On Thursday, five people were injured when a reaction between trimethylaluminum and water caused an explosion in a lab at Dow Chemical's electronic materials facility in North Andover, Mass., Massachusetts Fire Marshal Stephen D. Coan said at a press briefing.

Four of the injured were taken to a local hospital, then three of them were transferred to Boston hospitals. The injuries were burns and shrapnel wounds, Coan said.

The explosion occurred around 2:20 pm. Emergency responders spent the rest of the day securing the scene and ensuring it would be safe for investigators. There was significant damage to the lab where the explosion occurred--Coan said that some windows were blown out and that the HVAC system, hoods, ceiling panels, and lighting were damaged. Local news reports say that people living adjacent to the plant felt the explosion. The building, however is structurally sound and should be reoccupied once investigators are finished, Coan said.

A trimethylindium explosion at the same site in 2013 resulted in the death of production operator Carlos A. Amaral, 51. Dow concluded that in that incident:

• An employee sustained injuries as a result of the overpressure of a small stainless steel manufacturing vessel during an operation associated with a Trimethylindium (TMI) manufacturing batch.
• An undesired and unexpected reactive chemical event occurred within the vessel as the employee was transporting the vessel from the glove box to the next manufacturing unit for further processing.
• The overpressure resulted in a release of reacted and unreacted materials and a fire.
The most highly probable cause of the unplanned event was the ingress of cleaning liquid from the cavity space of the ball valve into the crude TMI. Due to the nature of the event, it is impossible to completely validate this conclusion.

The Occupational Safety & Health Administration initially fined Dow $28,000, then settled for $17,500. The citations included one serious one for failing "to ensure reactor pots were adequately designed and inspected to prevent or minimize chemical explosions."

Fire marshal Coan said that yesterday's incident was "much different" from the 2013 one, although I was watching the press briefings online and couldn't ask specifically what he thinks the difference was other than trimethylindium versus trimethylaluminum. Hopefully more information will come out once investigators can get into the lab and finish interviewing the people involved.

There were two press briefings yesterday with the fire marshal, one at 6 pm and the other at 10 pm Eastern. A Boston Globe reporter tweeted these videos from that briefing.