Biological Gas mask, protective gear, antibiotics to ward off or treat agents used. Helps to be in good physical shape and have your shots up to date. Watch for contamination through foodstuffs and water sources. Chemical Gas mask, protective gear, M8 or M9 test paper or other detection devices, materials to decontaminate with. Always try to move upwind of a chemical attack.
Most agents disperse in a relatively short period of time.
It is risky for armies to use these as wind changes can send it back to the sender. With terrorists, however, this wouldn't be a risk.