Most people who have encountered mercury have done so after breaking a thermometer. And no doubt we find it as one of the most appealing elements in the periodic table. Right!! We are talking about the most mysterious element-able to cure and to poison; able to bear the whole weight of the light, and being capable of shattering into a numerous uncountable particle in different directions.

The first written record of mercury comes from Aristotle, who in the fourth century BC described the metal as “liquid silver.” In 380 BC Theophrastus of Eresus was writing was is still the largest and most productive mercury mine in the world, at Almaden, Spain. In the first century AD Pling described the mine as “the most famous for the revenues of Roman nation.” Pling also mentioned that the mine was worked exclusively by Roman convicts and slaves, and wrote the hazards of poisoning.

To ask how poisonous is mercury is to ask “how high is up?” a lethal dose of mercury is less than the quarter of a gram. It would take 113 such doses to make up a single dose.

Mercury has been used in dental practice as amalgam for the last two centuries, as a dental restoration filling material. This material contains mercury in about 50% of its total mass and the remaining constituents are silver, tin, copper, zinc, and other trace metals.¹

According to Skinner, amalgam is a special type of alloy in which one of its constituents is mercury. In dentistry, it is common to use the term amalgam to mean dental amalgam.²

Though the dental amalgam is widely used, however, injudicious handling consequently leads to human health risk, particularly associated with occupational exposure and environmental damage from mercury emission.

Elemental

Mercury is able to cause myriads of adverse effects including:

Neurotoxicity (elemental mercury, mercuric salts such as mercuric chloride)

Exposure to mercury has been associated with over 250 symptoms in humans, resulting in complications for proper diagnoses. Mercury can be quickly removed from the blood. From here it is transported and sequestered into various tissues; in other words, there may not be a direct correlation between blood mercury concentration and the gravity of mercury poisoning.³ Low-level exposure to vaporized metallic mercury can be inhaled, causing mercury poisoning. The principal toxic effects of this exposure include excitability, tremors, and gingivitis. Exposure to vaporized metallic mercury can also be toxic to the immune system, nervous system, kidneys, cardiovascular system, gastrointestinal system, lungs, muscle, liver, blood cell count, skin, and eyes Figure 1.⁴ Human fetuses and small children who are exposed are more likely to have mercury concentrated in the brain and kidney.⁵Figure 1

Fate of inorganic mercury and potencial effects, Source: Philippe Grandjean.

Mercury in its nonionized form crosses the lipid barriers in the membranes of the brain and placenta and is removed slowly after getting oxidized. Maternal amalgam restoration can expose the fetus to low levels of elemental mercury and even removal of this restoration cannot help to escape the mercury exposure. However, no evidence has been found regarding adverse pregnancy outcomes or health deterioration in the new borns.

The mercury body burden of dental personnel is usually higher than in the general population. Mean urine mercury levels of 3–22 μg/L have been found in dental practitioner in contrary to general population group who have a range of 1–5 μg/L (Hörsted-Bindslev P. Amalgam toxicity-environmental and occupational hazards. J Dent. 2004 Jul; 32 (5):359-65). This increased level is due to dealing with dental amalgams whether it be mixing and placing the amalgam restoration or just removing the mercury amalgam filling.

General effects

Depend on form of Hg, dose, route of exposure, during foetal stage of development

Neurological effects

Emotional instability

Thermometers

Health-care facilities are one of the main sources of the release of mercury into the atmosphere

One gram of mercury is present in a normal thermometer

Healthcare workers work with mercury-based products on a routine basis and are in danger of inhaling toxic vapor when breakages or leakages happen

OSHA Mercury⁷ (PEL) of 0.1 mg/m³ (8 h [TWA]). Some state OSHA mercury vapour limit of 0.05 mg/m3 (8 h TWA).

In general, the toxicology of mercury is highly dependent on the route of administration, the exposure conditions, and the speciation of mercury. Mercury exposure through dental amalgam fillings may occur by mercury vapor inhalation released from the restorations in the mouth, released organic mercury ingestion, or swallowing small pieces of amalgam releasing mercury in the alimentary tract. The European Chemical Agency (ECHA) website indicates the following classification for mercury Table 1 (http:// apps. echa. europa. eu/registered/data/dossiers/ DISS 9d872986-171c 222a e044 00144f67d249/ AGGR 995f8d07 ac73 4081 8ad0 ed8b5616e7ee_DI SS 9d872986 171c 222ae044 00144f67d249.html).{Table 1}

Mercury vapor is lipophilic and can pass biological membranes, including the blood-brain barrier and placenta, thus resulting in deposition in the central nervous system, including the fetal brain. The vapor dissolved in the blood and tissues rapidly becomes oxidized due to catalase activity. Ionic Mercury becomes bound to some extent to metallothionein and accumulates in the kidneys. Excretion takes place via urine and to some extent through feces and sweat.

The World Health Organization confirmed that decreasing the use of dental amalgam is not only important in reducing human exposure, but also to lessen the considerable amount of mercury that is estimated to be released into the environment from this source. The use of dental amalgam and its applications, such as illegal sales and use in ASGM, improper waste management, or even through cremation, is contributing to the problem of global mercury pollution.⁴

Dental amalgams off-gas mercury vapor. The newer high copper amalgams are less stable and create a much greater release of mercury vapour.

In a 2016 document titled, “Dental Amalgam and Mercury Regulation,” the European Federation of National Associations of Water Services advocated for a ban on dental amalgam to decrease mercury in the sludge from the wastewater treatment plants.⁸

Nationally, dentists discharge about 5.1 tons of mercury into publicly owned treatment works, and most of this mercury will end up in the environment.⁹ Once the amalgam waste has gone through the sewage treatment plant, the remaining amalgam waste becomes sewage sludge. This sewage sludge is then disposed of in landfills, incinerated, or sold as fertilizer for agriculture purposes. These pathways of disposal of sewage sludge release mercury into groundwater or air.¹⁰ Dentists typically dispose of excess amalgam into specific medical waste containers, however, if this waste is incorrectly disposed of, the amalgam may be incinerated, causing the mercury to enter the air where it will eventually end up in the water or on land.¹¹

A substantial source of mercury pollution comes from cremation. Estimations of the amount of mercury released via this pathway vary considerably, due to the large number of dental restorations.¹² Cremation emissions add to both environmental pollution in areas close to the source and also countrywide emissions due to atmospheric transport. These emissions are deposited primarily through rain. Mercury is persistent and can change in the environment into methylmercury, which is extremely toxic.¹³ During cremation, mercury will enter the process, since it is not only from dental amalgam in teeth but also due to bioaccumulation of mercury in the body.¹⁴

Crematoriums have many risk factors, not just to the funeral workers, but also to the population in surrounding neighborhoods. Living near these environmental toxic exposures can have negative health effects, particularly in vulnerable subpopulations.¹⁵

The World Health Organization, the US EPA, and other public health experts consider any level, no matter how low, of emissions of mercury, dioxins, furans, and particulate matter from incineration to be a threat to human health. Vulnerable populations such as babies, children, women of childbearing age, and the elderly are particularly at risk from exposure to these toxins. Employees who work in these environments, as well as those populations who live near the source, are exposed to higher levels of these pollutants.⁴,¹⁶,¹⁷ The effects of mercury vapor exposure can last long after the exposure has ended. While typical symptoms and signs, such as tremors, gingivitis, and salivation may quickly disappear after exposure has stopped, mechanisms of long-lasting or remote effects have not been investigated.⁴

According to Esdaile and Chalker,¹⁸ the approximate amount of mercury released through ASGM is between 410 and 1400 tons annually, which is about 37% of total global mercury emissions. Easy access to mercury, along with its low cost and the soaring price of gold make this a sustainable livelihood for miners.

Artisanal small-scale gold mining is the largest source of mercury emissions worldwide. Around 15 million people are estimated to be working in this sector and about 5 million are women and children. Artisanal small-scale gold mining has devasting effects not only on the local inhabitants but also on the environment, especially rivers, due to mining locations. Research shows that populations in these areas, as well as those downstream, eat fish that are highly mercury toxic. These communities are also subjected to tremendously harmful levels of mercury vapor, causing neurological, kidney, and possibly immunotoxic/autoimmune effects from mercury exposure.¹⁹

An alternative to mercury in ASGM is the borax method. Gold is gravitationally separated by sluicing and panning, with iron shavings possibly removed by a magnet, then gold concentrates are mixed with an equal mass of borax. This mixture is heated and the gold solidifies in a relatively pure form when cooled. The borax complexes to silicate and oxide impurities.¹⁹

The set national objectives and the effective methods for minimising amalgam use has been documented in Table 2.{Table 2}

Promote mercury-free dental restorations, including raising public awareness of amalgam's mercury

Atraumatic restorative treatment

This is a nonmercury dental filling technique, that was developed in the 1980s in Tanzania as a minimally invasive way to fill teeth. Using atraumatic restorative treatment saves teeth that would have otherwise been extracted due to decay.

Advantages

Atraumatic restorative treatment requires no electricity, water, or conventional dental equipment. Only hand instruments are needed to clean the decay and a high viscosity glass-ionomer is then placed in the tooth.

Glass ionomer cement

Glass ionomer cements have organic acids like eugenol and bases like zinc oxide, and may also contain acrylic resins. Glass ionomer also have glass filler like some composite resins that release fluoride with time.

Advantage

Tooth-colored restorative material

Disadvantage

They are limited to use in small restorations.

Composite resin fillings

These restorations are the most common alternatives to dental amalgam and are referred as “tooth-coloured” or “white” fillings due to their color. Composites are acrylic resins reinforced with powdered glass filler. The color (shade) of composite resins can be customized to closely match surrounding teeth. These restorations are either self-cured or light-cured in layers using “blue light.”

Advantages

Blend in with surrounding teeth

Disadvantages

More difficult to place than dental amalgam

Mercury-free fillings are more minimally-invasive than amalgam

Dentistry is most outstandingly and foremost a healing vocation.¹ Today it has become very crucial to adapt eco-friendly approach in every aspect of our lives including dentistry. Dental practice not only has a huge impact on the environment due to metallic unwanted waste generated in various procedures but also due to excessive use of water and electricity which has altogether accentuated the move towards “Green dentistry”. Green dentistry is a nature-friendly perspective that reduces the environmental impact of dentistry and creates an amiable surrounding. It deploys the model of four R, i.e., rethink, reduce, reuse, and recycle. Eventually, one gets the best and pocket-friendly quality treatment. Green dental practice focuses on preserving water and energy, reduction of waste, and minimizing the use of hazardous toxins that have a detrimental effect on patients as well as the environment, and promote the user friendly “Green” products.

Being an ingenious environmentally friendly tool, green dentistry has proven to save money and time along with energy conservation and reduction in pollution with the use of the latest techniques and procedures. Green or holistic dentistry, therefore, protects the environment and humanity from the hazards of rapid urbanization, especially in developing countries like India. To conclude, we quote Ray Kroc ”As long you are green, you are growing. As soon you are ripe, you start to rot.” So let us go green today and save Mother Earth from biohazards for a better tomorrow.

”THE FOOD YOU EAT CAN BE EITHER THE SAFEST AND MOST POWERFUL FORM OF MEDICINE OR THE SLOWEST FORM OF POISON.”

Financial support and sponsorship

Nil.

Conflicts of interest

The authors of this manuscript declare that they have no conflicts of interest, real or perceived, financial or nonfinancial in this article.