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., NO 3 − , NH 4 + and SO 4 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/m 3 ) PM 2.5 and 1.73 (0.68, 4.41) per IQR (3.71 µg/m 3 ) SIA, 0.95 (0.71, 1.29) per IQR (0.05 µg/m 3 ) SOA, 1.22 (1.02, 1.46) per IQR (0.39 µg/m 3 ) BC, 1.02 (0.83, 1.26) per IQR (0.56 µg/m 3 ) 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.