The carcinogen chromium-6, once spotlighted by Erin Brockovich, is present at dangerous levels in the tap water of 218 million Americans, according to a report by the Environmental Working Group. The city of Phoenix, Arizona, had the highest level of the toxin in its water supply, at over 400 times the target safe level. The Environmental Protection Agency announced plans to evaluate chromium-6’s effects and to help communities and experts produce better cost-benefit analyses of ways to improve water quality.