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HYDROFLUOROCARBON-236FA 17 logical evaluations were conducted. Under the conditions of this study, the 4-hr no-observed-adverse-effect level (NOAEL) for narcosis in rats is at or above 134,000 ppm, and the 4-hr lowest-observed-adverse-effect level (LOAEL) for narcosis is at or below 189,000 ppm. In an acute inhalation study by Ulrich (1996), a group of five male and five female young adult rats was exposed (whole body) to HFC-236fa (purity >99.5%) at a nominal concentration of 471,000 ppm (actual mean concentration was 457,000 ppm) for 4 hr. Chamber oxygen was maintained at 21.5% ± 5.33% with an air flow of 18.7 L/min. Although chamber temperature ranged from 19.7 to 30.8°C, there was no apparent effect on the test animals. Shortly after initial exposure, the rats exhibited a brief period of hyperactivity before becoming prostrate. No toxicologically significant effects were observed 1 hr after cessation of exposure or during the 14-day post-exposure observation period. A transient loss of body weight was observed in two female rats (3 and 9 g, respectively, during the first 3 days after exposure), but terminal weights exceeded those of day 0. Necropsy revealed dark red lungs in one male and three females, renal cysts in two females, and an enlarged pituitary in one female. The pathological findings were not considered to be exposure related. On the basis of the results of this study, 457,000 ppm is considered a 4-hr NOAEL for HFC-236fa inhalation exposure in rats. Cardiac Sensitization Huntington Research Centre (HRC 1994) examined the cardiac sensitization potential of HFC-236fa in six young adult male beagle dogs. The specific epinephrine dose to elicit a minimal response, as determined by altered electrocardiogram (ECG) with a few ectopic beats, was determined for each dog and found to range from 2 to 12 µg/kg. Dogs were exposed by face mask to HFC-236fa at concentrations of 50,000, 100,000, 150,000, 200,000, 250,000, or 300,000 ppm. An epinephrine challenge injection was administered after 5 min of exposure, and ECG monitoring was continued for another 5 min after the challenge. Evidence for cardiac sensitization to epinephrine challenge was observed in two of six dogs following exposure to HFC-236fa at 150,000 ppm as shown by multifocal ventricular ectopic activity in one dog and a fatal ventricular fibrillation in another. Following a 5-min exposure of six dogs to 200,000 ppm, two of the dogs (one of which also exhibited a positive response at 150,000 ppm) exhibited multifocal ventricular ectopic activity. At higher concentrations, excitation or narcosis in
