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Mercury and dental teams
Mercury in Dentistry - Still a Problem
The production of metallic mercury is limited to about ten thousand tonnes each year, worldwide. Estimates of the amount used in dentistry suggest that about 150 tonnes are used in dental restorations each year, the average dentist using 2 or 3 pounds (1 to 1.5 kg) annually. This seems insignificant compared with up to 150 kilotonnes released each year into the biosphere by degassing of the earth's crust and by burning fossil fuels.
However, in the confined space of a contaminated dental surgery, the comparatively low partial pressure of free mercury means that at room temperature, saturation of air with mercury vapour can theoretically give rise to levels of 20 mg per cubic metre. This is a massive 400 times the recommended time-limited value (TLV) of 0.05 mg per cubic metre proposed by the World Health Organisation for occupational exposure. The TLV is the theoretical amount to which an adult can be occupationally exposed during an 8 hour day without supposed adverse effects on health.
By contrast, the normal atmospheric level of mercury vapour has been measured as between 1 and 4 ng (0.000001 to 0.000004 mg) per cubic metre and is the result of natural processes combined with pollutant emission and, for example, the release of mercury from dental fillings by cremation. It is easy to see, therefore, that unchecked mercury contamination of the dental surgery can theoretically give rise to vapour levels well in excess of the accepted working safety limit. In surveys of mercury concentrations in the atmosphere of dental practices, it has been established that at least 10 percent of surgeries have vapour concentrations greater than 0.05 mg per cubic meter and the occupants are therefore at risk of mercury toxicity. But how sure are we of the TLV guidelines ?
How Safe is 'Safe' ?
In 1992, a researcher in Singapore determined the mercury vapour level in the surgeries of 98 dentists in practice for an average of 5.5 years, and whom he then subjected to a battery of psychomotor and neurological tests. The average mercury vapour level was 0.014mg per cubic metre - about one-third of the TLV - however, the dentists scored an average of 14% worse in the tests than unexposed control subjects. The unavoidable conclusion of this important study is that even at one-third of the TLV for mercury vapour, measurable impairment of key psychomotor processes was demonstrated.
The wide variety of health problems associated with chronic exposure to mercury vapour are listed on another page of this site. Of all the symptoms listed, the most readily induced appears to be that of tremor, which has been reported to occur at vapour concentrations in excess of 0.1 mg per cubic meter, that is, twice the TLV. 'Tremor' in this context is defined as the inability to hold still a 15lb weight. It is worrying that so many of the symptoms might be confused with 'stress', which affects most dentists to some extent. How can we be sure that this is not actually mercury toxicity ?
The Future of Mercury Amalgam as a Filling Material
Mercury amalgam has been used in dental restorations since the 1830's and despite occasional debate it remains the most commonly-used restoration material, notwithstanding the growing popularity of more natural-looking polymer material for use in anterior teeth. Much has been made in the last ten years, especially in the 'popular health' press, of cases in which an allergy to amalgam fillings has been alleged to have caused serious health problems. In the individuals affected, it was considered necessary to replace all their mercury-containing restorations with composite and the reported results have been dramatic.
The safety of mercury-based amalgam restorations in teeth has been questioned from time to time; the earliest critiques in the literature are papers by a German chemist called Alfred Stock, published in the 1930s. As a result of Stock's work, a special clinic was set up in Berlin to investigate amalgam safety.
Amalgam fillings do slowly release minute quantities of mercury into saliva as they slowly corrode, and thence into the body (mercury vapour is detectable in the breath of persons with amalgam fillings). It has also been demonstrated that tooth grinding (bruxing) increases mercury release from the fillings; the concentration of mercury vapour in the mouth can exceed the TLV when chewing gum is used. These effects are transitory, however, except when the subject is a heavy user of gum and has more than six fillings, when mercury vapour in the oral atmosphere can exceed the TLV for several hours.
These doses are, in most cases, very small and are comparable with the intake of mercury from food. Nevertheless, the evidence that in some individuals these amounts of free mercury can compromise health is becoming more abundant. In the USA late in 1991, a specialist review panel of the Food and Drug Administration dismissed calls for a ban on dental amalgam, stating that there was insufficient evidence linking any specific disease with mercury from dental fillings and in Britain, the British Dental Association has continued to support the placement of mercury amalgam restorations.
National guidelines on mercury amalgam use vary considerably:
- Britain: the British Dental Association supports continued use of amalgam
- USA: the Food and Drug Administration pronounces mercury amalgams as having 'Generally Regarded as Safe' status
- Sweden: placement of amalgams to be phased out by 1997
- Austria: aims to be amalgam-free by year 2000
- Germany: the German dental association recommends alternatives for children under six, pregnant women and kidney patients and bans production of gamma-2 amalgam, which is more susceptible to corrosion in the mouth
- World Health Organisation: TLV values to be reviewed; a statement has been issued that 'no amount of exposure to mercury vapour can be considered totally harmless'
Do Amalgam Fillings Damage Health ?
However in the medical literature since 1990, there has been a steady stream of reported case histories from patients with a wide variety of symptoms ranging from insomnia to multiple sclerosis, all of whom have experience dramatic improvement in their condition once all mercury- containing fillings had been removed. This assertion is actually illogical, since the removal of multiple amalgam restorations in a short time causes a distinct pulse of mercury loading to the body, persisting for some weeks; under these conditions, one would expect the symptoms to worsen.
Parallel with these case histories have been a series of papers demonstrating that mercury can indeed interfere with the immune system and its components, giving credibility to the concept of the 'mercury sensitised' patient with classical allergy symptoms to mercury. Indeed, such was the public interest in the preventative removal of amalgams for health reasons, that when the Princess of Wales underwent this operation, it made the newspaper headlines in 1992.
In the last year, evidence has been published showing that true mercury sensitivity does occur in genetically-susceptible people, who exhibit immune system-modulated diseases, the severity of which can be altered by the mercury burden of the patient. These people are quite rare - perhaps one in every 50,000 of the population, but they respond to classical patch-testing tests using mercury compounds and frequently show cross-reactivity to other heavy metals such as cadmium.
Many dentists now offer amalgam replacement programmes for patients and guidelines are available from organisations such as the IAOMT (International Academy for Oral Medicine and Toxicology) for procedures to be followed during amalgam removal in order to avoid the pulse of mercury exposure which normally accompanies restoration removal. Typically, this will involve the following:
- nutritional support with high-dose antioxidant nutrients in the three months leading up to removal and for six months afterwards
- routine use of rubber dam
- separate air supply for patient, delivered by nose mask
- protective clothing for the patient
- use of high volume suction and on-tooth air scavengers
Mercury Sensitisation and Accumulation in Dentists
Dentists, too, can be among these people affected by mercury sensitisation. There have been many documented cases of contact dermatitis among dentists sensitised to mercury, but these are perhaps minor issues compared to the serious health risks of chronic occupational mercury exposure throughout the profession as a whole.
Evidence published in 1989 suggests that mercury is able to travel easily from the oronasal cavity to the floor of the cranial cavity, where it is concentrated into the pituitary gland. The pathway is thought to be the olfactory nerves or the cranial venous system; these bypass the detoxifying effect of the liver which ameliorates the effect of lung and gut doses of mercury. Pituitary glands taken post-mortem from Swedish dentists showed that the mercury level in the pituitary gland was, on average, 2.5 times (and in one case 169 times) that in the brain cortex.
Measuring the Mercury Burden in Dentists - Urine Monitoring
Regular monitoring of all hazardous materials is an accepted part of working practice under the Control of Substances Hazardous to Health Regulations (COSHH) 1988. In the absence of overt signs and symptoms of mercury toxicity in dental staff, a biochemical test which assesses the degree of exposure is required. Since tissue sampling is obviously not possible, and remote detection by, for example, x-ray fluorescence is not widely available, analysts are restricted to a limited range of biological samples.
Mercury levels in the blood do reflect the amount absorbed; however, the metal is quite rapidly cleared into the tissues (the half-life is 3 days) and one sample may not be representative of periodic exposure. Hair and nail tissue are quite good indicators of exposure to the organic forms of mercury such as methyl and ethyl mercury, but do not accumulate inorganic mercury sufficiently to form the basis of an accurate analytical method.
Mercury is however excreted in the urine in amounts which accurately mirror the total body burden; the long half-life in the body tissues (90 days) effectively smoothes out differences in day-to-day exposure. Since the amount of urine produced varies through the day, a standardisation process is needed for accuracy. Some analysts have opted for 24 hour urine collections, which tend to be impractical for the busy professional. Others correlate the mercury content of the urine to a standard specific gravity to compensate for the effects of dilution; sometimes it is linked to the creatinine content, which is relatively constant.
Air Sampling Methods
Measurement of the concentration of mercury vapour in the air of dental practices has been carried out in several surveys published in the literature. The simplest method uses a chemically-impregnated paper disk which is exposed to the practice air for a period then 'developed' to assess the exposure risk. This system has limited use in potentially exposing a health risk to their staff through elevated mercury vapour levels. While it can be useful in relating the practice exposure risk to the theoretical time-limited value (TLV), the method excludes mercury absorbed via the skin and by inhalation of aerosol material by the dentist; two major routes of absorption.
For those practices where personnel are found to have a high mercury burden, air sampling can help identify the source of the contamination. This is usually carried out by sophisticated 'sniffer' apparatus operated for 24 hours in various locations in the surgery during a working day. Mercury vapour is adsorbed on to a gold 'element' according to the concentration and flow rate. At the end of the fixed exposure time, trapped mercury is driven off by heating and assayed by atomic absorption spectrophotometry. Hire rates for this type of equipment are usually prohibitive for general practice, at around £600 per session.