High levels of E. coli in water can indicate contamination from sewage. Most strains are harmless and some live in the intestines of healthy people and animals. But others can be dangerous. Even a mouthful of contaminated water can lead to diarrhea, and the germ can cause illnesses such as infections in the urinary tract or in the intestines.

Daily water quality tests measure levels of the faecal bacteria E. coli. World Triathlon’s water safety guidelines and a 2006 European Union directive give a range of E. coli levels from “excellent” to “sufficient.” Anything beyond 900 colony-forming units per 100 milliliters is not considered safe or “sufficient”. However, experts stressed that these numbers are simply guidelines used to assess risk.

Plus, Aurélie Merle, the Paris 2024 director of sports, told reporters Tuesday that water samples are taken 21 and a half hours before decisions are made about the swim. That leaves uncertainty about their accuracy the day of the race.

Merle said Tuesday that testing results showed “we’re very close to the threshold of triathlon.” She noted that one of four test sites along the triathlon course was below the threshold for E. coli. Two other sites were just above the limit and one was more elevated, she said, citing a range of 980 to 1,553.

“We know the sun and the heat have a very strong impact on the quality of the water,” she said, hoping that a heat wave that hit most of France on Tuesday would help improve water quality enough for the swims to go ahead.

Organizers have said the marathon swimming races, scheduled for Aug. 8 and 9, could be relocated, if needed, to the Vaires-sur-Marne Nautical Stadium in the greater Paris region, which already hosts rowing and canoeing competitions and can accommodate up to 15,000 spectators.

Efforts to make the river suitable for swimming included the construction of a giant basin to capture excess rainwater and keep wastewater from flowing into the river, renovating sewer infrastructure and upgrading wastewater treatment plants.

Paris Mayor Anne Hidalgo very publicly took a swim in the river two weeks ago, along with Paris 2024 chief Tony Estanguet; Marc Guillaume, the top government official for the Paris region; and swimmers from local swimming clubs. Data released last week show that E. coli levels at the Bras Marie were at 985 units per 100 milliliters that day, slightly above the established threshold.