If you’ve been on the internet at all in the past week, you’ve probably seen a headline like this: “Urine (Not Chlorine) Causes Red Eyes in Pools.” And that’s simply not true.

When I was in high school, I worked for a swimming pool company. It’s not really a germane fact, it’s just an explanation of why I started getting twitchy when I saw this bit of wrong news getting repeated all over the place. It all started when someone from the CDC put together this “healthy swimming” pamphlet. On the last page you can see the following:

“Smell that “chlorine”? It’s not what you think. What you smell are actually chemicals that form when chlorine mixes with pee, poop, sweat, and dirt from swimmers’ bodies. Yuck! These chemicals—not chlorine—can cause your eyes to get red and sting, make your nose run, and make you cough.”

Then someone went on “Today” and said that “when we go swimming and we complain that our eyes are red, it’s because swimmers have peed in the water.”

But no, that’s not quite right. It’s probably pretty likely at an indoor public swimming pool, but it’s not the direct cause and it’s certainly not the only possible cause. As usual, there’s way more to it. I’ll explain.

What you’re smelling, that “chlorine” smell, isn’t chlorine. Well, most of it isn’t, anyway. It’s the smell of related molecules called “chloramines.” Chloramines form when free chlorine comes into contact with (and combines with) ammonia and nitrogen. Chloramines aren’t particularly good at cleaning your pool, and they irritate your eyes and skin, and they smell, well, like a public swimming pool.

Now, some of the sources of ammonia and nitrogen are human bodily fluids. Yes, urine, but also sweat — remember, you’re exercising when you’re in the pool, so you’re going to sweat a lot, in the pints per hour range for athletes — plus saliva, mucous, and so on.

But one of the biggest contributors to chloramine formation, at least in outdoor pools, is rain. Rain isn’t just pure water — it’s mixed with whatever’s in the atmosphere, and whatever’s on the ground before it runs into your pool. Rain introduces all sorts of nitrogen, as well as ammonia and phosphates from runoff, into the pool, and drives the formation of chloramines.

What I think the CDC was trying to get at is that a very chloramine-heavy pool is less likely to have much free chlorine in it — since much of the chlorine will have been converted to less-effective chloramines — and is therefore less likely to be sanitary. But that’s definitely not the same thing as saying that it’s pee in the pool that’s making your eyes red.

But I guess that’s not as catchy a headline.

***

Richard Ford Burley is a doctoral candidate in English at Boston College, where he’s writing about remix culture and the processes that generate texts in the Middle Ages and on the internet. In his spare time he writes about science and skepticism (and swimming pool chemistry) here at This Week In Tomorrow.