Mercury Exposure from Dental Amalgams: A Toxicological View & The Precautionary Principle

Toxicological Assessment of Daily Mercury Exposure from Dental Amalgams

Dental amalgam fillings, which are approximately 50% elemental mercury (Hg⁰) by weight, continuously release mercury vapor. The amount released and its subsequent impact can vary significantly.

Daily Mercury Vapor Release from Amalgam Fillings

The quantity of mercury vapor released is influenced by several factors:

• Number, size, and age of amalgam fillings.
• Mechanical stress from chewing and bruxism (teeth grinding).
• Temperature changes from hot foods or liquids.
• Oral hygiene practices like brushing.
• Potential galvanic corrosion from other metals in the mouth.

Quantitative Estimates of Daily Release/Intake

Scientific literature provides varying estimates for the daily amount of mercury vapor released and absorbed:

• General estimates range from 1 to 27 micrograms (µg) of mercury per day for individuals with multiple amalgam fillings. Many studies suggest an average of around 3 to 7 µg/day.
• The World Health Organization (WHO) estimated in 1991 an average daily intake of about 10 µg/day (range 3-17 µg/day) for a person with an average number of fillings.
• More recent reviews often cite figures in the range of 1-10 µg/day for the average person with amalgams, though this can be higher.

Absorption and Fate of Inhaled Mercury Vapor

• High Absorption: Approximately 80% of inhaled elemental mercury vapor (Hg⁰) is absorbed into the bloodstream via the lungs.
• Distribution: Being lipid-soluble, elemental mercury readily distributes throughout the body, importantly crossing the blood-brain barrier and the placenta.
• Oxidation and Retention: In tissues, particularly the brain, Hg⁰ is oxidized to its inorganic divalent form (Hg²⁺). This ionic form is less mobile across biological membranes (potentially becoming "trapped" in the brain) and is excreted more slowly. Hg²⁺ is primarily responsible for cellular toxicity.
• Accumulation: Due to slow excretion and continuous release, mercury can accumulate over time, especially in the kidneys, brain, thyroid, and pituitary gland.
• Long Half-life: The biological half-life of inorganic mercury in the brain can be exceptionally long, estimated from several years to decades. In blood, it's around 60-90 days.

Toxicological Impact Considerations

Assessing the impact involves comparing estimated intakes to reference levels and understanding mercury's known effects.

1. Reference Exposure Levels (RELs) / Tolerable Daily Intakes (TDIs)

Various agencies set guidelines, though they vary and have specific contexts:

• ATSDR (Agency for Toxic Substances and Disease Registry): Chronic inhalation Minimal Risk Level (MRL) for elemental mercury vapor is 0.2 µg/m³.
• EPA (U.S. Environmental Protection Agency): Reference Dose (RfD) for methylmercury (in fish) is 0.1 µg/kg body weight per day (e.g., 7 µg/day for a 70kg adult). While for a different form of mercury, this is a highly protective benchmark.
• JECFA (Joint FAO/WHO Expert Committee on Food Additives): Provisional Tolerable Weekly Intake (PTWI) for inorganic mercury (excluding methylmercury) was 4 µg/kg body weight, roughly 0.57 µg/kg/day (approx. 40 µg/day for a 70kg adult). This is for *ingested* inorganic mercury, which has much lower absorption than inhaled vapor.

2. Potential Daily Absorbed Dose from Amalgams

Using an estimated release of 5 µg/day from amalgams with 80% absorption: 4 µg/day absorbed dose (0.057 µg/kg/day for a 70kg adult).

Using a higher common estimate of 15 µg/day released with 80% absorption: 12 µg/day absorbed dose (0.17 µg/kg/day for a 70kg adult).

The lower estimate is below the EPA RfD for methylmercury, while the higher estimate exceeds it. It is critical to remember the EPA RfD is for methylmercury and aims to protect the most sensitive developmental endpoints.

3. Key Toxicological Factors

• Chronic, Continuous Exposure: Dental amalgams represent a long-term, continuous internal source of mercury.
• Individual Susceptibility: Genetic differences in detoxification (e.g., GSTM1, MTHFR), nutritional status (selenium, zinc, glutathione levels), and overall health (kidney/liver function, existing toxic burden) significantly influence how an individual handles mercury. What might be tolerated by one person could be detrimental to another.
• Neurotoxicity: The brain is a primary target. Chronic low-level exposure is linked to fatigue, memory issues, concentration difficulties ("brain fog"), mood changes (anxiety, depression, irritability), tremors, headaches, and insomnia. These symptoms can be subtle and often misattributed.
• Nephrotoxicity: Kidneys accumulate mercury, and chronic exposure can lead to damage.
• Immunotoxicity: Mercury can alter immune function, potentially leading to immunosuppression or autoimmune reactions in susceptible individuals.
• Endocrine Disruption: The thyroid and adrenal glands can be affected, potentially leading to hormonal imbalances.
• No Established "Safe" Level for All: For potent, bioaccumulative neurotoxins like mercury, many toxicologists argue against a universal "safe" threshold, as effects can occur at very low levels in sensitive individuals or during critical developmental windows.
• Body Burden vs. Excretion Markers: Standard tests like urine or blood mercury may not fully reflect the accumulated burden in tissues like the brain. Autopsy studies have shown higher mercury in tissues of those with amalgams.

Equation for Estimating Brain Concentration (Conceptual)

While complex, the concentration of mercury in the brain ($C_{brain}$) can be conceptually understood as proportional to several factors:

Cbrain ∝ (Daily Absorption Rate × Exposure Duration × Brain Partition Coefficient) / (Brain Elimination Rate + Brain Oxidation Rate to Hg²⁺)

This highlights the importance of the absorption rate, how long fillings are present, how readily mercury enters brain tissue, and how slowly it is removed or becomes trapped after oxidation.

Conclusion of Toxicological Assessment

The daily mercury intake from dental amalgams can contribute a measurable, and for some, significant amount to their total body burden. While many may not show overt clinical symptoms, a subset of the population—due to factors like a high number of fillings, genetic predispositions, compromised health, or poor detoxification capacity—could experience or have existing health issues exacerbated by this chronic low-level exposure. The ongoing scientific discussion and evolving regulatory advisories for vulnerable groups underscore the need for individualized consideration of these risks.

The Precautionary Principle and Mercury Exposure

The precautionary principle is a strategy for approaching issues where there is scientific uncertainty about potential harm from an activity or substance. It advocates for taking preventive action in the face of such uncertainty, especially when the potential harm is significant.

Core Tenets of the Precautionary Principle Applied to Mercury

• Taking Preventive Action in the Face of Uncertainty (and Known Harm): Mercury is unequivocally a potent toxin. While debates exist about "safe" low-dose chronic exposure levels for *everyone*, the known serious harms (neurotoxicity, nephrotoxicity, etc.) and the uncertainties regarding individual sensitivities trigger the precautionary approach. It argues for minimizing exposure rather than waiting for absolute proof of harm in every context.
• Shifting the Burden of Proof: Instead of requiring definitive proof of harm before acting, the precautionary principle can suggest that those proposing or continuing an activity involving a potentially harmful substance (like mercury in dental products) should demonstrate its safety, especially when effective alternatives exist. For dental amalgam, given available mercury-free options, the continued use of a mercury-based product faces a high burden of justification under this principle.
• Exploring a Wide Range of Alternatives: The principle mandates thorough exploration and preference for safer alternatives. Dentistry has numerous effective mercury-free restorative materials (composite resins, ceramics, glass ionomers), strengthening the argument for phasing out mercury-containing amalgams.
• Increasing Public Participation in Decision Making: Transparency and public involvement in decisions about potential health risks are key. Full disclosure about mercury in dental fillings, along with information about potential risks and alternatives, empowers individuals to make informed choices, aligning with a precautionary stance.

Specific Considerations for Mercury that Reinforce Precaution

• Bioaccumulation and Persistence: Mercury builds up in the body over time and persists in the environment. Even seemingly low daily exposures can lead to a significant cumulative body burden.
• Vulnerability of Specific Populations: Developing fetuses, infants, and children are exceptionally susceptible to mercury's neurotoxic effects. The precautionary principle strongly advocates for their protection from any avoidable mercury exposure. This is recognized in FDA advisories and EU restrictions.
• Potential for Irreversible Effects: Some neurological damage from mercury can be permanent. The risk of serious, irreversible harm is a critical factor for invoking precautionary measures.

Conclusion: "No Mercury is Better Than Any Mercury"

From the perspective of the precautionary principle, when a substance like mercury is a known potent toxin, bioaccumulates, poses particular risks to vulnerable groups, has the potential for irreversible effects, and has viable, safer alternatives for common applications (such as dental fillings), the most health-protective and ethically responsible approach is to avoid its intentional introduction into the human body. Therefore, the precautionary principle strongly supports the ideal that no avoidable mercury exposure is better than any mercury exposure at all.