Glycol ethers are readily absorbed by inhalation, ingestion and dermally (Browning and Curry, 1994). The rate of dermal absorption is inversely proportional to the molecular mass of the glycol ether. Generally, dermal absorption decreases with increasing molecular mass or decreasing volatility. This is true for both the ethylene glycol and diethylene glycol series. EGME is absorbed three times more readily than EGEE and EGEEA and ten times more readily than EGBE (Dugard et al., 1984). An in vitro study of isolated human epidermis demonstrated that rates of dermal absorption were slower for the diethylene glycols compared to their ethylene glycol equivalents (Dugard et al., 1984). In another study using several glycol ethers (EGEE, PGME, PGMEAc, 2PG1BE, EGDME, EGDEE, DEGDME) all were rapidly absorbed with a lag time of less than 1 hour and in most cases less than 30 minutes (Larese et al., 1999). Toxicity following skin absorption has occurred in animals (Carpenter et al., 1956; Duprat and Gradiski, 1979) and humans (Ohi and Wegman, 1978; Morton, 1990).
In a volunteer study comparing exposure by mouth (via a respirator) and dermal absorption (while only wearing shorts and breathing air through a respirator) it was found that the average blood concentrations of EGBE were 3-4 times higher after dermal exposure. The results of this study suggested that absorption through the skin may account for about 75% of the total uptake of EGBE (Johanson and Boman, 1991). However, this measure of dermal uptake is thought to be an overestimate. The sample collection technique used in this study probably affected the results; local absorption of EGBE near the sampling site may have lead to a falsely high blood concentration. A more recent study using a different method has shown that dermal absorption accounts for no more than 15-27% of the total uptake of EGBE in humans (Corley et al., 1997).
During exposure of only 25% of the body surface (more realistic in terms of the working environment), dermal absorption only accounted for 4.4-48.4% of the total (Corley et al., 1997). In another study it was estimated that when the whole body surface is exposed to vapour, dermal uptake could account for 55% of the total uptake of EGME and 42% of EGEE (Kezec et al., 1997). Rates of absorption of EGBE in a volunteer study were between 7 and 96 nmol/min/cm2 (Johanson et al., 1988).
The rate of absorption via the lungs depends on the concentration of the glycol ether and the respiration rate. In human volunteers, 76% of the inhaled dose of EGME was absorbed (Groeseneken et al., 1989). This figure was 64% for EGEE when at rest but the rate of uptake increased as the exposure concentration or the ventilation rate increased (Groeseneken et al., 1986a). Pulmonary uptake of EGBE in volunteers was 10 |imol/min or 57% of the inspired amount (Johanson et al., 1986).
It should be noted however, that many of these studies on rates of absorption have used almost pure glycol ethers. This does not reflect the situation in industry where solvent mixtures are used. The effect on absorption rates of other components in industrial mixtures is unknown. In an experimental study, the use of acetone as a vehicle decreased the lag time and increased the percutaneous absorption of the glycol ethers tested (Larese et al., 1999).
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