How did the iron lung treat coal gas poisoning?

The Iron lung was essentially a pressure chamber in which the the pressure could be increased quite a lot over atmospheric pressure as a means of forcing the absorbed coal gas back out. Its other common use was in treating polio, and also divers with the 'bends'.

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Coal-gas is poisonous by being chiefly Carbon Monoxide, which is fast acting as it binds much more readily than the intended oxygen to the blood's haemoglobin.

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Also, the "iron lung" formerly used to treat polio, is a very different device from the hyperbaric or recompression chamber used to treat the "bends". The iron-lung is a an artificial respirator, the recompression chamber is not. Instead, it is a vessel large enough to accommodate the casualty, and filled with air at the pressure of the dive, then released very slowly.