A new government report shows one in eight public swimming pools were shut down two years ago because of dirty water or other problems, like missing safety equipment.
Kiddie pools were most likely to be the germiest, from fecal matter and improper chlorination.
The report is based on more than 120,000 inspections of public swimming pools in 2008, including those in parks and hotels. It's the largest study of the topic ever done by the Centers for Disease Control and Prevention, which released the report Thursday.
Each year, there are about 15 or 20 outbreaks from stomach bugs blamed on pools, the CDC said. Studies suggest a quarter of them are caused by bacteria, viruses or parasites that should have been killed by proper pool treatment and chlorination.
Fecal particles are a common factor, especially in kiddie pools and fountains where children frolic. But urine is also a problem: It contains nitrogen that eats up chlorine in pool water, depleting the supply. Sweat and suntan lotion have the same effect.
And about one in five adults admit they have peed in the pool, according to a survey of 1,000 Americans done last year for a chemical industry advisory group, the Water Quality and Health Council.
Reports of pool-related illness have been on the upswing for much of the last decade, but it's not clear whether conditions are worse or whether there's more awareness and testing, said Michele Hlavsa, chief of the CDC's swimming pool program.
"We definitely need to focus on improving pool operations," she added.
Before you go swimming, the CDC suggests buying test kits from hardware stores and checking the water at public pools. Health officials also say people who've had diarrhea shouldn't swim; and everyone should avoid swallowing pool water.
The inspections were done in 13 states, and pool regulations and reporting varied. The CDC report did not give a breakdown of pool closures from water quality problems or other issues, such as missing life rings or other safety equipment.
The CDC did a similar study in 2002 in five states. That smaller study found 1 in 12 inspections resulted in immediate closure. Neither has enough data to give a precise picture of the nation as a whole.
The new study is published in a CDC publication, Morbidity and Mortality Weekly Report.