Bob Bowman, longtime coach of Olympic superstar Michael Phelps, says Rio's polluted waters pose a health risk to open-water swimmers that must be addressed before the 2016 Games.

Pollution at beach venues hosting open-water swimming and sailing won't affect swimmers like Phelps in the competition pool.

But Bowman said on Wednesday he has seen first hand the effects it can have on those who are exposed to it.

"In terms of the water quality of the open-water swimming, yes, I have great concerns about it," Bowman said before the US Swimming Championships, which started in San Antonio on Thursday.

"I happened to coach a young lady at the 2007 Pan Am Games there, and she came back and has been sick the rest of her life with a life-altering illness," he said.

Bowman was talking about Kalyn Keller, who finished fourth in the women's Pan Am 10 kilometer open-water race that year and was later diagnosed with Crohn's Disease, an inflammatory bowel disease.

The illness pushed her into a reluctant early retirement in 2008.

Bowman said Chip Peterson, who won the men's open water silver at the 2007 Pan Ams, also had ongoing health problems.

Within weeks of the race in Rio, Peterson learned he had ulcerative colitis, which gave him intermittent episodes of abdominal pain, fever diarrhea and blood loss.

"Same thing," Bowman said. "So I know first-hand that it's serious. They've got to do something about it."

International Olympic Committee president Thomas Bach insisted this week that an Olympic warm-up triathlon held over the weekend with swimming off Rio's Copacabana beach showed that "everything is going in the right direction".

But Peterson and Keller are convinced their health issues stem from their swims in the waters off Rio, although Peterson - who has undergone three surgeries - is still competing.

The 27-year-old won the Pan American Games 10K gold in Toronto last month and will be trying to book a return trip to Rio next year.