What you need to know about the water quality of the Seine during the Olympic Games in Paris

Swimming has been banned in Paris’s long-polluted Seine River for more than a century. So with Olympic swimming events scheduled for the river, the city invested $1.5 billion (1.4 billion euros) in an effort to clean it up.

With the Paris Games underway, officials are keeping a close eye on water quality. Athletes could suffer health effects from swimming in a river with higher than acceptable levels of E. coli or other bacteria.

Here are some important things you should know:

Paris, like many old cities around the world, has a combined sewer system, meaning that the city’s wastewater and rainwater flow through the same pipes. During heavy or prolonged rainfall, the pipes can become overloaded and untreated sewage flows into the river instead of to a treatment plant.

That could mean bacteria, viruses or parasites are getting into the water, said Dr. Nicole Iovine, an infectious-disease specialist at University of Florida Health in Gainesville. And it’s not just human waste.

“It’s also the wild animals — rodents, for example. If it rains a lot, all that from those animals can end up in the Seine,” she said.

In preparation for the Paris Olympics, the city built a large basin to collect excess rainwater and prevent untreated waste from entering the river. It also renovated the sewer system and modernized its water treatment plants.

The system could still be hit by heavy rainfall.

“The city of Paris has done a lot of work to clean up the Seine, but they have no control over the weather,” Iovine said.

City officials said rainfall during the opening weekend of the Olympics filled 20 percent of the basin, so contamination levels afterward likely came from upstream sewage.

Water quality tests in June showed unsafe levels of E. coli bacteria, though results showed improvement in early July. E. coli is found in human and animal digestive tracts and waste. Most strains are not harmful, Iovine said, but high levels can indicate dangerous bacteria.

A control group performs daily tests at four locations for the presence of E. coli and enterococci bacteria, which can indicate the presence of feces and potentially pathogenic bacteria.

The World Triathlon Federation has determined that an E. coli level of more than 900 colony forming units per 100 milliliters is unsafe.

However, it’s not a “magic” number, Iovine said, and officials likely take other factors into account — such as weather — when making decisions about water quality.

On a sunny day, the sun’s ultraviolet rays can kill the bacteria and reduce their concentrations.

Swimming in water with unsafe bacteria levels can lead to stomach and intestinal problems. Swimmers can inadvertently swallow the water or get infections through open wounds.

Even a sip of contaminated water can cause diarrhea. In addition, bacteria can cause diseases such as urinary tract or intestinal infections. In the worst case, there can even be life-threatening sepsis.

“The athletes are young and in the best shape of anyone, so they’re going to be more resilient than a lot of us,” Iovine said. “But that doesn’t mean they can’t get sick from these things.”

Bose reported from Jackson, Mississippi. AP reporter Kate Brumback contributed from Paris.

The Associated Press Health and Science Department receives support from the Robert Wood Johnson Foundation. The AP is solely responsible for all content.