Toxic metals, like lead and uranium, are exchanged with harmless zinc and copper. It removes chlorine, and reduces but does not eliminate bacteria. It does not accumulate pollutants, but rather converts them to copper and zinc compounds. The Needs catalog contains an insert which describes the CuZn filter in detail, comparing it toxin by toxin in a long list with a reverse osmosis filter. The list is inaccurate in some respects, because it does not remove calcium.
Carbon filters remove chlorine, organic chemicals, and tastes and smells.
They do not remove non-chlorine additives, some microorganisms, or toxic minerals. Distillers remove all microorganisms, toxic and beneficial minerals, radioactive substances, and additives, but not volatile organic compounds. Ideally, a distiller should have a demand pump, to move the water to the faucet. An automatic boiler flush is useful because it periodically replaces the boiler water to reduce maintenance due to boiler calcium scale. When you DO clean the boiler, be sure to use very hot water.