Citation: Babrauskas, V., Some Basic Facts About Ignition Events During Fueling of Motor Vehicles at Filling Stations, California Fire/Arson Investigator 16, 25 (Apr. 2005). Some basic facts about ignition events during fueling of motor vehicles at filling stations by Vytenis Babrauskas, Ph.D. The January 2005 issue of The California Fire-Arson Investigator contains a story (p. 20) on the legal aspects of a Louisiana case where the responsibility for a fire at a filling station was being litigated. The fire was a static-electricity-caused ignition of gasoline vapors, which led to burn injuries to an individual engaged in filling in his truck. While the story focuses mostly on the legal issues of that case, the presentation of the relevant physics will not leave the reader with a competent understanding of this—yet, to a fire investigator, understanding the fire physics should be at least as important as understanding legal issues. The problem is that the story presents a description of such ignition as a novel event and furthermore presents an erroneous account of static electricity. In fact, such an event is not novel and about 150 – 200 fires of this kind occur each year in the U.S. alone. In my recent Ignition Handbook 1 , I have devoted a section to explaining some of the technical issues involved with these fires and I would like to summarize here some of the research that has been done on this topic and the salient findings of that research. For an ignition to occur of gasoline vapors mixed in air, the mixture must be within its flammable range, which are approximately 1% to 7%. With rare exceptions of very cold climates, the interior of the fuel tank is above the UFL, while the outside atmosphere will be below the LFL, thus, once the gas cap is removed, a concentration must exist somewhere between the tank interior and the outside world which is within the flammable range. This flammable zone can surround the filler opening and extend some distance down the filler tube. If a spark occurs in that zone, a fire is likely to result, since an exceedingly small spark energy suffices to ignite gasoline vapors. In view of this innate hazard, there should be absolutely no surprise that accidents of this kind occur—perhaps we should be surprised that the probability of this occurrence is quite low. Electrostatic charging can occur due to the flow of gasoline, as noted in the original story. But with the right combination of weather, clothing, and car-seat materials, getting in and out of the vehicle seat can cause substantial charging of the person, and this charge is not necessarily dissipated by grabbing a door handle, especially since in cases in many cases the resistance between the vehicle and ground may be very high. In the latter case, a third cause of charging also exists: the movement and braking of the vehicle on the pavement. Most of the incidents occur under low-humidity conditions, consequently, they are more prevalent in cold weather. A disproportionate fraction of these incidents (55% of the incidents where the ignition details are known) have involved an individual who re-enters and re-exits the vehicle during the fueling operation 2 ; see Figure 1. But a substantial number of the fires (20%) have occurred while the person was initially in the process of removing the filler cap, prior to actual commencement of refueling. It was of course completely wrong for the Court conclude that the clothing and the car seat could not have been involved in electrostatic charging because they were determined to be “poor conductors of electricity.” Hopefully, most CCAI members will know that precisely the opposite is true—rubbing two objects that are good conductors of electricity (e.g., metal against metal) will not result in electrostatic charging, but rubbing two electrical insulators (e.g., fabric against fabric or plastic) can well do so. This charging effect explains why fueling is more likely to result in an accident if a person gets out of the vehicle, inserts the filler nozzle into the tank, then gets back into the vehicle, then gets out of the vehicle again to remove the nozzle. Since charging is caused each time by rubbing the clothing against the car seat, charging to a higher potential can occur if there are three actions of moving out, moving in, and moving out, instead of a single action of moving out. The individual who was injured in the case