Chromium 6 or Hexavalent Chromium is a man-made chemical that is a known carcinogen if inhaled in showers, especially by young children. But is viewed as not being as toxic if ingested in drinking water as it gets converted in the acidic stomach environment into Chromium 3, which is interestingly, a vitamin. Chromium 6 has been detected in major cities in the US to exceed the suggested US EPA limit of 10 parts per billion. The State of California has recently adopted this limit as a goal for regulatory mandate.
In April 2013, Pristinet tested public tap-water samples for Chromium 6 in San Jose, Ca, adjacent to an elementary school and found the levels to be far in excess of the then proposed goal of five parts per billion.
The Environmental Working Group (EWG 2009) tested tap water in 35 major US cities and their results are shown in the map below.
The infographic below from H2O distributors.com, explains the Chromium 6 issues very well.