Take a look at some of the chemicals used in hydroponic gardening. There are a whole lot of them I am not real interested in having around. Since everyone wanted to claim it was safe and didn't give what I considered a solid argument about it I decided to look it up. It seems in many vermiculite mines asbestos is also present. This issue has been resolved for the most part with those mines being idled since the early 90s, so it shouldn't be an issue for any newly produced items. There seem to be no other known health risks.
Of course, I think asbestos was held in high regard as a naturally occurring silicates with desirable properties and no health risk for about 75 years. Oops. How old are MSDS sheets? Can we dig up an old one for asbestos?
I have a small yard that is surrounded by concrete on all sides. I started pouring out the hand warmers there. I will see if there is any appreciable difference over time.