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Once EPA learns of a potential hazardous substance release, an established set of procedures is followed to investigate the site, evaluate the threat, and determine the best course of action. First, EPA designates an On-Scene Coordinator (OSC) to evaluate the incident and determine the appropriate response agency. If the OSC determines that EPA will take the lead in responding to the incident, the OSC will evaluate the urgency of the situation to determine the appropriate response alternative.

Prior to initiating an emergency response action, the OSC will conduct an off-site preliminary assessment of the release and the site's characteristics. The preliminary assessment helps to identify specific hazards and determine the appropriate response measures and safety measures needed to ensure the health and safety of the responders. The OSC may rely on a variety of methods to collect the necessary information, including interviewing witnesses, first responders, and others present at the site; reviewing records and documents at the facility or on the vehicle; analyzing photographs taken at the site; or conducting a visual (off-site) inspection using binoculars. Thanks to the Community Right-to-Know law, important information about the released substances is often easily and quickly obtained.

Once EPA determines the type of emergency response action needed, response personnel take special precautions to ensure that they are protected from the threats posed at the site. When entering the site, response personnel wear personal protective gear to shield or isolate them from the chemical, physical, and biological hazards that may be encountered on the site. The selection of the type or level of personal protective equipment is based on the identification of the hazards or suspected hazards, potential exposure pathways, and the performance of the equipment in providing a barrier to these hazards. Because there is often little known information on specific hazards during the initial phase of an emergency response, the OSC typically directs that the most protective equipment be used at first; as more information about the hazards and conditions become available, the OSC can decide to downgrade the level of protection to match the site hazards.

As response personnel enter the site of the release, they gather additional information and further evaluate the site risks and hazards present. Response personnel use this information to further refine their response activities and the safety measures being taken. Generally, response workers entering the site conduct a visual survey for potential hazards and may perform air monitoring for potential dangers to life and health. For example, a visual survey might note the condition of waste containers (e.g., rusted or other unusual conditions), determine potential exposure pathways, and identify other possible dangers, such as confined space and oxygen-deficient environments, ground subsidence, visible vapor clouds, or areas that contain biological indicators, such as dead vegetation or wildlife. Response personnel use direct-reading instruments and testing equipment when performing air monitoring. One important goal of monitoring during initial site entry is to establish work or safety zones at the site. As the emergency response action continues, response personnel conduct periodic monitoring to ensure that any new hazards are identified promptly and that appropriate controls are implemented to protect the responders and nearby communities.

Site investigations are essential to protect the health and safety of response personnel and others during emergency response actions. The information gathered is absolutely critical to enable responders to proceed confidently and safely, and to ensure that local communities receive accurate information about the potential for adverse health effects.