophrenia.

One study found that people in U.S. cities do have higher rates of depression than people in rural areas, while another suggested that the prevalence of mood disorders is 21 percent higher in urban areas than rural ones, while the prevalence of anxiety is 39 percent higher. As many as ten studies have found that some cases of schizophrenia may be linked to urban environmental factors.

Why do cities affect people's mental health so negatively? Researchers who published a study in Nature suggested that it's possible humans' brains develop slightly differently in urban areas, predisposing them to stress-related disorders. They found that the amygdala, which processes emotion, was more active in people who were currently living in a city. In addition, the cingulate cortex, which helps regulate the amygdala and processes negative emotion, was more active for people who were raised in cities. The researchers caution that this doesn't mean that cities drive people crazy; it just means that people in cities may be more predisposed to stress-related conditions.

City dwellers have troubled circadian rhythms.

Finally, as if you weren't stressed out enough, cities are likely to affect your circadian rhythms, or your body's internal clock that tracks day and night. Many studies have shown that lights at night confuse this internal clock, as does urban night shift work. There are a number of health consequences when your circadian rhythms get out of whack, including depression, insomnia, inflammatory diseases, diabetes, and even cancer.

But if you leave aside all these problems, city life turns out to be a pretty great deal.

Here's how city life is actually affecting your healthS

In cities, there's less of a risk that you'll become obese.

This is one of those health statistics that's backed up by a lot of evidence. One study found that 39.6% of people in rural areas were obese, compared to 33.4% in cities. People in cities today tend to be more physically fit and less likely to eat unhealthy foods that can cause obesity. But is this related to something inherent to city life, or is it more of an economic issue like asthma? Most studies suggest that it's the latter. People in rural areas are generally not as wealthy as their urban counterparts, and this means less access to health information about food — and less access to foods like fresh vegetables and fruit, which are far more expensive than dinner at McDonalds.

You're less likely to die of an accident in a city. And your risk of suicide is a lot lower, too.

Your risk of injury-related death, whether that's a car accident or falling off a roof, is 20% lower if you're in a city. Some researchers have suggested this is partly because emergency medical services are not as robust in rural regions as they are in cities. So somebody in a car accident in a remote area is more likely to die than in a city, simply because help won't arrive in time.

You're also at far less of a risk for committing suicide in a city. Indeed, one study found that men in rural areas were 54% more likely to commit suicide than men in cities. There has been such a huge spike in rural suicides among (mostly white) men that some sociologists have dubbed it a "culture of suicide." Likely this is related to widespread unemployment in these regions, as well as the rural spirit of hyper-individualism, where people are not encouraged to seek help for personal problems.

Your old age will be more pleasant and healthy.

People over the age of 65 reported a better quality of life in cities. Researchers believe that this may be because seniors in rural areas are more isolated and have less social connections than a city allows. It's also possible that you'll live longer in a city, though evidence for this claim is in dispute — mostly because better access to health care in a city can bias the evidence.

What isn't in dispute is that cities do offer more opportunities for seniors to socialize, which contributes greatly to happiness. And there are more health services for older people as well, which can help extend life.

Here's how city life is actually affecting your healthS

You likely have better TB resistance, and probably can eat cheese, ice cream and other milk products without gut pain.

A vast number of people on the planet today can eat milk products as part of their daily diet, all thanks to a recent genetic mutation that spread like wildfire through cities starting about 12,000 years ago. People who don't have this mutation are called lactose intolerant, and they experience very unpleasant gastric distress when they try to digest dairy.

Many scientists believe this odd, dairy-friendly mutation spread so rapidly because it conferred an incredible survival advantage on people who had left the nomadic life behind to settle in cities. One of the main sources of food in these cities would have been from farming, and dairy would have been a key source of nutrition. People who couldn't get nutrients from milk products would not have coped well with urban life. As cities spread, so too did this urban mutation, until nearly everyone in the western world was chomping on cheese and cream.

It's also likely that humans packed together in cities evolved greater resistance to tuberculosis than their country brethren. So cities were making us healthier even thousands of years ago (though today cheese might be counted on the "unhealthy" side of the ledger).

So which is better for humanity? A world of cities or a world that's gone back to nature? Ultimately, says the CDC, it's probably neither. One statistic that emerges from all these studies is that the healthiest people of all tend to live in suburbs — especially wealthy ones. People in those areas can afford the very best medical care, and thus they're healthier. What truly matters, no matter where you live, is a good healthcare system with lots of social support. Once you have that, you're going to be healthier — even if you're living on the Moon.

Annalee Newitz is editor-in-chief of io9. She is also the author of Scatter, Adapt and Remember: How Humans Will Survive a Mass Extinction.

Additional reporting on this story by Joseph Bennington-Castro.