TPQ calculations for hazardous substances in molten form
Facilities are subject to emergency planning and notification requirements under EPCRA (also known as SARA Title III) when an extremely hazardous substance (EHS) is present at a facility in an amount equal to or in excess of its TPQ. For some EHSs that are solids, two TPQs are given (e.g., 500/10,000 pounds). The lower quantity applies only if the solid exists in powder form or is handled in solution or molten form (40 CFR 355.15). Otherwise, the 10,000-pound TPQ applies. The amount of non-reactive solid in molten form must be multiplied by 0.3 to determine whether the lower TPQ is met (40 CFR 355.16(c)). What is the significance of multiplying by the fraction 0.3 and how was this fraction chosen?
Emergency planning for EHSs under EPCRA Section 302 is based on estimates of the quantity of an EHS released to the air. One of the factors that affect the quantity that actually becomes airborne is the physical state of the substance. At molten temperatures, significant amounts of vapor are not likely to be generated. The Agency examined the fraction of volatilization expected from the solids on the EHS list and found that the amount of chemical that actually volatilizes ranges from 0.008 to 0.3 pounds per minute per pound spilled. Since information was not available for all the solids on the EHS list, the Agency chose to incorporate the more conservative fraction of 0.3 for TPQ determination. As a result, a facility that handles an EHS in molten form must multiply the amount of non-reactive solid in molten form by 0.3 to determine the weight applied toward the TPQ.