Benzene

The benzene molecule including the hydrogen atoms. The controversy is that it was found in nearly all labs as a solvent until it was suggested that it causes leukemia. At the same time we inhale benzene by cigarette smoke, and it is a natural part of crude oil and gasoline. The maximum allowed tolerance is 1 ppm for an 8 hour workweek of 40 hours. Nowadays this means that you have to wear protective gear in the lab.

http://en.wikipedia.org/wiki/Benzene