Diving into a swimming pool is a lot like eating a hot dog — both are hallmark pleasures of summer, so long as no one talks about what’s in them. Whether you’re leaping into an indoor swimming pool where legions of kids are learning to swim or heading down to the timeless neighborhood institution known as the public swimming pool, it’s a given that the water will be full of sweat, Band-Aids, communicable diseases, and at least pone kid who is definitely peeing — or worse. Chlorine can help to keep swimming pool bugs under control, but chemicals come with other risks for young swimmers who are undoubtedly swallowing the dirty pool water. Anyone who’s ever second-guessed jumping in might want to trust that instinct.

Swimming pools can increase their chlorine content to the point of killing every parasite, but there’s evidence that this can increase the risk of asthma attacks and even cancer. However, if there’s not enough chlorine in the water, there’s the danger of parasites like cryptosporidium and bacteria like E. coli lurking below. Adding to that complexity in 2021 is the serious chlorine shortage, brought on by a pandemic-year boom in backyard swimming pools (up 23 percent over 2019) and a catastrophic fire at the Louisiana plant that supplies most of the nation’s chlorine tablets.

It would be refreshing to believe that there’s a clean way to resolve this problem. There isn’t. There is only the filthy truth.

When Your Pool Is Too Dirty

Epidemiologists love nothing more than tracking pool-based outbreaks. In 1954, for instance, researchers identified a new bacterial species, Mycobacterium balnei, just from rifling through swimming pool data. Swedish patients were suffering from skin lesions on their elbows, and all had curiously suffered minor scrapes in the same swimming pools. Since lab ethics weren’t really a thing in the ’50s, the researchers proved they had found the right bacteria by isolating a sample from of the infected pools and heroically injecting it into their elbows. But Sweden wasn’t alone. That same year, Washington, D.C., faced an epidemic of pharyngeal-conjunctival fever, again associated with unsanitary swimming pools. This time it was a virus, and the far more conservative scientists at the National Institutes of Health isolated it in sterile Petri dishes.

Lest we think of swimming pool epidemics as a thing of the past, bacterial skin disease is now so common among people who spend their days in swimming pools that one study suggested it should be recognized as an occupational disease for hydrotherapists, while another from Germany found that babies who stay away from pools have lower rates of diarrhea, ear infection, and airway infection.

Remember the water-birthing fad? We now know it leads to Legionnaires disease, a life-threatening form of pneumonia — because good luck finding water that isn’t infected with Legionella bacteria. And, in a fitting twist, in 2013, researchers described how relatives of the 1954 mycobacteria continue to thrive in indoor swimming pools, causing a telltale cough and fever in lifeguards, nicknamed “lifeguard lung.” The scariest part is that mycobacteria are everywhere — and notoriously resistant to chlorine. “Others have reported high numbers of mycobacteria in swimming pools and whirlpools and in hot tubs,” the review continues, citing numerous prior studies. “Mycobacteria are resistant to chlorine.”

Meanwhile, cryptosporidium, a chlorine-resistant parasite that spreads via feces and causes weeks of diarrhea, is so common in swimming pools that the Centers for Disease Control and Prevention created a special database just to keep tabs on it. In 2000, CDC reported that there was a problem, and that crypto cases had doubled in just one year. CDC began tracking crypto outbreaks and found that in the United States cases had jumped an average 13 percent every year, from 2009 to 2017 — and a third of those came from swimming pools.

Now, these pathogens may be chlorine-resistant. But that doesn’t mean we can’t always bump up the chemicals in our local pools. Enough chlorine or bromine can kill pretty much anything. So why not swim (or birth) in a pool full of chemicals? Which brings us to our next problem…

When Your Pool Is Too Clean

Chlorine is a double-edged sword. It kills bacteria and parasites, sure, but studies suggest it doesn’t exactly do wonders for our bodies. While we were off tracking diseases in under-treated pools, one 2002 study claimed we missed a larger threat. “Old and even more recent reports on indoor pollution do not deal with the air of chlorinated swimming pools,” the authors write. “Despite the generally obvious and readily noticeable irritant character of this type of environment, even in well-maintained pools.” They were talking about chlorine, of course, and how it can toy with our airways. Indeed, five years later scientists examined 800 adolescents who consistently visited indoor and outdoor pools, and found that those with asthma were more likely to suffer acute attacks the longer they spent in any body of chlorinated water.

More controversial is the claim that chlorine in swimming pools could be linked to cancer. It’s hard to know for sure. On one hand swimming pools are chlorinated with hypochlorite salts, which the International Agency for Research on Cancer does not consider carcinogenic. On the other hand, preliminary studies have suggested links between long-term exposure to chlorinated water and increased risk of bladder and colorectal cancer. That could be because, even though hypochlorite salts aren’t carcinogenic, chlorination by-products such as trihalomethanes (THMs) and haloacetic acids (HAAs) have been shown to cause cancer in laboratory animals.

How to Swim Without Getting Diarrhea (Or Dying)

It is important to note that none of the risks mentioned above are particularly commonplace. There were only 32 cryptosporidium outbreaks in the U.S. in 2016, and only two cases of Legionnaire’s disease recorded in infants via water birth in 2017. And while a 1992 study suggested that chlorinated water may account for up to 4,200 cases of bladder cancer in the U.S. each year, only about 0.03 percent of Americans get bladder cancer in the first place.

But neither risk — too much chlorine or too little — is going away. The authors of the 2002 study on chlorine risks conclude by suggesting that, until we know more about the dangers of long-term exposure to chlorine, we ensure that pools are well-ventilated and, perhaps, that bathers shower before entering the pool so there will be less of a need for chlorination in the first place. It’s a nice thought, but there will always be a population that skips showering. Which is why the authors of the 1992 cancer study say we’re still safer with chlorine than without it. “The potential health risks of microbial contamination … greatly exceed the risks [of chlorine],” they told The New York Times. So if you have to choose between crypto and chlorine — embrace the chemicals.

This article was originally published on June 9, 2017