Perinatal Exposures to Ambient Fine Particulate Matter and Outdoor Artificial Light at Night and Risk of Pediatric Papillary Thyroid Cancer

BACKGROUND: Pediatric thyroid cancer incidence has been increasing globally, with environmental exposures being a hypothesized risk factor. OBJECTIVE: We evaluated the association between pediatric thyroid cancer risk and perinatal exposure to ambient fine particulate matter (PM) with aerodynamic diameter ≤2:5 lm (PM_2:5 ) and outdoor artificial light at night (O-ALAN). Both are considered environmental carcinogens with evidence of thyroid function disruption, reported associations with thyroid cancer in adults, and concerns of distributive inequity. O-ALAN may also serve as a proxy for other outdoor air pollutants or urbanization. METHODS: We conducted a case–control study of papillary thyroid cancer nested within a California birth cohort that included 736 cases diagnosed at 0– 19 y of age and born in 1982–2011 and 36,800 controls frequency-matched on birth year. We assigned individual-level exposures for residence at birth for ambient PM_2:5 concentrations from a validated, ensemble-based prediction model and O-ALAN using the New World Atlas of Artificial Night Sky Brightness. We calculated odds ratios (OR) and 95% confidence intervals (CI) using logistic regression adjusting for potential confounders and stratified by age and race/ethnicity. RESULTS: We observed statistically significant associations between PM_2:5 exposure and papillary thyroid cancer risk overall (OR per 10-lg=m³ increase in PM_2:5 =1:07, 95% CI: 1.01, 1.14), among the 15–19 y age group (OR = 1:08; 95% CI: 1.00, 1.16), and among Hispanic children (OR = 1:13; 95% CI: 1.02, 1.24). For O-ALAN, we observed statistically significantly increased odds of papillary thyroid cancer in higher exposure tertiles in comparison with the reference tertile in the overall population (tertile 2: OR = 1:25, 95% CI: 1.04, 1.50; tertile 3: OR = 1:23, 95% CI: 1.02, 1.50) and when modeled as a continuous variable (OR = 1:07 per 1 mcd=m² ). In age-stratified analyses, significant associations were observed among the 15–19 y age group, but not the 0– 14 y age group. No significant differences were found by race/ethnicity. DISCUSSION: This study provides new evidence suggesting associations between early-life exposure to PM_2:5 and O-ALAN and pediatric papillary thyroid cancer. Given that O-ALAN may also represent other air pollutants or broader urbanization patterns, further research and refinements to exposure metrics are needed to disentangle these factors. https://doi.org/10.1289/EHP14849.