Asbestos.

This is coolbert:

"Sometimes the hastily-developed technology turns out to be immensely effective, but other times it can backfire, putting the user in as much or more peril as the danger from which it is supposed to be protecting them. One such example of a supposed advance that actually turned out to be dangerous to the user was the British civilian gas mask of the Second World War.... While the masks were effective in terms of being able to filter out poisonous gases like mustard gas, phosgene or chlorine gas, the filters in them contained a chemical that we now know is extremely harmful to humans: asbestos... Asbestos, which was widely used as a heat-resistant insulator... before it was discovered just how harmful prolonged exposure was. It causes asbestosis, pleuritis, and lung cancer, as well as a number of other lethal and incurable diseases." - -  Jay Hemmings.

See from the Internet blog of John Simkin this very sad account of good measures taken during wartime that resulted in unanticipated and very bad results decades later.

"Spartacus BlogGas Masks in the Second World War killed more people than they saved"

Gas masks and filters as distributed to British civilians [and presumably too military personnel] prior to and during WW2 effective against poison gas attack but injurious to the wearer in the long term.

A CONSTITUENT ELEMENT [ASBESTOS] USED IN THE FILTERS EXPOSING CIVILIANS AND MILITARY BOTH TO A VERY DEADLY CARCINOGEN AGENT. 

Deadliness of asbestos not known [?] at the time. Maybe only decades later was the link between diseases such as asbestos and mesothelioma understood. But too late for many.

The threat of widespread poison gas warfare in the European theater of battle during WW2 never materialized. But the threat as perceived did have to be taken into account and prudent measures taken in advance.

coolbert.