What is purpose of the list of extremely hazardous substances (EHSs) in regards to the emergency planning requirements of EPCRA and 40 CFR 355?

The EHS list and its threshold planning quantities (TPQs) are intended to help communities focus on the substances and facilities of most immediate concern for emergency planning and response. However, while the list includes many of the chemicals which may pose an immediate hazard to a community upon release, it does not include all substances which are hazardous enough to require community emergency response planning. There are tens of thousands of compounds and mixtures in commerce in the United States, and in specific circumstances many of them could be considered toxic or otherwise dangerous.

The list represents only a first step in developing effective emergency response planning efforts at the community level. Without a preliminary list of this kind, most communities would find it very difficult to identify potential chemical hazards among the many chemicals present in any community. Similarly, TPQs are not absolute levels above which the EHSs are dangerous and below which they pose no threat at all. Rather, the TPQs are intended to provide a "first cut" for emergency response planners in communities where these EHSs are present.