Toluene concentrations in the blood and risk of thyroid cancer among residents living near national industrial complexes in South Korea: A population-based cohort study

•Carcinogenic risk of toluene on thyroid is not well established.•Toluene levels in the blood are positively associated with risk of thyroid cancer.•This positive association tend to be stronger in residents living near a road.•Toluene may be involved in the development of thyroid cancer. Toluene is classified as a possible carcinogen, but its role on thyroid cancer is not well established. Vehicle emissions are one of the largest contributed sources of toluene, but no studies evaluating the influence of living near a road on the association between toluene and the incidence of thyroid cancer have been reported. Therefore, we examined potential associations between blood toluene concentrations and incidence risk of thyroid cancer, and an effect modification of living near a road. We conducted a prospective cohort study using data from South Korean “Monitoring Project for Exposure to Environmental Pollutants and Health Effects among Residents Living near Industrial Complexes” survey. Study participants living near national industrial complexes were recruited from January 2003 to 2011. Incidence and mortality cases of thyroid cancer (C73, ICD-10 code) were identified using the National Cancer Registry and Statistics Korea, respectively. Blood toluene concentrations were measured using gas chromatography mass spectrometry. We used Cox proportional hazards regression models to estimate the hazard ratios (HR) and the 95% confidence interval (CI) between blood toluene concentrations and thyroid cancer risk. During the follow-up (median 8.6 years), 33 cases of thyroid cancer were diagnosed. The geometric mean of the toluene concentration in the blood was 0.56 μg/L for cases and 0.29 μg/L for non-cases. After adjusting for potential confounders, a positive association between blood toluene concentrations and thyroid cancer was found (HR = 2.77, 95% CI = 1.00–7.65 in the highest tertile vs. the lowest tertile, p for trend = 0.044). This positive association was stronger in people living near a road (≤50 m). Blood toluene concentrations may be positively associated with the incidence risk of thyroid cancer. Moreover, this association may be stronger among people living near a road.