Comparison of hair, nails and urine for biological monitoring of low level inorganic mercury exposure in dental workers

Abstract

Creatinine-corrected urine mercury measurements in spot urine samples are routinely used in monitoring workers exposed to inorganic mercury. However, mercury measurement in other non-invasive biological material has been used in some epidemiological studies. Dentists and dental nurses remain a group of workers with potential exposure to inorganic mercury through their handling of mercury-containing amalgam, although changes in work practices have reduced the current, likely exposure to mercury. Therefore, dental workers remain an occupational cohort in whom the value of using different biological media to identify exposure to low level inorganic mercury can be investigated. Samples of head hair, pubic hair, fingernails, toenails and urine were analysed for mercury content from a cohort of UK dentists (n=167) and a socioeconomically similar reference population (n=68) in whom any mercury exposure was primarily through diet. The mercury content in all biological material was significantly higher in the dental workers than in the control population (p<0.0001). The geometric mean and 90th percentile mercury concentrations in the urine samples from dentists were 1.7 and 7.3 micromol mol(-1) creatinine, respectively, with only one sample having a value at around the UK's Health and Safety Executive biological monitoring health guidance level of 20 micromol mol(-1) creatinine. Receiver operator characteristic analyses suggested that the ability of the biological material to discriminate between dentists and referents were fingernails>urine approximately equal to toenails>pubic hair approximately equal to head hair. Further investigation is warranted as to why fingernails appear to be such a good discriminator, possibly reflecting some contribution of direct finger contact with amalgam or contaminated surfaces rather than systemic incorporation of mercury into growing nails. Good correlation between head hair and pubic hair mercury levels in all subjects was obtained (r=0.832), which was significantly improved when hair samples weighing <10 mg were excluded (r=0.868). Therefore, under these exposure conditions and using the described pre-analytical washing steps, there is little influence from atmospheric contamination on the level of mercury content of head hair. The choice of non-invasive biological materials for mercury analysis depends on a number of considerations. These include the toxicokinetics of urinary mercury excretion, the growth rates of hair and nail, the nature and time-frame of exposure, and the fact that urine mercury may not reflect the body burden level from dietary methyl mercury. However, the data from this study suggests that urine mercury remains the most practical and sensitive means of monitoring low level occupational exposure to inorganic mercury.

Scientific Opinion on the risk for public health related to the presence of mercury and methylmercury in food

“…Inorganic mercury in urine has been reported not to be strongly associated with fish consumption whereas it is strongly associated with dental amalgam fillings and occupational inorganic/elemental mercury exposure (Morton et al, 2004). In case of frequent tuna consumption (1 -7 meals per week) (Carta et al, 2003) or high fish consumption (> 4 carnivorous fish meals per week) (Passos et al, 2007) and the absence of occupational inorganic mercury exposure and dental amalgams, urinary total mercury has been related to carnivorous fish consumption.…”

Exposure to Mercury Vapors in Dental Workers in Poland

“…The recent literature data confi rm that Hg-U remains the most practical and sensitive marker of monitoring low-level occupational exposure to inorganic mercury [7]. In view of well established dose-effect and dose-response relationships between Hg-U concentrations and critical health effects, the recommended Hg-U levels are used as the main guideline for workers' health protection.…”

“…To assess low-level occupational mercury exposure of dental staff, urine mercury (Hg-U) concentration was used as an exposure index [4,[6][7][8][9][10][11][12][13][14][15][16]. The recent literature data confi rm that Hg-U remains the most practical and sensitive marker of monitoring low-level occupational exposure to inorganic mercury [7].…”

Comparaison des concentrations de 34 métaux et éléments minéraux dans les ongles des mains et des pieds chez 50 sujets volontaires sains

“…Si les ongles de par leur structure sont moins exposés à ce type de pollution, ils ne sont pas à l'abri de toute contamination principalement au niveau de la cuticule unguéale, surtout au niveau de la main. Ainsi, chez des dentistes exposés aux amalgames dentaires, il a été montré que la concentration moyenne en mercure dans les ongles de la main est plus élevée que dans les ongles du pied (respectivement 1,42 µg/g et 0,43 µg/g), alors que dans le groupe témoin de sujets non exposés les concentrations sont voisines (respectivement 0,24 µg/g et 0,18 µg/g) [2]. C'est la raison pour laquelle il nous a paru utile, en particulier dans un contexte de surveillance professionnelle, d'établir des valeurs normales pour les éléments minéraux dans les ongles de pied.…”