As in any emergency situation, the most important rule is to always think SAFETY. If you suspect that you have discovered a hazardous substance release, you should 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. Also, you should 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.
Once you are a safe distance away, you should attempt to isolate the incident and prevent others from coming into contact with the hazardous substances. If possible, you or someone nearby should 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. Even if you don't have specific information on the hazards involved, you should provide the 911 operator with as much information as possible, such as 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, they will establish safety zones and will initiate safe procedures to address the threat. (See Investigating Sites for further information on these procedures). As a witness, you should provide the responders with any information about the incident that may be helpful, but remember, they are trained first responders and will treat the incident as an emergency situation -- you do not want to obstruct their actions!