In Vitro Model for Decontamination of Human Skin

By: Hongbo Zhai, MD, University of California; and Howard I. Maibach, MD, University of California School of Medicine

Posted: April 1, 2009, from the April 2009 issue of Cosmetics & Toiletries.

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Results
The mean values, standard deviations (SD) and percent dose of [14C]-formaldehyde obtained from each group are summarized in Table 1. No statistical decontaminating differences were found among those groups, except in the case of isotonic saline at 3 min post-exposure in wash solutions, which showed a significant difference (87%, p < 0.05) when compared with tap water (62.6%) (see Table 1b and Figure 1). The percentage of formaldehyde evaporation was found to increase linearly (R2 = 0.94) with extended application times (see Figure 2). Evaporation = 10.8 + 0.433 time; the percentage doses of evaporation of formaldehyde were, respectively: 7.7%, 13.6%, 19.7%, 24.4% and 35.9% at correlating time intervals of 1 min, 3 min, 15 min, 30 min and 60 min.

Discussion
A previous study² indicated that skin decontamination of alachlor at 0 hr with soap and water removed 73% ± 15.8% (n = 4) of the applied dose with the first wash, and that the amount removed increased to 82.3% ± 14.8% with two additional washes. Decontamination after 1 hr was found to remove 87.5% ± 12.4% with three successive washes. After 3 hr the decontamination ability decreased, and after 24 hr only 51.9% ± 12.2% could be recovered with three successive washes. Using water only, at 0 hr, 36.6% ± 12.3% alachlor was removed with the first wash and the total increased to 56.0% ± 14.0% with two additional washes. And at 24 hr, the total amount decreased to 28.7% ± 12.2% for three successive washes. Continual successive washes (6–8, in sequence) recovered 80–90% of the skin-applied alachlor.

In the current study, at all time points, most formaldehyde was recovered in the wash solutions. By the 3-min mark, only isotonic saline provided statistically significant enhanced decontamination, compared with tap water. By 30 min, significance was lost. Results indicated that, overall, the three model decontamination solutions were almost equally effective in removing the applied dose of formaldehyde. However, isotonic saline provided a slight enhancement.

The volatility of the dose applied of formaldehyde to skin was determined and showed a linear trend with extending application time. Even with the rapid formaldehyde evaporation, penetration into skin occurred within 30 min post-application. As noted, obtaining total mass balance proved difficult, presumably due to volatility. Mass balance accountability is 48–99.6%, compared with the previous in vivo study with a less volatile chemical in which dose accountability was 80.6–95.2%.² Thus, a less volatile contaminant such as an irritant and/or allergen could simplify model development. Now that this practicable model has been developed, a factorial design approach can be utilized to improve decontamination methodology.

The present authors had previously developed a high throughput model utilizing ground callus and delipidized callus, noting a relationship of binding to callus and penetration of applied chemicals.^{9, 10} Relating the callus assay to the current model may be of value since both in vitro methods provide a system that might aid in the prediction of decontamination assessment.