Recent estimates published by WHO report that in 2016 outdoor air pollution caused 4.2 million premature deaths worldwide and urged urban planners, policymakers, and environmentalist to make health and wellbeing their number one priority when designing cities. In view of this, the current paper explores the effectiveness of trees and vegetation in dispersing air pollutants at pedestrian level by administering detailed fieldwork and spot measurement of both pollutants and microclimatic parameters close to one of the most polluted roads in London (Euston Road); followed by modelling a variety of real-life scenarios by using computational simulation application for validation and prediction. Whilst many studies agree in general on the mitigation of urban air pollutants by vegetation, the result of the current study contradicts this common understanding and demonstrates drastic increases in the concentration of particulate matters in the vicinity of trees. The results highlight that trees reduce wind velocity and air movement, causing pollutants to trap inside urban canyons. Therefore, planting more trees does not necessarily mean less pollution, at least locally. Instead, to alleviate air quality problems, more attention should be given to vegetation configuration, type, scale and most importantly, their locations and distributions within active urban pockets.