In fact, you're more likely to get sick swimming in a pool than a lake.
Jumping into a public pool is a classic summer activity, but new research from the Centers for Disease Control (CDC) suggests that perhaps you should think twice before taking the leap. The smell of chlorine that's closely associated with safety and cleanliness just isn't cutting it anymore, the CDC warns, as a number of pathogens have developed chlorine resistance.
Two recent studies by the CDC on treated vs. untreated "recreational water" have found that a person is more likely to get sick from swimming in a pool than in an natural body of water, like a lake or river. Chlorine-resistant bugs are now the leading cause of outbreaks, which are defined as "two or more persons contracting a similar illness from the same location and time of exposure."
"There were 493 outbreaks caused by treated recreational water from 2000 to 2014 (that’s from a pool, hot tub, spa, or water playground) resulting in 27,219 cases and eight deaths. Eight deaths. In that same time period there were only 140 outbreaks from nontreated water like lakes or rivers or oceans, and 4,958 cases of illness from those outbreaks. Two people died."
Of course, these numbers lack some context. It is impossible to know how many visits were made to natural swimming spots, compared to artificial ones. It's much easier to know how many public and private pools and hot tubs exist in the United States than it is to track all the beaches, lakes, and rivers where people swim. But one fact stands out -- that far more people get sick swimming at hotels than at beaches, where 32 percent of the outbreaks occurred. Hot tubs are even worse than the pool, representing 60 percent of the hotel-related outbreaks. Pools at public parks saw 23 percent of outbreaks and clubs or resorts were 14 percent.
The icky root cause is fecal transmission, a.k.a. human poop. From Popular Science again:
"About 80 percent of the illnesses contracted from treated water were caused by Cryptosporidium, a parasite that lives in animal intestines and spreads by shedding itself from feces into water sources. The illnesses from untreated water were more varied, but the main culprits still tended to come from poop. Norovirus was the single biggest contributor, at about 30 percent of all cases, and that’s spread through vomit and feces. The most common bacterial infection, Shigella, spreads through diarrhea."
How to avoid this? Stay out of pools if you can and head to the beach or lake instead. If that's not possible, at least stay out of hotel pools, particularly the hot tub. Carry a small pack of testing strips in your pool bag to ensure the chlorine levels are high enough. If you're a parent, don't let your kid swim if s/he has had diarrhea in the last few weeks. Be scrupulous about personal hygiene before entering the water. And please, please don't drink the pool water.