Miljøprojekt, 1088 – Test for human bioopløselighed af jordforureninger

Test for human bioopløselighed af jordforureninger

Summary and conclusions

The effects of soil contaminants upon both the soil environment and humans depend upon the bioavailability of the contaminants from the soil. On the other hand, maximum contaminant limits are generally based upon toxicity studies with animals or uptake studies with humans exposed to the contaminants in their most soluble form. Therefore, there is a need for methods to estimate the bioavailability of soil contaminants to humans after e.g: oral exposure.

A variety of in vivo methods have been used for estimation of the bioavailability of soil contaminants in experimental animals, but all are expensive, subject to considerable random and systematic errors, and may impose ethical considerations regarding the pertinence of using experimental animals for the purpose.

Also, a variety of in vitro methods have been introduced to estimate the bioaccessibility of soil contaminants in laboratory tests, e.g: the dissolution of the contaminants in the human gastrointestinal tract, as an indicator of in vivo bioavailability. These methods are also subject to random and systematic errors, but they are less costly and without the ethical problems. A crucial point for the applicability of the in vitro methods is that they must give bioaccessibilities that are well correlated to accepted in vivo bioavailabilities.

Initially, two methods designed to test the human bioaccessibility of soil contaminants were selected for implementation and validation in order to evaluate their use in controlling Danish soil quality criteria based upon oral uptake. The methods are based upon physiologically representative conditions and are designed to yield ”worst case realistic” (i.e.: cautious) estimates of bioaccessibility. Methods developed by RIVM mimicking the conditions in a fasting child have been selected for testing the selected heavy metals (cadmium, lead and nickel), and mimicking the conditions in a child after meal for the polycyclic aromatic hydrocabons benzo(a)pyrene and dibenzo(a,h)anthracene.

Test validation demonstrated a sufficient test quality for the intended purpose at most points. The test methods are enclosed as appendices (in Danish), and requirements for the quality and quality control of analyses and tests are presented in the report. The test performance was satisfactory and compliant with requirements set earlier, but with an indication of too high between-series variation, in particular for lead. Adjustments of the test method with respect to analysis of test solutions and calculation of bioaccessibility relative to a pH dependent reference were suggested.

In order to enable calculation of the bioaccessibility relative to a toxicological reference (the form of the contaminants used in toxicological studies behind the soil quality criteria), suggestions for such references are included.

The RIVM tests have been employed with contaminated soils from seven sites, each representing different sources of contamination. The bioaccessible concentrations were in almost all cases lower or significantly lower (up to a factor of 20) than the ”total” soil contaminant concentrations. The data did not allow for derivation of generic bioaccessibility factors for specific contaminants and/or sources.

The variability of the bioaccessible concentrations of soil contaminants over the sites was of the same order of magnitude as the variability of the ”total” concentrations of soil contaminants. Bioaccessible concentrations can accordingly be used similarly to ”total” concentrations in controlling soil quality criteria.

In previously published studies, a good correlation has been found for lead and cadmium between in vitro bioaccessibility and in vivo bioavailability. For nickel and PAH, only very few in vivo bioavailability data have been published.

In this study, soil samples with in vivo bioavailability data obtained in other studies have been tested for in vitro bioaccessibility of the metals: cadmium, lead and nickel (49 samples), and the PAH: benzo(a)pyrene and dibenz(a,h)anthracene (17 samples). Heavy metals were initially tested using the fasted state method and PAH using the fed state method, both developed by RIVM to simulate the physiological conditions in the human gastrointestinal tract and including mouth/oesophagus, gastric and intestinal steps.

For cadmium, the correlation in vitro to in vivo was satisfactory, but not for lead. For nickel and PAH, the in vivo data were insufficient to evaluate the correlation. The in vitro test results obtained in this study correlated well with in vitro data obtained in other studies with other methods, except for lead, where the RIVM fasted state method gave low results.

The poor in vitro to in vivo correlation for the RIVM fasted state method lead data were attributed to the high and insufficiently stable pH in the stomach and intestinal segment of the test, as well as to a lead-precipitating effect of other soil constituents.

Accordingly, two other test methods were subsequently applied to soil samples with in vivo bioavailability data from other studies: the RIVM fasted state without the intestinal step and the SBRC method (a simple test simulating stomach dissolution at low pH and high buffering capacity). Here the SBRC provided adequate correlation and high values for lead, but unsatisfactory correlation for cadmium. The RIVM fasted state without intestinal segment provided adequate correlation for both lead (only test results with pH in the test solution below the required maximum of 1.8) and cadmium, but high values for cadmium.

In conclusion, it is recommended to apply the in vitro RIVM fasted state test method for cadmium and the same test method without the intestinal step for lead for quantitative estimates of the bioavailability for oral exposure. If the pH values in the test solutions are not within specified limits, the test must be repeated with smaller amounts of soil. Furthermore, it is recommended to apply the in vitro fasted state method from RIVM and the RIVM fed state method for qualitative assessments of the bioavailability of nickel and PAH from soil, respectively.

An investigation program including as a minimum testing of five soil samples from each site is suggested, giving relative bioaccessibilities, “total” concentrations and bioaccessible concentrations of the contaminants for each sample (site-specific application of bioaccessibility). It is recommended to prepare a guideline for the use of bioaccessibility tests in site-specific risk assessments based upon the results obtained in this project.

In order to facilitate the implementation and quality control of the tests in practical use, it is suggested to identify stable and homogenous control material to be used by the test laboratories for routine quality control. It is also suggested to share the control material with laboratories outside Denmark in order to enable comparison of data among laboratories and potential correlation to bioavailability data obtained in animal studies. Finally, it is suggested to collect data from using the tests over a trial period in order to evaluate subsequently whether generic bioaccessibility factors can be derived for specified soil contaminants or contaminant sources. An evaluation of the variation of bioaccessibilities over sites with varying sources of soil contamination can be made simultaneously.

The general conclusion is that correction of soil cadmium and lead concentrations for relative bioaccessibility in evaluation of compliance with soil quality criteria and cleanup levels based upon reduced bioavailability/bioaccessibility of the contaminants may be recommended in site-specific risk approach. Conversely, the data available at present do not allow for general regulation of soil quality criteria and cleanup levels for specific contaminants, soil types or sources. One test can, with modifications, be used for measuring cadmium, lead and nickel bioaccessibility, and another, but similar, test for PAH. However for nickel and PAH, the bioaccessibilities can currently only be used for estimates of the relative risk associated with different soils.