Air purifier in action a man and a girl pedalling the air filtration plant in the gas proof shelter 2 February 1938 Britain preparing for war it was assumed that gas attacks would play a major part in any war The Yorkshire Penny Bank in Cheapside installed a gas proof shelter with an air filtration plant operated by two people working cycle pedals to purify gas polluted air as it entered the shelter from outside In the event gas was never used in the Second World War