The Bhopal accident in 1984 led to promulgation of an Act in 1986 to identify hazardous substances, notify the inventories and prepare for emergencies by planning, notification and reporting. It is stated that for chlorine, if the quantity is above 100 pounds, one has to comply with this Act and there are about 350 chemicals in the list.
Off-Site Emergency Response Plans: A Preparedness Tool - Fire Engineering
LINK