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Assigning Threshold Planning Quantities (TPQs) for Extremely Hazardous Substances

Ranking

The threshold planning quantities were developed based on a ranking approach. The ranking was based 

on an index of toxicity, physical state, and potential for the chemical to become airborne. Chemicals with 

the highest index rank were assigned a threshold planning quantity of 10,000 pounds. The statutory two 

pound limit established a reasonable lower limit for the most extremely hazardous substances on the list. 

Between two and 10,000 pounds, chemicals were assigned to intermediate categories of 100, 500 or 1000 

pounds based on the order of magnitude ranges of the index values. 

Index Value

The index was equal to level of concern (g/m3) divided by V where V is the extent to which the chemical 

can become airborne. Index = Level of Concern/V 

The more toxic the chemical (lower level of concern), the lower the index, and the lower the threshold. 

The more volatile the chemical, the lower the index, and the lower the threshold. Inversely, the lower the 

boiling point (more volatile), the higher the V or potential to become airborne.

V Determination

1. For a chemical that is gaseous at ambient conditions, V=1.

2. For a chemical that is a liquid,, V= 1.6 M0.67 / (T + 273), where M= molecular weight and T= 

boiling point in degrees C. V= G/Q = rate of volatilization (mass vaporized per time) per mass of 

liquid spilled. Assumes spill is 1 cm deep and has density of water.

3. For a chemical that is a solid in powder form with a particle size of less than 100 microns, V=1.

4. For a chemical that is a solid in solution, V=1.

5. For a chemical that is a molten solid, V=1. However, the amount molten should be multiplied by 

0.3 to obtain the actual amount to be compared to the TPQ for reporting. 

6. For a chemical that is a solid with a NFPA reactivity rating of 2, 3, or 4, V=1.

7. For a chemical that is a solid not meeting any of the above conditions 3, 4, 5 or 6, the threshold 

planning quantity is 10,000 pounds.

Level of Concern

The Level of Concern (LOC) was equal to the Immediately Dangerous to Life and Health (IDLH) levels 

in grams/cubic meter, published by the National Institute of Occupational Safety and Health (NIOSH).

For substances without a published IDLH level, the LOC was estimated from acute animal toxicity test 

data as follows with inhalation data preferableto other data, and median lethality data preferable to other 

types:
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USEPA, OFFICE OF EMERGENCY MANAGEMENT February 2008

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LOC = LC50 x 0.1 or LOC = LCLO

LOC = LD50 x 0.01 or LOC = LDLO x 0.1

LC50 is the Lethal Concentration 50% or the concentration of chemical in air at which 50% of the test 

animals died. (g/m3or mg/L)

LCLO is the Lethal Concentration Low or lowest concentration of the chemical in air at which any test 

animals died. (g/m3or mg/L)

LD50 is the Lethal Dose 50% or the dose of the chemical that killed 50% of the test animals.(mg/kg)

LDLO is the Lethal Dose Low or the lowest dose of chemical at which any test animals died. (mg/kg)