From Water World (Patrick Crow):
The decision to undertake a first-ever national standard for perchlorate reverses a 2008 Bush Administration decision and comes after EPA Administrator Lisa Jackson ordered EPA scientists to make a thorough review of the chemical. The agency said more than 4% of public water systems have detected perchlorate and between 5 million and 17 million people may be served drinking water containing perchlorate.
Perchlorate is both a naturally occurring and man-made chemical best known for its use as rocket fuel ingredient. EPA said research indicates that it may impact the normal function of the thyroid, which produces important developmental hormones.