Which is not a safety hazard associated with tank cleaning?

The answer identifies "Excess oxygen" as not being a safety hazard typically associated with tank cleaning. In the context of cleaning tanks, safety hazards are often linked to the presence of flammable substances, such as petroleum vapors, and the potential for fire and explosions due to these substances.

When tanks that previously contained flammable materials are being cleaned, the concerns include the buildup of flammable vapors and gases, which pose a significant fire and explosion risk. Proper ventilation and removal of these vapors are critical safety measures during the cleaning process.

In contrast, the presence of excess oxygen is not inherently a direct hazard in tank cleaning; rather, it is usually desired to ensure proper combustion and to aid in the process of fuel burning or vapor dispersion. Therefore, while oxygen is essential in combustion processes, in this context, it is not categorized as a safety hazard but rather an atmospheric factor that isn't itself dangerous. Thus, this distinction clarifies why "excess oxygen" is not associated with the hazards in the same way as the other options.