Chronic neurobehavioural effects of elemental mercury in dentists.
British Journal of Industrial Medicine (Now published as: Occupational and Environmental Medicine). 1992 Nov;49(11):782-90.
Ngim CH, Foo SC, Boey KW, Jeyaratnam J.
Source: Department of Community, Occupational and Family Medicine, National University of Singapore.
Neurobehavioural tests were performed by 98 dentists (mean age 32, range 24-49) exposed to elemental mercury vapour and 54 controls (mean age 34, range 23-50) with no history of occupational exposure to mercury. The dentists were exposed to an average personal air concentration time weighted average (TWA) of 0.014 (range 0.0007-0.042) mg/m3 for a mean period of 5.5 (range 0.7-24) years and had a mean blood mercury concentration of 9.8 (range 0.6-57) micrograms/l. In neurobehavioural tests measuring motor speed (finger tapping), visual scanning (trail making), visuomotor coordination and concentration (digit symbol), verbal memory (digit span, logical memory delayed recall), visual memory (visual reproduction, immediate and delayed recall), and visuomotor coordination speed (bender-gestalt time), the performance of the dentists was significantly worse than that of the controls. The dentists scored 3.9 to 38.9% (mean 13.9%) worse in these tests. In trail making, digit span, logical memory delayed recall, visual reproduction delayed recall, and bender-gestalt time test scores were more than 10% poorer. In each of the tests in which significant differences were found and in the block design time, the performance decreased as the exposed dose (product of the TWA of air mercury concentrations and the years of exposure) increased.
These results raise the question as to whether the current threshold limit value of 0.050 mg/m3 (TWA) provides adequate protection against adverse effects of mercury.
The organs most frequently affected by metallic mercury in chronically exposed subjects are the nervous system, kidney, and mucosal surfaces of the mouth.' Z The central nervous system is probably the most sensitive organ." Many studies on the neurotoxic of mercury have been reported?" The earlier ones were usually on subjects exposed to high concentrations (more than mg/m') of mercury.' 7 Some of the more recent studies generally lack adequate measurements on exposure to mer~
Neurobehavioural performance tests are sensitive methods used to detect subclinical eiTects."'° The purpose of this study was to measure early changes in performance of motor speed, manual dexterity, visual scanning, and visual and verbal memory among dentists exposed to mercury vapour at concentrations of less than 0-05 mg/In' in their work environment.
Even though the results of the intelligence tests showed that difierence in intelligence between the controls and dentists was not statistically significant, the dentists had a higher least square mean standard score and performed significantly better in the WAIS-R subtest of comprehension. The controls smoked more cigarettes and drank more alcohol; only a few of the dentists smoked cigarettes or drank alcohol. The controls were also more likely to take traditional Chinese medicinal products containing mercury and to have a higher number of dental amalgam fillings.
Dentists are exposed to vibration and physical load at the hands, wrists and arms when using drilling and grinding tools. This may affect motor speed (finger tapping speed) and manual dexterity (grooved peg board time). Furthermore, dentists are probably self selected persons with greater fine movement of the fingers and hands and would be expected to have a better basic performance, before exposure, in finger tapping and grooved peg board tests compared with the controls.
Despite these confounding factors (mostly in favour of the dentists), the performance of the dentists in most of the neurobehavioural tests was significantly worse than their controls anda dose was seen between cumulative dose and performance.
The exposure doses for the dentists were estimated by the product of exposure intensity (TWA personal air exposure concentrations) at the time of survey and the years of exposure, assuming that the exposure was constant throughout. In the present study, all the dental offices surveyed were air conditioned. Some older dentists may have worked in naturally ventilated ofiices and have had diiferent levels of exposure before air conditioning systems were installed. Historical exposure data were not available for dentists in Singapore. The average duration of practice, however, was only 5-5 years with a maximum duration of 24 years. Most dentists surveyed would have begun their practice in air conditioned offices with similar exposure concentations as measured in this study.
The exposure concentrations were low. The air TWA mercury concentrations averaged 0014 mg/m' and the end of exposure blood mercury concentrations averaged 9-8 pg/l. Generally there is a lack of published data relating environmental mercury and blood mercury concentrations around this concentration. Roels er al" published data showing that blood mercury concentrations from 7-5 to 15 pg/l correspond to air mercury concentrations from 0-O10 to 0-020 mg/m'. Our data compare favourably.
The neurobehavioural effects of mercury from earlier studies on workers exposed to air mercury concentrations around or below O-05 mg/m' are inconsistent. Studies by Roels et alf and Langolf et als did not detect any symptoms of chronic mercury poisoning or impaired psychometric tests. Changes in verbal intelligence and short term memory were reported by Piikivi er in a group of chlorine manufacturing plant workers with blood mercury concentrations of 15 pg/1 and an estimated air mercury concentration of O-O25 mg/m'. Soleo er administered the World Health Organisation neurobehavioural test battery to a group of workers at a fluorescent lamp factory and detected changes in short term auditory memory (digit span) in eight workers whose urinary mercury concentrations ranged from 30 to 40 l (blood mercury concentration roughly 18—24 pg/i). Finger tremor was detected by Verberk er al" in 21 workers with a mean urinary mercury concentration of 355 pg/g of creatinine (blood mercury concentration roughly 20 pg/1). In our study, significant differences in performance were found for most neurobehavioural tests carried out between dentists and controls.
A significant increase in aggressive mood measured by the profile of mood states was found between exposed and control groups. This increase in aggressive mood seemed to be related to dose, suggesting possible effects of exposure to mercury vapour on personality.
In the present study, no dentists, based on their current neurobehavioural test performances alone, and the fact that, at the time of study, they did not show any overt neurological and behavioural signs and symptoms on clinical examination or complain of difficulty in performing their jobs, would be classified as suifering from neurological damage or deficits as a result of exposure to mercury vapour in their work. The fall in performance in tests measuring memory and visuomotor performance could, however, be a sign of early damage to the central and peripheral nerves that may lead to presenile dementia and finger tremor if exposure is continued. Detailed neurophysiological study including nerve conduction velocity and evolved potential evaluation would provide further information on the neurological state of the dentists and is recommended.
The effects could not be due to bias in the selection of controls or to chance as they were not only found between the controls and dentists, but also between the high exposed and the low exposed dentists. The differences in digit span, visual reproduction, and Bender-gastalt tests between the dentists and controls could not be fully explained by non-normal data distributions alone as a general trend of gradual performance shift was seen in the test score distributions and there was no evidence that the differences were due to isolated extreme exposed dentists. The results of statistical analysis on the digit span and visual reproduction using cumulative logit models supported these findings. The effects were also not likely due to confounding factors as most of the important potential confounders were either adequately matched in survey design or statistically adjusted for during analysis. The effects of using vibrating hand tools by the dentists were not taken into account.
Significant differences in neurobehavioural performance tests were found in subjects exposed for a comparatively short period (5-5 years) and at concentrations (GM 0-014 mg/m') well below the current threshold limit value TWA recommended by the American Conference of Governmental Industrial Hygienists." Some of the results may well be significant by chance alone. The dentists scored consistently worse, however, in the neurobehavioural tests. Also, a dose effect was also noted in these tests. In view of the consistency of this finding, a current TLV-TWA value of 0-05 mg/m' as providing protection against adverse effects of mercury requires to be reviewed.