New research on the link between ultrafine particles and brain tumours

In urban areas, exhaust gases are important sources of ultrafine particles but their link with various cancers (lung, prostate, and breast) remains inconclusive despite studies.

The  Canadian Census of Health and the Environment cohorts provided key data for the study

In urban areas, exhaust gases are important sources of ultrafine particles (UFPs, < 0.1 μm). Little is known, however, about their long-term effects on the body. The link between UFP and various cancers (lung, prostate, and breast) has been studied, but results remain inconclusive.

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Many studies have recently focused on the ability of fine and ultrafine particles (UFPs) to reach the brain. Animal studies suggest that a fraction of inhaled UFPs migrate to the brain either via the olfactory nerve when deposited on the nasal epithelium, or by crossing the blood-brain barrier after entering the systemic circulation via the alveolar epithelium.

This transfer is favored by the size of the particles and the constitution of their coating (proteins and lipids). The migration of UFPs to the brain is slow, but elimination rates are also very slow, so they can accumulate over time. The deleterious effects of UFPs are either direct (retention of particles in the brain) or indirect when they induce the production of inflammatory mediators.

Epidemiological data on air pollution and brain tumors are limited. The most recent European study has shown that for fine particles (PM2.5 < 2.5 μm ) beta absorption (a marker of traffic-related air pollution) was most strongly associated with malignant brain tumors. However, no studies have been conducted to date on the relationship between UFPs and brain cells and how they reach them.

Measuring exposure

A cohort study investigated variations in environmental concentrations of UFPs within different areas of the cities of Montreal and Toronto, Canada. 1.9 million adults were included, with the data being obtained from four phases of the Canadian Census of Health and the Environment from 1991, 1996, 2001 and 2006.

At the beginning of the study, each person was assigned an exposure level based on the characteristics of his or her place of residence. Exposures were averaged over three years, with a one-year lag to account for mobility.

Measuring incidence

At the same time, the researchers followed the cohort of those individuals with malignant brain tumors (ICD-10 codes C71.0-C71.9) between 2001 and 2016. A total of 1400 incident brain tumors were identified during the follow-up period.


Measurements showed that PFUs were consistently associated with an increase in brain tumors incidence in all models examined. Tumors were more common in females (n=800) than in males (n=600). This risk ratio remained elevated after adjusting for the incidence of socio-demographic factors.

The results suggest that an increase in the average environmental UFP concentration of 10,000 / cm3 over three years contributes to approximately one new case of brain tumor per 100,000 people (assuming an age-standardized baseline incidence rate of 8 per 100,000 people).

Brain tumors, on the other hand, are not positively associated with spatial variations in the concentrations of the other air pollutants that are frequently monitored in urban areas such as fine particulate matter (PM2.5) and nitrogen dioxide (NO2). The prevalence of diesel vehicles (a major source of UFP) is low in Canada. As a result, some areas have high NO2 concentrations (from gasoline-powered vehicles) but no significant increases in UFP.

Study limitations

The exposure models used to estimate spatial variations in outdoor UFPs concentrations were based on data from the end of the study period (2001-2016). Therefore, this measure of exposure likely exhibited some original imprecisions:

- Temporal (fluctuating emissions over time).
- Spatial (local changes in infrastructure that may affect PM2.5 emissions).

In addition, the lack of individual data on other risk factors for brain tumors, such as family history, precluded consideration of these factors.


Pre-existing data on outdoor air pollution and brain tumors were not consistent. This study showed that environmental UFPs may represent a previously unknown risk factor for brain tumors in adults. However, these are of poor prognosis and have few specifically identified risk factors. It would, therefore, make sense to replicate such a study in urban areas with a high density of ultrafine particles.

Weichenthal, Scott & Olaniyan, Toyib & Christidis, Tanya & Lavigne, Eric & Hatzopoulou, Marianne & Van Ryswyk, Keith & Tjepkema, Michael & Burnett, Rick. (2019). Within-City Spatial Variations in Ambient Ultrafine Particle Concentrations and Incident Brain Tumors in Adults. Epidemiology. 1. 10.1097/EDE.0000000000001137.