Methyl methacrylate is used to make hard clear plastics like aircraft windows. When it overheats, it reacts with itself, building pressure that can rupture a tank.
The chemical is not regulated under the U.S. EPA’s Risk Management Program or California’s parallel system. That could mean the damaged tank was overseen under an alternate hazardous-materials program, which would leave regulators with fewer tools to manage its storage.
Facilities housing methyl methacrylate may also be left out of community emergency planning because the substance isn’t regulated under federal and state accident prevention programs either. That could leave nearby residents without information about the risks they face, or how officials would respond in the event of a catastrophe.