Exposure to outdoor ultrafine particle number concentrations and incidence of childhood cancer

Background: In urban areas, transportation and industry-related combustion processes release large quantities of outdoor ultrafine particles (UFPs; <0.1 µm). Due to their small size, UFPs can cross biological barriers, including the placenta, and reach vital organs. Exposure during critical windows of development may increase the risk of adverse health outcomes in children, including cancer. Methods: We conducted a cohort study of over 965,000 children born in Canada between 1999 and 2020 whose birthing parent resided in Montreal or Toronto during pregnancy. Vital status and cancer incidence were obtained from the Canadian Vital Statistics Databases and the Canadian Cancer Registry. High-resolution estimates of outdoor UFP number concentrations and UFP size (i.e., mean particle diameter for a given UFP number concentration) were assigned to children’s six-digit residential postal codes from annual tax records. We estimated hazard ratios (HRs) between outdoor UFP number concentrations and childhood cancer using Cox proportional hazards models, adjusting for UFP size, sociodemographic factors, and co-pollutants. Results: Children were followed from 0 to 14 years old for a mean of 9.1 years. In total, 1,625 children were diagnosed with cancer. For each 10,000 particles/cm 3 increase in prenatal UFP exposure, we observed positive associations with acute lymphoblastic leukemia (ALL) (HR: 1.12; 95% confidence interval [CI]: 0.94, 1.34) and neuroblastoma (HR: 1.15; 95% CI: 0.85, 1.57), although estimates were imprecise and included the null. Consistent associations were also observed for these outcomes across different exposure windows and multiple sensitivity analyses. In contrast, null associations were observed for central nervous system tumors, and an inverse association was observed for lymphoma. Conclusion: Overall, our findings varied depending on the cancer diagnostic group evaluated. Positive associations observed between outdoor UFPs and ALL and neuroblastoma warrant further research.