You pour tap water into the upper chamber, and it passes through a charcoal and paper filter as it drips into the lower chamber. The whole thing is meant to sit on your refrigerator shelf until you're ready to drink it. Even the least expensive filters, however, were shown to be effective at removing or reducing a wide range of contaminants. The NSF found that almost all the commonly available filters did a good job of reducing lead contamination. Tap water sometimes contains lead that leaches from lead or lead-soldered pipes.
Filters also do a good job of removing chlorination byproducts and improving taste.
The main drawback is that most filters don't remove all bacteria and parasites, although the most expensive carafe-style filters do. One common filter, the Pur Plus, passed the NSF test for the most troublesome parasites, giardia and cryptosporidium, but most others weren't reliable in this area. You need a different type of purifying system if these chemicals concern you. The magazine says that 20 percent of American households use filters, a figure expected to double within five years. To see how carafe-style filters affected the taste of local water, we invited five people to sample water filtered by four different manufacturers.