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Occup Environ Med. 2007 October; 64(10): 715–716.
PMCID: PMC2078394

Occupational exposure in dentistry and miscarriage: authors' reply

We would like to address the comments made by Dr Guzzi and Dr Pigatto, in their letter regarding our paper “Occupational exposure in dentistry and miscarriage”.1 They were concerned about our exposure assessment and policy implications, and pointed out that no monitoring data were used in the assessment of exposure to mercury. In a retrospective case‐control study such as ours, it is next to impossible to find environmental or biological monitoring data to cover relevant time windows for all the study subjects and for all the exposures. Therefore we assessed exposure to mercury based on the self‐reported number of amalgam fillings made over a one‐week time period. We also took into account exposure to mercury during removal of old amalgam fillings by determining their frequency. The number of amalgam fillings placed per week has been shown to be associated with urinary mercury concentration in dental personnel.2

There is also monitoring data available on mercury exposure among Finnish dental workers, as indicated in the discussion of our paper. The concentrations of inorganic mercury in blood measured during the study period remained below the reference value for the unexposed (25 nmol/l). The median concentration among the dental nurses was 6.0 nmol/l (range 1.8–17.4, n = 26) and among dentists 6.9 nmol/l (range 4.0–20.2, n = 23). All the concentrations of mercury vapour in the air of dental offices in 1994–2004 (range 3.0–5.9 μg/m3, n = 10) were also far below the current occupational exposure limit (50 μg/m3) in Finland.3 Thus both environmental and biological measurements indicate a low level of exposure in Finnish dental offices.

In addition, we feel that dental personnel are quite capable of correctly reporting the number of their amalgam fillings, although it may be more difficult for the pharmacists and healthcare secretaries. We believe that we have successfully been able to separate those with no fillings from those with a high number of fillings.

Drs Guzzi and Pigatto consider it surprising that we do not mention that the fetus may be exposed to mercury vapour. However, the potential exposure of the worker and the fetus was in fact our main reason for examining the association between mercury exposure and spontaneous abortion. The writers seem to have missed the part in our introduction saying “it has been shown that inorganic mercury accumulates in the placenta and a substantial fraction of maternal blood mercury reaches the foetus”.4

Consistent with our findings, most earlier studies among dentists and dental nurses have shown no clear association between mercury exposure and reproductive disorders.5,6,7,8,9 In addition, a recent study by Hujoel et al showed no evidence that mercury‐containing dental fillings placed during pregnancy increased low birth weight risk.10 Occupational studies have been carried out in the Nordic countries (Denmark, Finland, Norway and Sweden) and the US. Thus, their results cannot necessarily be generalised to working conditions in countries where exposure levels are clearly higher.

Based on our findings and other available evidence, we stand by our policy implication: in general, there is no need to restrict work in dental clinics during pregnancy. If the exposure level exceeds the reference limit for non‐exposed (inorganic mercury in blood 25 nmol/l or in urine 50 nmol/l in Finland), exposure of a pregnant worker should be prevented. We would also like to emphasise our second policy implication: it is important to conform to good occupational hygiene in dental work during pregnancy.

Footnotes

Competing interests: None declared.

References

1. Lindbohm M ‐ L, Ylöstalo P, Sallmén M. et al Occupational exposure in dentistry and miscarriage. Occup Environ Med 2007. 64127–133.133 [PMC free article] [PubMed]
2. Jokstad A. Mercury excretion and occupational exposure of dental personnel. Community Dent Oral Epidemiol 1990. 18143–148.148 [PubMed]
3. Hakala E. Elohopea (Mercury). In: Kemikaalit ja työ (Chemicals and work). Työterveyslaitos, Helsinki 2005. 69–72.72
4. Ask K, Åkesson A, Berglund M. et al Inorganic mercury and methylmercury in placentas of Swedish women. Environ Health Perspect 2002. 110523–526.526 [PMC free article] [PubMed]
5. Heidam L Z. Spontaneous abortions among dental assistants, factory workers, painters, and gardening workers: a follow up study. J Epidemiol Community Health 1984. 38149–155.155 [PMC free article] [PubMed]
6. Brodsky J B, Cohen E N, Whitcher C. et al Occupational exposure to mercury in dentistry and pregnancy outcome. J Am Dent Assoc 1985. 111779–780.780 [PubMed]
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8. Dahl J E, Sundby J, Hensten‐Pettersen A. et al Dental workplace exposure and effect on fertility. Scand J Work Environ Health 1999. 25285–290.290 [PubMed]
9. Dahl J E, Sundby J. Risiko for spontanabort blant norske, forstegangsfodende tannleger (Risk of spontaneous abortion among primiparous dental surgeons in Norway). Norsk Epidemiologi (Nor J Epidemiol) 1999. 951–55.55
10. Hujoel P P, Lydon‐Rochelle M, Bollen A M. et al Mercury exposure from dental filling placement during pregnancy and low birth weight risk. Am J Epidemiol 2005. 161734–740.740 [PubMed]

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