Dermal exposure is considered to be the result of possibly different mass transport processes, either directly or indirectly from a source to the skin. Basically, this has been distinguished in processes towards the skin (compartment) and from the skin, whereas overall pathways have been identified as direct contact, surface contact transfer and deposition of aerosols. These processes and pathways have been conceptually described (Schneider et al., 1999, however, neither all process parameters or determinants have been identified nor their influences have been quantified. Currently, research projects, e.g. RISKOFDERM, are carried out to develop predictive exposure models that are based on the concepts of dermal exposure processes. Many process parameters and exposure determinants are being and will be evaluated for their influence on the processes.

If dermal exposure needs to be quantified, a sampling strategy should be designed. The objective(s) of the sampling will affect the design of the sampling strategy very much. Basic questions as sampling location, timing, duration and frequency should be answered in view of both knowledge of the exposure processes and pathways, and the objectives of sampling. Timing of the sampling, and sampling duration may largely determine the impact on sampling results in cases where the mass transport rates away from the skin, e.g. by resuspension, evaporation or permeation, are high. Sampling for risk assessment may result in other choices of duration and frequency compared to those made in view of sampling for an intervention study. For an exposure pathway by surface-contact transfer the selection of sampling locations may differ from those for a deposition pathway.

The selection of sampling methods is also an integral part of sampling strategy. Again, exposure processes and pathways, and the objectives of sampling will affect the selection of the sampling method very much. In addition to customary criteria for evaluation of the performance of sampling methods, e.g. accuracy, precision, repeatability, resolution, robustness etc., the conceptual model of dermal exposure has been useful to evaluate current sampling methods, e.g. surrogate skin, removal, and in situ detection methods, in view of what processes are reflected by the results, and the suitability of the results for the objective(s) of sampling. For example, for risk assessment it is considered relevant to estimate uptake, i.e. biologically relevant (dermal) exposure. Cherrie and Robertson (1995) defined an exposure metric (Esk) as the integral of the concentration of the contaminant substance in the skin contaminant layer (Csk) over the whole surface of the skin and over the whole period of exposure. For a concentration Esk= Csk.s.t, where s is the area exposure and t is the duration of exposure, exposure metrics would be mg.kg^-1.cm².s, which shows that both determination of mass and area of exposure are relevant for the specific sampling objective.

Future research should be focussed on issues related to validation of presently available exposure sampling methods, e.g. by ‘benchmarking’methods to compare sampling performance. The need for harmonisation and standardisation has already been recognised by starting new working group within the European standardisation commission (CEN) dedicated to dermal exposure methods and strategies.

It is assumed that attention will be paid to develop new methods, since no sampling method is available that assesses dermal uptake, relevant for risk assessment, adequately. The knowledge on dermal exposure processes and exposure sampling methods also provides guidelines for the design of appropriate sampling strategies. The strengths of different methods may be combined to improve risk assessment.