Why Reduce Vaporization Rates

The release of a toxic or hazardous material usually results in minimum impact or presents little danger while it remains in anonairbome liquid state in which it can be more easily contained or controlled. When any or all of the material vaporizes and becomes airborne, however, it becomes hazardous to people both close to the release and in surrounding areas. Therefore any steps that can be taken to reduce the quantity of toxic/flammable material that vaporizes will serve to reduce the consequences of the release and the total number of people involved. 

The effects of the release of a toxic material are proportional to both its airborne concentration and the duration of the release that is, the greater the concentration or time of exposure, the greater the consequences to those who are exposed. The toxic effect can be expressed mathematically in a probit equation that calculates the probability of damage, Y (CCPS, 1989a). The equation is 

Dose is the product of concentration, C, and time, t. As the equation shows, when n is greater than 1.0, as it is for many compounds, concentration effects are actually the more significant component of the total dose. Therefore, reducing the vaporization rate reduces the toxic dose because of its impact on reduced concentration, even though the lower vaporization rate may increase the duration of the exposure. 

Values for the constants in the probit equation (3-1) for a few compounds can be found in Guidelines for Chemical Process Quantitative Risk Analysis (CCPS, 1989a, Table 2.9, p. 156). 

Other means of evaluating the toxic effects of vapors, and their potential impact on exposed people, can be found in the Emergency Response Planning Guidelines for Air Contaminants (ERPGs), issued by the American Industrial Hygiene Association (1992). The documents define values of exposure to toxic vapors needed to produce specific health effects. Three levels of vapor concentration are defined  

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