What characteristic must leather or approved non-leather boots have?

Leather or approved non-leather boots must have metal parts that do not come into contact with the skin to ensure safety and comfort. This characteristic is crucial in aviation and other industrial environments where employees may be exposed to various hazards. Metal components can conduct heat, cold, or electricity, and if they are in direct contact with the skin, they can cause injury or discomfort. Therefore, this design consideration helps prevent potential adverse effects while promoting the wearer's overall safety when working in environments where such risks may be present.

The other options relate to different aspects of boot design but do not match the essential safety feature required for aviation-related activities. While visibility may be important in certain situations, there isn’t a specific regulation concerning color-coding for safety boots within aviation contexts. Waterproofing could be beneficial but is not a mandated characteristic. Size adjustability, although important for comfort, does not directly address safety concerns related to direct contact with metal components.