Exposed subjects, now in particular, reported significantly more often respiratory, gastrointestinal, and skin symptoms than those belonging to the non-exposed group. Moreover, all work-related symptoms were significantly more often reported by exposed than non-exposed workers. As reviewed by Domingo et al. (2008) relatively few studies have investigated the health condition of compost workers [5]. This is the first study that investigated work-related health effects among compost workers in Flanders, Belgium. Composting of organic waste on a larger scale is a fairly new industrial activity in Flanders. Therefore, due to the limited number of workers active in this industry, the study population was small. This is a major limitation of this study since it could be the reason why some associations between exposure and health effects did not reach statistical significance.

However, the strength of the associations suggests that the odds ratios found in our study are genuine. Following Santos et al. (2008), the OR is one of the most frequently used measures of association between a risk factor and an outcome (e.g. health effect) in epidemiology [9]. The risk ratio (RR) and prevalence ration (PR) are important measures to quantify the strength of an association between a risk factor and a health effect [9]. Thompson et al. (1998), stressed that the OR overestimates the RR or PR when the health effect is common (i.e., prevalence higher than 10%) [10]. As argued by Santos et al., the major draw-back of using OR when an outcome is common, is related to its misinterpretation as PR [9].

Furthermore, there are some other limitations of the present study. We cannot be certain that no selection bias ��healthy worker effect (HWE)�� was introduced. The HWE refers to the phenomenon that workers must be relatively healthy in order to be employable in a workforce. As stated by Li et al. (1999), morbidity and mortality rates within the workforce are usually lower than in the general population. As a result, increases in both morbitity and mortality due to occupational exposure might be wholly or partially masked [11]. Another possible limitation of this study is the occurrence of reporting or recall bias. Citing Pearse and Checkoway (1988), ��recall bias may occur because a patient with a chronic disease may ponder the possible causes of their disease, and therefore they may be more likely to recall some past exposures than healthy controls�� [12].

In addition, the cross-sectional AV-951 design gives no information on the temporal sequence between exposure and outcome. Several studies illustrate that exposure to organic dust in compost workers is significantly associated with a higher frequency of health symptoms and diseases [2,6,7]. For example, a cross-sectional study by B��nger et al.