California chemical leak: What is methyl methacrylate, the chemical forcing 40,000 residents to evacuate?

A major chemical emergency in Garden Grove, Orange County, has forced around 40,000 residents to leave their homes after a storage tank at the GKN Aerospace facility began overheating and releasing hazardous vapours. The chemical inside the tank is methyl methacrylate, or MMA, a volatile industrial liquid used in plastics and resin manufacturing. Officials warned that the unstable tank could potentially fail, leading to a major spill or even an explosion, prompting large-scale evacuation orders across several nearby cities. Emergency crews have been racing to cool the tank and reduce the risk while keeping people out of danger.The incident began on Thursday at the GKN Aerospace facility in Garden Grove, where a storage tank containing methyl methacrylate overheated and began venting vapours. As the situation worsened, evacuation orders were issued and later expanded to cover parts of Garden Grove, Anaheim, Stanton, Cypress, Buena Park and Westminster.Methyl methacrylate (MMA) is a colourless industrial chemical used to make acrylic plastics, resins and coatings. It is commonly found in products such as acrylic glass, aircraft windows, paints, adhesives, dental materials and lightweight plastic components. The chemical has a strong fruity odour and evaporates easily, which means it can quickly release vapours during a leak or accident.MMA is considered hazardous because it is highly flammable and its vapours can ignite when exposed to heat, sparks or flames. In large storage tanks, the chemical must be carefully stabilised and temperature-controlled, as overheating can increase pressure inside the container and raise the risk of a fire or explosion.Health agencies including the US Environmental Protection Agency and the CDC warn that exposure to MMA vapours can irritate the eyes, skin, nose and throat. Higher levels of exposure may cause headaches, dizziness, nausea, coughing and breathing difficulties. Because the vapours are heavier than air, they can spread along the ground and travel toward ignition sources.Another major concern is MMA’s polymerisation risk, where the chemical can begin reacting with itself and generate heat. If that reaction accelerates uncontrollably, it can trigger thermal runaway, a dangerous process in which rising heat causes the reaction to speed up even further, rapidly building temperature and pressure inside the tank. In severe cases, this can cause a rupture, fire or explosion. That is why emergency crews in California have been continuously cooling the damaged storage tank to prevent the situation from escalating.The fear was not just that the tank could leak. Emergency officials warned it could also rupture or enter thermal runaway, a dangerous chain reaction in which heat and pressure keep building until the container fails. Reuters reported that the tank held about 7,000 gallons of MMA, while other reports said multiple tanks were present at the site and several had already been neutralised, leaving one unstable tank as the primary concern. Because the chemical is flammable and can give off hazardous vapours, even a partial failure could put nearby neighbourhoods at risk.Fire crews have been using remotely operated water hoses to cool the tank and bring down its temperature, while also placing sandbag barriers to prevent any spill from reaching storm drains or nearby waterways. Officials have also brought in hazardous-material experts to help determine the safest long-term fix. According to the latest reports, the tank’s temperature had stabilised, but crews were still treating the situation as dangerous enough to keep evacuation orders in place.Health officials warned that exposure to methyl methacrylate vapours could cause respiratory problems and nausea, which is why residents in the evacuation zone were told to leave quickly and stay clear until officials gave the all-clear. Reuters reported that around 15% of ordered evacuees had not left, even as shelters were opened in nearby cities. The broader message from authorities was simple: the tank remained unstable, and the safest move was to keep people away until the risk had been reduced.