What is the most crucial first step when an unknown substance is identified?

Identifying the source of an unknown substance is a fundamental step in ensuring safety and understanding the potential risks it poses. Investigating the source helps determine the substance's origin and potential hazards, which is critical for assessing how to handle it properly. This step informs subsequent actions, such as whether the substance requires special handling or disposal procedures, or if it poses an immediate threat to health or the environment.

Understanding where the substance came from can also guide decisions about containment and cleanup, helping prevent further exposure or environmental contamination. By determining the source, responders can take appropriate measures based on whether it's a benign material or something that could lead to an emergency situation.