The neurologic effects due to short-term exposure to xylene have been reported from laboratory studies of human volunteers. The results of these studies indicated that the odor threshold for xylene is approximately 1 ppm (32). Brief exposure to approximately 70 ppm xylene did not affect reaction time or short-term memory (33). Inhalation exposure at 90 ppm, however, caused deleterious effects on reaction time, manual dexterity, body balance, and EEG (34). Short-term exposure to xylene at 300 ppm has been associated with a decrement in performance on reaction time, memory span, and critical flicker fusion tests (35). Exposure at 100-400 ppm also caused an impairment ofbody balance and increased reaction time (36), and eye irritation occurred at 460 ppm (32).
In animals, repeated exposures to air concentrations of xylene in the range of 200-2000 ppm have been associated with effects on behavior and/or brain chemistry. Exposure to xylene for 4 hr/day for 3 consecutive days at 1600 ppm increased motor activity in rats (37). Changes in dopamine and noradrenaline levels and turnover in various regions of the rat brain have been observed after xylene exposure for 6 hr/day for 3 consecutive days at 2000 ppm (38).
Changes in neurotransmitters have also been reported after a 30-day exposure from 200 to 800 ppm (39). The earliest neurochemical effect of xylene, a decrease in brain glutathione, occurred after 5 days of exposure at 50 ppm (40).
A summary of the neurotoxic effects reported for xylene is shown in Table 4. The results of studies of humans and animals indicate xylene neurotoxic effects below the current TLV level of 100 ppm. The results would predict that occupational exposure at the TLV would adversely affect human performance. The long-term effects from repeated exposures to xylene remain uncertain.