SIFTING THROUGH THE FILTER JARGON

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Reverse osmosis filters force water through a semipermeable membrane that removes salt, heavy metals, most chemicals and many other microscopic impurities, including VOCs and nitrates. They do not reliably filter out bacteria.

Ion-exchange filters, as used in single-cartridge, faucet-mount filters and in large water softeners, expose water to plastic beads coated with ionized salts that capture nitrates, many minerals, including lead and mercury, and the iron, magnesium and calcium that can give water an off-color or off-taste. Some can be recharged - and contaminants washed away - in a disposable bath of strong brine, so they can be used for years. WARNING: One of our editors sees a dentist who warns against too-soft, overdemineralized water (such as steam-distilled "pure spring water"), as it deprives us of minerals that are essential to lifelong strong teeth and bones.

UV generators, included in some top-of-the-line, multistage home countertop, undercounter and central filters, expose water to natural ultraviolet (sunburn) light radiation that kills living parasites and bacteria, but not all cysts and eggs (these must be filtered out in another stage).

Save instruction sheets and be sure to clean, renew or replace filter or purification elements as indicated - some after a given number of months, others after a number of gallons of water.

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