Almost 500 years ago Paracelsus (1493-1541) wrote: "Dosis facit venenum." ("The dose makes the poison."). The relationship between dose and response (effect) is still one of the most fundamental concepts of toxicology (the science of poisons), but when we discuss environmental alarms and chemical health risks it is sometimes forgotten. A logical consequence of the dose concept is that all environmental risk analysis is more or less quantitative in nature.

A quantitative risk assessment is comprised of several components. The National Academy of Sciences in 1983 (Risk Assessment in the Federal Government: Managing the Process) identified the following four steps:

1. Hazard identification
2. Hazard characterisation (dose-response assessment)
3. Exposure assessment
4. Risk characterisation

The hazard identification is an assessment of whether a substance or a product can cause harm to health or environment (due to its inherent properties). This assessment is based on results from investigations of the biological effects, from individual species to ecosystems, together with physical and chemical properties (that influence the distribution and dispersion in the environment). This means that knowledge from several scientific fields must be considered and integrated, e.g. epidemiology, toxicology, ecotoxicology and environmental chemistry.

The hazard characterisation, or dose-response assessment, is primarily aiming to establish the dose/amount that can cause damages and how a change in dose will influence the likelihood or frequency of adverse effects. The traditional deterministic approach has been to establish the no-effect level (NOEL) or lowest-observed-effect level (LOEL), and subsequently divide by a uncertainty factor to obtain the tolerable or acceptable daily intake (TDI/ADI). Statistical methods are now increasingly used for analysis of the dose-response relationship, and to calculate the "benchmark-dose".

It is problematic to transfer dose-response relationships between species, e.g. from animal studies to humans, and different levels of exposure. However, physiologically based pharmacokinetic models (PBPK) can be used to improve the situation and accomplish a better description of intra- and interspecies variability in uptake and distribution.

The exposure assessment is of fundamental importance in the risk assessment, since "the dose makes the poison". A major portion of the exposure assessment is related to quantitative estimations of the size and magnitude of the exposure. Questions that need an answer are: Who are being exposed? Which are the likely routes of exposure (e.g. air, water or foodstuffs)? What is the dispersion in the recipient? What is the duration of exposure and what is the variability over time?

The risk characterisation is the last component of the risk assessment. Here the risk analyst presents his/her conclusions and describe the effects anticipated as a consequence of the exposure. The outcome of risk assessment should be put together in this last step and presented in a useful and understandable manner. Preferably, quantitative risk estimations are to be presented. Simulations of possible outcomes with Monte Carlo-methodology are useful and provide a good overview of the uncertainty involved. It is therefore anticipated that probabilistic risk analysis will gain even more popularity and importance in the future.

Semi-quantitative methods, e.g. risk matrices and risk indices, have also found use within the environmental field. The purpose is then to rank (classify) different alternatives and outcomes by probability and consequences. Criteria for environmental hazards and classification schemes for contaminated soil and water are examples where semi-quantitative methods have found use.

Risk communication, the exchange of information between stakeholders and the risk analysts, is an integrated part of the risk analysis process. This communication is substantially affected by the fact that risks are experienced and interpreted very differently depending on the perspective of the observer. Risk perception also varies significantly depending on the situation at hand. Therefore, it is important to plan and carry out the risk communication as a two-way process. The stakeholders are not only to be informed, but should also be involved in the risk analysis process.

• Burgman, M. (2005) Risks and decisions for conservation and environmental management. Cambridge University Press. [Read my review in Risk Analysis]
• Hoffman, D. J. et al. (1995). Handbook of ecotoxicology. Boca Raton, Lewis Publishers.
• Kammen, D. M. and D. M. Hassenzahl (1999). Should we risk it? : Exploring environmental, health, and technological problem solving. Princeton, N.J., Princeton University Press.
• Klaassen Curtis, D. (2001). Casarett and Doull's toxicology : The basic science of poisons. New York, McGraw-Hill.
• Paustenbach, D. J. (2002). Human and ecological risk assessment : Theory and practice. New York, Wiley.

A complete assessment of health- and environmental risks, from start to end, is a comprehensive and time-consuming task. Luckily, in many cases there is useful documentation to start from ("the wheel does not need to be reinvented"). Risk assessments and criteria documents are already at hand for many substances, products and activities, both in Europe and in North America. The task is then mainly to search the literature, collect, and evaluate these documents and estimate the exposure.

I fully support the "Principles for Risk Analysis" approved by the Society for Risk Analysis. I am always interested in finding new clients and I will be happy to take assignments within this field of expertise. Please review my resume and my Ph.D. thesis with applications in environmental science.