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Mercury
Sources and Health Effects
Sources. Mercury is present in healthcare facilities in various devices and products, including thermometers, blood pressure monitoring equipment, batteries, esophageal dilators, and fluorescent light bulbs (1). Other sources of atmospheric mercury include mining, coal combustion, and waste incineration (2). Medical waste combustion is the fifth largest identifiable source of mercury (3). Anthropogenic mercury releases account for about forty percent of total mercury input to the atmosphere (4). The remainder of mercury releases to the atmosphere arise from natural sources such as volcanic eruptions and rock weathering. Table 1 summarizes the main sources of mercury emissions created by human activity in the United States (3):
Table 1: Anthropogenic mercury emissions in the United States.
Pathways. Mercury exists in many forms. The exposure routes and risks are somewhat different for each form. The two forms of interest to this paper are elemental mercury and organic mercury compounds. Elemental mercury is pure mercury. Organic mercury compounds, such as methylmercury (CH3Hg), are composed of mercury, carbon, and other elemental species. Elemental mercury is absorbed through the lungs (5). Organic mercury is rapidly absorbed if swallowed and can be absorbed through the lungs and skin.
Acute exposure. Elemental mercury is a liquid at room temperature and pressure. Spilled elemental mercury, such as from a broken thermometer, can vaporize and disperse into the surrounding air. This situation, when it takes place indoors, can lead to exposure to mercury vapors through inhalation. The severity of the exposure depends on air temperature, amount of mercury spilled, air flow in the room, and the size of the person being exposed. For example, the relative "dose" a newborn would receive is potentially far greater than what an adult in the same room would receive.
Acute effects. In humans, acute elemental mercury exposure—even at low levels—can produce high blood pressure; impaired vision, speech, or hearing; neurological problems such as tremors and loss of hand-eye coordination; weakness; memory problems; headaches; insomnia; and personality changes such as extreme irritability, shyness, or nervousness (5, 6, 7). Mercury has been shown to be disruptive to neurological development. Young children and fetuses are particularly sensitive to the effects of acute mercury exposure, as their nervous systems are not completely developed (5). It can take up to several months for the body to rid itself of elemental mercury through urine and feces (5). Reported cases of acute poisoning from volatilized mercury are very rare, occurring in modern times primarily in gold mining operations (8). Exposure to high concentrations of elemental mercury can adversely affect the functioning of the kidneys and heart (9).
Chronic exposure. Like elemental mercury, organic mercury compounds can affect the human nervous system. Incineration of mercury-containing medical devices is of concern because it provides a pathway for formation of these compounds. Elemental mercury is oxidized to a reactive form in the atmosphere. Upon deposition to soil or water, this form of mercury is transformed by bacteria into a highly toxic organic compound called methylmercury. This compound attaches to algae or small particles, which are ingested by zooplankton. The zooplankton are then eaten by small fish, and large fish eat the small fish. In this way, methylmercury bioaccumulates, increasing in concentration with each movement up the food chain (3).
Humans, at the top of the food chain, can continually receive high levels of methylmercury exposure through their diets. In the United States, mercury pollution has prompted several thousand fish consumption advisories in fresh water lakes, rivers and streams. The advisories recommend bans or limitations on the consumption of fish, particularly for women of reproductive age and children (10).
Chronic effects. Serious incidents of high level exposure to methylmercury have occurred in Japanese fish and Iraqi grain. Both incidents have caused poisoning that resulted in fatalities (11). In both of these incidents, the fetus was noted as the most sensitive part of the population affected by methylmercury.
Offspring born of women exposed to methylmercury during pregnancy have exhibited a variety of developmental neurological abnormalities, including the following: delayed onset of walking, delayed onset of talking, cerebral palsy, altered muscle tone and deep tendon reflexes, and reduced neurological test scores (9).
Recent studies indicate that methylmercury has adverse effects not only on the developing human fetus but also on the adult reproductive system. In Hong Kong, the relatively high consumption of methylmercury-contaminated fish was positively correlated with male infertility. Regardless of age, men with 2-25 ppm of mercury in their hair were twice as likely to be subfertile than were men with lower levels of mercury in their hair (7).
Long-term studies have been undertaken in island communities where seafood is a major component of the diet. The purpose of these studies was to determine the behavioral effects of prenatal mercury exposure arising from ingestion of contaminated whale meat and fish. Exposure was determined by analyzing mercury levels in the mothers' hair. The results of one study indicated that "a doubling in mercury exposure may cause a developmental delay of approximately 2 months" in seven-year-old children (12). These effects were most pronounced in the areas of language, attention, and memory. A separate study found no significant harmful effects on development at the exposure levels studied (13). Other studies have found no evidence of a positive association between mercury levels in diet and health effects (11).
Consensus in the literature. There is little controversy surrounding the health effects of acute exposure to mercury. Current research focuses on fate and transport issues. It is widely accepted that elemental mercury is a potent neurotoxin. Organic forms of mercury, such as methyl mercury, are also accepted as toxic to the nervous system. For this reason, many healthcare facilities are endeavoring to remove or reduce the amount of mercury used in their operations. Almost 600 American hospitals and clinics have signed on to a single "Making Medicine Mercury-Free" campaign (14).
A recently released report from the National Research Council of the National Academies of Science and Engineering concludes that "neurodevelopmental effects are the most sensitive, well-document effects" for deriving a safe exposure dose (15). The report also concludes that
The population at highest risk is the children of women who consumed large amounts of fish and seafood during pregnancy. The committee concludes that the risk to that population is likely to be sufficient to result in an increase in the number of children who have to struggle to keep up in school and who might require remedial classes or special education (15).
However, more research is required to obtain consensus on the proven effects of methylmercury exposure on the reproductive system and the developing nervous system. Other issues that require further study include hypertension, cardiovascular disease, and immunological effects related to methylmercury exposure.
References
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