Biological monitoring and exposure to mercury

Occupational health professionals' interest in controlling mercury (Hg) exposure, and the use of biological monitoring in this context, has been ongoing for a number of years. Evidence from urinary Hg results in a number of UK firms who have undertaken some form of biological monitoring or occupational health surveillance suggest that exposure has decreased over the last 10-15 years. This decrease precedes the establishment in the UK of an advisory biological monitoring guidance value (HGV) for urinary Hg and the production of updated medical guidance from the Health & Safety Executive on Hg exposure (MS12 1996). This latter document recommends a urinary sampling interval for urinary Hg of between 1 and 3 months, which is consistent with the reported toxicokinetics of Hg excretion, but we highlight that urinary Hg represents integrated exposure over many previous months. Mercury is a recognized nephrotoxin and MS12 1996 mentions the use of regular dipstick protein estimations. We review our experience of investigating proteinuria and enzymuria in a large-scale cross-sectional occupational study. The incidence of Hg-induced renal disease is probably very rare at current exposure levels. Therefore acceptance of a high false-positive rate of proteinuria not related to Hg exposure needs to be considered in any urinary protein testing regime of Hg workers. The establishment of an HGV for urinary Hg has raised questions about the uncertainty associated with a urinary Hg result, including factors such as diurnal variation, whether urine correction by creatinine or specific gravity is preferable and the possibility of non-occupational sources of Hg contributing significantly towards breaching the HGV. Correction of urinary Hg results by creatinine or specific gravity and the use of a fixed sampling time, such as the beginning or end of the day, substantially reduce the uncertainty in a urinary Hg measurement. But even with good laboratory precision, an individual with a true urinary Hg excretion of 20 nmol/mmol creatinine could supply urine samples of between 14 and 26 nmol/mmol creatinine. The influence of dietary sources in the UK contributing to urinary Hg values approaching or exceeding the HGV is unlikely. The use of tribal or ethnic cosmetics and remedies needs to be considered if a urinary Hg result looks inappropriately high, as some such preparations have been found to contain Hg and can be absorbed through the skin. The ability of excessive chewers or teeth grinders who have a large number of dental amalgam fillings to breach the urinary HGV in the absence of substantial occupational Hg exposure has been reported in a few Scandanavian studies. We report here a likely case of this phenomenon. Since the establishment of the HGV, our biological monitoring Hg data from a number of industry sectors using inorganic or metallic Hg have suggested that a minority of samples (13%) are still greater than the HGV.