Last changed: 4/04/11
The examination of humanotoxic effects in the context of the critical loads concept focuses in respect of each particular heavy metal (Pb, Cd, Hg) on its transfer via soil and/or water into human nutrition or drinking water systems. Other sources of pollution with consequences for human health such as house dust or paints are not taken into consideration.
The heavy metals Pb, Cd and Hg are not part of the family of trace elements which in small quantities are essential for human health e.g. copper or zinc. If absorbed into the human organism in sufficient quantity they will cause damage. Nutrition is the principal path of pollution of the human organism with Pb, Cd and Hg. Depending on the heavy metal the direct transfer via deposition on nutritive plants or indirect paths via the soil pore water or surface waters are of varying significance. For Germany critical loads in respect of the protection of human health have been calculated and mapped for all three heavy metals. To this aim the WHO guiding valuesWHO (World Health Organization) 2004: Guidelines for Drinking-Water Quality. Volume 1: Recommendations. 3rd. ed., Geneva for Pb, Cd and Hg concentrations in drinking water were taken as effect indicators. For Cd an additional criterion was the observance of a limit of 0.1 mg/kg in wheat grain as an indicator of the assurance of food quality.
One of the effects of Pb is neurotoxicity. Even at relatively low levels of exposure the intellectual development of children can be affected. Recent studies have shown that harmful effects can manifest in children even with blood lead levels of less than the much-debated effect threshold (Pb level in blood of 100 µg/l). Absorbed Pb gradually accumulates in the bones. During pregnancy it is partially mobilised, passing through the placenta and polluting the unborn child. Infants can be equally at risk of exposure via breast milk. For this reason, ensuring that mothers are themselves subject to lower levels of exposure (from their own birth to that of their children) is decisive when it comes to the protection of children from Pb pollution.
Up to 90% of Cd in non-smokers is ingested through food. A smoker who smokes an average amount of tobacco ingests double the daily ingestion rate of a non-smoker. The absorbed Cd accumulates in the body and causes damage above all to the kidneys. People with existing kidney problems, such as diabetics, and in general the elderly are most at risk of disease. Cd is also one of those substances believed to be carcinogenic.
Hg has a neurotoxic effect and damages the kidneys. Biotic and abiotic processes in soils and water sources lead to the transformation of Hg into methyl Hg, which is particularly neurotoxic for unborn children. Exposure occurs through the ingestion of food on the part of the mother. Foodstuffs with high levels of methyl Hg include fresh-and salt-water fish and sea animals from particular areas, including Scandinavia and the Arctic. Food pollution occurs largely through atmospheric Hg- deposition, the subsequent mobilisation of Hg in soils and transport in water.
Neither animals, plants nor micro organisms require Pb, Cd or Hg for their metabolic processes. Once certain concentrations in the environment have been exceeded the three heavy metals have harmful effects. These range from individual deaths with the concomitant reduction in the number of individuals, stunted growth, visible leaf damage and reproductive disorders to changes in physiological processes and the impairment of microbiological metabolic processes. Most of the data on the effects come from laboratory research and have been partly confirmed by (very much more laborious) field trials. The results of investigations into the effects form the basis of the determination of critical concentrations in soil water in the critical loads concept. The determination of precautionary values for the Federal Soil Protection and Contamination Ordinance (Bundes-Bodenschutz- und Altlastenverordnung [BBodSchV]) is also based on ecotoxicological effects data.
The toxic effect only comes from the bioavailable fraction of the heavy metal, which can however change in accordance with changes in the soil milieu (pH, root exudation). In soils, heavy metals can bind very strongly to humic components or clay minerals and metal oxides. These metal fractions are only available to a limited degree for many organisms. In any case, soil-eating or digging organisms (earthworms, moles) or grazing animals can ingest them directly. Of greater ecotoxicological consequence are those heavy metals which dissolve in soil pore water. In such cases it is above all free ions which are bioavailable. Plant roots, insects and other invertebrates, and microorganisms can ingest them. In contrast, heavy metals in a chemical complex with dissolved organic matter, whilst being mobile (meaning that they can be transported into groundwater or surface waters), can hardly be ingested by living organisms. The proportion of bioavailable heavy metals in the total concentration in soils depends on chemical and biological values e.g. pH, humus content, root exudation and clay content.
Heavy metals demonstrate very different mobility characteristics in soil: Cd is very mobile, Hg and Pb less so. Decisive factors of influence are pH value and the clay and humus content. Cd will dissolve very strongly at pH values of less than 6 – 6.5, whereas Pb and Hg require values of pH 4 or below for their mobility clearly to increase.
Increase in mobility means on the one hand the transport of heavy metals into deeper soil levels and groundwater; on the other it leads to uptake by plants. In accordance with the above the transfer out of the soil into the plant is more marked in the case of Cd than it is in the cases of Pd and Hg. The behaviour of plants in respect of heavy metal absorption differs fundamentally according to the mode of land use, crop types, pH value and soil.texture. This is also taken into account in the determination of trigger values and intervention values related to the soil/plant path in the Federal Soil Protection and Contamination Ordinance.
Risks arise especially in the course of changes in chemical ancillary conditions e.g. by the degradation of organic substance in the soil which causes the organically bound heavy metals to dissolve, or through acidification which causes certain heavy metals (e.g. Cd) to mobilise.
Critical loads in respect of environmental protection have been calculated for Pb and Cd for agricultural and semi-natural ecosystems. Critical loads for Hg relate exclusively to strongly humic forest soils, because to date it is only these which show evidence of the harmful effects of Hg from the long-range transport of atmospheric pollutants.The solid and solute organic substance of the humus is principally responsible for the storage and transport of Hg in soils.
The value range of the critical loads can be taken from the chart, and information on the method of calculation and the critical concentrations in environmental media adopted as basis can be found in the Modelling and Mapping Manual.