Why are mercury, cadmium and lead and their compounds phase-out substances?
Mercury, cadmium, lead and their compounds are regarded as particularly hazardous. The use of these metals is already regulated in several areas (see several EG and Swedish regulations).
The problem of elevated mercury levels in the environment, principally in lake fish, is old and known to people in general in Sweden. Despite national actions, the deposition of mercury over Sweden is still high, approximately 4.2 tonnes per year, due to long-range air transport principally from Europe but also from other parts of the world. Annual Swedish emissions to air are estimated to amount to 0.7 tonnes. Mercury and its compounds, principally methylmercury, has adverse effects particularly on the nervous system and its development, as well as adverse effects on the cardiovascular system, immune system, reproductive system and kidneys. Disturbance to the development of the nervous system and toxicity to the central nervous system are the most sensitive and best-documented effects. Mercury is transformed to methylmercury by natural processes in the environment and is bioaccumulated in the food chain. Mercury is transferred to the foetus, crosses the blood-brain barrier and probably inhibits brain development even at low concentrations. Populations who eat large amounts of fish, shellfish and marine mammals are particularly at risk.
Mercury concentrations in lake fish exceed the WHO/FAO limit of 0.5 mg mercury/kg fish in half of the Swedish lakes (equivalent to around 50,000 lakes), which means that women planning to have children soon, are pregnant or are breastfeeding are recommended not to eat certain species of lake and marine fish in order to avoid effects on the foetus and newborn infant. The rest of the population should eat these types of fish no more than once a week, according to the Swedish Food Administration. An 80 % decrease in mercury concentrations in deposition is required to attain concentrations in the long term of no more than 0.5 mg mercury/kg fish. There are also indications that reproduction in fish-eating mammals and birds is affected by the high concentrations in fish.
Despite the fact that mercury deposition has decreased in recent decades, this is not sufficient to prevent the metal from accumulating. Concentrations are increasing by approximately 0.5% annually in the top layers of forest soil. In southern Sweden the concentrations are already above the levels that have been shown to effect soil-biological processes and organisms. This mercury also constitutes a source of methylmercury through leaching to water systems.
The greatest source of exposure in humans (the non-smoking part of the population) is the diet. Women with low iron depots generally have a higher cadmium burden than men due to a higher uptake of cadmium in the gastrointestinal tract. Thousands of women in Sweden and far more in the rest of Europe are estimated to have elevated cadmium levels in their kidneys.
Cadmium accumulates particularly in the kidneys, and it is also there that damage has principally been noted. At approximately the same exposure level at which damage to kidneys occurs, effects on bone density have been observed. The results of recent research suggest that effects may arise at lower levels of exposure (cadmium burden) than is stated in previous risk assessments of cadmium.
The supply of cadmium to arable land, principally through commercial fertilisers and digested sludge, has decreased. An average net increase in the cadmium content in arable land is, however, still taking place. The most important measure to limit supply to Swedish arable land is to limit deposition, which accounts for the predominant part of the total supply of cadmium.
The cadmium concentration in the surface layer of Swedish forest land increased steadily up to the mid-1980's, to concentrations that were three to five times higher than the estimated natural levels. In recent years, the reduced deposition combined with increased soil acidification has meant that more cadmium is transported away from the surface soil layers compared to what is added. This has meant that the concentration of cadmium in the soil (humus layer) has started to decrease over large parts of Sweden. At the same time, leaching to water has increased. No large-scale effects (similar to those caused by lead and mercury on microbiological activity) have been identified as a result of the cadmium content of the soil. Nor has it been possible to identify any effects on soil-living animals due to the cadmium concentrations in soil. The toxicity of cadmium in very soft waters, typical of Nordic conditions, needs to be studied more closely before conclusions for these waters can be drawn.
Exposure to lead can cause damage to the nervous system and give rise to impaired cognitive development and intellectual performance, foetuses and small children being particularly sensitive. Other effects are high blood pressure and increased incidence of cardiovascular diseases in adults. Levels of lead in the blood of children have fallen sharply since the lead in petrol was replaced by other substances in the early 1990's, from 60 µg/l at the end of the 1970's to approximately 20 µg/l at present. The margin between measured blood lead concentrations in pregnant women and small children and the levels at which health effects start to occur is relatively small - a factor of 2 to 5. Continued monitoring of these groups is therefore needed.
Concentrations of lead in forest soil in southern Sweden equals or are higher than the levels at which effects may be expected. There are therefore well-founded suspicions that lead is already causing adverse effects today in large parts of Swedish forest land. This may entail effects on the soil organisms living in the top layer of soil. The elevated concentrations also entail risks of the metal being accumulated by mammals and birds living in the forest landscape. The concentrations are generally higher in southern than northern Sweden, which indicates long-range transport. Air deposition has, however, decreased due to the phase-out of lead in petrol and reduced emissions from industrial processes and in the extraction of lead. In limited geographical areas, use or storage of metallic lead may result in elevated lead concentrations in soil and vegetation.