Blue Cross (Blaukreuz) is a World War I chemical warfare agent consisting of diphenylchloroarsine (DA, Clark I), diphenylcyanoarsine (CDA, Clark II), ethyldichloroarsine (Dick), and/or methyldichloroarsine (Methyldick). Clark I and Clark II were the main agents used.
Clark I was used with Green Cross munition earlier; however for the first time it was used as a standalone agent in the night from July 10 to July 11 1917 at Nieuwpoort, Belgium, during the operation Strandfest. The artillery munition used as a delivery vehicle contained a large amount of glass spheres closed with a cork and sealed with trinitrotoluene. Later N-ethylcarbazole was added. Depending on the caliber, the munition contained between 7 and 120 kilograms of the agent.
Blue Cross is also a generic World War I German marking for artillery shells with chemical payload affecting the upper respiratory tract. [1]