Levels of Mercury in Fish-Eating Children, With and Without Amalgam Restoration.

BACKGROUND: Mercury is a naturally occurring metal that exists in three forms: elemental (metallic), inorganic, and organic mercury. Amalgam, which is an alloy of inorganic mercury, is used as a restorative material in dentistry. Organic mercury gets ingested in the body mainly by the consumption of seafood. Mercury is also stated to cause various adverse health effects such as gastrointestinal disturbances, dermatitis, muscle weakness, and neurological disorders. In recent years, the use of amalgam has become a controversy stating the various adverse effects of mercury. Hence, the study was conducted to determine and compare the variation in levels of organic and inorganic mercury in fish-eating children before and after placement of amalgam restoration.
MATERIALS AND METHODS: Seventy-five subjects, 42 males (56%) and 35 females (44%) of age group ranging 7-13 years, living in South Canara district of Karnataka, India, were selected as a part of the study. Hair and urine samples were collected for estimation of organic and inorganic levels of mercury, respectively. Informed consent was collected from all the participating subjects.
RESULTS: On comparison between organic and inorganic mercury levels during the study period, the concentration of organic mercury in hair samples was greater irrespective of amalgam restorations present (1.172 and 0.085, respectively; P < 0.001).
CONCLUSION: Thus inorganic levels of mercury do not seem to pose a threat as much as the organic levels observed in hair, which remain fairly constant for a longer period of time. Hence in a coastal region where this study was undertaken and fish being a staple food, the risk could probably be attributed to more of an organic toxicity than an inorganic one. Thus amalgam is relatively safe to be practiced and the controversy against it should be reevaluated.

INTRODUCTION

Amalgam, an alloy of mercury, is regarded as the most versatile dental restorative material.[1] Mercury, which is liquid at room temperature, is alloyed with solid metals such as silver, tin, copper, and zinc. Amalgam has been used for restorative purposes for the past 150 years. It has got high strength, durability, and dimensional stability. Studies have proved amalgam has got higher survival rate than tooth colored restorative materials.[2] In spite of all this advantages, dental amalgam is losing its popularity as a restorative material because it contains mercury as its main ingredient.

Studies have proved that mercury is a poisonous metal. It was during 1800s the phrase “mad as hatter” was coined because of the chronic mercury exposure that the felters faced because mercury was used in hat making. In 1940s and 1950s, mercury became known as the product that caused acrodynia, also known as “pink disease.” Some of the more recent occurrences include exposures in Minamata Bay, Japan (1960), mercury-contaminated fish in Canada, and methylmercury (MeHg) treated grain in Iraq (1970). All this has raised questions whether to continue using dental amalgam or not. Mercury is present in different physical and chemical forms. The three forms are elemental (Hg0), inorganic (Hg+), and organic forms. Elemental form is raw form from cinnabar ore. Inorganic form is formed when it combines with metals such as sulfides and chlorides.[3] This inorganic form is converted to organic form when it enters the food chain. Seafood is the major source of organic mercury in the human body. MeHg is present as a result of the methylation of inorganic Hg by microorganisms usually present in sediments. It undergoes a remarkable biomagnification process and accumulates in the fish muscle tissues of long-lived predatory species such as pike in fresh water and sharks in ocean waters.

The degree of biomagnification depends on the location of the fish species in the food chain.[4] Organic mercury from seafood gets accumulated in hair strands and inorganic mercury from amalgam gets deposited in kidneys, which is later eliminated through urine.[5] Because mercury is universally present and its toxicity being a well-established fact, dental amalgam has become a very controversial subject. Hence, a study to estimate the mercury levels in fish-eating children having amalgam restoration has become necessary. This study aimed to determine the level of organic and inorganic mercury in fish-eating children.

MATERIALS AND METHODS

Seventy-five subjects (fish eating) either males or females, of the age group ranging from 9 to 14 years, living in the South Canara district of Karnataka, India, were included in the study. Informed consent was obtained from each of the subjects.

Procedures for scalp hair collection and measuring organic mercury

A single strand of hair was collected on the day of examination for baseline values and again 3 months later. It was then subjected to the atomic absorption spectrophotometer for estimating the level of organic mercury.[6]

Procedures for urine collection and measuring urinary inorganic mercury

Urine samples were collected from the control group subjects and before the start of any restorative procedure in the study group subjects for baseline values. Three-month post-filling urine samples were again collected from both the study groups and control groups to assess the inorganic levels of mercury. The urine sample (~10 mL, morning midstream sample) collected from each subject was subjected to the cold vapor technique together with atomic absorption spectrophotometer for analysis.[7] The samples were digested before analysis with nitric acid to a homogenous solution. This would release bound mercury as Hg2+ from protein–sulfur complexes.

RESULTS

Variation within each group at baseline and 3 months later in hair samples (paired t test)

The variations of mercury levels in hair between each of the three subgroups in this study were evaluated using the paired t test. Among the control group, the mean level of organic mercury increased from 1.12 to 2.33 µg/L. On comparing the organic mercury level from children having single restoration, the mean value increased from 1.27 to 2.606 µg/L whereas the mean value increased from 1.55 to 2.800 µg/L in children having multiple restorations [Table 1, Figure 1].

Table 1

Variation within each group at baseline and 3 months later in hair samples (paired t test)

		N	Mean (µg/L)	Standard deviation	Significance
Hair (baseline)	Group A: control group	25	1.12896	0.27252
	Group B: fish eating with one restoration	25	1.2764	0.307779	<0.001
	Group C: fish eating with 2–3 restoration	25	1.552	0.4814446
	Total	75	1.08602	0.403491
Hair (3 months)	Group A: control group	25	2.39948	0.591205
	Group B: fish eating with one restoration	25	2.6016	0.471486
	Group C: fish eating with 2–3 restoration	25	2.80088	0.717765	<0.001
	Total	75	2.25858	0.610927

Figure 1

Variation within each group at baseline and 3 months later in hair samples (paired t test)

Variation within each group at baseline and 3 months later in urine samples (paired t test)

The variations of mercury levels in urine between each of the three subgroups in this study were evaluated using the paired t test. Among the control group, the mean level of organic mercury increased from 0.18 to 0.216 µg/L. On comparing the organic mercury level from children having single restoration, the mean value increased from 0.29 to 1.23 µg/L whereas the mean value increased from 0.32 to 1.96 µg/L in children having multiple restorations [Table 2, Figure 2].

Table 2

Variation within each group at baseline and 3 months later in urine samples (paired t test)

		N	Mean (µg/L)	Standard deviation	Significance
Urine (Baseline)	Group A: control group	25	0.189	0.096548
	Group B: fish eating with one restoration	25	0.29208	0.177411	<0.001
	Group C: fish eating with 2–3 restoration	25	0.32092	0.148411
	Total	75	0.21668	0.184063
Urine (3 months)	Group A: control group	25	0.21408	0.09338
	Group B: fish eating with one restoration	25	1.2352	0.331192
	Group C: fish eating with 2–3 restoration	25	1.96868	0.456366	<0.001
	Total	75	1.06616	0.766075

Figure 2

Variation within each group at baseline and 3 months later in urine samples (paired t test)

DISCUSSION

Amalgam fillings currently comprise about 50% mercury, with the remainder principally silver, copper, tin, and zinc. Although other restorative materials are available, popularity of amalgam is maintained by its relative durability, ease of use, and lack of postoperative sensitivity. In many countries, amalgam is still the most commonly used filling material in posterior teeth.[8] However, dental amalgam has drawbacks as well. First and foremost, it is not esthetic. Second, the issue over mercury toxicity, even though there is overwhelming evidence of its safety, has always remained a contentious issue.

The oral cavity is consistently wet owing to the continuous secretion of saliva and the high humidity of exhaled air. Research now shows that amalgam is not entirely chemically stable after the initial set. In contrast to earlier studies, recent evidences suggest that amalgam in the oral environment constantly releases small quantities of cytotoxic corrosion products and Hg vapor.[910] The mercury levels (inorganic mercury) are greatly increased by mild abrasive actions such as chewing, brushing, and ingestion of hot beverages.[11] Investigators have demonstrated that people with amalgam restorations have higher oral levels of mercury vapor than people who do not have amalgam restorations.[12] Various other studies quote that the absorption of mercury through gastrointestinal tract is minimal, and the mercury from amalgam that is swallowed adds very little to the total body burden of mercury.[13] This is in accordance with the study by Mackert and Berglund,[14] which also stated that the extremely low doses of mercury attributable to amalgam restorations was insufficient to produce any detectable negative effect on general health. The current point of controversy is whether or not the levels released are great enough to be hazardous to the health of the patient.

Other than amalgam restorations, there are also other sources of inorganic mercury such as chemical factories, municipal waste, thermometers, electrical switches, fluorescent bulbs, and medicines. Microorganisms (bacteria, phytoplankton, and fungi) convert inorganic mercury to MeHg that gets accumulated in the food chain. Seafood, that is, salt water fish is the major contributors of MeHg (organic form of mercury) in the body of subjects who consume it.

No large studies have been completed that examine the effects of mercury exposure from dental amalgam fillings in children. This study was carried out among fish-eating children in the coastal region.

Organic mercury level assessment

Chemists first synthesized MeHg in the mid-19th century in the form of dimethylmercury.[15] However, nowadays human exposure occurs almost exclusively to MeHg from consumption of fish and marine mammals. MeHg contamination in fish poses a particular challenge to public health, because fish is a highly nutritious food, with known benefits for human health. Moreover, fish are culturally vital for many communities and constitute an important global commodity.[16] MeHg is present as a result of the methylation of inorganic Hg by microorganisms usually present in sediments. The degree of biomagnification depends on the location of the fish species in the food chain (e.g., in unpolluted ocean water, the herbivorous reef fish may have very low concentrations of MeHg, whereas in bigger fishes such as sharks it shows a higher level). Sample collected to assess the organic mercury content was occipital hair,[6] mainly because mercury has a half-life of 70 days and remains relatively stable. Also previous studies in literature have shown concentrations in hair to correlate well with concentrations in organs where mercury may accumulate.

The hair sample in our study was collected at baseline and 3 months later. The difference in organic mercury levels at 3 months and at baselines was statistically significant. The average values being 1.248 µg/L (Group A), 1.325 µg/L (Group B), and 1.2705 µg/L (Group C). The levels increased in all the fish-eating groups irrespective of the presence or absence of restorations. This is in accordance with studies by Salehi and Esmaili-Sari[6] and Fakour et al.[17]. Our study is also in correlation with the study by Babi et al.[18] who stated the increased concentrations of mercury level in hair are due to sea food consumption.

Inorganic mercury level assessment

Dental fillings made with mercury amalgam can be a source of human exposure to elemental mercury vapors for many populations. Amalgam surfaces release mercury vapor into the mouth and lung, depending upon the number of amalgam fillings and other factors. The estimated average daily absorption of mercury vapor from dental fillings varies between 3 and 17 µg mercury.[19] Thus amalgam restoration groups were used as study groups in this study to measure exposure levels as other sources of exposure are highly variable and would not be standardized.

The presence of mercury in urine[3] generally represents recent exposure to inorganic and/or elemental mercury. However, inorganic mercury can accumulate in the kidney and slowly get excreted through the urine. Therefore urine mercury levels can also represent exposures to elemental mercury and/or inorganic mercury that occurred sometime in the past. An individual’s level may vary greatly from day to day and even within a given day.[13] Hence, standardizing of sample collection is necessary (e.g., collect samples at the same time of the day). Dodes[13] stated that the most common way to measure mercury exposure is through urine samples, because it is easy to collect these samples. Hence in our study, midmorning samples of urine were collected at baseline and 3 months later, half-life being around 70 days.

The mercury levels in urine increased significantly in all the groups with amalgam restorations from baseline values to a 3-month follow-up period, irrespective of the consumption of sea food. The increase in mean value of inorganic mercury among salt water fish eaters was 0.9431 µg/L (Group B) and 1.647 µg/L (Group C). A proportional increase in urinary mercury levels has been observed with an increase in number of restorations in our study. The findings thus demonstrate a strong positive association between urinary mercury concentration and number of amalgam surfaces as seen in others studies by Woods et al.[7] and Ye et al.[20] In between control groups, the increase was 0.0251 µg/L (Group A). This increase in value could be due to various environmental exposures of mercury other than amalgam. The US EPA has developed Reference Doses (RfDs) for mercuric chloride (inorganic Hg) of 0.3 µg/kg body weight per day. An RfD is an estimate (with uncertainty spanning perhaps an order of magnitude) of daily exposure to the human population (including sensitive subgroups) that is likely to be without appreciable risk of deleterious noncancer effects during a lifetime.

The values obtained in our study stays well within the maximum permissible limits. These observations imply that the level of mercury exposure from all sources including amalgam restorations did not exceed the capacity for elimination via the urinary excretion in these subjects.

CONCLUSION

In coastal regions of Karnataka where the present study was conducted, the level of organic mercury from fish was higher than inorganic mercury from dental amalgam. Thus, stating the risk could probably be more of an organic form than an inorganic one. Thus amalgam is relatively safe to be practiced and the controversy against it should be reevaluated.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.