To keep water safe and drinkable, U.S. communities maintain an extensive network of wastewater treatment plants aimed at removing disease-causing germs and known toxic chemicals before the water flows into rivers and streams. While current purification standards prevent millions of illnesses every year, health experts say that it's important to remain vigilant about what's in our water, as demonstrated by the dangers from an increasingly common group of chemicals known as "endocrine disruptors."

At the Waste Water Treatment Plant in Boulder, Colorado, technician Dan Wadas leads nervous biology students toward what he says will be the smelliest part of their tour - the headworks. "The first stop's the worst," he tells them. "If you guys can get through the first stop, it'll be no problem. Don't fall in!" The college students laugh nervously and grip the metal handrail as their tour guide directs them past the barnyard smelling water. He points to the raw sewage, which - after traveling through underground pipes - emerges into the treatment plant as brownish sludge. "That's dirty and as you'll see, that's not good. We've got to clean it up, aesthetically, but also [to prevent] disease."

The water flowing through the treatment plant will end up back in Boulder Creek, to be reused by hundreds of cities downstream. Boulder works to clean it up because untreated sewage water can make people sick, if they drink it, bathe in it, or even water their gardens with it.

To help identify contaminants, the treatment plant's $10,000 microscope magnifies samples of sewage water, then displays them on a video monitor. This makes normally invisible, single-celled creatures appear to be the size of a human hand. The students gather around the viewing screen as a shimmery, bottle-shaped organism swims toward what look like tiny black dots.

Technician Mark Dane explains that the big one is an epistylus, and it's hunting bacteria. "He's actually trying to form a vortex. There!" he points to the screen, "he just ate something. And as he forms that vortex, it brings all the particles and little bacteria and everything in front of him. He's a protozoan. When he retracts real fast like that, he's grabbing a piece of food out of the water."

The "good" bugs, like epistylus, occur naturally in sewage water, where they gobble the bad stuff then excrete it as less dangerous particles. Once the microbes have done their job, technicians chemically neutralize any remaining contaminants. By the time the water flows out of the plant and into Boulder Creek, it smells and looks much cleaner than it did coming in. But Dan Wadas says that doesn't mean he would drink it. "I'll tell you what. Our water is much cleaner than the creek, though."

Cities downstream will purify it even more before it's added to their local water supply, but there's concern that this recycled water may not be safe enough. That's because today's sewage contains an ever-increasing load of chemicals that most treatment plants can't detect, much less eliminate.

These chemicals are called endocrine disruptors. They're in a variety of household products: everything from plastics, which release the chemicals as they degrade, to pharmaceuticals in which endocrine disruptors are part of the medical formula. Biologist Alan Vajda, a doctoral candidate at the University of Colorado, lists some of them: "supplementation by growth hormone, supplementation with anabolic steroids. Even things like Prozac and opiates like codeine, nicotine metabolites like cotinine."

Mr. Vajda says many of these chemicals end up in sewage when consumers excrete them in their feces and urine. Other times, leftover pills are flushed down the toilet. These endocrine disruptors include "estrogens," and Mr. Vajda warns that even tiny amounts can add up to serious problems. "It's the estrogen in the drinking water," he says, "added to all of the other sources in our diets, in many other sources, that are adding up together, and could very well be involved in the increases that we see in things like breast cancer, prostate cancer, testicular cancer and so forth."

His professor at the university, David Norris, points out that cancers aren't the only threats. "Alzheimer's, ADHD, depression, all have hormonal correlates identified. You don't have to live underwater to be swimming in a sea of estrogens."

Creatures that do live underwater are alerting scientists about the potential dangers from these chemicals. Just downstream from Boulder's wastewater treatment plant, scientists have discovered fish that carry both male and female reproductive organs -- an abnormality known as "intersex fish." These findings are similar to reports from Britain.

Now that more researchers are looking for them, intersex fish are being discovered downstream from many U.S. treatment plants. Professor Norris says these strange fish are a warning that more subtle problems may be happening in streams and among people exposed to endocrine disruptors, adding "I think it's time that we just recognize that we as people are responsible for what we do. This is not the sort of thing that you can point the problem at, other than us. We're the problem, and it's our responsibility to clean it up."

Professor Norris would like to see the United States follow the example set by Britain, where efforts to remove endocrine disruptors from sewage are already reducing the numbers of intersex fish. Scientists hope their findings will eventually reduce endocrine disruptors throughout the environment, making it safer for fish, people and other creatures.