The Environmental Working Group and U.S. Public Interest Research Group say that the problem is due to byproducts that form when adding chlorine to the tap water.
They admit that adding chlorine to tap water saves thousands of lives each year by reducing the number of harmful bacteria in the water. However, they say that this process itself actually creates hundreds of toxic chemicals called "chlorination byproducts," or CBPs.
According to the report, chlorine added to water interacts with organic matter, particularly the soil and plant material that comes from run-off by agriculture and urban sprawl.
And the problem seems to affect large and small cities alike. They note that a handful of large cities with a history of high CBP levels account for a large portion of the women at risk -- suburban Washington, D.C., and Pittsburgh, as well as urban centers like Philadelphia and San Francisco.
But more than 1,100 small towns (with fewer than 10,000 people) have also reported potentially dangerous levels of CBPs in their tap water over the past six years, according to the report.
They write that pregnant women living in small towns supplied by rivers and reservoirs are more than twice as likely to drink tap water with elevated levels of CBPs than women in large communities.
In total, the investigators list 42 cities across the U.S. -- both large and small -- that expose more than 500 pregnant women each year to trihalomethanes (THM), the most common chlorination byproduct.
A new standard put forth by the Environmental Protection Agency went into effect just this month that will lower the allowed levels of chlorination byproducts, including THMs.
However, the investigators list multiple cities with lower levels of THM in tap water that they say still expose thousands of women to potentially dangerous toxins for an entire trimester.
"It's not a big surprise," Joel Schwartz, PhD, tells WebMD. He is an associate professor of environmental epidemiology at the Harvard School of Public Health in Boston.
Schwartz says a number of recent studies have linked chlorine byproducts to reproduction risks. His group, for example, has found the substances could affect a baby's birth weight. Other research has pointed to risk of birth defects and miscarriage.
"There is potential to cause harm. But there are things we can do to reduce the risk," he says, adding that the following measures won't drop your risk to zero, but they can make things safer:
- First, don't kid yourself with the bottled water. There may be no way to tell how long it has been since the company last tested it.
- You may want to install filters on the sinks you use for your drinking water. Products that use charcoal can filter out the chlorine byproducts. The more charcoal a particular filter uses, the more contaminants can be sifted from the water before you drink it.
- You also may want something for your shower, since these contaminants could evaporate in the steam and be inhaled.
But Schwartz adds there are much bigger issues here that need to be handled at the community level. First, people need to decide how the water will be used. All the water going into your house doesn't really need to be fit to drink, when a lot of it is used to flush toilets, water the lawn, or do the wash. If a town doesn't have to pay to make all household water drinkable, it can then free up some resources to do a better job treating water that people will drink.
Getting rid of chlorine is not the answer, Schwartz says. But communities can use it more responsibly. It's helpful when treatment plants can tailor the amount of chlorine they use according to how much is actually needed. And if more of the particles and debris can be filtered out, germs have fewer hiding places and are easier to kill. That also means less chlorine.
It's also a good idea to clean the plumbing that brings water from the treatment center to your home. The pipes can get crusty with gunk. So facilities have to keep some chlorine in the water to treat it as it makes its way to you. But cleaner pipes mean less chlorine.
"These measures will not be free," Schwartz says. "But there are things we can do that will not bust the bank that will make things better."
The groups are calling for the federal government to take immediate action to clean up the lakes and rivers that provide tap water by reducing the soil erosion and the nutrient and animal waste from farms and feedlots that increase the need for chlorination. The farm bill currently being debated in Congress, they say, would be one step towards protecting America's tap water.
With reporting by David Flegel, MS