First result in Google...from wiki answers (just an fyi, not flamin):
Battery Out-gassing
Yes, vehicle batteries are usually of the lead-acid type, and these batteries give off hydrogen gas while they are either being discharged, or charged.
Hydrogen gas is a colorless, tasteless, and odorless gas, is highly flammable and when mixed in proper proportion with oxygen in the air, hydrogen can be explosive.
Smoking or any arc/spark within the hydrogen-oxygen gas envelope, or an internal spark inside the battery, can cause a massive explosion throwing sulphuric acid and disintegrated battery case plastic for some distance. This explosion of the battery out-gas can do very serious bodily harm, and has been reported to have resulted in deaths.
Therefore, it is critical that all possible safety precautions are followed when working with, on, or around vehicular batteries. Most modern auto batteries have decals on them listing the major hazards and recommended precautions