Reputable companies that sell water pure filters can back up their claims with independent laboratory analyses. Underwriters Laboratory, for example, will analyze the content of what comes out of "pure water filters". But, manufacturers must pay for the testing and, of course, if they know that their product is a rip-off, they don't want the water analyzed.

The hope of these companies is usually to get in and out of the market quickly. Sell lots of filters over the internet or on a home shopping network, bank the money and then go out of business.

There is one company, for example, that advertised a "maintenance free" countertop unit to provide pure water, saying never need replacement, easy to install, 30 day warranty, etc. A great many customers say that the unit only worked long enough to get beyond the 30 day period before failing. The products are no longer on the mark