Why is the Orange County chemical tank crisis so hard to fix?

A pressurized tank containing about 7,000 gallons of methyl methacrylate (MMA) in liquid form in Orange County is at risk of exploding or leaking. The polymer product is not toxic, but the liquid MMA monomer is hazardous if it enters the air, harming people at high concentrations and with chronic exposure. Officials are lowering the tank temperature to reduce the chance of uncontrolled heating. Fire crews lowered the temperature to around 61 degrees, with 50 degrees as the goal, to enable safer response options. If the tank exceeds a threshold, it can enter thermal runaway, and officials plan to evacuate people and keep them away until conditions are safe to return.

"Inside the tank is an estimated 7,000 gallons of a chemical used to make plastics called methyl methacrylate, or MMA, stored in liquid form. "It's durable, lightweight, transparent, so it could even be used as a substitute for glass," Elias Picazo, assistant professor of chemistry at USC, said of the final plastic product. The polymer can also be used in household goods as well."

"The polymer itself isn't toxic, but its liquid MMA predecessor - a monomer, essentially a bunch of single molecules - is. If it gets into the air, it can harm people at high concentrations and through chronic or extended exposure. "The other hazard is the explosion itself. And it sounds like it's already the reaction has already initiated, and that's where the worry comes in for the explosion," Picazo said."

"In chemistry, there's a concept known as "thermal runaway reactions. And those are really hard to control," Picazo said. If the temperature of the tank exceeds a certain threshold, Covey said, "we know the tank is going into thermal runaway, and we're going to pull everybody out of the area, make sure it's safe, and let the tank do what it's going to do.""

"By Friday night, fire crews were making progress lowering the temperature of the chemical tank, which is down to around 61 degrees, with 50 degrees being the goal, Craig Covey, an Orange County Fire Authority division chief, said. "That could enable crews to implement some solutions that don't involve an explosion or destructive leak. But there is still a ways to go before the area is safe for people to return.""

Read at Los Angeles Times