There are many chemical respiratory agents suitable for use by terrorists. They are the oldest chemical agents used and have caused the most casualties throughout the 20th century. Many are available in large quantities for industrial use and are susceptible to potential sabotage. This paper will concentrate on respiratory agents that are readily available and have the potential to cause a large number of casualties and panic. These agents have a lower rate of lethality when compared to other chemical agents but could produce many casualties that may overwhelm the emergency medical system.