A person I know had to get tested for toxins at work regularly and at some point showed really high arsenic levels. At which point they had to figure out what poisoned him. He worked in material science but no materials he handled would have given off arsenic to his body. After some investigating, they found out that the Pu'Erh tea he consumed was the culprit. They tested the tea in a lab and after he stopped drinking it, his levels became normal.
So that might not kill.you but I guess having these toxins in you body cannot be good either.
Since then I try to only buy teas which are lab-tested. When I lived in Germany I bought my teas at sunday. Tbey tell you harvest, growing conditions and many more things and they also claim to lab test the tea badges.
I haven't found anything comparable in the US yet.