diphenylchlorarsine

Toxic gas, used by the Germans (as Clark 1) in World War I. Inhaling causes nausea, vomiting, headaches, and chest pains but it is rarely lethal and then only in extremely high concentrations rarely achieved in field conditions.

It was introduced simultaneously with mustard gas as the agent to be used on offensive operations, since mustard was too persistent to be used against areas toward which German troops were advancing. It was specifically aimed at penetrating Allied gas masks, by distributing the substance as a fine dust which could not be neutralized by the chemicals or stopped by the coarse filters in the masks. However, a technical error in the design of the shells used to fire the gas meant the dust was too coarse to penetrate gas masks.