I haven't read or researched this anywhere, but.......I'd betcha a nickel or some cadmium.......
Being from the land of rules & regulations leads me to guess that it may have also had something to do with the "possibility" of degradation in the landfill.......
If they would only pay attention to what we're doing HERE.....there wouldn't BE any Redheads in the landfill to then "possibly" cause a problem......
The problem is when the cadmium is broken down to a small enough increment to enter the body & pile up in the tissues along with other problematic heavy metals......lead is the most noteworthy, as it's been used probably the longest in manufacturing.....
I asked a doctor once about screening for heavy metals & he told me lead is not a hard one, there's a standard testing, but all the other ones I'd have to get him the MSDS sheets for the chemicals & inks that I worked around so they had some idea as to what to look for.......
really wouldn't think there's any problem cooking on one at all......unless you're licking the outside surface & picking up excess cadmium, especially if the grill's heated up.....play it safe & just don't do it !!!!