EE was finishing blowing off load of Cyanide into ADR tank. Went to back of truck and Cyanide Crystals fell from top of Storage tank and hit EE, the trailer, and the ground. The wind was blowing toward back of tanker. EE was not sure if EE had inhaled any or not but had a co-worker watch EE disconnect then go get the Supervisor. EE then called mayday.