ACUTE, 2-WEEK, AND 13-WEEK INHALATION TOXICITY STUDIES ON DIMETHYLETHOXYSILANE VAPOR IN FISCHER-344 RATS

Dimethylethoxysilane (DMES), a volatile liquid, is used by NASA to wat erproof the heat-protective silica tiles and blankets on the Space Shu ttle. Acute, 2-wk, and 13-wk inhalation exposures to DMES vapor were c onducted in male and female Fischer 344 rats. In the acute study, rats were exposed to 4000, 2000, 1000, 500, or 0 (control) ppm DMES for 4 h and observed for 14 days. There were no deaths. Narcosis and ataxia were observed in rats of the two highest concentrations only. These si gns disappeared within I h following exposure. There were no DMES-rela ted gross or microscopic tissue lesions in rats of all exposure groups . In the 2-wk study, rats were exposed for 6 h/day, 5 days/wk to 3000, 1000, 300, 100, or 0 ppm DMES. During exposure, narcosis was observed in rats of the 3000 and 1000 ppm groups. There was a mild decrease in body weight gain in rats of the 3000 ppm group. A decrease in platele t count, an increase in bile acids, and reduced weights of the thymus, testis, and liver were observed in rats of the 3000 ppm group. Micros copically, hypospermatogenesis and spermatid giant cells were observed in the seminiferous tubules of the testes of rats exposed to 3000 ppm DMES. In the 13-wk study, rats were exposed 6 h/day, 5 days/wk to 200 0, 600, 160, 40, or 0 ppm DMES. During exposure, rats of the 2000 ppm group exhibited mild narcosis and loss of startle reflex. Recovery fro m these central nervous system signs was rapid. Body weights were mild ly decreased for rats of the 2000 ppm group. There were no exposure-re lated effects in hematology, serum chemistry, or urinalysis. Female ra ts of the 2000 ppm group had delayed estrous cycles (6 days compared t o 5 days in control rats). Noteworthy organ weight changes in rats of the 2000 ppm group included decreases in thymus, liver, and testicular weights; however, pathologic lesions were observed in the testes only . Sperm motility, epididymal sperm count, and testicular spermatid cou nt were dramatically reduced. Microscopic lesions included degeneratio n of the seminiferous tubular cells, pyknosis or absence of germ cells , and hypospermia in the epididymis. Rats of the 600 ppm group had a s light decrease in thymic weight and a transient decrease in body weigh t. Results of the acute, 2-wk, and 13-wk inhalation studies indicate D MES concentrations of 1000 ppm and higher produce narcosis that rapidl y disappears following exposure. Repeated exposure of rats to DMES at either 3000 ppm for 2 wk or 2000 ppm for 13 wk caused testicular atrop hy and hypospermia in male rats. Female rats exposed to 2000 ppm for 1 3 wk had delayed estrous cycles. Toxicological effects in rats of the 600 ppm group were minimal and equivocal. The 160 ppm concentration wa s a no-observable-effect level (NOEL) for 73 wk of exposure to DMES.