Evaluation of Equilbrium Toxicity Hazards USING “Infinite Points”

Equilibrium concentrations of vapor above toxic liquids can be orders of magnitude above the Threshold Limit Value or other vapor-toxicity criteria. Thus, it is important to know the vapor pressure – and, thus, the concentration in air – at the liquid temperature, to determine the toxicity hazard. However, for many liquids – and particularly for novel liquids and liquid mixtures – only one data point may be available, typically for the vapor pressure near room temperature. As is shown in this paper, the “infinite point” for that class of liquids can be used to establish an approximate vapor-pressure-versus-temperature relationship.

An “infinite point” for aliphatic hydrocarbons was observed in 1923 by E. R. Cox, and “infinite points” for many classes of chemicals were identified by R. R. Dreisbach in 1949. Through the use of a graph in this paper, the approximate vapor pressure that corresponds to room temperature can be determined from one data point. In addition, “transposition” of the Antoine equation can be used to derive the A, B, and C constants from these two data points, or from three data points (if available), for use in spreadsheet analysis of toxicity hazard versus liquid temperature.

Based on the ratio of equilibrium vapor concentration to toxic concentration, the type of breathing protection can be selected. The Occupational Safety and Health Administration recently [2006] published a tabulation of protection factors for twenty types of inhalation-protection equipment, and the Table is included in this paper.