Ambient ultrafine particles
(UFPs, <0.1 µm) can reach the human brain but to our knowledge epidemiologic studies have yet to evaluate the relationship between UFPs and incident brain tumors
We conducted a cohort study
of within-city spatial variations in ambient UFPs across Montreal and Toronto, Canada among 1.9 million adults included in multiple cycles of the Canadian Census Health and Environment Cohorts (1991, 1996, 2001, and 2006). UFP exposures (3-year moving averages) were assigned to residential locations using land use regression models with exposures updated to account for residential mobility within and between cities. We followed cohort members for malignant brain tumors
(ICD-10 codes C71.0-C71.9) between 2001 and 2016; Cox proportional hazards models (stratified by age, sex, immigration status, and census cycle) were used to estimate hazard ratios (HRs) adjusting for fine particle mass concentrations (PM2.5
), nitrogen dioxide (NO2
), and various sociodemographic factors.
In total, we identified 1400 incident brain tumors
during the follow-up period. Each 10,000/cm3
increase in UFPs was positively associated with brain tumor incidence (HR=1.112, 95% CI: 1.042, 1.188) after adjusting for PM2.5
, and sociodemographic factors. Applying an indirect adjustment for cigarette smoking and body mass index strengthened this relationship (HR=1.133, 95% CI: 1.032, 1.245). PM2.5
were not associated with an increased incidence of brain tumors
Ambient UFPs may represent a previously unrecognized risk factor for incident brain tumors
in adults. Future studies should aim to replicate these results given the high prevalence of UFP exposures in urban areas.