From the top of Rio de Janeiro’s Sugarloaf or the Christ the Redeemer statue that towers above the city, Guanabara Bay looks picturesque. But the ailing estuary, site of the Olympic’s sailing and rowing, is anything but pristine.

Roughly 16 million people live around the bay, making it one of the world’s most densely populated urban areas. Many neighborhoods lack proper sanitation, causing squalid water conditions, including raw sewage and extreme levels of disease-causing microorganisms in Guanabara Bay. Athletes have complained the water is littered with trash, and that it irritates their skin and causes stomach ailments. Some teams have instructed Olympic rowers to avoid splashing water on each other and to carry hand sanitizer onboard their boats.

These water pollution problems aren’t unique. According to the United Nations, up to 90 percent of wastewater in developing countries flows untreated into waterways used for bathing, drinking, or fishing.

Yet it isn’t often that such water pollution captures media attention on an Olympic scale.

Where’s all the pollution coming from?

A lot of the pollution comes from raw sewage. Roughly half of the houses in the Guanabara Bay drainage basin—Rio de Janeiro and the surrounding cities—remain unconnected to sewage treatment plants. That means waste from millions of people flows untreated into the bay. Experts say the percentages of households without a sewage system may be much higher in slums and shantytowns where some houses have no access at all to toilets or tap water. Sewage overflows worsen during the rainy season from October to April, when rainfall can overwhelm the sewers that carry waste to sewage treatment plants.

View Images Belgian sailor Evi van Acker in action during a race on day five of the Rio Olympic Games. Van Acker says she suffers from a stomach ailment that she suspects she got from racing in Rio's contaminated bay. Photograph by Clive Mason, Getty

Urban runoff and industrial wastewater also are major sources of pollution. About 17,000 industries surround Guanabara Bay, including pharmaceuticals, refineries, and oil and gas terminals. Every day an estimated 150 metric tons of industrial wastewater flows into the bay—enough to fill about 7 large tanker trucks.

What’s in it that could be risky?

Human sewage can carry a number of pathogens, including viruses and bacteria. Enteric viruses—excreted in feces and found at high levels in untreated sewage—are a major concern in Guanabara Bay, says Kristina Mena, an associate professor at the University of Texas Health Science Center at Houston School of Public Health. She studies water quality and human health risk, and independently reviewed a 2015 Associated Press study that found extreme levels of an enteric virus called adenovirus in Guanabara Bay.

“Adenovirus can cause a lot of different problems—from diarrhea to respiratory, eye, and skin infections,” says Mena.

Located at the mouth of Guanabara Bay, Copacabana Beach, where the marathon swimmers and triathalon athletes will compete, generally has better water quality than the bay. But some antibiotic-resistant bacteria have turned up in high levels at Rio’s beaches. A strain of “super-bacteria” called carbapenemase-producing bacteria (CPB) was detected at five beaches, including Copacabana.

Renata Picão, an environmental microbiologist at the Federal University of Rio de Janeiro, says her study is the first to find CPB—usually associated with hospital waste—in recreational waters. While infection can spell trouble for hospital patients with compromised immune systems, “we have no idea about the risks outside of the hospital environment,” says Picão.

How big of a threat does water pollution pose to the Olympians?

That’s hard to know, says Mena. Not everyone exposed to waterborne pathogens—which can enter the body through the nose, eyes, mouth, or even cuts on the body—will become infected. And not everyone who becomes infected will get sick, she says.

Several athletes fell ill while training in Rio, including 13 U.S. rowers at a test event last summer. Last week Belgian sailor Evi Van Acker, who won a bronze medal at the London Olympics, said she became sick with stomach problems after sailing on Rio's bay. Yet it’s impossible to know whether these gastrointestinal illnesses were the result of Guanabara Bay exposures.

View Images Guiana dolphins swim near fish farm pens in Rio's Guanabara Bay. Despite the untreated sewage and other pollution that flows into the bay, 34 Guiana dolphins live, breed and feed there. Photograph by Renata Brito, AP

Nevertheless, the viral counts in Guanabara Bay are some of the worst Mena has ever seen. “This is such an extreme situation both in the range of microorganisms present and the extent of exposure. There are going to be health risks,” she says.

What about Rio residents?

Waterborne illnesses are a major problem for Rio residents, especially Rio’s poorest people who live near the most polluted parts of the bay and have the least access to sanitation, says Ricardo Igreja, an infectious disease doctor at the Federal University of Rio de Janeiro.

Eating seafood contaminated with heavy metals, industrial chemicals including PCBs, and hydrocarbons from petroleum products, also causes long-term health concern for Rio residents practicing subsistence fishing in Guanabara Bay, says Abílio Soares Gomes, a marine biologist at the Federal Fluminense University in Niterói, a Rio suburb.

What about marine life?

Historical records show that thousands of fish, dolphins, and whales once swam in Guanabara Bay. “If you look back 100 maybe 150 years, there were so many fish that people would say hitting them was a problem for boats,” says Fabiano Thompson, an oceanographer at the Federal University of Rio de Janeiro.

Not so anymore, says Thompson.

Numbers of marine mammals have dwindled over the last couple decades. A small population of gray Guiana dolphins remains in the bay. Their bodies contain high levels of heavy metals, including mercury and other industrial chemicals. The possible effects on the dolphins remain unknown, though some of these chemicals have been linked to altered hormones and problems with the immune system in marine mammals.

An influx of nutrients caused by sewage and urban runoff can lead to excessive plant and algae growth in parts of the bay. Sometimes the algae use up too much oxygen in the water, causing fish to die. A string of oil spills in Guanabara Bay over the past two decades also have led to massive fish kills.

How do Brazil’s clean water rules compare to the U.S. and elsewhere in the world?