I can relate an actual "chemical spill" that occurred in SoCal - I don't recall the precise date, but it was sometime in the late 1980s. We were unloading people at Anacapa Island when the boat captain gave five blasts on the horn and announced on the PA that everyone on the island was to get back on the boat immediately. A rocket on the launch pad at Vandeenberg (Pt Conception 50 miles or so to the west) had blown up on the launch pad, releasing a devil's brew of chemicals into the atmosphere. The park islands were downwind of the launch pad, and the word went out - everybody off.

The boat captain got on the horn, and only half jokingly inquired as to which direction should we head - further out to sea, or return to the mainland to die with everyone else. There was definitely some confusion and "fog of war" in the air along with the nasty rocket propellants. We had people on San Miguel Island, much closer to Vandenberg, and getting them off was a significant problem. They were enveloped by the chemical cloud and minimized it by retreating to the ranger station and sealing the windows and doors. We immediately sent a plane and got them off, although they still had to hike about half a mile to the airstrip. Medical treatment and monitoring was necessary, but everyone recovered.

With strong prevailing winds (think Santa Anas) a considerable distance and large populations could be involved, especially if you are dealing with highly toxic substances. And the circumstances will not be immediately clear. Anacapa was essentially unaffected, we learned later.

Living next to Vandenberg AFB can be exciting. A few years later, a rocket launched from Vandenberg blew up, dropping rocket parts on both San Miguel and Santa Rosa Islands. Good thing we had hard hats readily available.....