In toxicology, there is four forms of mercury. These are liquid, vapour, inorganic compound and organic compound.
Surprisingly, elemental mercury has little or no toxic effect when swallowed. Droplets of liquid metal mercury if swallowed are poorly absorbed and most liquid mercury passes harmlessly through the digestive tract.
This is the nasty stuff. Mercury vapour is significantly toxic when inhaled and this accounts for most occupational and accidental exposures. Mercury vapour is rapidly absorbed into the blood from the lungs, and can cross into the brain. It can accumulate over long periods and exists the body slowly. Mercury inhibits enzymes and interferes with the normal functioning of cells. Chronic mercury vapour exposure effects the central nervous system, and increasing concentrations may result in insomnia, irritability, memory loss, headaches, depression and muscle tremors. Diagnosis of mercury intoxication requires urine testing in which 10µg is normal, 100µg is a significant exposure, and 300µg is when symptoms appear. Patients are exposed to levels of mercury vapour well below any level of risk, and dental staff are only at a small risk if mercury is used and handled in a irresponsible manner.
Inorganic mercury compounds
Dental amalgam contains inorganic mercury compounds, for example, silver-mercury. It is of very low toxicity, harmless if swallowed, and excreted effectively. When substances form a compound they behave very differently from their individual components, and other compounds. For example, the mercury compound mercuric chloride is caustic to the gut and is a violent poison (but luckily that’s not found in fillings!) Elemental sodium is a highly reactive and explosive metal, but we eat it every day in the form of sodium chloride (table salt).
Organic Mercury Compounds
Some organic compounds of mercury are highly toxic at low concentrations. None of this type of mercury forms in the mouth from dental amalgam. However, waste amalgam from dental practices, if not using special separators, goes into waste water and sewerage, and out into our waterways. Certain micro-organisms can ‘eat’ mercury and turn it into the harmful organic mercury comounds. Mercury than then become concentrated as it is passed up the food chain, until we eat large contaminated fish and ingest a form a mercury that is harmful to our health. This is one of the main reasons that some countries have phased out the use of amalgam fillings, not due to perceived direct health hazard of ‘mercury’ fillings. Although not yet mandatory in Queensland (it is for Victoria) we at Cornerstone Dental do conduct environmentally responsible dentistry, and have installed amalgam seperators that achieve 99% reduction in waste water mercury content.
Mercury and Amalgam Fillings
Amalgam fillings do release very small quanities of mercury vapour. To understand how that relates, for a person with no amalgam fillings, and a low fish diet, the daily intake of mercury from air, water and food is about 10-20µg. The average daily dose when a patient has a whopping 12 or more amalgam fillings is only increased by up to 3µg. Mercury release has also been measured for certain procedures:
- 1-2µg when mixing amalgam
- 6-8µg placing an amalgam filling
- 44µg dry polishing an amalgam filling (bad practice)
- 2-4µg wet polishing using water spray and suction (good practice)
- 15-20µg removing an amalgam filling using water spray and high speed suction
- 2-4µg 30seconds after the amalgam is removed while suction is present
Threshold levels of mercury exposure are based on industry levels and allow for safety and continuous exposure. It is customary to restrict the use of amalgam in who are perceived to be at-risk groups (children, pregnant women, those with compromised immune systems). The World Health Organisations’ no observed effect threshold level for the general public is 5µg/m3, and 1µg/m3 for risk groups. However, to achieve 1µg/m3 from amalgam fillings you would need to absorb 20µg/day of mercury vapour into your lungs a day and 400µg/day from your saliva/gut. To compare, 27 amalgam fillings would produce a daily inhalation dose of 1.7µg by inhalation and 1.3µg by your gut.
The National Health and Medical Research Council (NHMRC) has released a report, the Working Party on Dental Amalgam and Mercury in Dentistry. It includes medical and scientific experts in the fields of immuniology, toxicology, epidemology and therefore has excellent credibility. The report confirms that amalgam is safe and effective material, though is being used less frequently in modren dentistry by choice.
It summarises: “Dental amalgam continues to be a useful direct restorative material. While low levels of mercury are released and absorbed from dental amalgams, there is no convincing evidence of adverse health effects at these levels with the exception of rare cases of contact hypersensitivity”.
Significant clinical studies on the effects of mercury have concluded that there are no adverse effects of amalgam on general health (Ahlquist et al), no difference in mental function (Saxe et al), and removal of amalgams does not improve general health (Ahlquist et al). Some dental practitioners engage in the removal of amalgam restorations (for profit), and publish papers supporting amalgam removal from diagnostic information and clinical procedures for which there is a lack of scientific evidence. The NHMRC also states that “no evidence exisits to justify the removal of dental amalgam restorations to relieve certain symptoms or treat certain conditions, other than hypersensitivity”.
We also cannot ignore small studies that claim to demonstrate mercury toxicity from amalgam. However, the quality of data available in published reports on the adverse health effects of low level mercury is a matter of concern. A number of studies identified with ‘suggestive’ or positive findings of sub clinical effects are methodologically flawed or have interpretational problems that fail to add a ‘weight-of-evidence’. “Although it is not possible to completely rule out adverse effects in a minority of susepeble patients, it is concluded that there is insufficent evidence to justify the claims that mercury from dental amalgam restorations has an adverse effect on the health of the vast majority of patients” (Eley and Cox, 1988).
A direct allergy to amalgam fillings is rare, and can be identified by a rash or hives on placement of a filling, or a long term lesion directly next to each filling on the cheeks and tongue. This can be confirmed by patch testing by an allergist or dermatologist, and is an indication to replace amalgam fillings. A similar reaction can occur for some people to white fillings also.