The Rio Olympics are kicking off on August 5th, but Rio de Janeiro’s waterways show alarming levels of pollution that could be dangerous for athletes and tourists alike. The Olympics and Paralympic aquatic venues are teeming with dangerous viruses and bacteria from untreated sewage, according to a 16-month-long study commissioned by The Associated Press.

The findings prompted biomedical expert Valerie Harwood to give this advice to visitors, according to the AP: "Don't put your head under water."

Rio de Janeiro’s waterways show alarming levels of pollution

Samples of the Rodrigo de Freitas Lagoon, where Olympic rowing will take place, showed 248 million adenoviruses per liter as of June. By comparison, adenovirus concentrations in the thousands per liter are considered alarming in California. Adenoviruses are common causes of respiratory illness, according to the Centers for Disease Control and Prevention. Though adenoviruses rarely cause serious illnesses or death, they can cause pneumonia, diarrhea, and bladder infections. Children and people with weak immune systems are of course more susceptible to falling ill.

Very high levels of adenoviruses (37 million per liter) were also detected at the Gloria Marina, where the sailing races will start off. With those levels of pollution, if athletes swallow just three teaspoons of water, they’re almost certainly going to be infected with the viruses, according to the AP. Whether or not they’ll get sick depends on how strong their immune system is, of course.

Some athletes are taking precautions, such as taking antibiotics, bleaching oars, and using special suits and gloves to avoid contact with the water, the AP reports. But antibiotics only work against bacteria, not viruses. And tourists who are going to attend the games are still exposed to the risks of the contaminated waters.

Olympics officials still say that Rio’s waterways are safe

Despite the alarming results, Olympics officials say that Rio’s waterways are safe. "We would never, ever risk the health or the condition of any athlete for a competition," Mario Andrada, chief spokesman for the local Olympic organizing committee, told The Associated Press. "So the health of the athletes is our first priority. And the athletes don't run a risk sailing in Guanabara Bay."

The Olympics committee has previously said that bacterial testing has shown the waterways to be within state guidelines, the AP reports. But bacterial tests don’t really give the whole picture; viruses are known to stick around longer, for weeks and even months. And they can be dangerous as well.

For some athletes, there’s not much to do other than accepting the situation. "There's been a lot of talk about how dirty the water is and all the viruses," Finnish team sailor Noora Ruskola told the AP. "I'm mentally prepared for this. Some days the water is totally OK, and some days there are bad days."