Background and aim: Many studies that examined blood cadmium levels by exposure routes, and distribution difference by sex and age have been done. However, studies that examined the association between social economic status such as income level, education level, and occupation level and cadmium level, also regarding the confounding effects of smoking were relatively rare. Therefore, our study aim was to examine the association between socio-economic variables and the blood cadmium level with the confounding effect of smoking.
Methods: Data were drawn from the two independent cross-sectional waves of KNHANES (2007-2009, 2010-2012) and involved years that were containing blood cadmium and cotinine data (2008, 2009, 2010, 2011). We investigated the income level by comparing the cadmium level between the highest income quartile group (Q4) and the lowest income quartile group (Q1). Education level was investigated by comparing the blood cadmium level between the college graduate group and under elementary graduate group. Association with the occupation was investigated by using the occupation classification variable of KNHAENS, comparing the cadmium level between professional and managers, non-manual, skilled, and semiskilled workers, unskilled workers, and unemployed group. Smoking variable was investigated by categorizing smoking status (never, former and current), pack-year into quartile variables. Additionally, urinary cotinine level was used to guarantee the validity of smoking variables. Regarding dietary factor, association between blood cadmium level and intake of rice and barely was analyzed by frequency of rice and barely intake per week.
Results: Blood cadmium level increased by increasing age groups. Participants earning low income, and less education had higher blood cadmium levels. Significant differences were found between never (0.87 μg/L) versus former (0.77 μg/L) and current (1.12 μg/L) smokers. After controlling sex and smoking in men and women, the association between income and cadmium level became substantial decrease. In additional controlling for education, the associations were disappeared. However, there was strong negative association between education and cadmium level even after controlling for smoking history, pack-year, or urinary cotinine. Additionally, blood cadmium increased by intake of rice and barely intake, when rice and barely intake was added in the final multivariable regression model.
Conclusion: In cross sectional data from KNHANES, blood cadmium levels were significantly higher among those with low income and less education levels. These associations were contributed by smoking history, but still remained.
Keyword: Cadmium, smoking, socio-economic status, path analysis