Exposure to methyl methacrylate, a highly flammable chemical used to make plastics, can cause serious respiratory and neurological problems. GARDEN GROVE, Calif. — About 16,000 people remain under evacuation orders in Southern California as an overheated chemical tank still poses a risk to homes closest to the site, and officials Tuesday gave no word on when they might be able to return home. The crisis forced 50,000 people to evacuate in and around the Orange County city of Garden Grove last week. A crack that formed by chance on the tank relieved pressure and helped avert a catastrophic explosion, allowing…