According to
www.snopes.com, the story of welders being killed when a spark ignited their Bic lighter is false; no such fatal accident has ever been recorded, as far as they could discover. However, there have been several cases of such lighters exploding due to overheating (a salesman in Belgium lost his car when 500 lighters, that he had left in his car on a hot day, ignited) .
The only known fatality due to an exploding Bic lighter was a 66-year old woman, whose family sued Bic for $11 million, claiming that the lighter had blown up in her hand (presumably it was defective). The case was settled out of court, and the results of the settlement were sealed.
So, on the one hand, it seems to be safe to carry them in your pocket while welding; on the other, it seems to be quite dangerous to store them in your car, or even carry them in your pocket, on a hot day. <img src="/images/graemlins/frown.gif" alt="" />
http://www.snopes.com/horrors/techno/lighters.htm