PROJECT:  RESEARCH PROJECT
TASK 1 WITH AN EXAMPLE
      BLOCK CONTENTS                         7 STEP PROJECT                                     BOTTOM OF PAGE
 

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Objective:
   

Learning activities:   

AN EXAMPLE:  THE NERVOUS SYSTEM EFFECTS OF OCCUPATIONAL  EXPOSURE ON WORKERS IN A SOUTH AFRICAN MANGANESE SMELTER

There has been increasing interest in the chronic nervous system effects of  long term occupational Mn exposures  below  the American Conference of Governmental Industrial Hygienists (ACGIH) Threshold Limit Value (TLV) of  0.2mg/m3  (ACGIH, 1996),  and  in environmental exposures  at or above the  United States Environmental Protection Agency reference concentration (US  EPA RfC) of 0.05 mg/m3 (IRIS, 1999) sparked by increasing use of the gasoline additive Methylcyclopentadienyl Manganese Tricarbonyl (MMT) (US EPA, 1994). This goes to the purpose of the study.

 Iregren (1999)  reviewed 13 studies of Mn exposed workers, showing mostly  motor effects with tests for finger tapping, diadochokinesometry and  Luria-Nebraska (LN) items, along with postural tremor and sway abnormalities.   Pegboard, memory, reaction time and cognitive tests were less conclusive. 

More recently,  Lucchini et al.  (1999) found  Mn effects with  World Health Organisation Neurobehavioural Core Test Battery (WHO NCTB) and LN tests in smelter workers at levels around 0.1mg/m3.   Mergler et al (1999) and Beuter et al (1999)  found subtle neurobehavioural effects  and exposure-age interactions  with environmental exposures measured as  blood manganese (MnB). As part of the same study  Bowler et al  (1999)  found mood effects, while Hudnell et al. (1999) located these exposure-response relationships at environmental concentrations around the USEPA RfC.  Roels et al. (1999)  in a prospective study,  showed reversibility of effects  below 0.1mg/m3.  These studies have raised concerns that the ACGIH TLV  might be too high to protect against neurobehavioural effects. 

On the other hand,  no effects were found by either Gibbs et al.  (1999)   at occupational exposures averaging 0.18mg/m3 total  dust,  or by   Deschamps et al (2001)  at exposures averaging below 0.15 mg/m3 over a 20 year period. 

Problems with the relatively few existing studies include small numbers of exposed subjects (the largest study had 141 exposed),  and  non standard   possibly insensitive neurobehavioural tests.  Furthermore,  there is relatively poor   consistency across studies for exposure effects and for the character of exposure-response relationships.  There is still insufficient scientific information relating to subjective symptoms and mood effects, especially from  prospective study designs at low exposure levels,  and to the  individual clinical implications of group effects found.  

This study sought to address some of these problems by studying a large number of subjects within a long term average exposure spectrum ranging from near zero to 25 times the ACGIH TLV. I

t was not possible in this study to meaningfully disentangle chronic from acute effects of manganese exposure and accordingly the relationship between the cumulative exposure index (CEI) an integrated intensity-duration exposure measure across jobs and a number of neurobehavioural endpoints was investigated.   Selection of endpoints was based upon those reported in the state-of-the-art review of the scientific literature by Iregren  [1999] which lists tests with evidence for Mn effects.  A companion paper (Myers et al., 2002a) details atmospheric and biological exposure/effect measures and  their correlations,  and the implications of these for exposure and medical surveillance in the workplace.

This study aimed  to clarify some of the inconsistencies in the literature and particularly to better understand the nature of the exposure-response relationship. 

“Exposure-response relationship” denotes the existence of structure beyond a simple  difference between external referents and all the exposed and/or  between the internal referents and the rest of the exposed, while   “continuity of response” denotes  continuing incremental impairment in function as exposure increases.   

Scientific understanding of the impact of Mn exposure on function leads to the  hypothesis (or elaborated aim) that the exposure-response relationship should show no impact on function (or a threshold) at low exposures, given that Mn is an essential element in human nutrition,  followed by a continuity of response at higher levels of exposure  beyond that threshold. 

References:

The critical appraisal checklist is useful to follow and to assess whether  you are progressing with your project in such a way as to include all the necessary methodological steps.

Section B.  Epidemiological research methods and protocol development.  In "Epidemiology:  A manual for South Africa."  Eds Katzenellenbogen JM, Jubert G, Abdool Karim SS.  Oxford University Press, Cape Town 1997, pages 49-100.

Evaluation: 

Steps 1 through 6 will count 3 marks each (3x6 = 18), the final report 3 marks, and the final presentation oral 3 marks. This gives a total of 24 marks.

Marks will be allocated as follows:

3 = Complete, 2 = Progress, 1 = Markedly incomplete, 0 = no hand-in