A new angle that nobody has ever considered much, despite it being a well known product of volcanism in the past. It is possible they had not made the connection because volcanos pour out so many toxins it is difficult to pin the blame on any particular poison. Sulfuric acid is obvious and its effects clear to the naked eye whereas mercury is far more subtle and also far deadlier than acid rain could ever be.
You may be present on a paradigm buster at Vault-Co. In ten years, I don't think we ever factored this into the extinction cycle. After all, it is the most lethal toxin on earth even in very small concentrations. With mercury you're lucky if you only go blind and sterile.
Obviously, the only place you'd be safe from this kind of mercury is in a Vault. Anywhere else would be unsafe. Of course, mercury exposure makes humans violent and crazy when it builds up in the body. "Mad as a hatter," in fact. So you'd have to watch out for the mercury zombies.