The exposure of students to traffic-emitted carbon monoxide (CO) in their daily walk to school is evaluated, with a particular emphasis on the effect of trees and route choice. The study is focused on the city centre of Aveiro, in central Portugal. Time evolution of the georeferenced location of an individual is tracked with a GPS for different alternative walking routes to a school. Spatial distribution of CO concentration is simulated with a computational fluid dynamics (CFD) model. An exposure model is developed that associates the georeferenced location of the student with the computed air quality levels (at an average breathing height) for that specific grid cell. For each individual, the model calculates the instantaneous exposure at each time frame and the mean value for a given period. Results show a general benefit induced by the trees over the mean exposure of the student in each route. However, in the case of instantaneous exposure values, this is not consistent along the entire period. Also, the variability of the estimated exposure values indicates the potential error that can be committed when using a single value of air quality as a surrogate of air pollution exposure. 1. Introduction According to the United Nations Environment Programme (UNEP) information in 2009 [1], at least 1 billion people are exposed to outdoor air pollution that exceeds the air quality standards of the World Health Organization (WHO). Poor air quality in urban areas, mainly associated with the transportation sector, has been directly linked to almost 0.8 million premature deaths annually, especially in sensitive groups as children [1]. In urban areas, motor vehicles are a significant source of air pollution, and, despite the achieved technological progress in reducing motor emissions, there is a continuous growth in the world’s motor-vehicle fleet and consequently an increase in the total traffic-related emissions. Motor vehicles emit large quantities of air pollutants (such as carbon monoxide (CO), hydrocarbons (HC), nitrogen oxides (NOx), particulate matter (PM), benzene, formaldehyde, and acetaldehyde) that have a negative impact on human health and contribute to the formation of by-products such as ozone (O3) and secondary aerosols. Noncombustion emissions from these vehicles should also be taken into account since road dust, tire, and break wear contain chemical compounds that may pose risk to human health. Due to the high amount of air pollutants released by traffic and the difficulty in quantifying them, taking all into account, it is common to use
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