Which of the following is NOT a possible source of fires on a ship?

Excessive humidity is not considered a direct source of fires on a ship. While high humidity can create conditions that may eventually contribute to issues such as corrosion or mold, it does not ignite or sustain combustion. Fires typically require a combustible material, an ignition source, and sufficient oxygen to sustain combustion. In contrast, smoking, electric equipment, and welding or burning are all activities that can produce sparks, heat, or open flames, which can easily ignite flammable materials found aboard ships. Therefore, while excessive humidity may lead to other maintenance issues, it does not function as a catalyst for fire.