Toxicity-Hazard Index and the "Infinite Point"

TOXICITY-HAZARD INDEX AND THE “INFINITE POINT”

Richard W. Prugh, Principal Process Safety Engineer

Chilworth Technology, Inc., a Dekra Company

113 Campus Drive, Princeton, New Jersey 08540

Abstract

Recently, the Occupational Safety and Health Administration published Assigned Protection Factors for several types of breathing-protection devices, as part of the OSHA standard in 29 CFR 1910.134(d)(3)(i)(A).  Proper use of such devices would allow a person to enter an environment that was at the Maximum Use Concentration of the toxic gas or vapor of interest.  Thus, it is essential that the concentration of the gas or vapor be known prior to protected entry into that environment.

For many toxic vapors, the “worst-case” equilibrium concentration above a spill of the liquid at a given temperature can be readily calculated from the Antoine vapor-pressure equation – with three coefficients – or, with somewhat greater accuracy, using an equation with five coefficients.  Thus, for example, the maximum volumetric concentration of vapor above a spill at “room temperature” can be easily obtained as the ratio of the vapor pressure divided by atmospheric pressure.  Then, the type of respirator to protect against that calculated concentration can be determined.  Also, the ratio of the maximum equilibrium concentration divided by the OSHA Permissible Exposure Limit could be considered to be a Toxicity-Hazard Index, and this Index would be numerically equal to the required Protection Factor.

However, with novel or complex compounds, the coefficients for a vapor-pressure-versus-temperature equation may not be available.  If one set of vapor pressure and temperature values is known, an equation can be derived through use of the “infinite point”, at about 1,625^oC and 55,500 psia [about 2.9x10⁶mmHg].

The purpose of this paper is to show how Toxicity-Hazard Indexes can be obtained and how the “infinite point” can be used to estimate room-temperature concentrations of toxic vapors.