Chromium-6, popularized in the movie, Erin Brockovich, is a well-known cancer-causing chemical found in a vast majority of the country's tap water.  (To find your county's chromium-6 rating, click on the chromium-6 interactive map).  Although the chemical can be found naturally, industrial pollution is a major contributor.  Even the slightest exposure to Chromium-6 can cause skin burns, pneumonia, pregnancy complications, and stomach cancer.  A reverse osmosis system filters down to the micron level, effectively removing the chemical.