Authors:
Frank Swartjes | National Institute for Public Health and the Environment (RIVM) | Netherlands
Dr Joanna Wragg | British Geological Survey | United Kingdom
Mark Cave | British Geological Survey | United Kingdom
Prof. Stefan Trapp | DTU | Denmark
Prof. Renato Baciocchi | University of Rome Tor Vergata | Italy
Iason Verginelli | University of Rome Tor Vergata | Italy
Roberto Pecoraro | Versalis | Italy
Jeroen Provoost | Independent researcher | Finland
PhD Yvonne Ohlsson | Swedish Geotechnical Institute (SGI) | Sweden
PhD Paula Marinho Reis | Universidade de Aveiro | Portugal
Frank Swartjes1, Joanna Wragg2, Mark Cave2, Stefan Trapp3, Renato Baciocchi4, Iason Verginelli4, Roberto Pecoraro5,*, Jeroen Provoost6, Yvonne Ohlsson7, Paula Marinho8
1National Institute of Public Health and the Environment (the Netherlands),2British Geological Survey (UK), 3Tech. University of Denmark (Denmark), 4University of Rome Tor Vergata (Italy), 5Versalis (Italy), 6Independent researcher (Finland), 7Swedish Geotechnical Institute (Sweden), 8University of Aveiro (Portugal), *Section on VAPOR INHALATION only
INTRODUCTION. A very practical possibility for assessing human exposure is that of calculating human exposure, using an exposure model. Such a model enables the calculation of the rate of soil contaminants that enter the human body, blood stream, or target organs. Exposure models consider direct contact with the soil and intake of so-called contact media that include soil-borne contaminants. 25 Years ago, the first generation of human exposure models was published. Today, human exposure models are widely available and worldwide used on a large scale, often without much review or criticism. Therefore, the question is warranted after 25 years if human exposure models are ‘finished’ or if serious knowledge gaps remain. From a survey focused on the state of the art in the European Union it was concluded that the most important exposure pathways are exposure through soil ingestion, vegetable consumption and vapour inhalation. Other exposure pathways may be of importance in specific situations. This session will include presentations on these critical pathways and other relevant issues, whose contents are briefly summarized below. SOIL INGESTION. Exposure through soil ingestion is controlled by soil and dust ingestion rates and the relative bioavailability factor in the human body. There is consensus on soil and dust intake rates for children (except for the fact that the behaviour of children regarding time use, including time spent outdoors, changed considerably the last two decades), less for adults. For the calculation of the actual uptake in the blood stream the oral bioavailability has to be determined. It depends on the release of contaminants in the gastronomical tract (the bioaccessible fraction), absorption in the small intestine and the fraction metabolized in the liver. One of the best options to assess oral bioavailability is using the Unified Barge Model. PLANT UPTAKE. Simple, efficient and well-tested plant uptake models exist. The problem is that the processes and the exposure estimations through vegetable consumption are not simple: many different crops exist (roots, leafy, grains, fruits, tubers and others) and also the soil and growth conditions vary widely. Experimental results for the same contaminant often show a large variation, and so does exposure estimates. Models can principally be adapted to these different crops and dynamic field conditions. Uptake estimates, however, are often highly uncertain. VAPOR INHALATION. Exposure through vapour inhalation is influenced by contaminant, soil and building properties and how contaminants move from the source to the outdoor or indoor environment. Vapour inhalation models have a tendency to overestimate the soil gas, the outdoor air and indoor air concentrations, especially the former. Integration of more refined models, including for instance biodegradation, with soil gas data, collected through nesty probes or even flux chambers, provides more accurate (less conservative) estimates. The experience gained so far is that a more refined assessment often leads to a relevant reduction of the exposure estimates and that in specific cases it could even be possible to exclude the health assessment of this pathway, based on the type, concentration and depth of the source. PRACTICE. In practical applications, it is often assumed that generic data and assumptions in exposure models are overly conservative. Site specific adjustments result in more realistic estimations of exposure and thereby the need for risk reduction. However, when (spatial and temporal) variability and uncertainties of model parameters and limitations in the mathematical representations of a process, or quality requirements on site data, false-positive or negative errors can be observed. Identified common mistakes demonstrate a shortcoming in describing and communicating limitations and the validity range of exposure models. MONITORING. For many of the contaminants accumulated in the body, the health impact is still unknown. Therefore, identifying relationships between exposures and health effects over a lifetime, or for a given age group (e.g. through childhood), is a key point in understanding the mechanisms underlying these causal associations. In a recent study, techniques such as solid-phase fractionation, bioaccessibility testing and biomonitoring were coupled to identify relationships between house dust metal contents and contaminant levels in toenail clippings. The complexity of the task is acknowledged and contributions of causal environmental factors cannot be separated unless confounding factors such as age, gender, vitamin intake and others are considered in the evaluation of the exposure-biomarker relationship. CONCLUSIONS. An exposure model includes many uncertainties, but is an extremely useful tool, when smartly applied within the appropriate applicability domain. A truth cliché is that theoretical knowledge of and experience with exposure models improve the quality of the assessment. A few persistent knowledge gaps, however, in particular related to exposure through vegetable consumption and vapour inhalation need to be further investigated.