11 Skin wash and temporary skin deposition

Dermal absorption of pesticides - evaluation of variability and prevention

The reservoir effect has previously been described (Chapter 3.2.5) and is defined as a substance staying and accumulating in the skin instead of passing directly through to the bloodstream. The substance may then be released to the bloodstream with a certain delay. The absorption of the substance into the bloodstream will continue from the application site. This happens at a gradually declining pace, giving the appearance of prolonged elimination (Cnubben et al., 2002). Another possibility is that the substances remaining in the upper layers of the skin are removed by washing the skin or by desquamation after end of exposure.

When it comes to pesticide exposure and risk assessment it is important to know if it is possible to terminate the absorption of some of the pesticide residue by simple handwash; e.g. if the amount of pesticide remaining in the stratum corneum should actually be thought of as absorbed or whether it can be washed out. A guideline describing experimental methods in skin penetration expresses the fact that the amount of pesticide remaining in the skin at the end of an experiment should be considered absorbed (OECD, 2000). There may, however, be differences in hydrophilic and lipophilic substances just as there may be differences when it comes to damaged skin or intact skin as previously described (Chapter 10).

11.1 Skin deposition

After end of dermal exposure a large part of the pesticide is deposited in the skin. Part of the substance is placed in the upper layers (stratum corneum/epidermis) and some in the lower dermis. Regulations have up till now been based on the assumption that all pesticide residing in these layers is considered absorbed. A study from 2003 carried out on rats tested 19 different pesticides and found an ongoing absorption after skin wash in 17 of 19 pesticides (Zendzian, 2003), but only half of the pesticides gave rise to an increased systemic concentration (Zendzian, 2003). Transfer of these observations in rats to the human exposure situation is complicated by the fact that rats have a faster metabolism than humans.

A study made by Nielsen et al. in 2006 on 6 different pesticides showed that as much as 4% of the administered amount of pesticide remained in the epidermis by the end of the experiments after 48 hours and that the relative deposition in the dermis at the same time varied from 0.2% and up to more than 25% (Nielsen JB et al., 2006)

The variation depends of the solubility characteristics of the substances, as the most lipophilic test substances have the greatest relative deposition in the dermis as well as in the epidermis (Nielsen et al 2006).

A study testing chlorpyriphos by applying it to the hands of volunteers and afterwards washing the hands using a standard technique demonstrated that ethanol removed 30% of the chlorpyriphos on the skin at loadings of about 7ug/cm² (Fenske & Lu, 1994). Prewashing with ethanol increased removal efficiency. A 10% isopropanol/distilled water wash removed 43% immediately following exposure and 23% one hour post-exposure with skin loadings of 12ug/cm2 (Fenske & Lu, 1994).

More relevant in relation to normal working conditions is the amount of pesticide deposit in the skin after a working day of 6 hours and also if it is possible to stop the absorption when removing the residue by simple hand wash and the use of a mild soap as is recommended after working with pesticides.

A test substance with a long lag-time showed in the case of methiocarb an increased donor recovery given that the pesticide is removed after 6 hours compared to 48 hours. The difference between the two recoveries could be an estimate of the amount of pesticide that penetrates the skin within the past 42 hours. Since the recovery of methiocarb is 43% after 6 hours and 17% after 48 hours, respectively, the absorption between 6 – 48 hours is 25% of the administered amount. When it comes to dimethoate, which is known to have a lag-time exceeding 20 hours there is no penetration observed after 6 hours. A conclusion could therefore be that a test substance with a long lag-time and a limited affinity towards the more lipophilic compartment has no measurable absorption through the skin and no temporary deposition in the skin after short exposure time (in this study 1/3 of the lag-time). The reason is most likely that it takes more time and also requires a continuous concentration gradient favouring penetration into the skin to overcome the resistance that is due to the high hydrophilicity. Also aspects other than the hydrophilicity are of significance to the skin penetration, which is exemplified by caffeine having a comparable hydrophilicity and a slightly smaller molecular weight and showing clearly different penetration characteristics (Nielsen JB et al., 2006).

11.2. Does skin wash-off remove pesticide residue from the skin surface?

In an in vivo study by Fenske et al. (1998), the efficiency of captan removal from hands by handwashing was evaluated. Removal of 77.8% of the captan transferred to hands was achieved in the group for whom handwashing was done immediately; whereas efficiency was reduced to 68.4% after one hour’s residing on the hands (Fenske et al., 1998). Compared to the results by the same study group on removal of chlorpyriphos, it was demonstrated that removal of this pesticide was significantly lower (43% at time = 0 and 23% at time = 1 hr) (Fenske & Lu, 1994). The variations may be due to the differences in solubility of the two pesticides. Chlorpyriphos is more lipophilic than captan (logPow = 4.96 for chlorpyriphos and logPow = 2.35 for captan)., but also the differences in formulations may have influence on the permeation of the two pesticides, captan being formulated as a wettable powder while the chlorpyriphos formulation used was a liquid concentrate with an emulsifying agent (Fenske & Lu, 1994).

The results found by Nielsen et al. clearly demonstrate that wash-off after 6 hrs removes the predominant part of the administered amount of the test substance and by that reduces the possibility of subsequent skin deposition. At the end of the experiments where the skin has been washed after 6 hrs there is a significantly reduced skin deposition (Nielsen JB et al., 2006). Nielsen et al. also showed that the reduction of skin deposition varied according to solubility. The most hydrophilic test substances (glyphosate and caffeine) demonstrated a reduction in deposition of 80 – 90%, whereas the more lipophilic test substances (malathion and methiocarb) showed a reduction in deposition of only 35% (Nielsen JB et al., 2006).

When the amount of test substance is reduced by wash-off, the consequence is a lower concentration gradient between the skin and the receptor chamber which again as expected causes a decrease in flux. This was confirmed by the reduced receptor recovery seen in the data above. In the most hydrophilic substances a reduction in absorption to 1/3 is seen (expressed by receptor recovery) while the absorption of benzoic acid and methiocarb are reduced to 60-80% of the values seen without wash-off. The penetration coefficients change in agreement with the changes in penetration rate and the amount penetrating.

All together it must be concluded that simple handwashing or wash-off after end of exposure significantly reduces the total absorption of test substances. When it comes to the most hydrophilic substances the reduction is around 67% and for the lipophilic substances the reduction is a little less (Nielsen JB et al., 2006).

This documentation has clear preventive implications regarding wash-off procedures after a working day with dermal exposure to toxic substances.

A study from 2000 showed that after 90 min of pesticide exposure (glyphosate, alachlor, methyl parathion or triflualin) to pig skin it was possible to remove about 50% of the administered dose of pesticide by simple wash using propanol or a soap solution (Campbell et al., 2000). This American study also showed that soap was more efficient in removing pesticides from the skin than e.g. polyethylene glycol (Campbell et al., 2000). The reason for this observation could be that polyethylene glycol has been used as a detergent elsewhere with the purpose of making hydrophilic substances more soluble in water containing solutions and therefore may act as an enhancer to e.g. the absorption of the more hydrophilic pesticide glyphosat. This has been documented in earlier studies which showed the relation between detergents and the absorption of pesticides across skin (Nielsen JB, 2004).

According to current guidelines by OECD dealing with experimental methods in skin penetration, it is prescribed that the amount of pesticide that is not recovered in the donor chamber following exposure should be considered potentially accessible to systemic toxicity (OECD, 2000). This also accounts for the amount of pesticide that is deposited in the skin. The studies from Nielsen et al. show that absorption from the skin to the receptor chamber will continue after end of exposure. For some substances the absorption will even continue more than 24 hours following exposure. Thus, there is no doubt that a part of the residue in the skin will reach the systemic circulation sooner or later, but how much of the residue that actually leaves the skin surface is still not described in general terms but depends on observations from a few test compounds. In an experimental situation this would be the part of the residue that returned to the donor chamber. When testing methiocarb Nielsen et al. have indicated that about 8% of the administered dose was collected after 48 hours in the groups where the cells were washed after 6 hours. Since it is no more than 15% of the administered methiocarb dose that is found in the skin after 48 hours the 8% constitutes 1/3 of the amount of residue that would have been deposited in the skin if there had been no backward diffusion to the donor chamber. These data indicate that a considerable part of the residue that is temporary deposit in the skin returns to the donor chamber. Data representing the lipophilic substances show that there is a slight overestimation when approximating a reliable absorbable amount of substance by counting all the substance that is not retrieved in the donor chamber as absorbed. The overestimation, however, is not large and for the lipophilic substances the procedure used so far seems of practical use (Nielsen JB et al., 2006).