A further challenge, even more difficult to assess, comes from emerging contaminants. We are introducing compounds into our environment that did not exist only a few decades ago. To take one example, millions of people ingest pharmaceutical products every day, treating a range of conditions from arthritis to depression. Our bodies excrete residue from the drugs into the sewage system, and unused medications are often flushed down the toilet. As a result,they turn up in our tap water. One study found evidence of 56 pharmaceuticals or their byproducts in treated drinking water, including in metropolitan systems that together serve more than 40 million people.
These drugs are designed to change human body chemistry, and the risks they pose in the water supply may be real, but they are hard to quantify. The concentrations are extremely low, sometimes in parts per billion or even parts per trillion — far, far below the level of a prescribed medical dose. Nor are there any documented cases of pharmaceutical traces in drinking water leading to harm, but with such small doses it is difficult to assess effects that may be subtle or distant in time. As EPA scientist Christian Daughton has described, such contaminants are “at the outer envelope of toxicology.”