The federal Occupational Safety and Health Administration (OSHA) and a state commission are on site to help determine what happened, Sullivan said, "so we understand exactly what did occur and if there's any learning there."
Sullivan confirmed Tuesday the small explosion was a natural gas ignition. At the time of the incident last week, it was still being determined whether it had been an explosion or boiler malfunction.
The workers were getting ready to start the boiler around noon when the small explosion occurred. The brewery was shut down and the building evacuated. About 100 employees were on site at the time.
....Two men had been hurt slightly and were treated at the scene. One had smoke in his eyes, the other had a bump on his head.
http://uppermacungie.patch.com/articles/cause-of-small-explosion-at-sam-adams-brewery-still-under-investigation