Enlarge / You know why that kid is smiling. (credit: Getty | Imgorthand)

As Ars has reported before, those disturbing warm spots create harmful chemicals. Urine and chlorinated water react to form trichloramine and cyanogen chloride. At certain doses, these can cause respiratory and eye irritation. At high doses, cyanogen chloride can kill—it’s considered a chemical warfare agent. The levels found in pools, however, are nearly negligible. Long-term exposure by professional swimmers and pool workers may be linked to asthma, but otherwise it’s unlikely to cause many problems. And as Ars has also reported, you’d need a hellish scenario of two-parts water, one-part chlorine, and the wee of three million people in a pool to get to a lethal situation.

Still, it’s hard not to be curious about how much pee we let slip while taking a dip—and now we have the answer.