I’m glad we’ve never had to find out. I had a great uncle who died in the 1920’s after being gassed while serving in France with the 42d Infantry Division win WWI.
I was part of a team to work out the details with the Soviets to demil/destroy our respective stockpiles.
Sorry to hear about your great uncle - they didn't know it at the time but Mustard (HD) is a very active carcinogen - so you can maybe survive the blistering and the blinding - but it will kill you in the end.
My Great Grandfather was a Captain in WWI and lost his helmet during a fight and grabbed one he found in a trench. It was upside down and mustard had settled in it. It burned his head and he lost a bunch of his hair. It grew back eventually, but when he went bald in his 70’s he jokingly blamed the gas.