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After years of decline in usage and image in the Western world from the 1950s onwards, the bicycle is now emerging as a centre of interest for many researchers, policymakers, practitioners and advocacy groups in the health and environmental fields. The proportion of journeys taken by bicycle still varies greatly, from around 1% to 2% in the US up to 30% in The Netherlands. In the past decade, bicycle promotion policies have begun to flourish in Europe and North America, in particular the highly popular city bike sharing programs as seen in Paris, London or Barcelona.
The increasingly recognised benefits of cycling are driving the support for such initiatives.1 Cycling as a means of transportation facilitates the integration of healthy physical activity habits into daily routines, and has been shown to promote such benefits as cardiovascular health, fitness and healthy weight status.2 3 As an alternative to motorised transportation, the bicycle can also contribute to improving air quality, lowering greenhouse gas emissions and reducing congestion. Other quality of life, health and environmental benefits may also accrue from policies and transport interventions that promote cycling.4 5
Nevertheless, cycling may bring about detrimental impacts as well, such as the risk of traffic injuries, increased inhalation of air pollution6 and increased exposure to ultraviolet (UV) rays and noise. A growing and not yet conclusive body of research shows that cyclists are in general exposed to lower concentrations of traffic-related contaminants than other travellers,7 in particular if they choose lower traffic roads,8 but they still experience higher levels of pollutants than that measured by routine ambient monitoring stations.7 Strak et al's study (see page 118) represents an additional piece of evidence on the importance of route choice to reduce exposures, particularly interesting for its measurements of the understudied ultrafine particulate and soot.9 The authors measured cyclists' exposures simultaneously on a high-traffic and a low-traffic route in Utrecht, The Netherlands, on 16 days during morning rush hour. They found no difference for PM10 concentrations, but 39% and 59%, respectively, greater soot and particle number concentrations on the higher traffic route.
Pollutant concentrations alone, however, are only partial indicators of the potential harm associated with transportation choices. Indeed, active modes of transportation (walking, cycling) generally result in higher inhalation rates and hence greater pollutant intake rates than passive modes (car, mass transit). Very few studies have integrated estimates of intake rates in assessing cyclists' exposures.10 11 Furthermore journey times are generally longer for the same distance travelled.
Where Strak et al's study represents a remarkable contribution to the exposure and health field is in its attempt to examine the potential health effects of cycling commutes under differing traffic conditions. In their small study of 12 healthy volunteers they performed tests for lung function and markers of inflammation before and after each cycle trip. Associations between the changes in health markers measured 6 h after the ride and air pollution were mostly in the expected direction (ie, detrimental effects of exposures), although not statistically significant. Few studies have examined the health effects of being in a transport environment,12 13 particularly for cycling. Understanding how exposure level and duration and inhalation rates in different transportation environments may affect acute and chronic health endpoints is critical for developing successful health and planning policies. These journeys, generally characterised by short durations but with high peaks can potentially represent the greatest contributor to overall daily exposures, and therefore chronic disease.14
There is a critical gap in the environmental health literature in understanding hazardous exposures and associated health effects in transportation environments, but such studies are hindered by the logistical and technical challenges they represent. The relatively small sample size of Strak et al's study is understandable, given the complex parameters that were (neatly) incorporated in the design: minimising and controlling pre-exposure and postexposure to prevent effect modification of outcomes measured at various time intervals in the day, handling an array of specialised monitoring and health equipment, realising the experiment in a real life setting with its vagaries and safety issues, and finally finding sufficient exposure contrasts and health markers to detect measurable (albeit small) short-term health effects. In light of such challenges, Strak et al's study represents an important first step in filling the gap. Further explorations are needed, where more health markers can be incorporated (for example, markers of cardiovascular disease, gene activation, etc), outcomes tested at various time intervals (rather than only immediately or 6 h afterwards), more pollutants measured and finally different travel modes assessed as well.
The results of this study, and others to come, are essential pieces of information that can then be incorporated in overall assessments of planning, transportation and health policies. Active transportation is promoted as a solution to ailments ranging from physical activity to climate change; it is therefore timely to start considering the potential health risks together with the benefits to help design optimal policies. If increased health risks are shown, solutions can be sought with strategies that reduce ambient air pollution and noise levels and also by locating bicycle infrastructure and promoting routes on low-traffic (or no-traffic) roads. To understand the importance and solution of potential unintended consequences of cycling activities, the planning, health and environmental fields must unite to open new fields of multidisciplinary research and incorporate the missing links (such as Strak et al's findings) into comprehensive integrated health impact studies examining risks and benefits.15
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