Just how much pee is in that pool?

You know that sharp odor of chlorine from the swimming pool you can recall from earliest childhood? It turns out it’s not just chlorine, but a potent brew of chemicals that form when chlorine meets sweat, body oils, and urine.

But up until now, just how much urine has been difficult to measure, says chemist Xing-Fang Li of the University of Alberta. Li and her colleagues report they can now tell roughly how much pee is in a pool by measuring the artificial sweeteners carried in most people’s urine. Certain sweeteners can be a good proxy for pee, she says, because they’re designed to “go right through you” and don’t break down readily in pool water.

The scientists calculated that one 220,000-gallon, commercial-size swimming pool contained almost 20 gallons of urine. In a residential pool (20-by-40-foot, five-feet deep), that would translate to about two gallons of pee. It’s only about one-hundredth of a percent, but any urine in a swimming pool can be a health concern for some people, not to mention that smell that never quite goes away.