Rationale for list of regulated substances under Canadian Environmental Protection Act: appendix B

Appendix B: Technical Details for Determination of Threshold Planning Quantities Under EPA Risk Management Program

This Appendix is extracted from Federal Register,Vol. 51, No. 221, Monday November 17, 1986 page 41580.

Physical state and volatility were used to derive an index of the chemical's potential to become airborne and disperse. The two indices were combined to produce an overall risk "ranking factor" defined as:

Index = Level of concern/V


  • Level of concern = IDLH; IDLH is the Immediate concentration dangerous for life and health published in 1990 by NIOSH, alternative level of concern where used when IDLH 1990 was not available for a specific substance, see page 19; and,
  • V = index of potential to become airborne and disperse.

For gases and solids V equals 1.000, meaning all chemical once released can be potentially airborne. For liquids, V is calculated by estimating the rate of volatilization (mass vaporized per unit of time) per mass of liquid spilled. The V may be generated as follows using equations from Clements (1981) (see also TRC, 1986).

The evaporation rate of a liquid into stagnant air may be estimated by:

G = (1.74 x 10-4 MKAP) / (RT)


  • G = Generation rate (pounds/minute);
  • M = Molecular weight (grams/mole);
  • K = Mass transfer coefficient (cm/sec);
  • A = Surface area of spill (cm2);
  • P = Vapour pressure of the chemical (mm Hg);
  • R = Universal gas constant (82.05 (atm x cm3) / (g-mole x °K)); and,
  • T = Temperature of the liquid (°K)

mass transfer coefficient may be approximated by referencing the unknown chemical to water.

K = 0.83(18/M)0.33

Combining equations gives:

G = (3.78 x 10-4 M2/3AP)/RT

The surface area of a spill (or pool) is primarily a function of spilled quantity, provided the spill occurs on a flat non absorbing surface. The depth of the pool is assumed to be 1 cm, although if the area around a storage vessel is diked or not flat where puddling could take place, deeper levels could occur for the same surface area of spilled material. In the absence of specific information about the size of diked area for each liquid, we assume that the spill is 1 cm deep and has a density about that of water (1gm/cm3);

Area cm2
= 454 (gm/lb) x Q (lb) / ( (gm/cm3) x ( 1 cm))
= 454 Q


  • Q = Mass of liquid spilled (pounds)

Substituting and assuming the liquid is at its boiling temperatures (P = 760 mm Hg)
T > boiling point:

G/Q = V = 1.6 M0.67/ (T+273)

Where G/Q represents the rate of volatilization per mass of spilled liquid. Note that V was estimated for liquids at their boiling point rather than at ambient temperatures. Conditions during accidental releases are likely to vary and involve heat (e.g. fires, exothermic runaway reactions or reactions with air or water) causing more rapid volatilization of the liquid. EPA recognizes that spills at ambient temperatures are also likely and that the rate of volatilization may be impacted by heat from surroundings, subcooling due to evaporation and flashing from superheated conditions. However, for purposes of developing a relative ranking between substances volatilization at boiling points was utilized and consideration of other conditions for all chemicals is not expected to greatly reorder the ranking of chemicals.


TRC 1986. "Evaluation and Assessment of Models for Emergency Response Planning prepared for CMA", TRC Environmental Consultants, Inc. April 1986.

Clements, 1981. "Mathematical Models for Estimating Workplace Concentration Levels: A Literature Review" U.S.EPA, Clement Associates, 1981.

NIOSH (1990), (1994) and (1997) "Pocket Guide To Chemical Hazards", U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control and Preention, National Institute for Occupational Safety and Health, Washington, DC 20402

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