Assessment of the environment and health impacts arising from mercury-free dental restorative materials
A lot is known about the behaviour of mercury-free dental materials in the oral cavity and their limitations clinically (Alexander et al., 2014) but we are less informed about their behaviour in the environment (Chen et al., 2000; Erdal and Orris, 2012). The Minimata Convention on Mercury (UNEP, 2013) is a global treaty, with the objective to protect human health and the environment from anthropogenic emissions and releases of mercury and mercury compounds (UNEP, 2013). The convention becomes legally binding for all its parties from 16 August 2017. It seeks to reduce the amount of mercury in use. The Convention is relevant to dentistry and oral health in respect of the necessity for the phase down of mercury use in a number of products and processes. Mercury in dentistry is a relatively small contributor with proportion of the amount of the mercury mined estimated at 3-4% (Chin et al., 2000). However, the way in which it is used in dentistry can lead to its release into the environment either through waste water from dental clinics where amalgam (silver), (mercury containing) fillings are used or through cremation of bodies with amalgam fillings. It is imperative that the environmental impact of mercury-free dental materials is addressed so that one environmental concern is resolved but another arises. Therefore, with the Minimata Convention (UNEP, 2013) soon to be adopted through EU Regulation (European Parliament and Council, 2017).
Alternatives to amalgam fillings have been developed and have evolved over the last four decades with the application of innovative new chemicals and substances. From an emerging risks perspective, we need to be vigilant towards these new chemicals and substances (EPA, 2016) and ensure that we do not replace one environmental concern with another. The mercury-free dental materials can be grouped under the broad headings of:
- Resin composite
- Glass ionomer
- Resin modified glass ionomer
- Polyacid modified resin composite
Often collectively referred to as ‘white fillings’ by the lay public. While tooth-coloured restorative materials are generally more expensive than amalgam, they offer an aesthetic alternative to traditional amalgam fillings. However, there are concerns about their longevity and wear particularly in areas subjected to masticatory forces (WHO, 2009), microhardness and polymerisation (Sharkey et al., 2001).
To date much of the concern for the environmental impact of resin composites has centred on the probable toxicity of uncured resin and the use of a derivative of bisphenol-A; BisGMA in resin composites. The most commonly used resin system is the result of the combination of epoxy and vinyl chemistry, where two moles of glycidyl methacrylate are reacted with one mole of bisphenol-A to yield Bowen’s Resin Bis-GMA. The molecule (which features a number of isomers) contains a terminal vinyl group at each end and may be polymerised quickly, via a free-radical mechanism, to yield a cross-linked polymer. Bis-GMA is an extremely viscous liquid which needs to be thinned, before processing into commercial products, with diluting co-monomers such as triethyleneglycoldimethacryate (TEGDMA) and/or bisphenol-A dimethacrylate (Bis-DMA) being used. What we don’t know is how these materials behave when, excess is evacuated through the air and water systems or when removed from the oral cavity (Schmalx and Galler, 2017). Removal from the oral cavity may occur when fillings need to be replaced, because of clinical failure or the recurrence tooth decay. The filler in both composite and glass ionomer is frequently an alumina silicate or quartz and the acid used in glass ionomers a polyacrylic acid.
The planned research will be conducted in three Workpackages, where the OHSRC will lead on Workpackages 1 and 3, these Workpackages will centre on reviews of the literature and engaging with dental practitioners and with industry that supports the practice of dentistry. The ERI/BEES will lead Workpackage 2, which is focussed on the scientific determination of particulate matter in waste water streams, initial toxicity screening of waste streams.