1. SCIENTIFIC RATIONALE
Mercury and its compounds are highly toxic to humans, ecosystems and wildlife. Therefore, the Community Strategy Concerning Mercury was adopted in January 2005 with the key aim of reducing mercury levels in the environment and to reducing human exposure.
This Opinion addresses the issue of whether the replacement of mercury-containing, blood-pressure measuring devices (sphygmomanometers) would (i) endanger proper health care including health care for specific groups of patients, and/or (ii) compromise long-term translational epidemiological studies for public health. For this purpose the availability and quality of alternative methods for blood pressure measurements have been evaluated.
For this Opinion, evidence from a wide variety of sources, including peer-reviewed scientific and medical literature and published reports of institutional, professional, governmental and non-governmental organisations has been considered. In accordance with the practice of SCENIHR and its Working Groups, no reliance has been placed on unpublished work or publicly available opinions that are not scientifically based. Single case or anecdotal reports were generally not considered in establishing this Opinion. To review as much evidence as possible, especially where the available data are limited, attention has been given to some less rigorous studies where no other information was available. During the course of the deliberations and drafting the document, a Call for Information was issued by the Commission and the submissions have all been considered.
1.3 Mercury Toxicity
As previously described in the Opinion of SCENIHR on the use of dental amalgam (SCENIHR 2008), mercury is a metallic element that occurs naturally and also in the form of several types of ore, the mercury burden of the environment being derived predominantly from natural sources. Input into the earth’s atmosphere occurs regularly through emissions from volcanoes, soil erosion and the combustion of fossil fuels. Widespread utilisation of mercury and its compounds in a number of industries over the last several centuries has resulted in the release of large amounts of mercury into the atmosphere, increasing the total amount in the ecosphere. Of special importance has been the accumulation of some mercury compounds in the aquatic food chain and the use of mercury compounds in a variety of medical and cosmetic products including dental amalgam (SCENIHR 2008).
It is also important to note that there are several different forms of mercury. First, there is elemental mercury itself, a volatile form of the liquid metal, referred to as Hg0. Second, mercury is stable in two other oxidation states (Hg1+ and Hg2+) and is able to form inorganic compounds, of either monovalent or divalent form, including mercuric chloride (HgCl2), mercurous chloride (Hg2Cl2), mercuric sulphide (HgS), and mercuric selenide (HgSe). Third, mercury is able to form a variety of organic compounds, including methylmercury. There is a clear connection between all these forms with respect to the global cycle of mercury (Nielsen et al. 2006). Elemental mercury may be converted to soluble inorganic forms, which may be methylated in water, especially by microorganisms, and which enter the food-chain and accumulate in the tissues of large predatory fish. The ratio of methylmercury in these fish to the mercury concentration in the water can be as high as 105.
Due to the widespread use of mercury in industrial settings, a large and detailed database on human effects of elemental mercury inhalation is available. A number of reviews addressing the toxicity of elemental mercury have been published (ATSDR 1999, BAT 1997, IRIS 2002, MAK 1999, UNEP 2002). Each form of mercury has its own toxicological profile, although, in general terms, the organic mercury compounds have the highest toxicity, followed by elemental mercury and inorganic mercury compounds. This is important when considering different exposure routes to these forms. Elemental liquid mercury is used in measuring devices such as sphygmomanometers, and previously thermometers.
The assessment of elemental mercury toxicity is mainly based on observations in occupationally exposed humans. Inhalation of extremely high concentrations of elemental mercury, in excess of 10 mg/m3, may produce bronchitis and pneumonia, in addition to symptoms of the central nervous system. After long-term elemental mercury exposure in occupational settings and under occupational hygiene conditions considered as poor by present standards, the major effects of elemental mercury reported are on the central nervous system. The major manifestations of mercury poisoning from inhalation of elemental mercury are increased excitability and tremors. Characteristic symptoms after long-term high dose exposures (the inhalation of concentrations above 0.5 mg/m3 for many years) are muscle tremors in fingers, eye lids and lips, which may progress to chronic spasms of the extremities. After chronic occupational exposure to mercury vapour, proteinuria and even a nephritic syndrome have been described in humans. The glomerular damage may progress to interstitial immune-complex nephritis. Gingivitis and hypersalivation with a strong metallic taste are considered to be further symptoms of chronic inhalation exposure to elemental mercury.
Occupational allergies to mercury were rare, even with widespread exposures to elemental mercury at the workplace and the use of mercury in medicinal preparations (including the use of Hg2+ due to its bactericidal activity) and consumer products (Kanerva et al. 1993).
Mercury is a serious non-degradable environmental pollutant, which eventually accumulates on the sea bed and contaminates marine life (Langford and Ferner 1999). After discharge in the environment, natural transformations and environmental pathways of mercury are very complex and greatly affected by local conditions. There are two main types of reactions in the mercury cycle that convert this metal into its various forms: oxidation-reduction and methylation-demethylation. In oxidation-reduction reactions, mercury is changed from the relatively inert Hg0 to the more reactive Hg2+. The oxidation of elemental mercury Hg0 in the atmosphere is an important mechanism involved in the deposition of mercury on land and water. Hg0 can volatilize relatively easily and be transported in the atmosphere. In contrast Hg2+ has a short atmospheric residence time due to its solubility in water, low volatility and reactive properties. Hence after this conversion, mercury can be rapidly taken up in rain water or adsorbed onto small particles and be subsequently deposited in the environment (Nielsen et al. 2006).
In the environment mercury is transformed into methyl mercury when the oxidized, or mercuric species (Hg2+) gains a methyl group (–CH3).This methylation is primarily a natural, biological process resulting in the production of highly toxic and bioaccumulative methylmercury compounds (MeHg+) that build up in living tissues and increase in concentration in the food chain from microorganisms like plankton to fish and humans. Rates of biomethylation are a function of environmental variables affecting ion availability as well as the population sizes of methylating microbes and pH (acidic conditions are more favourable). Humans are exposed to methylmercury almost entirely by eating contaminated fish, seafood and wildlife that are at the top of the aquatic food chain.