1. Variability of exposure and estimation of cumulative exposure in a manually operated coal mine.(Mamuya SH et al,2006;Ann Occup. Hyg.)

2. Introduction There is an increasing trend towards studying quantitative exposure-response relationships for occupational health outcomes.The difficulty lies in finding appropriate ways of grouping workers for epidemiological studies.Grouping workers based on job titles, has been proposed as a solution.

3. The grouping scheme is based on the high contrast in exposure between subgroups and low attenuation.Attenuation is described as the bias in estimating linear exposure- response relationships towards zero.Severe attenuation can occur when exposure is highly variable from day to day and a limited number of measurement are available Occupational epidemiological studies usually apply a grouping scheme for exposure estimates based on task and location.

4. A sample of workers is selected for exposure monitoring and the mean of the results is applied to the the whole group. Grouping schemes have been used in mechanized coal mines but data on the variability of dust exposure and on efficiency of grouping schemes in manually operated coal mines is lacking.

5. Objectives To estimate the variance components of exposure to respirable dust and quartz. To assess whether the grouping by job team is appropriate for studying the association between respirable dust and quartz exposure and chronic respiratory symptoms.

6. Methods Settings Study was conducted in the Kiwira labor-intensive colliery, employing 556 surface and underground coal miners.It had been operating for 15 years when the study began. 299 men of a study population of 318, were used for statistical analysis. All the workers provided a job history including the number of years worked in the respective job teams and the number of samples per worker ranged from 1 to 3.

7. Personal respirable dust was collected on cellulose acetate filters in SKC cyclone using an SKC sidekick pump with a flow rate of 2.2L/min. The cyclone was clipped to the workers collar to collect dust in the breathing zone. Full-shift samples were taken during the day shift lasting for 5-10hrs. Respirable dust samples were quantifies by gravimetric analysis with a limit of detection(LOD) of 0.01mg/m^3. The respirable dust were analyzed for quartz by X-ray diffraction using NIOSH method 7500.The LOD for quartz was 0.005mg/m^3. The analyzing labs passed the intercalibration test

8. Dust-sampling strategy Personal dust samples were taken in two periods;June to August 2003 and July to August 2004. Samples were distributed into low,medium and high exposure groups by proportions of 1:3:5 respectively. There were 70 underground miners sampled under four job teams that were:development,mining transportation and maintenance. The 26 surface miners were sampled under the following job teams: washing, boiler and turbine,ash and cinders and administration.

9. Statistical analysis. -A two-way ANOVA (random-effect model) was used to estimate the variance components within groups,between groups and within workers for respirable dust and quartz from exposure-response slopes.The workers identity and groups were treated as random effects. -Individual cumulative exposures (CE) in mg yr m¯3 was determined from: -CE=∑(µ)(t) were µ= mean exposure conc.per job team and t=number of years a worker spent in that particular job team. -Non-detects were included in the analysis by dividing the LOD by two. -SPSS version 12.0 was used in all statistical analysis.

10. Results -The geometric mean exposure values to respirable dust and quartz was determined for each job team. -For underground workers, it was highest among the development team with values of 1.8 and 0.073 mg yr/m3 for respirable dust and quartz respectively. -For surface workers, it was highest among the ash and cinders team with values of 0.73 and 0.02 mg yr/m3 for respirable dust and quartz respectively. -The mean cumulative exposure for workers was 38.1 mg yr.m3 for respirable dust and 2.0 mg yr/m3 for quartz.

11. Results -The years of employment ranged from 0.3 to 34 years,with a mean of 10.2 years. -The within team worker variance was larger than between teams because of the large day-to-day variation in exposure within teams. -There was an attenuation of 5.7% for respirable dust and 17.7% for quartz.

12. Discussions -Only workers in the development team had markedly higher exposures than the surface teams. The surface workers had a range of mean exposure levels similar to those reported by analogous coal mines in the U. -The high day-to-day variability within teams is explained by different time spent in performing different tasks and the rotation between these tasks. -Geological differences in the rocks structure from site to site might also explain this difference

13. Strengths of the study. -The study clearly measured the respirable fraction of dust which is important for the development of lung disease. -Gravimetric analysis for the respirable dust and X-ray diffraction analysis for quartz were done in laboratories that were tested for proper calibration. -Cyclone personal samplers to measure exposure in the breathing zone,is the most suitable sampling strategy to measure the association between inhalation route and respiratory effects. -The personal sampler allowed for complete shift exposure measurements. -The study met the first objective of estimating variance components of exposure to respirable dust

14. Weaknesses of the study. -There was no detailed description of the setting and the mineralogical characteristics of silica dust and probable co- exposures.For example, the percentage of tridymite and cristobalite in the rocks. -The higher variability within job teams will lead to attenuation of the estimated impact on disease outcome. -Information was lacking on background respirable dust exposure levels. -The non-detects in the analysis might have over- or underestimated the exposure levels. -Equal variance within and between teams is assumed in the analysis that is not the case. -They failed to meet the second objective in this article that was meant to show that grouping by job teams is appropriate to study the association between cumulative dust exposure and chronic respiratory symptoms.