Which factor increases ignition risk when a fuel truck is not full?

• A. Fully loaded trucks pose the highest risk due to maximum fuel
• B. Because there is a greater mixture of fuel fumes and oxygen
• C. Because the tank is lighter
• D. Because there is less vapor space

Answer: (B)

Ignition risk is tied to the presence of a flammable vapor-air mixture, not just the amount of liquid fuel. When a fuel truck isn’t full, there’s more headspace for fuel to evaporate and mix with the surrounding air. That increases the chances that the vapor concentration reaches the flammable range (fuel vapors plus oxygen) and, if an ignition source is present, it can ignite. If the tank were full, there’s less headspace, so fewer vapors can accumulate and mix with air, reducing the likelihood of reaching a combustible mixture. The tank being lighter or the idea of “less vapor space” being present are not the correct drivers of ignition risk in this scenario.