The proportion of the population exposed to

The proportion of the population exposed to ionizing radiation was estimated by multiplying the number of industries using radioisotopes (from statistics of the Korea Radioisotope Association [27]) with the average number of workers without applying the correction factor. Thus, the exposed population could be either overestimated or underestimated.
In Korea, industrialization started in the 1960s, and the industries using chemicals greatly expanded and operated actively during the 1970s. During these years, the benzene exposure level might have been 5 to 10 times higher than the exposure limits [31]. However, since the 1980s, the working environment has improved, with several laws enacted for occupational safety and health, and most of the important carcinogens are now controlled by exposure limit. From 1980 onward, exposure to carcinogens decreased to a level lower than that associated with the past of other developed countries. Therefore, the PAFs calculated using IARC RRs may be overestimated.

Introduction
Of the various occupational lung diseases, those induced by inhalation of dusts such as asbestos, crystalline silica, and coal are most prevalent. Inhalation of dusts may cause a variety of lung diseases such as coal workers\’ pneumoconiosis (CWP), progressive massive fibrosis, chronic alveolitis, and crizotinib [1]. Notably, crystalline silica has been classified as a class I carcinogen by the International Agency for Research on Cancer [2]. CWP is a lung disease caused by inhalation of coal dust. Once a silica threshold has been exceeded, silica-induced pulmonary disease may progress without further exposure to silica. Clinical detection of CWP, however, is currently dependent on radiological and lung function abnormalities, which are both late diagnostic tools. Identification of accurate and reliable biomarkers would enable earlier detection before irreversible radiological changes in the lung occur [3,4].
Cytokines influence various biological events such as inflammation, metabolic mechanism, cell growth and proliferation, morphogenesis, fibrosis, and homeostasis. Major sources of cytokines in the lung are epithelial cells, endothelial cells, fibroblasts, and inflammatory cells [5]. In previous reports, the relationship between pulmonary inflammation and dusts, and cytokines has been demonstrated for mediators of various toxicological and pathological effects, and several cytokines related with coal dust [6–10]. In one study, the initial concentrations of tumor necrosis factor-alpha (TNF-α) were related to later progression of CWP. Miners who showed abnormally high dust-stimulated release of TNF-α had an increased risk of progression in CWP. TNF-α in pneumoconiosis induced by coal dust was reported to be a powerful tool to estimate individual prognosis of pneumoconiotic disease, even after the end of occupational exposure [11]. Interleukin-8 (IL-8) is a chemokine secreted by a variety of cells types including fibroblasts in response to IL-1 and TNF-α [12]. IL-8 is an important activator and chemoattractant for neutrophils, and has been implicated in a variety of inflammatory diseases [13]. IL-8 is important in the lung inflammation produced by crystalline silica. Both TNF-α and IL-8 were reported to be increased in the supernatant of spontaneous or dust-stimulated monocytes isolated from peripheral blood and in sera of CWP, which did not include progressive CWP [14].
While various studies have addressed the increased production of IL-8 following exposure crystalline silica or coal mine dust in macrophage and fibroblasts, only one human validation study has been published [1]. The available data suggest the importance of IL-8 and TNF-α in CWP. However, little information exists concerning the in vivo relevance of predictive discrimination between the levels of cytokines and progression of CWP. To determine the significance of serum cytokines regard to progression of CWP, a longitudinal design is necessary. However, follow-up studies dealing with the prognostic usefulness of cytokines are scarce in Korea.