Chevron refinery explosion

Why is there a lockdown? What if fire spreads but there's still a lockdown?

Chemicals in the air mainly. We'd be exposed if we had windows and doors open or if we were just outside. Train stations are shut down, the bridges are all closed, and no one is advised to go outside. I don't think we'll ever be in a position where the fires will spread with their procedures, quick response from the entire fire department (or so it seems), and other things.