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HYDROFLUOROCARBON-236FA 16 biphasic with a rapid equilibration phase of up to 30 min followed by a slow linear uptake phase. The partition-coefficient data and the data obtained from the gas-uptake experiments were used in a physiologically based pharmacokinetic (PB-PK) model in an attempt to describe mathematically the disposition and metabolism of HFC-236fa. The PB-PK model was unable to describe adequately the loss of the test material from the animal chamber. Data from the gas-uptake experiments were inconsistent with metabolism-mediated disappearance of HFC-236fa. Samples of blood, urine, or feces were collected from rats exposed to HFC-236fa after 6 and 24 hr of exposure (Vinegar et al. 1995). The samples were extracted with hexane or cyclohexane and analyzed by gas chromatography and mass spectrometry (GC-MS); some samples were methylated before GC-MS analysis to detect organic acids. Although HFC-236fa was detected in samples of blood, urine, or feces, no fluorocarbon metabolites of HFC-236fa were detected by GC-MS, either in the total-ion- current mode or in the single-ion-monitoring mode. Moreover, GC-MS analysis revealed no compounds with retention times consistent with seven fluorocarbons proposed as possible metabolites of HFC-236fa. Valentine (1995) found no fluoride ions in the urine of rats exposed to HFC-236fa concentrations as high as 50,000 ppm for 6 hr per day, 5 days per week for 2 weeks, thereby indicating no significant metabolism of the compound. TOXICITY INFORMATION Acute Toxicity Currently available data indicate that HFC-236fa has low acute toxicity by the inhalation route. Keller (1994) exposed young male rats to concentrations of HFC-236fa at 150,000 or 200,000 ppm (purity 99.06%) for 4 hr. Actual mean concentrations were approximately 134,000 and 189,000 ppm, respectively. During the whole-body exposure, the oxygen concentration was maintained at 21% ± 3% and air flow was approximately 2 liters (L) per min with a total chamber volume of 13 L. No rats died during exposure or during an additional day of observation following the exposure. Exposure to HFC-236fa at 134,000 ppm produced no observable effects, but rats exposed at 189,000 ppm exhibited narcosis (nonresponsive to sound) that persisted for approximately 30 min after cessation of exposure. No additional effects were observed during the 1-day post-exposure period, and no patho