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.
https://www.pharosproject.net/blog/show/40/mercury-contamination-drywall
The EPA says this is safe!?
https://www2.buildinggreen.com/article/epa-finds-coal-fly-ash-safe-concrete-and-gypsum-wallboard
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.
https://www.propublica.org/article/american-made-drywall-emerges-as-potential-danger
So how do you tell if your drywall is Natural Gypsum or Coal Fly Ash? You may not be able to distinguish the difference.
https://www.gypsum.org/press-roomfgd-gypsum-board/
http://www.motherearthliving.com/the-good-life/mercury-and-formaldehyde-found-in-drywall.aspx