Heavy metals is a general collective term which applies to the group of metals and metalloids with an atomic density greater than 4 g/cm³. Although it is a loosely defined term (Duffus, 2002), it is widely recognised and usually applies to the widespread contaminants of terrestrial and freshwater ecosystems. The heavy metals which are included in APIS are cadmium, chromium, copper, mercury, lead, zinc, arsenic, boron and the platinum group metals, which comprises Platinum, Palladium, Rhodium, Ruthenium, Osmium, and Iridium.. Unlike almost all organic pollutants, such as organochlorines, heavy metals are elements which occur naturally in the Earth’s crust. They are therefore found naturally in soils and rocks with a subsequent range of natural background concentrations in soils, sediments, waters and organisms. Anthropogenic releases can give rise to higher concentrations of the metals relative to the normal background values.
Sources of atmospheric emissions
Although heavy metals differ in their chemical properties, they are used widely in electronic components, machinery and materials. Consequently, they are emitted to the environment from a variety of anthropogenic sources to supplement natural background geochemical sources. Some of the oldest cases of environmental pollution in the world were caused by heavy metal extraction and use, for example, copper, mercury and lead mining, smelting and utilisation by the Romans.
The amounts of most heavy metals deposited to the surface of the Earth are many times greater than depositions from natural background sources. Combustion processes are the most important sources of heavy metals, particularly, power generation, smelting, incineration and the internal combustion engine (Battarbee et al 1988; Duce et al. 1991; Galloway et al. 1982; Hutton & Symon 1986; Nriagu 1989; Nriagu & Pacyna 1988).
Concerns about Heavy Metals
The Dangerous Substances Directive of the European Union (76/464/EEC) defines dangerous chemicals as those which are toxic, persistent and / or bioaccumulative. As they are elements, they cannot be broken down, therefore heavy metals will persist in the environment. Unlike many organic pollutants, which eventually degrade to carbon dioxide and water, heavy metals will tend to accumulate in the environment, especially in lake, estuarine or marine sediments. Metals can be transported from one environment compartment to another. The information in APIS is primarily focused on metals which are transported via the atmosphere. Many of the heavy metals are toxic to organisms at low concentrations. However, some heavy metals, such as copper and zinc are also essential elements. Concentrations of essential elements in organisms are normally homeostatically-controlled, with uptake from the environment regulated according to nutritional demand. Effects on the organisms are manifest when this regulation mechanism breaks down as a result of either insufficient (deficiency) or excess (toxicity) metal. Whether the source of heavy metals is natural or anthropogenic, the concentrations in terrestrial and aquatic organisms is determined by the size of the source and adsorption/precipitation in soils and sediments. The extent of adsorption depends on the metal, the absorbent, the physio-chemical characteristics of the environment (e.g. pH, water hardness and redox potential) and the concentrations of other metals and complex chemicals present in the soil water, river or lake. The concentration of metal in bioavailable form is not necessarily proportional to the total concentration of the metal.
Heavy metals accumulate in organisms as a result of direct uptake from the surroundings across the body wall, from respiration and from food. Uptake via food is most important in terrestrial organisms and it may also be important in the aquatic environment. Dietary uptake can include heavy metals adsorbed on particulates present on the surface of leaves etc, which have not been absorbed by the plant.
The free ion is generally the most bioavailable form of a metal, and the free ion concentration if often the best indicator of toxicity. However, there are exceptions, such as the well known case of mercury, where the organic form, (methylmercury) is more toxic than the inorganic ion. Metals exert toxic effects if they enter into biochemical reactions in the organism and typical responses are inhibition of growth, suppression of oxygen consumption and impairment of reproduction and tissue repair.
Critical levels and loads
Most critical levels for heavy metals have been developed to protect humans, although some have been suggested to protect aquatic organisms and agricultural soils which receive sewage sludge. There are no critical loads for heavy metals at present, but researchers have started to develop effects-based critical levels and critical loads for some heavy metals in soils and freshwaters.
Most critical levels derive from the European Union's Dangerous Substances Directive (76/464/EEC) and the US EPA (US EPA 1998) for water and the EU Air Quality Framework Directive (96/62/EEC) and the World Health Organization (World Health Organization 1987) for air. The first Air Quality Daughter Directive sets critical levels for lead and another in preparation will sets limits for cadmium, arsenic, nickel and mercury. The general aim of the Air Framework and Daughter Directives is to avoid, prevent and reduce harmful effects of air pollutants on human health and the environmental as a whole. The National Air Quality Strategy is the means by which this European Union legislation is being implemented in the United Kingdom. The EU sludge to land directive (86/278/EEC) sets critical levels for heavy metals in agricultural soils that receive sewage sludge.
Critical levels and loads for heavy metals should be available in the United Kingdom and Europe in a few years and they should be well founded effects-based standards which should protect target organisms in terrestrial and freshwater ecosystems. There are four developments which should lead to the production of critical levels and loads for heavy metals. As deposition from the atmosphere is an important source of heavy metal contamination of terrestrial and freshwater ecosystems, then the critical levels developed will be relevant to the management of emissions to air.
The UK is obliged under the Dangerous Substances Directive to set Environmental Quality Standards (EQSs; critical levels) for selected (List I and II) chemicals in water which are of national concern and the Environmental Protection Act 1990 requires EQSs for further chemicals (the Red List) in order to protect the aquatic environment. Critical levels for mercury and cadmium are available and one for arsenic is imminent.
The proposed European Union Water Framework Directive will require that EQSs are established for important chemical pollutants in the aquatic environment. While an ecotoxicological approach based on alga/macrophyte, Daphnia and fish is described in the draft directive, the United Kingdom recommends a Weight of Evidence approach. These EQSs, when available, will be critical levels for water and ones for cadmium, copper, mercury and arsenic, at least, are being developed. The draft directive also introduces the important concept of the ecological quality of water (rivers, lakes, transitional waters (estuaries) and the near shore ocean). In this, the major biological groups (phytoplankton, macrophytes, invertebrates and fish) are used to classify the water body into five ecological classes (high, good, fair, poor and bad) and this is supplemented with physical and chemical information. The objective of the directive is that all water bodies achieve good ecological status and this ecological approach to monitoring is likely to increase in importance.
At the Bad Harzburg Workshop in 1997 (UN ECE Convention on Long-range Transboundary Air Pollution, Task Force on Mapping 1998), representatives from many Member States of the European Union agreed approaches to set Critical Limits/Loads and effects-based standards for heavy metals in freshwaters and soils. For both soils and freshwater, a critical limits approach is recommended for cadmium, mercury and lead for key receptors. The relevant receptors in freshwaters are fish for mercury and water for cadmium and lead and invertebrates, food crops and groundwaters (and possibly soil micro-organisms) in soils. There is sufficient understanding of the behaviour of cadmium, mercury and lead in the environment to develop critical loads for freshwater and terrestrial ecosystems. Further work is needed in some areas and UK scientists are contributing to this.
Finally, the United Nations Economic Commission for Europe adopted a Protocol on Heavy Metals in 1998. The ultimate objective of this is to eliminate any discharges, emissions and losses of cadmium, lead and cadmium to air.
For more on the Protocol on Heavy Metals see:
For more on the EU Directive on Dangerous Substances see:
For more on the Convention on Long-range Transboundary Air Pollution see: