Figures I have are:
The British army (including the British Empire) had 188,000 gas casualties but only 8,100 fatalities amongst them. It is believed that the nation that suffered the most fatalities was Russia (over 50,000 men) while France had 8,000 fatalities. In total there were about 1,250,000 gas casualties in the war but only 91,000 fatalities (less than 10%) with over 50% of these fatalities being Russian. However, these figures do not take into account the number of men who died from poison gas related injuries years after the end of the war; nor do they take into account the number of men who survived but were so badly incapacitated by poison gas that they could hold down no job once they had been released by the army.
http://www.historylearningsite.....ar_one.htm

So you had a lot of survivors of gas attacks live to tell the tale and in the retelling was probably where threat outstripped reality. One of those survivors was Adolf Hitler who ordered no first use of chemical weapons in WW2.