Fabricated gypsum can contain as much as 2 parts per million of mercury. If I did the math right this could mean a 600 square ft apartment might have 2 grams of mercury. Let me know if I'm wrong.
The EPA says this is safe!?
It's interesting to note that this sheet rock poisoning article does not mention mercury. However, be sure to check the comments section for the discussion about mercury.
So how do you tell if your drywall is Natural Gypsum or Coal Fly Ash? You may not be able to distinguish the difference.