Urine-indicator dye is a substance that is supposed to be able to react with urine to form a colored cloud in a swimming pool or hot tub, thus indicating the location of people who are urinating while they are in the water. A 2015 report from the National Swimming Pool Foundation called this "the most common pool myth of all time", with nearly half of Americans surveyed by researchers believing that the dye existed.
Urine is difficult to detect as many of the naturally occurring compounds within urine are unstable and react freely with common disinfectants like chlorine, creating a large number of disinfection by-product (DBP) compounds from the original organic chemicals in urine.
In an article published in 2000, Snopes confirmed such a dye did not exist. However, a study published by the University of Alberta in 2017 identified urine in hot tubs and swimming pools based on other markers such as acesulfame potassium, used extensively as an artificial sweetener, being passed chemically unchanged in urine, and not suffering from DBP-related changes in water.
Rumours of the origin of urine indicator-dye go back at least as far as 1958, and the story is commonly told to children by parents who do not wish them to urinate in the pool. A 1985 biography of Orson Welles describes him using such a dye as part of a prank in 1937.
- Mikkelson, Barbara (December 14, 2000). "Piscine of the Crime". Snopes. Retrieved September 18, 2012.
- O'Neil, Lauren (25 June 2015). "Pee, not chlorine, causes red eyes from swimming pools: CDC". CBC News. Retrieved 26 June 2015.
- Lindsay K. Jmaiff Blackstock; et al. (1 March 2017). "Sweetened Swimming Pools and Hot Tubs". Environmental Science & Technology Letters. 4 (4): 149–153. doi:10.1021/acs.estlett.7b00043.