The biggest problem with passthroughs is the cord. Since a lot of them can be damaged after continual use. There are higher end PT's out there. For example:
VP-Passthru BB
Yes, even safe chemistry batts can blow. As well as batts in cell phones, etc. Nothing's completely safe but, there's very little chances of a these types of batteries can blow. Also, I just found out that the person that was injured used a e-cigar and not a mod.
What's the voltage on the BB passthrough?
Did the e-cigar take other batts or was it one of the built in ones?