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HYDROFLUOROCARBON-23 29 No published information is available on the distribution, metabolism, or pharmacokinetics for HFC-23. TOXICITY INFORMATION Human Studies Fagan et al. (1995) conducted a human exposure study to assess the feasibility of using HFC-23 as an indicator for the measurement of human cerebral blood flow in nuclear magnetic resonance spectroscopy. Five subjects were exposed in a blinded fashion to HFC-23 at concentrations ranging from 100,000 to 600,000 ppm. Up to eight pulsed exposures lasting 3 min each, interspersed with 2 min of air exposure between pulses, constituted the exposure regimen. The subjects underwent extensive physiological and neurobehavioral monitoring throughout the study. The maximum tolerated concentration was considered to be 300,000 ppm. Effects such as lightheadedness, tingling and numbness of the extremities, and hyperacusis (abnormal acuteness of hearing) were noted at concentrations of 300,000 ppm and above. These effects were seen in only one subject at 300,000 ppm, and no effects were observed at concentrations of 100,000 ppm and 200,000 ppm. No clinically significant physiological effects (e.g., heart rate and rhythm, respiratory rate, and oxygenation) were noted, nor were abnormal clinical laboratory or neurobehavioral effects observed. A small but statistically significant retention of carbon dioxide occurred at 300,000 ppm. The effects of HFC-23 were very short in duration and all subjects were back to baseline at the 30-day evaluation. Acute Toxicity HFC-23 has low acute toxicity by the inhalation route. Its approximate lethal concentration (ALC) in rats is greater than 663,000 ppm after a 4-hr inhalation exposure. Kennedy and Nash (1980) exposed groups of six male albino rats to HFC-23 for a single 4-hr period at concentrations of 18,900 ppm, 186,000 ppm, and 663,000 ppm. Oxygen concentrations were maintained at about 20%. Clinical signs observed during exposure to the two higher concentrations included reduced response to sound, gasping, labored breathing, sluggishness, and gnawing. Mild weight loss was observed 24-48 hr after exposure, but normal weight gain was achieved thereafter.
