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How did EPA determine TPQs for EHSs?

How did EPA determine threshold planning quantities for extremely hazardous substances?

The Agency assigned chemicals to threshold planning quantity (TPQ) categories based on an index that accounts for the toxicity and the potential of each chemical, in an accidental release, to become airborne.  This approach does not give a measure of absolute risk, but provides a basis for relative measures of concern. 

Under this approach, the level of concern for each chemical is used as an index of toxicity, and physical state and volatility are used to assess its ability to become airborne.  The two indices are combined to produce a ranking factor.  Chemicals with a low ranking factor (highest concern), based on the Agency's technical review, are assigned a threshold planning quantity of one pound.  It is believed that the one pound threshold planning quantity represents a reasonable lower limit for the most extremely hazardous substances on the list.  Chemicals with the highest ranking factors, indicating lower concern, were assigned a threshold planning quantity of 10,000 pounds.  This ensures that any facility handling bulk quantities of any extremely hazardous substances would be required to notify the State commission.  Between the limits of one pound and 10,000 pounds, chemicals were assigned to intermediate categories of 10, 100, 500, or 1,000 pounds based on order of magnitude ranges in the ranking factors.  The selection of the intermediate categories was based on standard industrial container sizes between one and 10,000 pounds. 

The Agency believes that limited State and local resources should be focused on those substances that could cause the greatest harm in an accidental release.  The TPQs developed in this approach meet the objective such that substances that are most likely to cause serious problems (extremely toxic gases, solids likely to be readily dispersed, or highly volatile liquids) have lower TPQs than those that might be toxic but are not likely to be released to the air (non-reactive, non-powdered solids).