Matti Jantunen, PhD, KTL (National Public Health Institute), Kuopio, Finland (Corresponding Author)

The term exposure has many quite different meanings, which sometimes result in confusion when people representing different curricula communicate. The situation is not different from many other scientific terms and their usage.

Outside of science exposure as a term is not commonly, except as a picture image on a film, or the visibility that this picture can give to a politician when it appears in print.

In epidemiology exposure has, rather than many meanings, an extremely broad meaning. It is used to name practically anything that a disease or symptom could be associated with. The term might mean e.g. "being born in a dairy farm", "working in rubber industry" or, more specifically "7 years of skin exposure through diazinon spraying in an apple farm". These are all useful definitions - depending on the study design. But they are also quite different from each other, on one extreme qualitative classifications relating to no specific route of entry and no specific agent and on the other quantitative measures of air concentrations.

In toxicological studies exposure can mean, e.g., the concentration in the blood or fluid surrounding a specific tissue and exposure time can mean the time between injection of an agent and sacrifice of the animal for pathologist's investigation.

In occupational hygiene exposure often refers to an 8 hour TWA concentration in the working zone, in radiation hygiene the term refers to ionisation of air by radiation (in contrast to dose, which in radiation refers to energy absorbed from radiation to tissue).

On one hand it is clear that the mentioned uses of the term, exposure, are legitimate in their own contexts, on the other hand it should be equally clear that no scientific curriculum for Exposure Analysis can be built around a concept, which can have a practically unlimited number of little related quantitative and qualitative meanings and dimensions. Therefore exposure analysis has to produce its own definition(s) for exposure.

Within the science of exposure analysis the most widely accepted definition for exposure is that of Wayne Ott: "the existence of a person and an agent (contaminant) in the same microenvironment at the same time (in potential contact with each other)". This definition is still rather broad. It does not distinguish between exposure integrated over time, or exposure as an average concentration in the microenvironment (over some time).

Other definitions of exposure within Exposure Analysis locate the role of exposure in the chain of events - or flow of molecules - from source via environmental concentration, exposure and absorbed dose to dose in the target organ (Lioy, and Georgopoulos Lioy).

In a comprehensive synthesis exposure has been given a strict mathematical definition beginning with "instantaneous concentration at a point of contact" (Zartarian and Ott), and then integrating, as necessary, from an instant and a dimensionless point over time, (skin) area and volume. This definition has, however, been criticised for adding theoretical complexity with little practical benefit (Mage and others).

In my own mind the term exposure should be allowed certain flexibility to maximise its practicability. To keep it within a single concept, however, this flexibility should not extend outside of the original definitions of Ott and Lioy. I.e. exposure requires a contact between man and the contaminant, and the dimension of time must always, implicitly or explicitly, be there. The shortest definition of exposure, in accordance with Ott's and Lioy's definitions, is the interface between man and the environment. Quite suitable for skin, isn't it?

Still one new concept is useful, the microenvironment. Also this definition allows flexibility. The contaminant concentrations measured in a microenvironment are considered as representative for the exposure of an individual or group of interest. For dermal exposure the microenvironment, depending on the study design, can be the contact surfaces of one's home or workplace, or it could be the boundary layer immediately above the skin (e.g. between cloths and skin).

Natural exposure model
The natural exposure (and risk) model literally follows the molecules of interest from their sources to the affected organs. These tracks are called exposure pathways. The routes of entry from the environment into a human body are ingestion, inhalation and through the skin.

As the figure shows, skin can act as the route of exposure (e.g. for dioxins and solvents), as the target organ of interest (e.g. UV-sensitivity from Tetracyclin treatment), or in both roles at the same time (e.g. skin contact allergens, nickel, latex).

There are some interesting complications, though. A part of the dermal exposure may also end up as non-dietary ingestion exposure (this is why our mothers told us to wash our hands before dinner). For small children, who put their hands on the ground, handle toys and then put their hands into mouth, this is a major source of exposure to pesticides, lead etc. In this case skin acts as a reservoir (immediate microenvironment contamination) for ingestion exposure.

Dermal exposure can form a variable part of aggregate exposure, which refers to the sum effect of exposures via all routes of entry to a single chemical (agent). An example is the use of solvents or volatile pesticides where exposure occurs via inhalation and dermal absorption. Inhalation exposure depends on the volatility of the solvent, temperature, ventilation rate and proximity. Dermal exposure depends on the exposed skin area, frequency of contact, and the molecular weight, volatility and lipophilic characteristics of the chemical. As these characteristics are mostly independent of each other, neither route can be automatically excluded until both have been assessed. Aggregate exposures can only be assessed and different routes of entry ranked at the whole body dose or target organ dose level.

Finally, dermal exposure route can also be a significant part of cumulative exposure, which refers to all agents and all routes of entry having a common effect in the same target organ. The significance of each agent and route of entry for cumulative risk can only be compared and summed up via the target organ effect/risk.

For both aggregate and cumulative exposure and risk assessment skin is more likely to play the role of the route of entry into the body, than the actual target organ of concern.

Administrative exposure model
The administrative risk assessment and management model differs from the natural, because it follows, not the logic of molecules, but the logic of decision making. Exposure assessment has two distinctly different roles in this process. As there is no risk without exposure, measured exposure data is a crucial element of risk assessment. On the other hand, because risk management options can rarely influence dose/response, the only way to compare policy options is comparing their impacts on exposure - which can only be done by exposure modeling.

Something that seems often to be too obvious to be remembered is that environmental health regulations can in long term only be justified by their public health benefits - although sometimes, in short term, public trust on decision makers and the regulatory agencies is the legitimate driving concern. Public health benefits can only be achieved, if the original risk characterization is sufficiently correct to lead the policy makers towards the right direction. Options evaluation and policy selection - to maximize the benefits and minimize the costs and opportunity losses due to general regulation and focussed intervention - must be based on qualitatively (the agent of concern is the true causal agent) and quantitatively (the predicted exposure reductions are of correct magnitude and those of the evaluated options correctly ranked). Finally, policy selection should be based on defined public health or public health proxy (exposure) objectives, and implementation should be accompanied with follow up and feedback loops, which ensure early detection of failure and correction of the course, if needed. Needless to say, if these conditions are not met, public health benefits of invasive and expensive programs may never materialize, may never be verified, and may even be negative.

Dermal Exposure and Dose models
Dermal exposure (to a contaminant in soil) is

Calculation of the dose from dermal exposure depends on whether the ultimate concern is whole body (systemic effects) dose through skin, dose of the skin and subcutaneous tissue (skin effects), or transfer from skin to mouth. Other presentations in this Conference will go deeper into these issues.

Conclusions
Exposure is the interface between man and the environment. Exposure analysis studies the phenomena affecting this interface.

Exposure assessment is a key element in all environmental health risk assessment. Exposure analysis, however, is also necessary for risk management. Exposure modeling is the only way of comparing the risk reductions achievable by alternative risk management options. Accordingly policy selections should be based on public health gain targets, which can rarely be measured directly. Exposure provides the most relevant indirect information. Carefully focussed exposure monitoring and modeling campaigns should therefore also accompany all environmental health policy implementations in order to provide early feedback for policy review when needed.

Skin is one of the 3 major exposure routes, often acting in combination with the other two, inhalation and ingestion. In exposure and risk analysis skin can independently act as a contaminated microenvironment, route of exposure and target organ.

Literature cited:

Georgopoulos PG, Lioy PJ. Conceptual and theoretical aspects of human exposure and dose assessment. J. Expos Anal Environ Epidemiol, 1994; 4:253-286.

Lioy PJ. Assessing total human exposure to contaminants- A multidisciplinary approach. Environ. Sci. Technol., 1990; 24:938-945.

Mage DT. Commentary: Defining Exposure and Related Concepts. J. Expos Anal Environ Epidemiol, 1998; 8:117-122.

Ott WR. Total Human Exposure-An emerging science focuses on humans as receptors of environmental pollution. Environ. Sci. Technol.,1985; 19:880-886.

Ott WR. Human Exposure Assessment: The Birth of a New Science. J. Expos Anal Environ Epidemiol, 1993; 5:449-472.

Zartarian VG, Ott WR, Duan N. Feature article: A Quantitative definition of Exposure and Related Concepts. J. Expos Anal Environ Epidemiol, 1997; 7:411-439.

OEHHA. Air Toxics "Hot Spots" Program Risk Assessment Guidelines. Part IV: Exposure Assessment And Stochastic Analysis. Technical Support Document, Part 6. Dermal Exposure Assessment. Office of Environmental Health Hazard Assessment, Sacramento, CA. October 27, 2000