Birth-control pills could add 10 million doses of hormones to our wastewater every day. Some of that estrogen may wind up in our taps.

• Hormones from birth-control pills can travel through showers, toilets, and washing machines to local wastewater facilities.
• In his book, "Troubled Water," activist Seth Siegel writes that birth-control pills add more than 10 million doses of synthetic estrogen to US wastewater every day.
• From there, the hormones could get discharged into rivers and lakes that serve as sources of drinking water.
• Only a tiny portion of the estrogen in wastewater makes its way to US taps, but Siegel still thinks we should remove it.
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After we swallow a pill, it doesn't take long for its ingredients to enter local water systems. That's because our bodies generally only absorb a small portion of the dose. The rest of the pill - up to 90% - is excreted, either into the toilet as waste or through sweat that gets washed down shower drains or washing machines.

Sometimes, people even choose to flush unused pills, sending a full dose down the toilet.

The hormones then travel to a wastewater plant, and from there, they can get discharged into nearby waterways such as rivers and lakes - some of which are used as sources of drinking water.

That means the prescriptions we take most often, like birth control, could eventually wind up in tap water in trace amounts. In his new book, "Troubled Water," activist Seth Siegel says scientists don't yet understand the health effects of this potential contamination. But Siegel thinks all pharmaceutical compounds should be removed from our taps right away.

It's hard to tell how much estrogen from birth control winds up in our taps

In the US, around 15 million women regularly take birth-control pills, which typically rely on a synthetic form of estrogen known as EE2. Since it's an endocrine disruptor, EE2 can interfere with reproductive hormones and development if consumed in excess or by vulnerable individuals like infants.

According to Siegel, birth-control pills add more than 10 million doses of synthetic estrogen to America's wastewater every day. That estimate is based the number of women who take oral contraceptives in the US, and assumes those women take the pill 21 days of the month and excrete around 90% of the dose into wastewater.

Very little of that estrogen actually makes its way into our taps, since most water-treatment systems filter it out along with other contaminants. A 2010 study determined that birth-control pills account for less than 1% of the total amount of estrogen found in US drinking water. But since local water systems don't test for EE2, the study's authors noted, it's hard to say for sure how much of the hormone is in our water.

Most of the estrogen in drinking water comes from other sources, such as livestock that excrete natural estrogen, which can travel through soil and into local groundwater.

But Siegel argues in his book that livestock can't solely be responsible for the estrogen in our wastewater, since other pharmaceutical compounds, like those used to make antidepressants, make it there as well.

"No one is giving a cow Zoloft," Siegel told Business Insider. "The only possible source of that is through the wastewater. That's why it's so relevant, so scary."

That doesn't mean women should stop taking the pill

Some water experts, Siegel said, abide by the adage that "the solution to pollution is dilution." In other words, dumping water that contains pharmaceutical compounds into larger bodies of water dilutes the concentrations of hormones enough to minimize their threat to humans or the environment.

But Siegel said that even in the purest bodies of water, like the Great Lakes, scientists have found residues of antidepressants like Prozac in the bodies of local fish.

In the northeast, scientists have discovered that estrogen in rivers and lakes can cause male fish to develop female biomarkers like ovaries. Other studies have shown that exposure to EE2 has led fish to become less fertile across generations.

Scientists don't know if these findings have any implications for human health, but Diana Aga, a chemical-pollution expert at the State University of New York at Buffalo, told Siegel that it's possible for similar effects to crop up among "more complex creatures."

"There's every reason to believe that estrogen and the pharmaceutical compounds that we're ingesting in micro-quantities are having an effect," Siegel said. "Why wouldn't it be possible that a newborn or fetus, or a three-year-old getting an irregular dosage, might not see some effect on their brain function or brain development?"

The solution, he said, isn't to stop making or taking medications. It's to make sure these compounds are deliberately removed at water-treatment plants.

"Pharmaceutical products have done great things for America's health. I am not saying that we should ban all these things," Siegel said. "What I am saying is that we should get all the benefits of these products and also get pure drinking water. We can do both."