What are factors of skin absorption?

Received 15 August 2002; in final form 6 November 2002

INTRODUCTION

Although skin absorption of organic solvents in the liquid phase is well recognized, fewer data on absorption of solvent vapours through the skin are available. We have previously published a study (Brooke et al., 1998) investigating the extent of dermal absorption of vapours for a range of organic solvents. This study showed that the extent of skin absorption can vary widely and depends on the chemical. For some chemicals, the extent of absorption was significant, e.g. for 1-methoxy-2-propanol (a glycol ether) dermal absorption accounts for up to 14% of the total absorbed dose. Other authors (Johanson and Boman, 1991; Corley et al., 1997; Kezic et al., 1997) have also studied the dermal absorption of glycol ether vapours. Johanson and Boman (1991) estimated (from a whole body exposure) that up to 75% of body burden for butoxyethanol could be due to dermal aborption of vapours. This estimate was disputed by Corley et al. (1997) who estimated (using PBPK modelling from a single arm exposure) the dermal absorption contribution to be 15–27% depending on environmental conditions. Kezic et al. (1997) also used an arm exposure (forearm) to assess the dermal absorption of methoxy- and ethoxyethanol vapour. They estimated that 40–55% of the body burden could be due to dermal absorption of vapours at very high vapour concentrations (1000 and 1250 p.p.m. versus UK exposure limits of 5 and 10 p.p.m.). Mraz and Nohova (1992) investigated the dermal absorption of N,N-dimethylformamide vapours and showed that this could account for 13–36% of the body burden. They observed that temperature and humidity strongy influenced the extent of absorption.

Studies have shown that temperature, humidity and occlusion all have an influence on the extent of skin hydration and permeability (Wiechers, 1989; Vanakoski et al., 1996; Boman and Maibach, 2000). Vanakoski et al. (1996) suggested that high temperature (82°C in a sauna) increased skin absorption through enhanced skin blood flow. Schafer et al. (2002) studied the effects of occlusion and environmental conditions on the forearms of volunteers. Lower temperatures and humidity (20°C and 30%, respectively) had little impact on skin surface water loss or the relative humidity in the microclimate between the skin and the occlusive article but did reduce skin hydration. Higher temperatures and humidity (30°C and 75%, respectively) increased both the relative humidity of the microclimate and skin hydration. In addition, the use of a vapour-impermeable occlusion reduced skin surface water loss. Meuling et al. (1997) studied the dermal absorption of the pesticide propoxur at 30°C under various humidities (50, 70 or 90%). The percentage body burden attributable to dermal absorption increased from 13% (at 50% relative humidity) to 63% (at 90% RH), indicating that skin moisture is important in dermal absorption of propoxur.

Despite studies showing that temperature, humidity and occlusion influence dermal absorption and that higher temperatures and humidities can dramatically increase the penetration of liquids through the skin, there has been little work on the effect of such factors on the dermal absorption of vapours. Dermal absorption of vapours can be significant although there are some conflicting data. Johanson and Boman (1991) reported 75% dermal absorption for 2-butoxyethanol whereas Corley et al. (1997) estimated a maximum of only 27% under various environmental scenarios. The study reported here has investigated the effects of temperature, humidity and clothing on the whole body dermal absorption of 2-butoxyethanol in order to clarify some of the data and determine the potential consequences of dermal absorption of vapours to workers.

The study reported here was conducted in a similar manner to our previous study (Brooke et al., 1998) of whole body absorption of chemical vapours under a series of controlled environmental conditions.

MATERIALS AND METHODS

Volunteers

The experimental protocol was approved by the Health & Safety Executive Research Ethics Committee and all volunteers provided written informed consent before participating in the study. Four volunteers (2 male, 2 female, aged 28–33) took part in the study and were exposed on nine separate occasions, each occasion separated by at least 3 weeks. All the volunteers were in good health at the time of the study, did not suffer from respiratory disease and were not on medication. A medical assessment was made immediately before the start of each experiment by a medical supervisor to ensure that each volunteer was fit to participate in the study. The medical supervisor was present throughout the exposure period and assessed each volunteer afterwards to ensure they were fit for discharge. All volunteers were asked to refrain from taking alcohol for 12 h before and after exposure.

Exposure protocol

In total there were nine exposure sessions, two of which were conducted under ‘baseline’ conditions. All exposures were 50 p.p.m. 2-butoxyethanol for 2 h (8 h TWA OES 25 p.p.m.). Exposures were performed in the Health & Safety Laboratory Controlled Atmosphere Facility, a purpose-built room of ∼8 m3 volume. Experimental atmospheres were generated by introducing 2-butoxyethanol vapour into the room by purging solvent-filled bubblers with compressed air. The atmospheric concentration within the chamber was then monitored continuously by two independently calibrated Miran infra-red spectrophotometers. One infra-red spectrophotometer was calibrated by an internal closed-loop system and the other by a dynamically generated standard atmosphere (MDHS3) (HSE, 1990).

Under the ‘baseline’ conditions, volunteers were exposed at rest ‘whole body’ and ‘skin only’ on two separate occasions. The ‘baseline’ conditions were 25°C, 40% relative humidity with volunteers wearing shorts and T-shirt. For ‘skin only’ exposure, the volunteers wore air-fed half-masks (clean air at a flow rate of 80–100 l/min) so that the inhalation route was excluded as a source of uptake. Inward leakage into the masks was assessed using the methods described in EN140 (1989) and was confirmed to be <0.02% for all volunteers. For the ‘whole body’ exposures no masks were used and thus uptake was via inhalation, ingestion and dermal routes. In this way, by comparing the differences in measures of body burden between the two exposure conditions, the contribution via dermal absorption could be assessed. For the subsequent exposures, volunteers also wore air-fed half-masks and a single parameter was changed: humidity (60% or 65%), temperature (20°C or 30°C) and clothing: either minimal (shorts only for male volunteers, shorts and bra-top for female volunteers) or overalls (all-in-one boiler suits, Tyvek Pro-Tech Chemical Protective Clothing conforming to CE 0120). Finally, a ‘industrial scenario’ was conducted where volunteers wore Tyvek overalls over their shorts and T-shirts and environmental conditions reflecting high temperature and high humidity (30°C, 60%), such as might be encountered in a tank-cleaning operation or similar.

Biological monitoring

2-Butoxyacetic acid is the major urinary metabolite of 2-butoxyethanol in humans. Rettenmeier et al. (1993) reported that up to 64% of the total 2-butoxyacetic acid formed could be excreted as a glutamine conjugate. This was confirmed by Sakai et al. (1994), who stated that ‘total’ 2-butoxyacetic acid gave a better correlation with 2-butoxyethanol exposure than free 2-butoxyacetic acid. The use of ‘total’ 2-butoxyacetic acid is preferred as it compensates for individual differences in the glutamine conjugation pathway. The use of ‘total’ butoxyacetic acid in urine has been used in this study to assess the absorbed body burden after exposures to 2-butxoyethanol vapour.

For all exposures, volunteers provided urine samples before and after each exposure (0, 4, 6, 8, 10, 12, 22, 26, 30 and 34 h). Urine volume was recorded and samples stored at –20°C until used for analysis.

At the end of the exposure period for ‘skin only’ exposures, volunteers took a final breath of solvent-free air via the mask and were asked to hold their breath prior to leaving the chamber in order to reduce the potential for inhalation. Clothing was then changed to ensure that uptake from contaminated clothing was unlikely.

All urine samples were analysed in duplicate for total 2-butoxyacetic acid using the following procedure. An aliquot of sample (50 µl) was hydrolysed with 50 µl concentrated hydrochloric acid at 90°C for 1 h in sealed tubes. After cooling, 1 ml acetone was added along with internal standard (propoxyacetic acid), anhydrous potassium carbonate and pentafluorobenzyl bromide (50 µl). Samples were then capped and derivatized at 90°C for 1 h. After cooling, an aliquot was transferred to GC vials prior to analysis. Analysis was by GC-MS (HP 5973, Agilent) with negative ion chemical ionisation using methane as reagent gas. Aliquots (1 µl) were injected splitless (30 s) at 350°C into a ZB-1 column, 30 m × 0.32 mm i.d., 1 µm film (Phenomenex). The oven programme was 100°C (held for 1 min), then ramped at 10°C/min to 200°C then ramped at 20°C/min to 220°C where it was again held for 1 min. The ions monitored were m/z 117 for propoxyacetic acid (internal standard) and m/z 131 for 2-butoxyacetic acid.

Physiological monitoring

To record any physiological changes in the volunteers (altered breathing or pulse rate) under the different conditions, physiological monitoring equipment (MP100, BioPac Systems Inc.) was used. The parameters monitored were breathing rate, pulse rate, skin surface temperature and skin resistance (a measure of perspiration).

Calculations and statistics

Urinary elimination is expressed as the cumulative total analyte excreted over the post-exposure collection period, calculated by summing the products of the analyte concentration and urine volume for the individual time points. The results obtained for ‘skin only’ exposure are expressed as a percentage of the ‘whole body’ measurement (inhalation and dermal exposure) in order to obtain an estimate of the uptake via the dermal route. In making these comparisons it is assumed that following uptake into the body the distribution, metabolism and excretion of these substances is identical under both sets of exposure conditions.

Each volunteer acted as their own control, i.e. their dermal absorption body burden for each exposure is compared to their own body burden for the ‘whole body’ exposure. Any differences in the percentage dermal absorption under different scenarios was assessed using the paired t-test; however, it should be borne in mind that there were only four volunteers when assessing the statistical significances found.

RESULTS

The actual environmental conditions for each exposure are given in Table 1. All exposures were within 5% of the target concentration. All temperatures were within 4% of the target value with a coefficient of variation of <2%. The humidity was slightly more difficult to control, however, all humidities were within 5% of the target value with a coefficient of variation of <14%.

Figure 1 shows the mean and range of estimates of the dermal absorption contribution to total body burden under the different environmental conditions. The mean ‘baseline’ (25°C, 40% RH) percentage dermal absorption was 11% (range 9–14 %) of the ‘whole body’ burden. Low temperature did not significantly affect the percentage dermal absorption. In the high temperature study, the percentage dermal absorption was significantly increased (P = 0.03) with a mean of 14% (range 12–15%). Increasing the humidity increased the percentage dermal absorption although not significantly. Clothing had little effect on the percentage dermal absorption. The overalls did not attenuate absorption significantly, with a mean percentage absorption of 10%. In the ‘industrial’ scenario (30°C, 60% RH and overalls), skin absorption as a percentage of the ‘whole body’ burden was significantly increased (P < 0.005) when compared to the baseline dermal study and when compared to any single parameter change (e.g. high temperature alone, minimal clothing alone). Under these conditions, skin absorption could account for up to 42% of the total body burden (mean 39%).

No significant differences (P > 0.05) in any of the physiological parameters were seen between the studies. However, it should be borne in mind that the skin temperature and resistance measurements were taken on the back of the hand and on the middle two fingers, respectively. In the high temperature, high humidity, overalls and industrial scenario studies, the skin temperature and sweatiness on other parts of the body are likely to have been increased.

DISCUSSION

These studies have shown that under ‘normal’ conditions (25°C, 40% RH) skin absorption of 2-butoxyethanol vapours contributes a mean of 11% (range 9–14%) of total body burden. This is higher than found for 1-methoxy-2-propanol (mean 4% from urine analysis, although the maximum seen in blood and breath was 14% for one volunteer each) in the previous study which suggests that 2-butoxyethanol vapour is more readily absorbed through the skin than 1-methoxy-2-propanol vapour.

The studies have shown that both increased temperature and increased humidity increase the percentage dermal absorption and this is statistically significant for increased temperature (P = 0.03, 30°C). This is likely to be due to increased surface blood flow (as reported, for example, by Vanakoski et al., 1996), increased skin hydration (as observed by Schafer et al., 2002) and perspiration [aiding dissolution of 2-butoxyethanol, forming a solution on the surface of the skin which may increase the apparent permeability coefficient (Wilkinson and Williams, 2002)] and opening of skin pores under conditions of increased temperature and/or humidity.

The studies showed that clothing had a minimal effect on the dermal contribution to total body burden with neither minimal clothing nor overalls having a significant effect on the amount absorbed through the skin. This could be because the rate of gas exchange through the clothing exceeds the absorption rate of 2-butoxyethanol through the skin.

Combining high temperature, high humidity and the wearing of overalls had a significant impact on the percentage dermal absorption, resulting in a mean dermal contribution to total body burden of 39% (range 33–42%). This may be due to the overalls generating a micro-climate next to the skin (as observed by Schafer et al., 2002) where the environment is significantly hotter and more humid than the ambient environment.

Our study shows good agreement with the PBPK estimations of Corley et al. (1997) under baseline conditions (our study: 25°C, 40% RH, mean absorption 11%; Corley et al.: 23°C, 29% RH, predicted absorption 15%). However, under conditions of higher temperature and humidity (33°C, 71% RH), Corley et al. (1997) estimated the dermal absorption of vapours to be 27% of total body burden, assuming that 100% of the skin was exposed. This is substantially lower than our observed values of 37–42%, even though we used a lower temperature and humidity. This is likely to be due to the microclimate induced by the wearing of overalls which increases the dermal absorption of vapours. Corley et al. (1997) also reduced their estimate to 8% when assuming only the head and arms were exposed (25% of the no clothing scenario). Our study shows that some types of clothing (such as studied here) are unlikely to attenuate exposure by such an extent and that certain clothing may exacerbate absorption.

The combination of conditions in the ‘industrial’ scenario has illustrated that, for 2-butoxyethanol, conditions of high temperature and high humidity may lead to significant body burden via the dermal route. In addition, and as our study shows, the use of some types of overalls will not attenuate this body burden and may in fact increase the amount absorbed through the skin. In environments of high vapour concentration, where reliance is on the use of personal and respiratory protective equipment, it is possible that a worker could receive a significant exposure of which they are unaware.

Acknowledgements—The authors would like to thank Dr D. Fishwick, Dr C. Barber and Dr J.A. Burton for providing medical cover throughout the exposure periods and pre- and post-exposure medical check ups of the volunteers. The authors would also like to thank Mike Clayton, HSL for conducting the inward leak testing of the air-fed masks and the volunteers for participating. This work was funded by the Health & Safety Executive, UK.

Open in new tabDownload slide

Fig. 1. Effect of environmental conditions on dermal absorption (% mean with range, N = 4). *Statistically significant (P < 0.005).

Table 1.

Environmental conditions for each 2h exposure period

References

Boman A, Maibach HI. () Percutaneous absorption of organic solvents.

Int J Occup Environ Health

Brooke I, Cocker J, Delic JI et al. () Dermal uptake of solvents from the vapour phase: an experimental study in humans. ; : 40.

Corley RA, Markham DA, Banks C, Delorme P, Masterman A, Houle JM. () Physiologically based pharmacokinetics and the dermal absorption of 2-butoxyethanol vapor by humans. ; : 30.

Dugard PH, Walker M, Mawdsley SJ, Scott RC. () Absorption of some glycol ethers through human skin in vitro. ; : 7.

EN140 () Half masks and quarter masks for respiratory protective devices. BS7356: 1990.

HSE. () Method for determining hazardous substances 3. Generation of test atmospheres of organic vapours by the syringe injection technique. HSE Books.

Johanson G, Boman A. () Percutaneous absorption of 2-butoxyethanol vapour in human subjects. ; : 92.

Johanson G, Kronborg H, Naslund PH, Byfalt Nordqvist M. () Toxicokinetics of inhaled 2-butoxyethanol (ethylene glycol monobutyl ether) in man.

Scand J Work Environ Health

Kezic S, Mahieu K, Monster AC, de Wolff FA. () Dermal absorption of vaporous and liquid 2-methoxyethanol and 2-ethoxyethanol in volunteers. ; : 43.

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

Mraz J, Nohova H. () Percutaneous absorption of N,N-dimethylformamide in humans.

Int Arch Occup Environ Health

Rettenmeier AW, Hennigs R, Wodarz R. () Determination of butoxyacetic acid and N-butoxyacetyl-glutamine in urine of lacquerers exposed to 2-butoxyethanol.

Int Arch Occup Environ Health

Sakai T, Araki T, Morita Y, Masuyama Y. () Gas chromatographic determination of butoxyacetic acid after hydrolysis of conjugated metabolites in urine from workers exposed to 2-butoxyethanol.

Int Arch Occup Environ Health

Schafer P, Bewick-Sonntag C, Capri MG, Berardesca E. () Physiological changes in skin barrier function in relation to occlusion level, exposure time and climatic conditions.

Skin Pharmacol Appl Skin Physiol

Vanakoski J, Seppala T, Sievi E, Lunell E. () Exposure to high ambient temperature increases absorption and plasma concentrations of transdermal nicotine. ; : 15.

Wiechers JW. () The barrier functon of the skin in relation to percutaneous absorption of drugs. ; : 98.

Wilkinson SC, Williams FM. () Effects of experimental conditions on absorption of glycol ethers through human skin in vitro.

Int Arch Occup Environ Health

1Health & Safety Laboratory, Broad Lane, Sheffield S3 7HQ; 2Health & Safety Executive, Bootle, Liverpool L20 3QZ, UK