Toxicology Letters, 644% (1992) 503-510 0 1992 Elsevier Science Publishers B.V., All rights reserved 03784274/92/J 5.00
503
Hazard assessment of chemical contaminants in soil C.L.M. Poels and W. Veerkamp Shell Internationale Petroleum Maatschappi The Hague (The Netherlands)
BV; Health, Safety and Environment
Division,
Key words: Contaminated soil; Human health; Hazard assessment; Human exposure to soil pollutants model
SUMMARY Disposal practices, accidental spills, leakages and local aerial deposition occurring in the past have led to local soil pollution in many cases. Especially in situations where people live on or nearby such locations this has created concern about possible adverse effects on human health. A stepped approach to the hazard assessment of polluted soil, as developed by a Task Force from the European Chemical Industry Ecology and Toxicology Centre (ECETOC), is described. In an early phase in the assessment process the potential exposure of humans is estimated. The Human Exposure to Soil Pollutants (HESP) model can be applied for this purpose. The model calculates the total exposure of adults and children resulting from pollutants present in soil, via 10 different exposure routes. The estimated exposure can be used to indicate the potential significant exposure routes and to carry out a preliminary hazard assessment. The model is also able to predict pollutant concentrations in soil which do not exceed accepted maximum exposure levels for humans in both standardised and site specific situations. The stepped approach is cost-effective and provides an objective basis for decisions and priority setting.
INTRODUCTION
Since the 1970s there has been increasing concern about possible effects of contaminated soil on human health and the environment. Making a balanced response to this concern and deciding on the extent of remedial measures have been hampered by a lack of objective and systematic methods to establish the extent of any hazard posed in a given situation. In response, a Task Force of the European Chemical Industry Ecology and Toxicology Centre (ECETOC) has developed a practical system for hazard assessment of chemical contaminants in soil. Correspondence to: C.L.M. Poels, Shell Internationale Petroleum Maatschappij BV, Health, Safety and Environment Division, P.O. Box 162,250l AN The Hague, The Netherlands.
504
In the past much attention has been given to the toxicological effects of chemicals. The results of toxicological tests permit the establishment of so called Maximum Tolerable Exposure Levels (MTELs) using appropriate uncertainty factors or safety factors. This paper concentrates on the assessment of the exposure of humans to chemical contaminants in soil which is the other essential component of hazard assessment. This paper deals especially with exposure of humans to soil contamination in relatively restricted areas as a result of former more localised sources such as accidental spills, leakages from pipelines and storage tanks, disposal of effluents via soak-aways, leaching of chemicals from landfills, local aerial deposition and application of sewage sludge to land. The exposure is strongly dependent on the usage of the land, e.g. urban, agricultural, recreational or industrial. Pollution resulting from localised sources is often characterised by the long term presence of chemical contaminants such that it is a reasonable approximation to assume equilibrium between the different environmental compartments within the contaminated zone. This assumption is an important basis for models calculating exposure. HAZARD ASSESSMENT
Hazard assessment in the context of this paper is defined as “the estimate of adverse effects which may result from the toxicity of substances and site specific exposure when they are present in soil”. Because of the dependence of the hazard of chemicals in soil on the site specific characteristics, only a general approach to hazard assessment can be given and not a specific procedure. The general principles discussed here should be applied to hazard assessment on a case by case basis. Knowledge of the intrinsic toxicological properties of a chemical and of the exposure conditions are the principal requirements for a hazard assessment. All desirable data are, however, rarely available and estimates or theoretical assumptions often have to be made. The degree of realism in these estimates then determines the reliability of the final hazard assessment; this requires expert judgement. Concept of hazard assessment Essentially the hazard assessment of chemical substances described here is based on a comparison of the Maximum Tolerable Exposure Level (MTEL) with estimated or measured Environmental Exposure Level (EEL). The main parameters to be determined are thus the MTEL for each exposure route (e.g. inhalation, ingestion and dermal absorption) or the total MTEL and the EEL for the same routes or all routes together.
505
Fig. 1. Some important exposure routes of man to soil contaminants.
Maximum tolerable exposure level The MTEL is defined as the dose of chemical taken up by a human, or a concentration to which a human is exposed, which, if not exceeded, does not lead to an adverse effect over prolonged exposure periods. Examples of MTELs are the Acceptable Daily Intake or AD1 [II for the oral exposure route and Air Quality Guidelines for inhalation [21 Exposure Assessment Exposure assessment can be defined as “the process of measuring or estimating the intensity, frequency and duration of exposure to a hazardous agent”. (Adapted from Ref. [31). It is essential to determine each route via which a human is exposed to a chemical substance present in soil. Figure 1 presents a scheme indicating the exposure routes potentially relevant to human beings. An early step in exposure assessment is the characterisation of the contaminant source, In addition to information on the concentration and distribution of the contaminant such source characterisation may involve the assessment of climatological factors, information on the local environment, its use by man (e.g. nature conservation, residential, agricultural or industrial area) and the degradation potential for organic contaminants. This information is important for the selection of the relevant exposure routes and the exposure duration. The next step involves estimation of the relative contribution of each exposure route to total exposure and the quantification of the exposure via each route. As a result, the EELS are estimated or determined experimentally. Finally, the estimation of exposure duration and the exposure pattern is required. Apart from exposure resulting from contaminated soil, it is possible for
506
humans to be exposed to the same contaminant via air, food and drinking water (background levels). Although background exposures can be relatively low they should be taken into account. The stepped approach to hazard assessment for humans Applying the concept, a stepped approach is desirable to arrive at conclusions in a systematic way and to optimise the use of resources. Although the concept is simple in principle, in reality it is rarely straightforward because the required data set often is not complete and realistic assumptions have to be made. The stepped approach to hazard assessment is outlined in Figure 2. It involves three steps: Initial Exposure Assessment: In this step it is ascertained whether or not exposure of humans is possible in the given circumstances. If so, the next step should be made. Preliminary Hazard Assessment: If the initial evaluation indicates a potential for exposure, the next step is the comparison ofthe potential EELS, with the MTELs for the relevant exposure routes. Potential EELs can be estimated using a model which computes exposure on the basis of relationships for the different exposure routes determined by observation or experi-
Fig. 2. The stepped approach.
ment. A model developed for this purpose is the Human Exposure to Soil Pollutants (HESP) model, which is fully described ‘elsewhere [4,51. The HESP model uses a limited number of site specific input data and a large set of, prior agreed, conservative but realistic assumptions, e.g. regarding housing, human behaviour, food consumption and climate. The standard model uses parameters relevant for Dutch circumstances agreed with specialists of the Dutch State Institute for Public Health and Environment (RIVM). The parameters can be readily adapted to situations in other countries. If all EEL’s are lower than their respective MTEL’s, no hazard will exist because of the conservative approach taken in their estimation. Consequently no further work is required. If one or more EEL’s exceed their respective MTEL’s, a potential hazard may exist and a definitive hazard assessment is required. Definitive Hazard Assessment: The definitive hazard assessment should be based on actual measurements of relevant exposure levels. These measurements should be restricted to those exposure routes which, according to the exposure assessment model, are significant. When comparing the measured EELS with the MTELs, two situations which may lead to a significant hazard, can be distinguished: (i) an EEL exceeds the MTEL, indicating that a potentially significant hazard exists; (ii) there is more than one exposure route, and although the EEL of each does not exceed their individual MTEL, the combination of routes leads to a total exposure which exceeds the potentially hazardous level. This second possibility requires specialists consideration to assess whether or not a potentially hazardous level is exceeded since exposure duration, intake versus uptake and metabolic and pharmacological data may have to be taken into account. Expert judgement is also required where more than one contaminant is present in the soil and may exert a toxic effect. DDT AS AN EXAMPLE OF HUMAN HAZARD ASSESSMENT
The Acceptable Daily Intake (ADI) for DDT is 0.02 mg/kg [61.The background exposure to DDT is considered to be insignificant in general because this compound was phased out for most uses some 20 years ago. Taking the urban situation as an example, using the HESP model it is possible for each exposure route and for all routes together to calculate the exposure of an adult and a child for a given level of DDT in soil (see Table I). Relevant exposure routes are “ingestion of soil/dust”, “ingestion of vegetables” and “dermal uptake via soil/dust”. Figure 3 shows the total exposure
508 TABLE I ESTIMATED HUMAN EXPOSURE TO DDT IN SOIL Chemical: DDT
Soil concentration: 20 000 mg/lcg
Urban soil use
Standard soil, 10% organic matter
Intake route (mg_/kg.d)
Adult
Child
Inhalation Vapour
5.433 -7
1.24E -6
Dust
1.23E -4
2.953 -4
Shower
2.12E -9
3.763 -9
Ingestion SoiVdust
1.42E -2
2.00E -1
Vegetables
8.89E -3
2.223 -2
Water
1.20E -7
4.673 -7
Meat/Dairy*
E
E
Fish*
E
E
Soil/dust
1.63E -3
8.543 -3
Water
2.493 -7
4.973 -7
2.493 -2
2.31E -1
Dermal
Totals
*Exposure route not relevant for urban soil use. E: power of 10.
Average daily intake of DDT by an adult in an urban area. Dashed line: AD1 for DDT, see Ref. i.61. J?ig. 3.
of an adult to DDT with increasing DDT concentrations in soil in relation to the ADI. From this figure, while considering the assumptions made in the model, a concentration of DDT in soil not exceeding the AD1 can be derived. For adults this concentration is estimated at 15700 mg/kg and for children at 1270 mg/kg. DISCUSSION
Results from HESP and other exposure models indicate that the total exposure estimates are generally dominated by 2 or 3 routes. For example, for highly volatile substances such as benzene and toluene, the inhalation of vapours is the major route. For water soluble substances, poorly adsorbed by soils, the ingestion of crops often dominates over other routes while for relatively persistent and lipophilic substances (e.g. DDT) the direct ingestion of soil and dust accounts for the largest part of the total exposure estimate. Inhalation of particulate matter and dermal absorption generally seem to contribute little to the total intake for all types of substances. An important result of model calculations is that an exposure route analysis frequently reveals that the child is the dominant receptor of soil contaminants. Direct consumption of soil is particularly important for small children in the age group between 2 and 6 years [7,81. Although the hazard assessment example presented concerns an urban area with respect to the exposure routes included, the HESP model is flexible in that it contains various other standard exposure route scenarios (e.g. agricultural, industrial and recreational) and allows for the selection of any exposure route combination to match a particular situation. If the hazard of chemical contaminants on an industrial premise is to be assessed, the exposure routes for vegetables and for meat and dairy products can generally be disregarded. In that case it is also not necessary to assess the exposure for a child. Using the stepped hazard assessment scheme outlined above it is possible to estimate in a systematic, objective and cost effective manner whether, in view of the nature of the contaminant(s) and site specific exposure, a potential hazard exists. The approach indicated makes it possible to determine the possible hazard resulting from the presence of chemical contaminants in the soil on a more objective basis than before. It may be appropriate to decide on remedial measures even if there is no hazard. In such cases the reasons for such decisions should be clearly recognised. The stepped hazard assessment is, when properly applied, a tool to make objective, informed choices and set the right priorities. It should be realised that the HESP model represents nothing more or less than an approximation to reality. Although many assumptions are
510
based on empirical data and some of the model relationships have been validated in field experiments, it is nevertheless necessary that the model calculations are further validated. Due to the conservative but realistic values chosen “false negatives” will be avoided while “false positives” will be prevented by the Definitive Hazard Assessment. Where applied discerningly HESP has proved to be a useful tool to discuss the hazard of contaminants in soil with authorities, to assist in the selection of the appropriate site rehabilitation method and, where deemed necessary, to estimate realistic soil clean-up criteria. REFERENCES Anonymous (1983) Toxicological evaluation of certain food additives. 26th Report of the joint FAO/WHO Expert Committee on Food Additives. WHO Food Additives Series, No. 17. Anonymous (198’7) Air Quality Guidelines for Europe. WHO Regional Publication, European Series No. 23. Anonymous (1982) Risk Assessment in the Federal Government: Managing the Process. Committee on the Institutional Means for Assessing Risks to Public Health. US National Research Council. National Academy Press, Washington, D.C. Poels, C.L.M., Gruntz, U., Isnard, P., Riley, D., Spiteller, M., ten Berge, W., Veerkamp, W. and Bontinck, W.J. (1991) Hazard assessment of chemical contaminants in soil. ECETOC Technical Report No. 40. Veerkamp, W. and ten Berge, W. (1992) Hazard assessment of chemical contaminants in soil. ECETOC, Technical Report No. 40, Revised Appendix 3. Anonymous (1986) Codex maximum limits for pesticide residues. Codex Alimentarius Vol. 13. Joint FAO/WHO Food Standards Programme. Kimbrough, R.D., Falk, H. and Stehr, P. (1984) Health implications of 2,3,‘7,8-tetrachlorodibenzo-pdioxine (TCDD) contamination in residential soil. J. Toxicol. Environ. Health 14,47. Van Wijnen, J.H. and Stijkel, A. (1988) Health risk assessment of residents living on harbour sludge. Int. Arch. Occup. Environ. Health 61,77-87.
503
Hazard assessment of chemical contaminants in soil C.L.M. Poels and W. Veerkamp Shell Internationale Petroleum Maatschappi The Hague (The Netherlands)
BV; Health, Safety and Environment
Division,
Key words: Contaminated soil; Human health; Hazard assessment; Human exposure to soil pollutants model
SUMMARY Disposal practices, accidental spills, leakages and local aerial deposition occurring in the past have led to local soil pollution in many cases. Especially in situations where people live on or nearby such locations this has created concern about possible adverse effects on human health. A stepped approach to the hazard assessment of polluted soil, as developed by a Task Force from the European Chemical Industry Ecology and Toxicology Centre (ECETOC), is described. In an early phase in the assessment process the potential exposure of humans is estimated. The Human Exposure to Soil Pollutants (HESP) model can be applied for this purpose. The model calculates the total exposure of adults and children resulting from pollutants present in soil, via 10 different exposure routes. The estimated exposure can be used to indicate the potential significant exposure routes and to carry out a preliminary hazard assessment. The model is also able to predict pollutant concentrations in soil which do not exceed accepted maximum exposure levels for humans in both standardised and site specific situations. The stepped approach is cost-effective and provides an objective basis for decisions and priority setting.
INTRODUCTION
Since the 1970s there has been increasing concern about possible effects of contaminated soil on human health and the environment. Making a balanced response to this concern and deciding on the extent of remedial measures have been hampered by a lack of objective and systematic methods to establish the extent of any hazard posed in a given situation. In response, a Task Force of the European Chemical Industry Ecology and Toxicology Centre (ECETOC) has developed a practical system for hazard assessment of chemical contaminants in soil. Correspondence to: C.L.M. Poels, Shell Internationale Petroleum Maatschappij BV, Health, Safety and Environment Division, P.O. Box 162,250l AN The Hague, The Netherlands.
504
In the past much attention has been given to the toxicological effects of chemicals. The results of toxicological tests permit the establishment of so called Maximum Tolerable Exposure Levels (MTELs) using appropriate uncertainty factors or safety factors. This paper concentrates on the assessment of the exposure of humans to chemical contaminants in soil which is the other essential component of hazard assessment. This paper deals especially with exposure of humans to soil contamination in relatively restricted areas as a result of former more localised sources such as accidental spills, leakages from pipelines and storage tanks, disposal of effluents via soak-aways, leaching of chemicals from landfills, local aerial deposition and application of sewage sludge to land. The exposure is strongly dependent on the usage of the land, e.g. urban, agricultural, recreational or industrial. Pollution resulting from localised sources is often characterised by the long term presence of chemical contaminants such that it is a reasonable approximation to assume equilibrium between the different environmental compartments within the contaminated zone. This assumption is an important basis for models calculating exposure. HAZARD ASSESSMENT
Hazard assessment in the context of this paper is defined as “the estimate of adverse effects which may result from the toxicity of substances and site specific exposure when they are present in soil”. Because of the dependence of the hazard of chemicals in soil on the site specific characteristics, only a general approach to hazard assessment can be given and not a specific procedure. The general principles discussed here should be applied to hazard assessment on a case by case basis. Knowledge of the intrinsic toxicological properties of a chemical and of the exposure conditions are the principal requirements for a hazard assessment. All desirable data are, however, rarely available and estimates or theoretical assumptions often have to be made. The degree of realism in these estimates then determines the reliability of the final hazard assessment; this requires expert judgement. Concept of hazard assessment Essentially the hazard assessment of chemical substances described here is based on a comparison of the Maximum Tolerable Exposure Level (MTEL) with estimated or measured Environmental Exposure Level (EEL). The main parameters to be determined are thus the MTEL for each exposure route (e.g. inhalation, ingestion and dermal absorption) or the total MTEL and the EEL for the same routes or all routes together.
505
Fig. 1. Some important exposure routes of man to soil contaminants.
Maximum tolerable exposure level The MTEL is defined as the dose of chemical taken up by a human, or a concentration to which a human is exposed, which, if not exceeded, does not lead to an adverse effect over prolonged exposure periods. Examples of MTELs are the Acceptable Daily Intake or AD1 [II for the oral exposure route and Air Quality Guidelines for inhalation [21 Exposure Assessment Exposure assessment can be defined as “the process of measuring or estimating the intensity, frequency and duration of exposure to a hazardous agent”. (Adapted from Ref. [31). It is essential to determine each route via which a human is exposed to a chemical substance present in soil. Figure 1 presents a scheme indicating the exposure routes potentially relevant to human beings. An early step in exposure assessment is the characterisation of the contaminant source, In addition to information on the concentration and distribution of the contaminant such source characterisation may involve the assessment of climatological factors, information on the local environment, its use by man (e.g. nature conservation, residential, agricultural or industrial area) and the degradation potential for organic contaminants. This information is important for the selection of the relevant exposure routes and the exposure duration. The next step involves estimation of the relative contribution of each exposure route to total exposure and the quantification of the exposure via each route. As a result, the EELS are estimated or determined experimentally. Finally, the estimation of exposure duration and the exposure pattern is required. Apart from exposure resulting from contaminated soil, it is possible for
506
humans to be exposed to the same contaminant via air, food and drinking water (background levels). Although background exposures can be relatively low they should be taken into account. The stepped approach to hazard assessment for humans Applying the concept, a stepped approach is desirable to arrive at conclusions in a systematic way and to optimise the use of resources. Although the concept is simple in principle, in reality it is rarely straightforward because the required data set often is not complete and realistic assumptions have to be made. The stepped approach to hazard assessment is outlined in Figure 2. It involves three steps: Initial Exposure Assessment: In this step it is ascertained whether or not exposure of humans is possible in the given circumstances. If so, the next step should be made. Preliminary Hazard Assessment: If the initial evaluation indicates a potential for exposure, the next step is the comparison ofthe potential EELS, with the MTELs for the relevant exposure routes. Potential EELs can be estimated using a model which computes exposure on the basis of relationships for the different exposure routes determined by observation or experi-
Fig. 2. The stepped approach.
ment. A model developed for this purpose is the Human Exposure to Soil Pollutants (HESP) model, which is fully described ‘elsewhere [4,51. The HESP model uses a limited number of site specific input data and a large set of, prior agreed, conservative but realistic assumptions, e.g. regarding housing, human behaviour, food consumption and climate. The standard model uses parameters relevant for Dutch circumstances agreed with specialists of the Dutch State Institute for Public Health and Environment (RIVM). The parameters can be readily adapted to situations in other countries. If all EEL’s are lower than their respective MTEL’s, no hazard will exist because of the conservative approach taken in their estimation. Consequently no further work is required. If one or more EEL’s exceed their respective MTEL’s, a potential hazard may exist and a definitive hazard assessment is required. Definitive Hazard Assessment: The definitive hazard assessment should be based on actual measurements of relevant exposure levels. These measurements should be restricted to those exposure routes which, according to the exposure assessment model, are significant. When comparing the measured EELS with the MTELs, two situations which may lead to a significant hazard, can be distinguished: (i) an EEL exceeds the MTEL, indicating that a potentially significant hazard exists; (ii) there is more than one exposure route, and although the EEL of each does not exceed their individual MTEL, the combination of routes leads to a total exposure which exceeds the potentially hazardous level. This second possibility requires specialists consideration to assess whether or not a potentially hazardous level is exceeded since exposure duration, intake versus uptake and metabolic and pharmacological data may have to be taken into account. Expert judgement is also required where more than one contaminant is present in the soil and may exert a toxic effect. DDT AS AN EXAMPLE OF HUMAN HAZARD ASSESSMENT
The Acceptable Daily Intake (ADI) for DDT is 0.02 mg/kg [61.The background exposure to DDT is considered to be insignificant in general because this compound was phased out for most uses some 20 years ago. Taking the urban situation as an example, using the HESP model it is possible for each exposure route and for all routes together to calculate the exposure of an adult and a child for a given level of DDT in soil (see Table I). Relevant exposure routes are “ingestion of soil/dust”, “ingestion of vegetables” and “dermal uptake via soil/dust”. Figure 3 shows the total exposure
508 TABLE I ESTIMATED HUMAN EXPOSURE TO DDT IN SOIL Chemical: DDT
Soil concentration: 20 000 mg/lcg
Urban soil use
Standard soil, 10% organic matter
Intake route (mg_/kg.d)
Adult
Child
Inhalation Vapour
5.433 -7
1.24E -6
Dust
1.23E -4
2.953 -4
Shower
2.12E -9
3.763 -9
Ingestion SoiVdust
1.42E -2
2.00E -1
Vegetables
8.89E -3
2.223 -2
Water
1.20E -7
4.673 -7
Meat/Dairy*
E
E
Fish*
E
E
Soil/dust
1.63E -3
8.543 -3
Water
2.493 -7
4.973 -7
2.493 -2
2.31E -1
Dermal
Totals
*Exposure route not relevant for urban soil use. E: power of 10.
Average daily intake of DDT by an adult in an urban area. Dashed line: AD1 for DDT, see Ref. i.61. J?ig. 3.
of an adult to DDT with increasing DDT concentrations in soil in relation to the ADI. From this figure, while considering the assumptions made in the model, a concentration of DDT in soil not exceeding the AD1 can be derived. For adults this concentration is estimated at 15700 mg/kg and for children at 1270 mg/kg. DISCUSSION
Results from HESP and other exposure models indicate that the total exposure estimates are generally dominated by 2 or 3 routes. For example, for highly volatile substances such as benzene and toluene, the inhalation of vapours is the major route. For water soluble substances, poorly adsorbed by soils, the ingestion of crops often dominates over other routes while for relatively persistent and lipophilic substances (e.g. DDT) the direct ingestion of soil and dust accounts for the largest part of the total exposure estimate. Inhalation of particulate matter and dermal absorption generally seem to contribute little to the total intake for all types of substances. An important result of model calculations is that an exposure route analysis frequently reveals that the child is the dominant receptor of soil contaminants. Direct consumption of soil is particularly important for small children in the age group between 2 and 6 years [7,81. Although the hazard assessment example presented concerns an urban area with respect to the exposure routes included, the HESP model is flexible in that it contains various other standard exposure route scenarios (e.g. agricultural, industrial and recreational) and allows for the selection of any exposure route combination to match a particular situation. If the hazard of chemical contaminants on an industrial premise is to be assessed, the exposure routes for vegetables and for meat and dairy products can generally be disregarded. In that case it is also not necessary to assess the exposure for a child. Using the stepped hazard assessment scheme outlined above it is possible to estimate in a systematic, objective and cost effective manner whether, in view of the nature of the contaminant(s) and site specific exposure, a potential hazard exists. The approach indicated makes it possible to determine the possible hazard resulting from the presence of chemical contaminants in the soil on a more objective basis than before. It may be appropriate to decide on remedial measures even if there is no hazard. In such cases the reasons for such decisions should be clearly recognised. The stepped hazard assessment is, when properly applied, a tool to make objective, informed choices and set the right priorities. It should be realised that the HESP model represents nothing more or less than an approximation to reality. Although many assumptions are
510
based on empirical data and some of the model relationships have been validated in field experiments, it is nevertheless necessary that the model calculations are further validated. Due to the conservative but realistic values chosen “false negatives” will be avoided while “false positives” will be prevented by the Definitive Hazard Assessment. Where applied discerningly HESP has proved to be a useful tool to discuss the hazard of contaminants in soil with authorities, to assist in the selection of the appropriate site rehabilitation method and, where deemed necessary, to estimate realistic soil clean-up criteria. REFERENCES Anonymous (1983) Toxicological evaluation of certain food additives. 26th Report of the joint FAO/WHO Expert Committee on Food Additives. WHO Food Additives Series, No. 17. Anonymous (198’7) Air Quality Guidelines for Europe. WHO Regional Publication, European Series No. 23. Anonymous (1982) Risk Assessment in the Federal Government: Managing the Process. Committee on the Institutional Means for Assessing Risks to Public Health. US National Research Council. National Academy Press, Washington, D.C. Poels, C.L.M., Gruntz, U., Isnard, P., Riley, D., Spiteller, M., ten Berge, W., Veerkamp, W. and Bontinck, W.J. (1991) Hazard assessment of chemical contaminants in soil. ECETOC Technical Report No. 40. Veerkamp, W. and ten Berge, W. (1992) Hazard assessment of chemical contaminants in soil. ECETOC, Technical Report No. 40, Revised Appendix 3. Anonymous (1986) Codex maximum limits for pesticide residues. Codex Alimentarius Vol. 13. Joint FAO/WHO Food Standards Programme. Kimbrough, R.D., Falk, H. and Stehr, P. (1984) Health implications of 2,3,‘7,8-tetrachlorodibenzo-pdioxine (TCDD) contamination in residential soil. J. Toxicol. Environ. Health 14,47. Van Wijnen, J.H. and Stijkel, A. (1988) Health risk assessment of residents living on harbour sludge. Int. Arch. Occup. Environ. Health 61,77-87.
