Saturation vapor concentration

Hexachloroethane is a solid that sublimes at ambient air temperatures. At 20 C the saturated vapor concentration is 670-700 ppm (Weeks et al. 1979) thus, there is a limitation on the vapor concentration that can be used in studies using the inhalation route of exposure. In circumstances where the saturation threshold is exceeded, microcrystalline hexachloroethane forms in the atmosphere and is inhaled by the exposed animals along with the volatilized hexachloroethane. 

Fig. 10. Sulfuric add vapor abundances as obtained by passive chemical ionization mass spectrometry. Curves 1, 2, and 3 are model predictions of Turco et d. [58]. Broken curves are equilibrium saturation vapor concentrations for 45 latitude summer (S) and winter (W). (Figure from Qiu and Arnold, Ref. [26]).

The temperature at which the vapor concentration is equal to the UFL is referred to as the upper temperature limit of flammability (UTL). Above the UTL, the saturated vapor concentration is greater than the UFL, i.e., the gas mixture becomes nonflammable. 

Inhalation Exposure. Intermittent exposure to near-saturation vapor concentrations of heated Aroclor 1242 (8.6 mg/m ) over 24 days was not lethal in rats, mice, rabbits, or guinea pigs, and no signs of intoxication were reported (Treon et al. 1956). Pneumonia, apparently unrelated to PCB exposure, caused death in some of the test and control animals except those exposed to 8.6 mg/m Aroclor 1242. The vapor concentrations are unknown, as the technique used to estimate them has since been shown to be invalid possible CDF contamination was not reported because CDFs had not then been discovered. Similar exposures to lower concentrations of heated Aroclors 1242 and 1254 were also found not to produce lethality in these species. No data were located regarding lethality or decreased longevity of animals due to acute or chronic inhalation of PCBs. 

Knowledge of the saturation concentrations of the organic condensable species remains incomplete. These concentrations are expected to vary significantly with temperature. The few available relevant measurements include the saturation vapor concentrations of mono-carboxylic and dicarboxylic acids (Tao and McMurry, 1989) and the )3-pinene aerosol products (Pandis et al., 1991). Saturation vapor concentrations of condensable products from the oxidation of some aromatic hydrocarbons (toluene, m-xylene, and 1,3,5-trimethylbenzene) were estimated to lie in the range of 3 to 30 ppt (Seinfeld et al., 1987). 

McMurry and Grosjean (1985) estimated saturation vapor concentrations for condensable products from the oxidation of 1-heptene (0.14-1.28 ppb), o-cresol (0.06 to 1.6 ppb), and nitrocresol (1.7 to 2.2 ppb). Odum et al. (1996) fitted their smog chamber data to (13.39) and derived effective vapor pressures for the aerosol products of m-xylene, 1,2,4-trimethylbenzene, and a-pinene. Available relevant vapor pressures are shown in Table 13.13. 

The exposure potential from the use of NMP in paint stripping is completely different from its use in semiconductor processing as a photoresist stripper. In fab use, NMP is isolated from the fab worker in processing equipment that is a closed system. The low vapor pressure of NMP (0.342 mm Hg at 25°C) limits the saturated vapor concentration to about 450 ppm at room temperature. It has been reported that hydrolysis in air at relative humidities of 40-60% (a typical level found in a semiconductor fab area) can reduce maximum concentrations to approximately 130 ppm.1 1 The potential inhalation exposure hazard is low, but NMP is readily absorbed through the skin, and repeated or prolonged skin contact with the liquid can cause severe dermatitis. 

The industrial hygienist can use information concerning the vapor pressure of a chemical to estimate the highest airixtme concentration attainable for that material. This so-called saturated vapor concentration (SVC) is estimated using the following equation ... 

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