Report No FR/D0020



March 1995


With the advent of the 1990 Environmental Protection Act and the subsequent application of the Integrated Pollution Control (IPC) regulations, releases of heavy metals to the environment have been significantly reduced. In particular, mercury emissions from industrial processes have been reduced, so that the contribution from dental premises has become more significant.

Historically, process emissions of mercury from the chlor-alkali industry were in excess of 50 tonnes/annum. Since IPC has come into force, emissions from this industrial sector have been much reduced to maybe around 25 tonnes/annum. In this study mercury emissions from dental premises to sewer have been estimated at between 5.3 and 7 tonnes per annum.

The contractors investigated the current UK situation according to the study brief and within the short timescale set. They contacted a wide cross-section of companies, dentists' trade and professional organisations and received a large amount of informed comment, but relatively little hard data.

The investigations confirmed the difficulties of reliably estimating mercury consumption and releases from dental premises in the UK encountered in previous studies. In addition, key companies active in amalgam material supply did not cooperate with the contractors. Therefore the main deliverable from this study is a revised assessment of mercury use and release based largely on refined estimates derived from the contractors' investigations. The contractors are sure that industry statistics exist, but these data were not released to them.

Based on interviews with dentists, and limited supply data, it is concluded that previous studies have overestimated the mass of mercury in restoration materials. A 'typical' mercury consumption is two spills, equivalent to 600 mg mercury per filling. This value considerably reduces the mercury consumption and potential release from dental premises, but should be confirmed by more detailed investigations.

Approximately 40 companies were contacted to obtain supply-side data. Insufficient data were provided to estimate quantities supplied to the UK dental trade. However, from marketing information supplied by one company, it was possible to produce some estimates of quantities involved; these are at best only indicative. Problems of double-counting, whereby one retail company supplies another, and the fact that many buy by mail-order from overseas further increase the inaccuracies in any estimates.

Scrap amalgam is collected from dental premises by companies or one-man bands. Indeed, dentists are alleged to be continually asked about the possibilities of collecting amalgam. The waste status of the scrap amalgam is not clear: ie. is the material special waste; does the duty of care apply; should collectors be licensed?

The contractors concluded that collectors are primarily interested in the silver content of clean scrap amalgam from the new amalgam preparation, rather than odorous, infectious amalgam debris recovered from suction system filters and traps. It is suspected that some amalgam debris collected from points after the spittoon ultimately reaches the sewer. This study has estimated the quantity of amalgam used annually in dentistry in the UK in a range from 19.2 tonnes up to a maximum of 30 tonnes. Recycled amalgam has been found to be approximately 9 tonnes/year. The contractors estimated that potentially between 5.3 and 7 tonnes of mercury are released to sewer from dental premises each year.

Amalgam is a dense material which sediments readily in pipes and traps forming a viscous matrix with organic debris and bacteria. Considerable quantities of amalgam deposition are known to occur in suction lines and wastewater pipes requiring unblocking by dental technicians, plumbers or drain contractors. Therefore pipelines can act as amalgam accumulators, reducing continuous particulate amalgam release, but leading to intermittent shock loads on emptying and a potential leaching of mercury ions. These amalgam deposits may continually leach mercury especially if aggressive disinfectants and cleaning agents are used in the surgery. There appears to be potential for biotransformation into organic mercury although substantiating data do not exist. The finest material, which accounts for the largest proportion of a sample by mass, is most easily transported and also has the highest specific surface area. It is therefore most liable to ionisation of mercury under favourable conditions.

It is clear that good methods exist for reducing mercury pollution from dentistry by >95% of the unabated situation quantified here. Effective and publicly acceptable methods of recovering the spent amalgam and recycling its mercury and other metal contents are at least as important as the separation technology installed at the surgery.

Possible future studies are suggested to refine the estimates made and to decrease the uncertainties encountered. The most beneficial are: a survey of dentists, monitoring of releases from a number of surgeries and an in vitro study of mercury solution chemistry and hence releases from amalgam. The cost of these studies has been estimated in the range 100 000 to 120 000.

Copies of the report are available from FWR, price 25.00, less 20% to FWR Members.