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Weighing new evidence against potential neurotoxicity
The new behavioural study among dentists by Ritchie et al, reported in this issue of Occupational and Environmental Medicine, found no adverse effects associated with low exposure to mercury in urine, hair, or nails, that assess subjective symptoms as well as more objective measures of psychomotor performance. The new study laudably addresses the World Health Organisation's request for chronic Hgo exposure studies that are useful in defining a threshold of effect below 25 μg/l and are not limited by insufficient statistical power and inconsistent measures of exposure and outcome.1 The study highlights the potential for neurotoxicity from low exposure to Hgo among dentists who work with amalgam restorations containing 50% Hgo, and stimulates discussion on methodological issues in study design that might increase sensitivity to detect adverse effects.
Though few associations were noted, the negative results must be weighed with positive evidence for a lower threshold even applicable to the general population, because dentists incur exposures that are comparable to that in the general population. For example, in a US national sample of 6925 dentists participating in the American Dental Association Health Screen Program (1990–96), the distribution of urinary Hg was skewed; 90% had concentrations under 6.0 μg/l.2 In a Washington State sample of 2196 dentists (1998–2000), the mean urinary concentration was 2.5 μg/l (range 0–67). The new study also reported mean urinary concentrations of 2.56 μg/l that agree with previous reports. It is noteworthy that two estimates among the general population are comparable to that of dentists with broad ranges (0–34 and 1–18 μg/l) and similar mean urinary concentrations of 3.1 and 9.0 μg/l respectively.
The study also undertook a more comprehensive evaluation of exposure …