How'd it get in there? Most treatment plants dose tap water with chlorine to kill things like parasites and viruses found naturally in freshwater sources.
Harmful effects: In small doses, chlorine is literally a lifesaver: It zaps dangerous contaminants. But studies show that long-term exposure to chlorine by-products can lead to miscarriage or birth defects, says Gina Solomon, M.D., a senior scientist at the Natural Resources Defense Council.
Your action plan: Chlorine by-products evaporate naturally, so let your glass of tap sit for five minutes before drinking it. Another option: Buy an inexpensive charcoal filter (a freestanding pitcher, a faucet attachment, or a built-in fridge device).
How'd it get in there? Water facilities strip out nearly all traces of the heavy metal, but if you live in a home built before 1975, chances are you have pipes that contain lead, which can leach into your water.
Harmful effects: Even a small amount is bad news; lead can trigger irritability, anemia, and nerve damage. Women trying to conceive should be especially careful: Lead poisoning can cause miscarriage and a host of fetal problems.
Your action plan: Lead can seep into water that's been sitting stagnant in pipes, "so run your tap for two minutes to flush it out," says James M. Symons, M.D., author of Plain Talk About Drinking Water. (Rather than waste that water, use it to wash your hands or the dishes.)
How'd they get in there? Gnarly bacteria like E. coli can make their way into water from human and animal waste that runs into reservoirs from broken pipes and sewage systems.
Harmful effects: "Just a tiny bit of bacteria can make you sick," says Suzanne R. Steinbaum, D.O., an internist in New York City. The last time you had a "24-hour stomach bug," it was likely caused by waterborne bacteria. Most people recover quickly, but pregnant women can be laid up for days.
Your action plan: Purifiers nix most bacteria. But if your immune system is compromised, first boil your water for five minutes.
HORMONES AND DRUG BY-PRODUCTS
How'd they get in there? Every time you pop a pill—whether it's a Tylenol or Xanax—traces of it come out in your pee. And (ick alert!) that urine can eventually find its way back into our tap water. Scientists haven't yet figured out how to remove most drug by-products from water.
Harmful effects: "Traces of things like aspirin probably aren't a big deal," says Solomon. However, the presence of prescription drugs "may contribute to the growth of antibiotic-resistant illnesses like MRSA, and hormones from meds like birth-control pills could be affecting pregnant women and their babies in ways we don't know yet."
Your action plan: Filters are fairly useless against drug residuals. So for now, just sit tight: The Environmental Protection Agency is assessing what regulatory action to take.