Airborne ultrafine particle concentrations and brain cancer incidence in Canada's two largest cities

Malignant brain tumours are rare, but are important to study because survival rates are low and few modifiable risk factors have been identified. Existing evidence suggests that outdoor ultrafine particles (UFPs; particulate matter < 100 nm; sometimes referred to as nanoparticles) can deposit in the brain and could encourage initiation and progression of cancerous tumours, but epidemiological data are limited.

High-resolution estimates of outdoor UFP concentrations and size were linked to residential locations of approximately 1.5 million people in Montreal and Toronto, Canada from 2001 to 2015. Cox proportional hazards models were used to estimate associations between annual average outdoor UFPs and malignant brain tumour incidence while adjusting for potential confounding factors including other outdoor air pollutants.

In total, 1365 incident brain tumour cases occurred during follow-up. Consistent positive associations were observed between long-term exposures to outdoor UFPs and brain tumour incidence with increased risk ranging from 10.5% (95% CI: -1.4, 24.0%) to 15.3% (95% CI: 0.4, 32.5%) per 10,000 particle/cm³ increase. Long-term exposures to oxidant gases, black carbon, or fine particulate matter (PM_2.5) were not associated with increased brain tumour incidence.

Our results suggest that long-term exposures to outdoor UFPs are associated with an increased risk of developing malignant brain tumours. On an absolute scale, the magnitude of this risk translates into approximately 24 additional cases per year per 10,000 particle/cm³ increase in annual average outdoor UFPs in a hypothetical city of 3-million people.

Canadian Institutes of Health Research (CIHR) Foundation Grant and The United States Health Effects Institute (HEI).