http://www.guinnessworldrecords.com/index.asp
Largest Death Toll From Chemical Weapons Attack
The greatest number of people killed in a single chemical weapons attack is estimated at 4,000, when President Saddam Hussein of Iraq attacked members of his country's Kurdish minority at Halabja, Iraq, in March 1988. The attack was ostensibly punishment for the support the Kurds had given to Iran in the Iran–Iraq war.