[...]

And if you look closely enough, the residue is there all right. In the late 90s Swiss scientists found cholesterol drugs in their groundwater; when they started checking lakes and streams, they found the stuff almost everywhere. The United States Geological Survey measured levels of pharmaceuticals, hormones, and other pollutants in 139 streams across the U.S. in 1999 and 2000 \ufffd 80 percent of the time they found at least one of the 95 chemicals they were screening for. The medical compounds seen most regularly were steroids and nonprescription drugs, though sex hormones such as estrogen and testosterone turned up too; in a subsequent USGS study, hormones and steroids were detected in more than 40 percent of groundwater samples. German scientists have found antibiotics, beta blockers, and even antiepilepsy medicine in their water; in Canada researchers have found cancer drugs and anti-inflammatories; in the UK they\ufffdve found everything from ibuprofen to antibiotics in streams and sewage alike.

The good news is that we're talking about some very, very small quantities of drugs here \ufffd the levels found so far have typically been measured in parts per billion or even trillion. And after three decades of study, no one's yet produced any evidence that drug residue puts humans at risk. The less-good news runs a little longer. First, there may be long-term cumulative effects we haven\ufffdt identified yet, especially considering how persistent some residues are \ufffd clofibric acid, one by-product of a cholesterol drug, can hang around for two-plus decades. Second, the extremely low concentrations of residue in water make it hard to distinguish any such effects from ones caused by the zillion other chemicals we\ufffdre constantly exposed to. Then there\ufffds the ongoing boom in prescription drug use: a 2004 report points out that while the U.S. population grew 13 percent between 1993 and 2003, the amount of prescription drugs we bought went up 70 percent over the same period.

Another possible concern is that we don\ufffdt know how all these drugs may interact. Some studies have found that combining different sex hormones in doses so tiny as to be theoretically insignificant can have a measurable (if small) effect. According to one USGS fish pathologist, something like this may be going on with pollutants in tributaries of the Potomac River, where in testing last year more than 80 percent of male largemouth and smallmouth bass were found to be producing eggs, which isn\ufffdt normally part of their repertoire. Residue can also accumulate in the tissue of fish and other wildlife and get back to us via the food chain. Meanwhile, there\ufffds always the potential problem of unmetabolized antibiotics entering the environment and helping breed a new generation of super-resistant bacteria.

[...]