ARTICLE NO. TX982483 Quantitative Exposure of Humans to an

There is potential for human exposure to cyclic siloxanes by the respiratory route. To determine the pharmacokinetics of octam-ethylcyclotetrasiloxane (D4), a material commonly found in per-sonal care products, the respiratory intake and uptake of D4 were measured in 12 healthy volunteers (25-49 years) on two occasions. Subjects inhaled 10 ppm D4 (122 jAg/liter) or air (control) during a 1-h exposure via a mouthpiece in a double-blind, randomized fashion. Inspiratory and expiratory D4 concentrations were con-tinuously measured. Exhaled air and plasma D4 levels were mea-sured before, during, and after exposures. Individual D4 uptakes were measured under steady-state conditions during three rest periods (10, 20, and 10 min, respectively) alternating with two 10-min exercise periods. Mean D4 intake was 137 ± 25 mg (SD) and the mean deposition efficiency was equivalent to 0.74/(1 + 0.45 VE), where Ve is the minute ventilation. No changes in lun