Particulate matter (PM) is the most harmful pollution component widely present in the environment, with no known level at which adverse health effects do not occur. Current estimates are of a 4% increase in mortality, 6% for cardiovascular mortality and 8% for lung cancer mortality per 10 mg/m3 of PM10 (PM10 = < 10 mm). However, analysis of causal links between particulate pollution and specific health impacts has been hampered by poor reliability of human exposure data. Exposure assessments tend to be available only at coarse spatial resolution. A new approach, magnetic biomonitoring, may provide a robust, cost-effective means to achieve measurement and sourcing of PM10, at unprecedented spatial resolution. Leaves from roadside trees form natural, widely-distributed pollution collection surfaces, at pedestrian-relevant heights, requiring no power source or protection. The leaves can be analysed magnetically, providing a quantitative proxy for PM10 concentrations, from vehicle combustion and/or from industry sources.