Issued to every warden and stored at wardens' posts, the gas rattle was to be used to inform the public that a gas bomb had gone off. Gas bombs would be of three 'poison' possible types - true gases (e.g. phosgene), vapours (mustard) and smokes (certain arsenicals). Thankfully, no rattle was ever called upon to be used.
All respirators, including the civilian mask issued to the general public, were equipped to deal with all three types. The activated carbon of the large container handled the true gases and the vapours, whilst the "contex", a particulate filter, coped with the smoke poisons.
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