In 2014, renal cancer was the 11th most common cancer in Korea; 4,471 new cases were diagnosed, and 944 deaths were recorded. The incidence of renal cancer per 100,000 people has increased from 3.0 in 1999 to 5.7 in 2014 [1]. Renal cancers are classified as tumors originating from the renal parenchyma or from the renal pelvis. The majority of tumors that originate from the renal parenchyma are primary tumors. Among them, renal cell carcinoma (RCC), which is a malignant tumor, accounts for approximately 85% [2]. Cigarette smoking, obesity, and hypertension have been reported to be risk factors of RCC, but their reported contributions are relatively small [3]. A number of epidemiological studies on the relationships between occupational exposure and the development of RCC have been conducted over the past 30 years. Epidemiological and animal studies conducted in Germany have shown that exposure to trichloroethylene (TCE) increases the risk of RCC development significantly [456]. In addition, many studies have recently been conducted on the relationship between sunlight exposure and the level of vitamin D in the body in relation to various types of cancer, including renal cancer [78910].

Vitamin D is ingested in food or synthesized naturally from cholesterol in skin exposed to sunlight (ultraviolet B; UVB). Foods, such as milk, butter, and fish, are relatively rich in vitamin D, but people obtain most of their vitamin D through exposure to sunlight [1112]. In vivo, vitamin D plays important roles in the metabolisms of calcium and phosphorus as well as in skeletal formation; osteoporosis, osteomalacia, or rickets can develop when its levels are insufficient [13]. Recent studies have shown that vitamin D reduces the risk of cardiovascular disease, metabolic syndrome, inflammatory disorders, and even malignancies [714]. Vitamin D interferes with carcinogenesis by inhibiting cell proliferation, promoting cell differentiation, and suppressing tumor invasiveness, angiogenesis, and metastasis [8915].

Despite the important functions of vitamin D, recent studies have shown that adolescents and young adults are at risk of vitamin D deficiency. According to a report on vitamin D deficiency in Koreans released in 2013, only 34.2% of males and 22.4% of females had normal vitamin D levels, and that vitamin D deficiency was more serious in younger females [16]. Because skin exposure to sunlight is the main source of the vitamin [11], the cause of vitamin D deficiency in Koreans is likely to be associated with reduced outdoor activity. In a study based on data collected during The Korean National Health and Nutrition Examination Survey conducted in 2009–2013, a positive correlation was found between outdoor physical activity and the serum vitamin D levels [1718]. Considering that most of the daylight hours on weekdays are spent at work, it is estimated that individual sunlight exposure is affected significantly by occupational sunlight exposure. Thus, a lack of occupational sunlight exposure may increase the risk of developing a range of cancers, including RCC. This study examined the relationship between occupational sunlight exposure and the incidence of RCC.

The cases and controls were compared according to sex, age, education, smoking history, alcohol history, exercise, hypertension, urinary stone, and family history of renal cancer (Table 2). The incidence of RCC was similar regardless of age and family history of renal cancer. On the other hand, there were significant differences in the incidence of RCC between the case group and control group in relation to education, smoking history, alcohol history, exercise, hypertension, and urinary stone.

The mean sunlight exposure of the case and control groups was 0.728 kJ/m² (standard deviation [SD]: 0.153) and 0.757 kJ/m² (SD: 0.173), respectively. This indicates that the case group had significantly lower mean sunlight exposure than the control group (Table 3).

Table 4 lists the changes in the risk of RCC according to occupational sunlight exposure. After dividing the sunlight exposure by quartile, the RCC risk for each quartile compared to the first quartile was shown before and after adjusting for education, smoking history, alcohol history, exercise, hypertension, and urinary stone. Compared to the first quartile, the RCC risk was higher in the second and third quartiles, but the difference was not statistically significant. The RCC risk was significantly lower only in the fourth quartile. The p for trend, which indicates the tendency for a change in the risk of RCC in each quartile, showed that the risk of RCC tends to decrease with increasing sunlight exposure.

Many studies have reported an association between sunlight exposure and renal cancer. Karami et al. [22] conducted a hospital-based case-control study on the relationship between occupational exposure to sunlight and the incidence of RCC. They reported that RCC has significant linear inverse associations with occupational exposure to sunlight among males. Boscoe and Schymura [23] conducted regression analysis of more than 3 million incident cancer cases against the daily satellite-measured solar UVB levels in the continental United States. They observed an inverse association between solar UVB exposure and cancer incidence for 10 sites (bladder, colon, Hodgkin lymphoma, myeloma, other biliary, prostate, rectum, stomach, uterus, and vulva). Mohr et al. [24] examined the association of UVB exposure, cloud cover, and intake of calories from animal sources with the incidence of renal cancer using multiple regression in 139 countries. They reported that the UVB irradiance was inversely associated with the incidence of renal cancer, whereas cloud cover and intake of calories from animal sources showed an independent positive association. Many other studies have explained the relationship between sunlight exposure and the incidence of renal cancer.

The method of assessing the amount of sunlight exposure is important in terms of the accuracy of the study. In Karami et al.'s study [22], interviewers surveyed the participants' occupations and tasks, and industrial hygienists or occupational health professionals categorized them and estimated the relative sunlight exposure between occupations. This study has limited accuracy in that it estimates and applies the relative difference in sunlight exposure between occupations. Boscoe and Schymura [23] used local UV doses measured by satellites in their study. They surveyed the residences of the participants diagnosed with cancer and used the UV data for the region, measured by satellites. Mohr et al. [24] analyzed the incidence of cancer according to latitude with the idea that sunlight exposure would decrease with increasing latitude. Their study also has a limitation in ignoring the individual differences and applying the amount of sunlight exposure in the area of residence collectively. On the other hand, this study surveyed the participants' past occupations and working periods. In addition, the sunlight exposure data were applied for each occupational group studied in France [21] and the weighted average occupational sunlight exposure was calculated considering the working period. Previous studies have estimated sunlight exposure by experts, or applied it equally according to the residential area or latitude. In contrast, although indirect, the present study used the UV data measured by satellites. On the other hand, because the referenced sunlight exposure was investigated according to the occupational group, the actual sunlight exposure within the same occupational group differed depending on individual occupations. Although the median of occupational sunlight exposure was used to reduce the deviation, this study also lacked accuracy.

The inverse correlation between sunlight exposure and RCC incidence is likely to have been affected by the lack of vitamin D in the body. Because most of the vitamin D in the body is produced by sunlight exposure of the skin, a lack of sunlight exposure causes a deficiency of vitamin D in the body [1225]. Vitamin D, which is produced by food intake or by sunlight exposure on the skin, is converted to the active form (1,25-hydroxy vitamin D) through the metabolism in the liver and kidney [26]. The activated vitamin D works through 2 pathways in the target organs: one is a fast pathway to activate the voltage-dependent calcium channels through the non-genomic action; the other is a slow pathway that causes genomic action by binding to the vitamin D receptor [27]. Vitamin D receptors are found in most human tissues, in addition to the organs of vitamin D known previously, such as bones, gut, and kidneys [28]. Activated vitamin D also binds to the Vitamin D receptor of cancer tissues, differentiates cells through gene regulation, induces the apoptosis of tumor cells, and inhibits angiogenesis and the proliferation and metastasis of tumor cells [2930]. Therefore, a vitamin D deficiency due to a decrease in sunlight exposure has not promoted apoptosis and inhibited the growth of tumor cells, thereby increasing the risk of RCC.

This study tested the hypothesis that the risk of RCC is lower in people with high sunlight exposure occupations. The Q4 group's OR for the Q1 group and p for trend supported the hypothesis of this study, but the Q2 and Q3 groups' OR showed no significant results. This study might have incomplete results due to some limitations. First, sunlight exposure through leisure activities, not work, was not considered. To evaluate the individual's exposure to sunlight more accurately, it is necessary to investigate and reflect the outdoor activity time on holidays as well as the commuting methods and time. Second, inaccuracy in exposure assessment may lead to misclassification bias. The difference in exposure to sunlight by different occupations in the same occupational group was not reflected. In the classification according to ISCO-08, the occupational classification of welders and sewing workers is bound to the same occupational group as No. 7 (craft and related trades workers), but the level of sunlight exposure will be significantly different in those who perform welding work outdoors and those who perform sewing work in a factory. Therefore, a future study will need to reduce the errors in exposure assessments with more detailed occupational classifications. Third, sunlight exposure data from France were used because there are no domestic data on sunlight exposure in the occupational groups. The use of domestically measured sunlight exposure data will provide results that are more accurate. Fourth, it does not reflect some personal risk factors and hazardous substances that affect the incidence of RCC. In RCC, smoking, obesity, and hypertension are known as personal risk factors, and TCE is known as an occupational risk factor [3456]. Smoking and hypertension were adjusted but obesity and occupational TCE exposure. The risk factors above were overlooked because the participants were not investigated directly but used the data surveyed from other studies. The lack of adjusting the 2 risk factors is a weakness of this study. Fifth, it is possible that memory decay bias and no-response bias influenced the results of the study. The average age of the participants was approximately 55, so it can be difficult to remember exactly 30 years of occupational history. In particular, in the case of non-regular workers or day laborers, the survey on the occupational history may have been inaccurate due to frequent turnover. In addition, jobs with low social prestige may have either refused to participate in the survey or intentionally missed some of their occupational history. In particular, because the survey was conducted through a questionnaire, the accuracy might have been lower than that of the interview.

Despite these limitations, this study is meaningful in that it analyzed the relationship between occupational sunlight exposure and the incidence of RCC in Koreans and found a partial but significant inverse correlation. Future studies need to use occupational sunlight exposure measured in Korea, refine the occupational group, and reflect sunlight exposure in daily life to obtain more reliable results.

Higher occupational sunlight exposure tends to reduce the risk of developing RCC. Therefore, people who work indoors or have less occupational exposure to sunlight should be exposed to more sunlight through outdoor activities.