As in any emergency situation, the most important rule is to always think SAFETY. Following basic safety precautions helps to keep you and other bystanders safe, and assists the trained first responders in identifying and controlling the release.
If you suspect that you have discovered a hazardous substance release:
Immediately move away from the suspected release site and keep a safe distance away.
Always treat any unknown materials as hazardous -- never touch, ingest, or inhale any materials at the incident site.
Do not rush to or touch victims at the incident site. By doing so, you may come into contact with or inhale the hazardous materials and, inadvertently, become a victim yourself. More often than not, you can do more good by staying away from the incident and providing information to professional responders.
Before the first responders arrive:
Once you are a safe distance away, attempt to isolate the incident and prevent others from coming into contact with the hazardous substances.
Attempt to report the incident to appropriate authorities. A call to your 911 operator will alert local fire and police departments to the incident, and they will notify other responders as necessary.
Provide the 911 operator with as much information as possible, even if you do not know the exact nature of the hazards involved. If possible, give the location and nature of the incident, a description of any materials, foul odors, or gases present, the number of victims and their reactions to the unknown materials, and any other information that may be helpful to responders.
Once professional responders arrive at the scene:
As a witness, you should provide the responders with any information about the incident that may be helpful.
Remember, they are trained first responders and will treat the incident as an emergency situation -- you do not want to obstruct their actions!