The incident triggered an evacuation of Valley Center (approximately 6,000 residents); destroyed the tank farm; and significantly interrupted Barton’s business. An investigation by the U.S. Chemical Safety and Hazard Investigation Board (CSB) has concluded that the initial explosion occurred inside a vertical above-ground storage tank that was being filled with Varnish Makers’ and Painters’ (VM&P) naphtha. VM&P naphtha is a National Fire Protection Association (NFPA) Class IB flammable liquid1 that can produce ignitable vapor-air mixtures inside tanks and, because of its low electrical conductivity, can accumulate dangerous levels of static electricity.2