Introduction
We have previously reported (Brooke et al, 1998) that solvent vapours can be absorbed through the skin and that the extent varies markedly and depends on the chemical. For some chemicals, the extent of absorption is significant e.g. for 1-methoxy-2-propanol dermal absorption accounts for up to 14% of the total absorbed dose after 8h exposure at the Occupational Exposure Standard. Other studies have shown that increased temperature (Vanakoski et al, 1996) and increased humidity (Meuling et al, 1997) can increase dermal absorption and that these conditions can cause physiological changes in the skin barrier function (Schafer et al, 2002).

We have conducted a second study using 2-butoxyethanol to investigate the influence of temperature, humidity and clothing on the dermal absorption of vapours. As for the first study, the extent of dermal absorption was determined by biological monitoring to measure the resultant body burden of the chemical.

Methods
Four volunteers were exposed on nine occasions. For eight of these exposures they wore air-fed half-masks to supply clean air for the inhalation route. The ‘baseline’ conditions (one ‘whole body’ and one ‘skin only’ exposure) were 25oC, 40% relative humidity with volunteers wearing shorts and T-shirt. For each subsequent exposure, a single parameter was changed: humidity (60%, 65%), temperature (20oC, 30oC) or clothing (minimal, Tyvek overalls). Finally, a 'industrial scenario' was conducted where volunteers wore overalls over their shorts and T-shirts and environmental conditions reflected high temperature and high humidity (30oC, 60%), such as might be encountered in a tank-cleaning operation or similar.

Body burden in each of the exposures was determined by the measurement of butoxyacetic acid excreted in urine after exposure. ‘Total’ butoxyacetic acid was measured after acid hydrolysis of any conjugated butoxyacetic acid, followed by derivatisation (using pentafluorobenzyl bromide) and analysis by GC-MS.

Results
Results show that ‘baseline’ dermal absorption of 2-butoxyethanol vapour was, on average, 11% of the total absorbed dose. Higher temperature (30oC, mean 14%, p=0.03) and greater humidity (65% RH, mean 13%, p=0.1) increased dermal absorption. The wearing of whole-body overalls did not attenuate absorption (mean 10%). By combining several factors together in the ‘industrial scenario’, dermal absorption of vapours was significantly increased (p<0.005) with a mean of 39% of the total absorbed dose.

Figure 1. Mean and ranges (N=4) for the % body burden attritubable to dermal absorption.

Conclusions
The work has shown that dermal absorption of vapours can be significant and that environmental conditions can affect the absorption. This is likely to be due to increased surface blood flow (as purported by Vanakoski et al, 1996), increased skn hydration (as observed by Schafer et al, 2002) and perspiration (aiding dissolution of 2-butoxyethanol, forming a solution on the surface of the skin) and opening of skin pores under conditions of increased temperature and/or humidity.

Some types of protective clothing may not be suitable to reduce absorption and may encourage a high humidity microclimate between the skin and the overall exacerbating dermal absorption. The possibility of significant absorption of vapours through the skin should be considered, particularly for workers in high vapour concentration conditions where control of exposure relies on respiratory protection.

References
Brooke I, Cocker J, Delic JI, Payne M, Jones K, Gregg NC, Dyne D (2002) Dermal uptake of solvents from the vapour phase: an experimental study in humans, Ann Occup Hyg. 42(8):531-40.

Meuling WJ, Franssen AC, Brouwer DH, van Hemmen JJ (1997) The influence of skin moisture on the dermal absorption of propoxur in human volunteers: a consideration for biological monitoring practices, Sci. Total Environ. 199(1-2):165-72.

Schafer P, Bewick-Sonntag C, Capri MG, Berardesca E (2002) Physiological changes in skin barrier function in relation to occlusion level, exposure time and climatic conditions, Skin Pharmacol. Appl. Skin Physiol. 15(1):7-19.

Vanakoski J, Seppala T, Sievi E, Lunell, E (1996) Exposure to high ambient temperature increases absorption and plasma concentrations of transdermal nicotine, Clin. Pharmacol. Ther. 60(3):308-15.