A Practical Guide to Dermal Exposure Assessment

INTRODUCTION

It is now well recognized that dermal exposure can lead to systemic toxicity far away from the point of contact. Skin is no longer seen as an impervious barrier and we know that any liquid substance will eventually pass through it, even water. A plethora of validated methods exist for inhalation exposure assessment, however, there is a lack of robust methodologies for assessing dermal exposures.

In this article, I offer a practical guide for practicing occupational hygienists, which has a reasonable level of precision. The method utilizes interception methodology (dermal patches) combined with some simple calculations and the use of IH SkinPerm mathematical exposure modeling.

THE CASE STUDY

A comprehensive health risk assessment was carried out for an offshore Oil & Gas operator. At the basic characterization stage during the site visit, it was noted that drilling crew members had skin exposure to oil-based mud. A qualitative dermal exposure assessment was carried out using the AIHA methodology, identifying Roughnecks and Roustabouts as having the highest potential for skin exposure. Oil-resistant gloves were in use at all times throughout the 12-hours shift.

Due to high exposure potential, it was decided to confirm the effectiveness of the existing PPE program and to determine the absorbed dose of the contaminant for the following comparison against the dermal OEL. A quantitative dermal exposure assessment exercise was granted to answer these questions.

METHODOLOGY

Step 1 - Establish Dermal OEL

First thing first we need to establish the benchmark for acceptable dermal exposure. We can do it by deriving the equivalent dermal OEL from the inhalation OEL. For oil-based mud, the in-house inhalation OEL of 250 mg/m3 (12-hour TWA) was in place. To calculate absorbed internal dose at this level we need to know the daily inhalation volume of an average healthy adult male worker at a moderate work rate (typical offshore worker scenario). A study by the California Environmental Protection Agency shows that an average adult male exercising at a moderate rate breathes in 35 liters of air per minute. In a 12-hour shift, a worker would inhale approximately 25 cubic meters of air (35 l/min x 720 min).

The Dermal OEL equivalent is calculated by multiplying the inhalation OEL by the volume of inhaled air in the working shift (250 mg/m3 x 25 m3/day). Dermal OEL = 6250 mg/day. We can further apply a safety factor of 1.5 to account for susceptible individuals and lower the Dermal OEL to 4150 mg/day.

Step 2 - Evaluate Dermal Loading

In order to determine the dermal loading of the contaminant, we need to measure it by interception and extrapolate the results onto the entire surface of exposed skin. SKC PERMEA-TEC pads were used for this purpose. Dermal pads were attached to a clean glove in the representative areas of the palm: thumb, middle finger, palm and back of the hand. Roughnecks and Roustabouts wore clean undergloves with affixed skin patches underneath the second set of protective gloves for the entire duration of the shift (12 hours). Any oil-based mud that penetrated the first set of gloves was trapped on the charcoal sorbent pad affixed underneath allowing for the collection of a representative sample.

All obtained samples were submitted for the GC-MS analysis for the evaluation of total hydrocarbons. The results indicated detectable levels of hydrocarbons on absorbent patches. One set of samples indicated that patches obtained from the Roughneck had 1200 μg of contaminant on the middle finger, 850 μg on the thumb, 1000 μg on the palm and 1800 μg on the back of the hand. The average concentration of the contaminant was 1213 μg (4850 μg / 4 samples) or 1.2 mg per sample.

The area of an absorptive patch is 3.57 cm2. Thus, we measured the dermal loading of 1.2 mg per 3.57 cm2 of the skin. According to US EPA "Exposure Factors" handbook, the area of an average male hand (exposed area) is 535 cm2 or 1070 cm2 for both hands. By extrapolating the average dermal loading of 1.2 mg per 3.57 cm2 onto the entire area of exposed hands (1070 cm2) we can calculate the dermal loading for both hands which is 360 mg per 12 hours of exposure (1070 cm2 / 3.57 cm2 x 1.2 mg).

Step 3 - Determine the Dermal Absorption

It is highly unlikely that 100% of the deposited contaminant will be absorbed through the skin. Each chemical has its own absorption rate which depends on molecular weight, water solubility, LogKow, and other factors. To determine the absorption rate AIHA IH SkinPerm mathematical exposure modeling tool was used.

A new entry for the main component of the oil-based mud - Distillates (Fischer-Tropsch), C8-26, branched and linear was made. The required parameters of the contaminant that will affect the absorption were entered manually. These were sourced from the chemical dossier located in the ECHA database. The IH SkinPerm model estimates that the dermal absorption for this substance is 6.24% of dermal loading meaning that only 22.4 mg of this substance will be absorbed through the skin (systemic toxicity).

Step 4 - Compare Absorbed Dose to Dermal OEL

The final stage of the dermal exposure assessment is the comparison of the calculated Absorbed Dose with the derived Dermal OEL. In our case, the absorbed dose is 22.4 mg/day and Dermal OEL is 4150 mg/day. The absorbed dose of the contaminant is <1% of the Dermal OEL, thus the risk of systemic toxicity from the dermal exposure to oil-based mud is negligible. Selected gloves were proved to be effective, however, not 100% impermeable. Therefore, the risk of local toxicity (contact dermatitis) still remains.

Furthermore, we can combine the obtained monitoring data for dermal and inhalation exposure. The personal inhalation exposure sample that was taken alongside the dermal exposure sample indicated the exposure level of 50 mg/m3 (12-hour TWA). This sample resulted in an internal dose of 1250 mg/day from inhalation exposure (50 mg/m3 x 25 m3). The dermal route of exposure would add another 22.4 mg/day of contaminant, thus the total body burden from both routes of exposure resulted in 1272.4 mg/day of internally absorbed dose.

SUMMARY

At the moment there is no gold standard for the dermal exposure assessment. Methods of different complexities exist, however, there is no universal agreement on which method is the best. The described methodology is relatively simple, inexpensive to use, transparent and most importantly it is scientifically sound. I encourage my fellow occupational hygienists to employ this method where the qualitative risk assessment cannot answer posed questions until the academics and authorities come up with a well-validated and prescribed method for the quantitative dermal exposure assessment.

N.B. No real exposure data was used in this article to preserve the anonymity of the client and exposed individuals.?

REFERENCE MATERIAL

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