From verso of card: "Home Office mobile gas vans . . . are used for the testing of respirators and for the purpose of training men and women under the conditions of an actual gas attack. The vans are so built that a gas cloud can be put up in the body of the van; the white canopies at the back are airlocks to prevent the escape of the gas when the door of the van itself is opened. The picture shows a group undergoing training at Hendon Police College. The respirator in use is the Service type, which is designed for use by the fighting services. . . ."