Minimum Ignition Energy is usually expressed in units of ______.

• A. Joules
• B. Calories
• C. Microjoules
• D. Millijoules

Answer: (D)

Minimum ignition energy is the smallest energy needed to ignite a fuel–air mixture. In fire hazards practice, this energy is reported in millijoules because it keeps the values in a practical, comparable range for most materials. Many ignition energies for dusts and vapors fall between the microjoule and millijoule scales, so using millijoules provides a convenient standard for testing and comparison. Using joules would yield numbers that are typically too large to be useful, while microjoules would be too small for routine reporting, and calories aren’t the common unit used in this field. So, millijoules is the standard unit.