It is important to make sure that gas supply lines contain nothing but gas before the equipment they feed is started. Unfortunately, there seems to have been little guidance or regulation on how this should be done, and sometimes the pipes are purged (in pipeline work the process is called blowdown) into the interior of a building. With all that gas around just a spark can blow the place up. The U.S. Chemical Safety Board (CSB) cites a number of explosions dating back to 1997 caused by improper purging - practices that were in most cases allowable under prevailing regulations.