Comment by OkayPhysicist

Here in the states, the main WWI issues are stockpiles of chemical weapons that were buried and then forgotten about. They pop up from time to time, and we've only recently started cleaning them up. Ironically, the ones that were stored somewhat deliberately with safety in mind end up being some of the worst to dispose of, because the poorly-stored ones can be largely thrown into a blast-resistant washing machine, popped open with a small charge, and then scrubbed with all sorts of fun chemicals to neutralize the toxins.

Fun fact: One of the best analogs available for phosgene that's been sitting in cold water for a hundred years is Wintergreen DOTS candy, if you can keep the engineers and military brass from eating all of them.