Let’s just start with the premise that the next pool you dive into will contain pee. Because it probably will. There’s also probably going to be a wee bit of fecal matter, a dollop of human sweat and some guy’s skin cells floating around.
Swimming pools are basically huge blue toilet bowls. We’ve all peed in them — be honest — and a new study is stirring up our guilt by showing that urinating in a chlorinated pool creates a toxic chemical called cyanogen chloride. Cyanogen chloride forms when chlorine from the pool reacts with nitrogen in urine. It acts like tear gas, roughing up the eyes, nose and lungs, and it’s classified as an agent of chemical warfare.
Predictably, the study quickly resulted in headlines like “Why peeing in the pool is chemical warfare.”
But in the grand scheme of bad things that can happen to you in a swimming pool, how bad is this? Do you really need to worry about dangerous urine-induced chemicals when you take that next dive?
In the new study, researchers mixed uric acid (found in urine) with chlorine in the laboratory. In their worst-case-scenario cocktail of substances mimicking both urine and sweat mixed with high levels of chlorine, the researchers found about 30 micrograms per liter (or parts per billion) of cyanogen chloride. That’s still well below the World Health Organization guideline of 70 parts per billion as a maximum cyanogen concentration in drinking water.
Again, that was a maximum in the lab, not in real swimming pool water. In an interesting thought experiment, Casey Johnston at Ars Technica calculated how much pee it might take for an Olympic-sized pool to produce cyanogen chloride at a level that would quickly cause “coma, convulsions and death”: 2,500 parts per billion. Her answer:
“In the end, we need a pool that is two parts water to one part chlorine and would probably burn the eyeballs out of your sockets and make your skin peel away from your bones.… If you and three million other people could get at this pool and unload your pee into it before your bodies melted, before the crowd crushed you to death, and before you drowned from the massive tidal wave of pee … yes, you could feasibly die of cyanogen chloride poisoning.”
The authors of the study don’t seem to know quite what to make of this. Ernest Blatchley of Purdue University said by e-mail that the team hasn’t had time to fully evaluate the calculations, and Jing Li of China Agricultural University in Beijing questioned the assumptions in the math, including the concentration of uric acid that set the rate of cyanogen chloride production.
I think it’s fair to say that details aside, it would take a heck of a lot of urine to turn a pool so toxic that it would kill you outright. But could the much smaller amounts generated in an actual pool do any harm? The researchers point out in their paper that the overall mix of by-products created by pool chlorination “have been linked to both acute and chronic adverse health effects among swimmers.” And it would be easy to eliminate essentially all of the cyanogen chloride: Just don’t pee in the pool.
Tucked in the paper’s supplemental material are calculations the researchers did for a small residential swimming pool used by 20 people. With each swimmer urinating an average of 50 milliliters, or about enough to fill a shot glass, the pool would contain only about 12 micrograms per liter of cyanogen chloride. That’s 12 parts per billion; not much, but more chemical warfare agent than one might ideally like in a swimming pool.
The pee-chlorine combo also makes trichloramine (NCl3), another lung irritant, and just a touch of chloroform. In recent years scientists have wondered whether exposure to trichloramine is linked to rising rates of childhood asthma (so far, a 2010 review reports, the link is “suggestive but not conclusive.”)
If you have asthma or swim frequently, pool chemicals may cause respiratory problems, even without the urine effect. But for someone like me who swims infrequently and doesn’t have other health problems, it’s probably not going to keep me out of the pool.
The study was published February 25 in Environmental Science & Technology. I used to write for the news section of this journal, where I learned daily that we’re surrounded by toxic substances, so you have to take a risk-management approach to life. When weighing the risks of exposure to small amounts of chemicals, consider the likelihood of other things that could actually kill you, or at least make you sick, in a swimming pool:
Drowning. This is by far your biggest risk, especially if you’re male. In the United States, about two out of every million men ages 20-24 drown in swimming pools each year.
Your poolmates killing you for peeing in the pool. Could happen.
Fecal coliform bacteria. Kids wearing diapers in the pool: enough said.
- The other 100 disinfection by-products in pool water, if you really want to worry. Some are known mutagens, meaning they cause mutations, which could lead to cancer. But one review found that even with all these chemicals, swimming pool water is about as mutagenic as drinking water.
I do not, however, recommend drinking the water in the swimming pool. It is, after all, full of chlorine and pee.
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