The Associated Press conducted a five-month investigation and found that drug residues have been detected in the drinking water of 24 major U.S. metropolitan areas, which serve roughly 41 million Americans. Concerns about these drug residues have largely focused on wildlife, as estrogen from birth control pills and other hormonal drugs has been interfering with fish reproduction (see past post here). Now, though, the AP is highlighting the number of people exposed and the potential for human health effects:

To be sure, the concentrations of these pharmaceuticals are tiny, measured in quantities of parts per billion or trillion, far below the levels of a medical dose. Also, utilities insist their water is safe.

But the presence of so many prescription drugs and over-the-counter medicines like acetaminophen and ibuprofen in so much of our drinking water is heightening worries among scientists of long-term consequences to human health. …

And while researchers do not yet understand the exact risks from decades of persistent exposure to random combinations of low levels of pharmaceuticals, recent studies which have gone virtually unnoticed by the general public have found alarming effects on human cells and wildlife.

“We recognize it is a growing concern and we’re taking it very seriously,” said Benjamin H. Grumbles, assistant administrator for water at the U.S. Environmental Protection Agency.

The number of U.S. prescriptions has risen to a record 3.7 billion, and more veterinary drugs are being doled out to pets and to livestock. Current treatment systems aren’t well-equipped to handle the influx:

Some drugs, including widely used cholesterol fighters, tranquilizers and anti-epileptic medications, resist modern drinking water and wastewater treatment processes. Plus, the EPA says there are no sewage treatment systems specifically engineered to remove pharmaceuticals.

One technology, reverse osmosis, removes virtually all pharmaceutical contaminants but is very expensive for large-scale use and leaves several gallons of polluted water for every one that is made drinkable.

Another issue: There’s evidence that adding chlorine, a common process in conventional drinking water treatment plants, makes some pharmaceuticals more toxic.

As for what the effects of the drug residues might be, we don’t know a whole lot:

Recent laboratory research has found that small amounts of medication have affected human embryonic kidney cells, human blood cells and human breast cancer cells. The cancer cells proliferated too quickly; the kidney cells grew too slowly; and the blood cells showed biological activity associated with inflammation. …

Some scientists stress that the research is extremely limited, and there are too many unknowns. They say, though, that the documented health problems in wildlife are disconcerting.

“It brings a question to people’s minds that if the fish were affected … might there be a potential problem for humans?” EPA research biologist Vickie Wilson told the AP. “It could be that the fish are just exquisitely sensitive because of their physiology or something. We haven’t gotten far enough along.” …

So much is unknown. Many independent scientists are skeptical that trace concentrations will ultimately prove to be harmful to humans. Confidence about human safety is based largely on studies that poison lab animals with much higher amounts.

Given that we consume lots of water every day for our entire lives, though, very small exposures can add up, and certain groups (like pregnant women, the elderly, and the very ill) may be particularly sensitive. Unlike many other pollutants, drugs are specifically designed to act on the human body in low doses, and drug safety testing usually looks at a substance’s effect over a relatively short time period, not a lifetime.

In short, we know enough to know that drug residues in sewage and water supplies deserve attention from researchers, municipal water and sewage entities, and the EPA.