Residential exposure to PM_2.5 components and risk of childhood non-hodgkin lymphoma in Denmark: A nationwide register-based case-control study

In a recent study, we observed an increased risk of childhood non-Hodgkin lymphoma (NHL) associated with exposure to fine atmospheric particulate matter (PM_2.5) and black carbon (BC). In this nationwide register-based case-control study, we focus on specific components of PM_2.5 in relation to childhood NHL in Denmark (1981–2013) by identifying all incidents of childhood NHL cases in the Danish Cancer Registry (n = 170) and four (cancer-free) randomly selected controls matched by date of birth and sex. We applied PM_2.5 concentrations and the following sub-components: secondary organic aerosols (SOA), secondary inorganic aerosols (SIA; i.e., NO₃ ⁻, NH₄ ⁺ and SO₄ ²⁻), BC, organic carbon (OC) and sea salt. We calculated a time-weighted exposure average from birth to index-date at all addresses. Odds ratios (ORs) were adjusted for register-based socio-demographic variables. We observed adjusted ORs and 95% confidence intervals (95% CI) of 2.05 (1.10, 3.83) per interquartile range (IQR, 4.83 µg/m³) PM_2.5 and 1.73 (0.68, 4.41) per IQR (3.71 µg/m³) SIA, 0.95 (0.71, 1.29) per IQR (0.05 µg/m³) SOA, 1.22 (1.02, 1.46) per IQR (0.39 µg/m³) BC, 1.02 (0.83, 1.26) per IQR (0.56 µg/m³) OC and 1.01 (0.79, 1.30) per IQR (0.87 µg/m3) sea salt, respectively. The estimates were attenuated after adjustment for PM_2.5, whereas the OR for PM_2.5 remained increased regardless of adjustment for specific components. The findings indicate that the previously observed relation between PM_2.5 and childhood NHL may be related to BC (as reported in our previous study) but also partly to SIA, but the role of specific chemical components of PM_2.5 remains ambiguous.