Comments on article "Toxicity and carcinogenicity of chromium compounds in humans" by Costa and Klein.

The recent article by Costa and Klein (2006) contains several inaccuracies about the federal drinking water standard for chromium. The federal drinking water standard for chromium is 100 ppb, not 50 ppb as stated by Costa and Klein, and it is based on non-carcinogenic effects, not a one in one million cancer risk level. The question of whether or not hexavalent chromium is carcinogenic via the oral route is the focus of much current interest and has major implications for regulation of chromium in drinking water and soil. The chronic drinking water study of hexavalent chromium currently being conducted by the National Toxicology Program will provide further information that will be useful in addressing this question.