The concern that silver-colored dental restorations, often mistakenly called “silver crowns,” might pose a health risk due to toxicity has persisted for decades. These restorations are made of dental amalgam, not pure silver. Public concern centers on one of amalgam’s main components: mercury. This article examines the scientific evidence regarding amalgam’s composition, how it interacts with the body, and the consensus on its systemic health effects.
Defining the Material: Dental Amalgam
Dental amalgam is a mixture of liquid mercury and a powdered alloy, used to fill cavities for over 150 years. The final material is approximately half liquid mercury by weight, mixed with a powder primarily composed of silver, tin, and copper. Silver often makes up the largest proportion of the alloy powder, giving the restoration its metallic, “silver-colored” appearance.
Tin and copper are included to enhance the material’s strength, corrosion resistance, and setting properties. Mercury binds the alloy particles together, creating a putty-like substance that hardens into a strong, durable, and long-lasting restoration.
How Mercury Vapor is Released
Once dental amalgam has set, a small amount of elemental mercury vapor can be released into the mouth over time through evaporation from the filling’s surface.
The rate of mercury vapor release increases significantly due to mechanical and thermal stresses commonly experienced in the mouth. Actions like chewing, tooth grinding, and toothbrushing cause friction and wear, stimulating vapor release. Temperature fluctuations from consuming hot beverages can also increase the vapor pressure within the amalgam, leading to a temporary spike in release.
When released, this elemental mercury vapor is absorbed primarily through the lungs, where it enters the bloodstream and is distributed throughout the body, including the kidneys and brain. The absorbed amount is low, typically estimated to be between 1 and 3 micrograms per day.
Scientific Findings on Systemic Health Risks
The question of whether low-level mercury exposure from dental amalgam leads to systemic illness has been the subject of extensive scientific review by major health organizations. The consensus among bodies such as the U.S. Food and Drug Administration (FDA) and the American Dental Association (ADA) is that the exposure levels are generally not harmful to the overall health of the general population. Multiple systematic reviews have concluded that there is insufficient evidence to support an association between the mercury released from amalgams and an increase in systemic diseases in the general adult population.
The total absorbed dose of elemental mercury from dental amalgam is considered low, falling well below levels that would cause overt toxic effects. For instance, a person would require an estimated 450 to 530 amalgam surfaces to reach the urinary mercury level associated with the most subtle, pre-clinical effects in sensitive individuals, according to a World Health Organization threshold. The exposure is generally 5 to 30 times lower than occupational exposure limits set for workers in environments handling mercury.
While systemic effects are generally not observed, localized adverse reactions are known, though they are quite rare, occurring in less than 1% of the population. These reactions are typically hypersensitivity or allergic responses to the mercury or other components in the amalgam, often manifesting as oral lichenoid lesions or inflammation of the gums. Removing the restoration usually resolves these symptoms.
Despite the overall positive safety assessment for the majority of people, regulatory bodies advise caution for certain high-risk groups who may be more susceptible to the effects of mercury exposure. The FDA specifically recommends that alternatives be considered for the following groups:
- Children under the age of six.
- Pregnant women or women planning to become pregnant.
- Nursing women.
- Individuals with pre-existing neurological conditions, such as Parkinson’s or Alzheimer’s disease.
- People with impaired kidney function.
These recommendations are based on the principle of minimizing exposure in vulnerable populations, even though a definitive link to adverse health outcomes in these groups has not been firmly established by the science.
Modern Options for Restorations
Several modern materials offer effective alternatives to traditional amalgam for those concerned about mercury or requiring new dental work. The most common choice is composite resin, often called “white fillings” due to their tooth-colored appearance. Composite resins are a mix of plastic resin and ceramic or glass particles cured with light, offering a highly aesthetic solution.
Other non-amalgam options include glass ionomer cements, a combination of acrylic and a glass component. These materials have the added benefit of releasing fluoride, which can help prevent further decay around the filling. For larger restorations, dentists may use porcelain or ceramic, which are extremely durable and blend aesthetically with the natural tooth.
While these alternatives are mercury-free and cosmetically superior, they have considerations regarding cost and durability. Composite resins are more technique-sensitive during placement and may not last as long as amalgam in high-stress areas. Gold alloy restorations also remain a long-lasting and durable option, though they are typically the most expensive.