What key factor determines the classification of a hazardous waste?

The classification of hazardous waste is fundamentally determined by its characteristics, which include factors such as toxicity, reactivity, ignitability, and corrosivity. Among these factors, the pH level is crucial since it directly influences the corrosivity of a waste. Waste is considered hazardous if it exhibits a pH lower than 2 or higher than 12.5, indicating that it can corrode materials and possibly harm human health or the environment.

While other factors like the physical state, odor, or source of generation play roles in identifying hazardous waste, they do not have the same regulatory significance as pH level. For example, a solid waste can be hazardous due to its pH level but may not necessarily be classified as hazardous solely based on its state or smell. The source of generation can provide context but does not inherently determine whether the waste is hazardous. Thus, pH level is a definitive and measurable criterion used in the classification process.