In 2003, California became the first U.S. state to ban methyl tert-butyl ether (MTBE), a gasoline additive used to reduce air pollution. The ban followed widespread concerns about MTBE contaminating groundwater supplies, particularly in areas with leaking underground storage tanks.
MTBE was introduced in the 1990s to help gasoline burn more cleanly and reduce smog-forming emissions, as required by the federal Clean Air Act. However, it was found to be highly soluble in water and slow to degrade, meaning even small spills could contaminate large volumes of drinking water.
Studies linked MTBE exposure to potential health risks, including cancer in laboratory animals, though human health effects remain debated. By 2003, California officials determined that the environmental and health risks outweighed the air quality benefits, leading to a phased ban that took full effect in 2004.
Other states followed California's lead, and by 2006, the U.S. Environmental Protection Agency had not banned MTBE nationally but encouraged states to take action. Today, ethanol is the primary oxygenate used in reformulated gasoline in the U.S.
