A descriptive framework for classification of dermal exposure measurements
M. Roff
Health and Safety Laboratory, Buxton, United Kingdom
This poster expands on the existing classifications and principles of dermal exposure measurement to clarify the assumptions behind the sampling and measurement processes and to align the principles more closely with the dermal exposure conceptual model.
The two “pure” strategies of measuring Actual Dermal Exposure and Potential Dermal Exposure have been in common use for many years and adopted in guidelines for measurements of pesticides such as OECD (1997) and WHO (1982). In 1993, Fenske introduced collective terms for three groups of measurement techniques: Surrogate Skin techniques for patch dosimeter and whole-body sampling, Removal techniques for washing, wiping and stripping, and In-situ techniques for visualisation and remote sensing. In 2000, Schneider et al proposed a conceptual model to describe the clothing and skin layers as a set of compartments: Outer Clothing Compartment Layer, Inner Clothing Compartment Layer and Skin Contaminant Layer. The conceptual model also describes in detail the transport mechanisms or pathways that affect gains and losses to these compartments, and through which dermal exposure may arise. These are: Deposition, where the agent lands directly on the compartment from the air as an aerosol or particle, Direct Contact (including immersion) where the compartment comes into direct contact with the contaminant, and Surface Contact, where the compartment comes into contact with contaminated surfaces and the agent is transferred to (and from) the compartment.
It is only when the four systems of Strategies, Techniques, Compartments and Pathways are put together that it can be seen that they form a framework that not only encompasses the dermal exposure processes (the conceptual model) but may be used to describe dermal measurement processes precisely. However, the terms require modification and expansion to form such a complete framework.
The two “pure” strategies of measuring Actual Dermal Exposure and Potential Dermal Exposure are inadequate to describe practical measurement strategies because they are seldom used as pure strategies. They are useful to distinguish parts of the mixed strategy, for example Potential body exposure and Actual hand exposure. A full description is required of the various parts of the mixed strategy to identify the compartment that is being measured for each part.
The term Interception techniques is proposed here to supercede and directly replace the term Surrogate Skin techniques, to better describe the sampling action and thus make it consistent with the titles of the other two groups. The dosimeter is placed above the compartment of interest and intercepts what would otherwise have landed on the compartment. Interception techniques should be divided into two parts, infinite sink and finite sink to reflect the retention characteristics of the samplers. Subdivisions of finite sink are proposed, namely high capacity, clothing–equivalent, skin-equivalent and uptake-equivalent that allow the different underlying assumptions behind the sampling process to be made clear, and these last two are the only two that may truly be described as the “surrogate skin” that was formerly our starting point. Most measurements are made using high capacity finite sink samplers to simulate infinite sink samplers.
When each sampler is considered for each of the three transport mechanisms or pathways that give rise to dermal exposure, they are found to measure different pathways, a fact that is not always understood by the occupational hygienists making the measurements. For example, removal methods and skin-equivalent interception samplers measure the amount retained on the skin after gains and losses of contaminant, whereas infinite sink interception samplers measure gains without accounting for the losses. By using the above definitions for samplers, the pathways are clarified.
There is an expectation in standard protocols (OECD, WHO) that sampling must capture everything that would otherwise land on the clothing and exposed skin, irrespective of whether it would be retained on skin. The OECD protocol specifies in Annex I that “Patches should be replaced immediately if they become saturated or torn”, thus making an infinite sink sampling principle out of finite sink samplers. But there is also an expectation that these same methods must sample in a manner representative of the occluded and exposed skin. The OECD guidelines call for skin-equivalent samplers to be developed. By using the above definitions for samplers, the type of sampler involved is specified and its sampling characteristics clarified.
Different time periods apply to measurement and exposure. Exposure starts at the start of a shift and ends after the residues are removed, which may be long after the shift has ended. However, measurement usually occurs during a small part of the working shift and takes no account of residence afterwards. Models based purely on extrapolation over the whole day from measurements of a small interval within it, will only scale up the highest rates of accumulation, and even those are based upon gains and not losses! The differences between sampling time, residence time and exposure time should be clarified as in the diagram of Figure 1.
Figure 1 Illustration of different types of time relevant to dermal exposure. The loading of the skin reflects losses during the work shift. The sampling occurs in a window within the exposure period. Different windows may reflect different accumulation rates during the work shift.
Conclusion
This framework to exactly describe methods of measuring dermal exposure, should be adopted for use in databases so that measurements can be properly compared between studies. The use of the framework will help occupational hygienists to specify their measurement strategies more clearly, and understand the implications of the measurements methods that they adopt. This framework will also be of use for harmonisation of methods. The terms in the framework may be expanded should new methods of measuring exposure be found or, as here, envisaged.
It is little wonder that we cannot reconcile biological monitoring with dermal exposures if the pathways are not clear, even to the professionals, if the sampling rate is dissimilar to skin, if maximum capacity is unrealistic, and if the sampling period is unclear. To that end, a skin-equivalent finite sink sampler must be the correct way to go. The Occupational Hygiene community must agree whether to adopt this type of sampler or not, should one be developed, as it would fundamentally change what we measure and would have a substantial impact on the way that risk assessments are carried out.
References
Fenske RA (1993) Dermal exposure Assessment techniques. Ann Occup Hyg; 37 687-706.
OECD (1997) Environmental Health and Safety Publications Series on Testing and Assessment No. 9: Guidance Document for the Conduct of Studies of Occupational Exposure to Pesticides during Agricultural Application. OCDE/GD(97) 148, OECD, Paris, France.
Schneider T, Vermeulen R, Brouwer, DH, Cherrie JW, Kromhout H, Fogh CL. (1999) Conceptual model for assessment of dermal exposure. Occup Env Med; 56: 765-773.
WHO (1982) Field surveys of exposure to pesticides. Standard Protocol Reference VBC/82.1. World Health Organisation, Geneva, Switzerland.
Content last modified: 24 May 2005
