Harvard Kennedy School

By Bill Walker

A driver entering an enclosed parking garage in California is greeted by a 20-by-20-inch sign declaring in 72-point type:

WARNING: Breathing the air in this parking garage can expose you to chemicals including carbon monoxide and gasoline or diesel engine exhaust, which are known to the State of California to cause cancer and birth defects or other reproductive harm.

The signs are known as Prop. 65 warnings, after the ballot proposition number of the state Safe Drinking Water and Toxic Enforcement Act, which in 1986 was approved by almost two-thirds of California voters.

The law established a registry that is regularly updated with substances that include hazardous chemicals found in common household products, electronics, pesticides, food, drugs, dyes, additives, construction materials and automobiles. All must carry warning labels if they contain threshold levels of a listed chemical. The law also prohibits the discharge of listed chemicals into sources of drinking water.

In the most recent update, the Prop. 65 registry, which is maintained by scientists at the state Office of Environmental Health Hazard Assessment (OEHHA), has grown to 874 chemicals and compounds. Chemicals are added only after exhaustive reviews by independent expert panels of studies from authoritative national and international public health agencies.