Executive Order 11850--Renunciation of certain uses in war of chemical herbicides and riot control agents. was signed on April 8, 1975, by United States President Gerald Ford. The executive order restricted the use of herbicides, and riot control agents, including tear gas. Each and every use would require the explicit approval.
On April 11, 2007 Joseph Benkert, a George W. Bush political appointee, informed the Senate Armed Services Committee that the Bush Presidency felt it could reinterpret the Executive Order and loosen the restriction on the use of gas as a riot control agent.
References
Executive Order 11850 Wikipedia(Text) CC BY-SA