What does 'hazardous substances' refer to?

'Hazardous substances' refers specifically to materials that can cause harm to humans or the environment due to their chemical properties. This definition encompasses a wide range of substances that can pose risks in various forms, such as toxicity, flammability, reactivity, and environmental impact. Understanding the impact of these materials is crucial for safety protocols in handling, storage, transportation, and waste disposal to protect individuals and the ecosystem.

The other options do not accurately capture the essence of hazardous substances. Handling costs do not determine a substance's hazard level, and materials deemed safe or those without regulations are not classified as hazardous regardless of their management considerations. Thus, the correct answer effectively highlights the potential risks posed by certain materials, which is a fundamental concept in hazard recognition and safety training.