Tens of thousands told to evacuate after a 34,000-gallon tank of methyl methacrylate leaked at an aerospace plastics facility, sending potentially toxic vapors into the air.

A damaged chemical tank in Southern California cracked over the weekend, which authorities were hopeful would relieve pressure and reduce the risk of an explosion.

The storage tank at GKN Aerospace with about 6,000 to 7,000 gallons (22,713 to 26,498 liters) of methyl methacrylate overheated last week and began leaking vapors into the air around Garden Grove, a c...