Do water filters do anything, or am I just fooling myself?

A filter is only as good as its biggest pore, says Dickson Despommier, Ph.D., professor of public health and microbiology at the Joseph L. Mailman School of Public Health at Columbia University. What determines whether one filter is better than another depends upon how small the filter's holes are and what they're designed to filter out. A filter system that contains activated carbon (for example, a Brita pitcher) will remove chlorinated compounds—which is an issue if your water supply is treated, he says. Besides removing the taste of chlorine, Despommier says such a filter is useful because "there's an emerging field looking at whether chlorinated hydrocarbons could cause cancer." He advises reading the filter box to see if it removes chemicals such as lead, mercury, or copper. You can also look to see if your filter will remove strains of bacteria such as Giardia and Cryptosporidium. Generally, the more your filter claims to remove, the smaller its pores are. Remember, the more your filter removes, the more expensive it will be. And because the pores are smaller, it will need to be replaced more often. "About two-thirds of America gets its drinking water from either reservoirs or standing bodies of water," says Despommier. "So we're essentially drinking lake water."

Though the water that flows from your kitchen tap may taste like the bottom of a lake, it's generally pretty safe. In fact, according to the Environmental Protection Agency, the U.S. has one of the safest water supplies in the world. In the event that your local water supply becomes contaminated with a substance that could cause an acute illness, the local water utility is required to alert you within 72 hours. If the contamination isn't acute but might threaten your health after continued exposure, the water utility must send out a warning on the next water bill.