By the time Brady, Gardner and the Brady Corps were shooting their war images they were using wet-plate collodion. Mercury was used in processing Dags, which Brady did use earlier in his career. Their biggest danger was mistaking their potassium cyanide glass for their whiskey glass or accidently allowing their developer (acetic acid) and potassium cyanide to come in contact with each other. This would produce a nice little toxic cloud inside of their Whatsit Wagons that would kill any bug and the photographer too. I heard a story not long ago about the title "The Mad hatter" apparently mercury was used in the production of felt hats. The workers would eventually go mad from the mercury fumes over time, thus the term. I guess this was before we had OSHA.