title: Comparison of air pollution exposures in active vs. passive travel modes in European cities: A quantitative review.
abstract: Transport microenvironments tend to have higher air pollutant concentrations than other settings most people encounter in their daily lives. The choice of travel modes may affect significantly individuals' exposures; however such considerations are typically not accounted for in exposure assessment used in environmental health studies. In particular, with increasing interest in the promotion of active travel, health impact studies that attempt to estimate potential adverse consequences of potential increased pollutant inhalation during walking or cycling have emerged. Such studies require a quantification of relative exposures in travel modes.
  
  
  
  The literature on air pollution exposures in travel microenvironments in Europe was reviewed. Studies which measured various travel modes including at least walking or cycling in a simultaneous or quasi-simultaneous design were selected. Data from these studies were harmonized to allow for a quantitative synthesis of the estimates. Ranges of ratios and 95% confidence interval (CI) of air pollution exposure between modes and between background and transportation modes were estimated.
  
  
  
  Ten studies measuring fine particulate matter (PM2.5), black carbon (BC), ultrafine particles (UFP), and/or carbon monoxide (CO) in the walk, bicycle, car and/or bus modes were included in the analysis. Only three reported on CO and BC and results should be interpreted with caution. Pedestrians were shown to be the most consistently least exposed of all across studies, with the bus, bicycle and car modes on average 1.3 to 1.5 times higher for PM2.5; 1.1 to 1.7 times higher for UFP; and 1.3 to 2.9 times higher for CO; however the 95% CI included 1 for the UFP walk to bus ratio. Only for BC were pedestrians more exposed than bus users on average (bus to walk ratio 0.8), but remained less exposed than those on bicycles or in cars. Car users tended to be the most exposed (from 2.9 times higher than pedestrians for BC down to similar exposures to cyclists for UFP on average). Bus exposures tended to be similar to that of cyclists (95% CI including 1 for PM2.5, CO and BC), except for UFP where they were lower (ratio 0.7).
  
  
  
  A quantitative method that synthesizes the literature on air pollution exposure in travel microenvironments for use in health impact assessments or potentially for epidemiology was conducted. Results relevant for the European context are presented, showing generally greatest exposures in car riders and lowest exposure in pedestrians.
authors.affiliation: Centre for Environmental Policy, Imperial College London, 14 Prince's Gardens, South Kensington, London SW7 1NA, United Kingdom. Electronic address: anazelle@imperial.ac.uk.
authors.forename: Audrey
authors.initials: A
authors.lastname: de Nazelle
authors.affiliation: Centre for Environmental Policy, Imperial College London, 14 Prince's Gardens, South Kensington, London SW7 1NA, United Kingdom; Grantham Institute, Climate Change and the Environment, Imperial College London, Exhibition Road, South Kensington, London SW7 2AZ, United Kingdom.
authors.forename: Olivier
authors.initials: O
authors.lastname: Bode
authors.affiliation: Centre for Environmental Policy, Imperial College London, 14 Prince's Gardens, South Kensington, London SW7 1NA, United Kingdom.
authors.forename: Juan Pablo
authors.initials: JP
authors.lastname: Orjuela
mesh_terms: D000393:Air Pollutants / Q000032:analysis*; D000397:Air Pollution / Q000032:analysis*; D001336:Automobiles; D001642:Bicycling; D002947:Cities; D004781:Environmental Exposure; D005060:Europe; D006801:Humans; D018986:Motor Vehicles; D014195:Travel; D016138:Walking
chemical_list: D000393:Air Pollutants
keywords: Bicycle; Black carbon; PM(2.5); Transportation; Ultrafine particles; Walk