No danger that I know of. I have never seen any of the glass jars crack or break. A friend of mine once told me that on one Friday afternoon he was cleaning his 6 jars from the Toyota. All 6 jars were already cleaned thus almost invisible, standing in line at the curb. His neighbor didn't see them and backed her car right on top of them. They were thrown to the road with great noise. But when he picked the jars up from under her wheels, none of them was cracked, chipped or damaged in any way! Not a scratch!
However, if you are concerned about using glass a VERY durable plastic substitute or stainless steel container can be used.