4.1. Main Findings
Occupational and environmental exposure to cement dust and their effects on human health is a leading respiratory health problem. Exposure to cement dust can cause various acute and chronic respiratory diseases including respiratory function impairment. Literature is available on respiratory function and cement dust, but the majority of the studies were conducted without considering the association with a long term duration-response effect between years of exposure and pulmonary function and were not elucidated by promising physiological factors which greatly influence the lung function such as age, height, weight, ethnicity, smoking and socioeconomic status.
The present study found a duration response effect and shows that long term exposure to cement dust prominently decreased the pulmonary function. Cement mill workers with duration of exposure greater than 10 years showed a significant reduction in FVC, FEV1, PEF and MVV relative to their matched controls. In contrast, Shamssain and Thompson  reported that the mean values for FVC and FEV1 were not significantly decreased in cement mill workers. They also did not show a significant relationship between the length of exposure in cement industry and lung function parameters. A possible reason for this difference could be that, Shampassian and Thompson  did not appropriately match the anthropometric parameters especially height, which were significantly different (p < 0.025) between groups. In addition, they did not report about calibration of the apparatus, which is an essential and important factor to obtain the precise results.
Al-Neaimi et al.  demonstrated that the ventilatory function (FVC, FEV1, and PEF) were significantly lower in the cement mill workers compared with unexposed subjects. Meo et al.  conducted a study on lung function and Surface Electromyography of intercostal muscles in cement mill workers. The study population were closely matched in terms of anthropometric variables and found significant reduction in lung function parameters, FVC, FEV1, PEF and MVV in cement mill workers compared with controls, however, they did not report association between lung function and duration of exposure to cement dust. Similarly, Mwaiselage et al.  investigated ventilatory function in cement factory workers and reported that exposed workers had significantly lower FVC, FEV1, and PEF than controls. Nordby et al.  reported that Forced Expiratory Volume in the first second (FEV1) reduced with an exposure-response relationship in the highest compared with the lowest exposure level of cement dust. Concurrently, Zelke et al.  found that FVC, FEV1 were significantly reduced among the cement production workers but not among the controls. The reduction in lung function was probably associated with high cement dust exposure.
In parallel to our findings, Merenu et al.  investigated the effect of cement dust exposure on 56 cement factory workers with a mean of 10 years exposure to cement dust on lung function. They found that the vital capacity and forced expiratory volume in one second were significantly lower in cement factory workers than in control subjects. Their results suggest that chronic cement dust exposure impairs lung function. Similarly, El Badari and Saeed  reported a significant reduction in FVC, FEV1 and PEFR in cement dust exposed workers compared to control. The lung function indices were found to be reduced with increasing duration of exposure to cement dust. Our results are in conformity with these results.
Olerue  reported that the lung function parameters FVC and FEV1 were decreased with duration of employment in cement industry, but this level was not statistically significant. Our results, however shows a significant decrease in FVC and FEV1 with increased duration of occupational exposure to cement dust (Table 1, Table 2 and Table 3). A possible reason for this difference is the selection criteria. Olerue  selected 76 cement mill workers from a cement plant that had started production of cement only 6 years ago and grouped the cement mill workers with very little duration of exposure, into 2 groups, namely 6-36 months and 37-72 months. This duration of occupational exposure was small and it could be the reason for the non-significant difference in the lung function parameters FVC and FEV1.
At the differences, Fell et al.  found that the mean pulmonary function indices were similar for cement mill workers and control group. There was no duration-response-related decrease in lung function indices. However, in the present study we found a decline in lung function parameters with period of exposure to cement dust. The main reason for this difference is the selection protocol of the cement mill workers and the control group. Fell et al.  selected 119 cement mill workers, from them only 19 were non smokers; remaining all were either smokers or ex-smokers. They selected the control group from an ammonia producing industry. They compared the lung function parameters of cement mill workers with a control group of subjects employed in ammonia industry. It is an established fact that FVC and FEV1 decrease significantly among cigarette smokers  and exposure to ammonia . However, in the present study we entirely excluded the smokers and our control group was composed of non-smoking clerical staff, shopkeepers and salesmen, all subjects were individually matched for age, height, weight, and socioeconomic status. Based on these reasons Fell et al.  results are in contrast to our findings.
Alakija et al.  showed that cement mill workers had a consistent decline in FVC, FEV1 and PEF with prolonged years of service in the cement industry. They also reported that workers who had less than five years of occupational exposure to the cement dust had a significantly higher FVC, FEV1 and PEF than the workers who had more than 15 years of exposure. The results of the present study are in agreement with the outcomes of Alakija et al. .