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Exposure to ultrafine particulate air pollution (UFP) contributes to adverse health effects in sensitive population groups such as children. There is a need to explore UFP exposure in terms of respiratory dosage, which leverages the effect of activity-specific ventilation rates. Commute-related dosage, which describes the dosage that occurs during travel between fixed locations on a routine schedule (i.e. morning school commute), is often underrepresented in air pollution studies. School commutes commonly utilize active transportation modes, and modelling dosage during such commutes requires the development of an approach that captures the influence of travel mode and terrain-enforced ventilation rates. The concept of low-dosage routing is discussed as a solution to UFP dose mitigation; however, it requires an in-depth understanding of the factors that control how dosage is accumulated at different points in the commute. This paper presents a modelling workflow for examining UFP dosage while walking to school. We apply a GIS-based approach that simulates school commute routes for 296,862 homes in Toronto. We use a physical exertion–dependent dosage model that reflects broad-scale variability in physiology, travel velocity, travel gradient, and ambient UFP concentration to evaluate route-specific commute dosage. Our results reveal the patterns of within-route variation in dosage and demonstrate the effects high regional UFP concentration and accelerated physical exertion (i.e. where ventilation rate increases during uphill travel) on creating sections of greater UFP dosage within a route. We use the model to examine UFP dosage under shortest-distance and lowest-dosage routing solutions and find that 13.4% of homes had low-dosage alternative routes, while 86.6% had shortest-distances route that were already the lowest-dosage. Our findings were consistent with studies in the literature that implement a dosage modelling approach. This study highlights the importance of dosage models that account for the dynamic nature of walking speed, ventilation rate, terrain, and pollution concentration along a school commute. • Travel-gradients affect respiratory dosage of ultrafine particulate matter. • Dosage is accelerated during school-commutes with greater physical exertion. • Some children who walk to school could benefit from low-dosage routing optimization. • The shortest route is likely the lowest-dosage route for most children in Toronto. [ABSTRACT FROM AUTHOR]