Commuting mode and pulmonary function in Shanghai, China

Discussion

Motor vehicles release a complex mixture of pollutants and serve as an important cause of spatial differences in air pollution within the urban environment [1]. In addition to the potential risks of residential TRAP exposure, it is plausible that commuting TRAP exposure may have additional adverse effects. This is supported by data demonstrating that commuting exposures may trigger both systemic and pulmonary inflammation. For instance, short-term traffic and transport exposures have been associated with nonfatal myocardial infarction [24], increased exhaled nitrous oxide [25], elevated blood fibrinogen levels [10], reductions in FEV₁ and FVC, increased biomarkers of neutrophilic inflammation in sputum, reductions in exhaled breath condensate pH [12], and increases in bronchoalveolar lavage cell counts [26]. However, one study performed in Australia found that lower versus higher pollution cycling routes resulted in short-term differences in subject symptoms but not in differences in peak flow or sputum neutrophilia, suggesting that not all differences in pollution exposure have immediate biological effects [27].

The pollution exposure faced by a commuter can be conceived as the result of a complex interaction of multiple factors including site characteristics (e.g. background ambient pollution), mode of transportation, vehicle characteristics such as ventilation system [28–30] and fuel type [8], and route characteristics (e.g. traffic density and route length). Given these multiple factors, it is not surprising that there is some inconsistency on the relationship between mode of transport and pollutant concentrations in the literature. One 2007 review, for instance, found that commuting within a vehicle, as opposed to via cycling or walking, seemed to be associated with higher levels of particulate matter exposure [7]. Cyclists in particular have had lower pollution exposures than those in cars and buses in several studies [13, 22, 23, 31]. Although presumably exposed to the same ambient environment, the relative exposure of walkers seems more variable. In a review relying on data from 47 studies, for instance, Knibbs et al. [13] found higher ultrafine particle exposures for pedestrians than for those using rail, automobile, bus or bicycle (which had the lowest). A number of recent studies, including one in Hanoi, Vietnam, have similarly found pedestrians to be exposed to higher levels of particulate air pollution than those in cars or buses [28, 32, 33], though one earlier study found the opposite [31]. The pollutant concentration itself, however, is not the only factor. Because of the higher minute ventilation required for cycling (or walking), it is also possible that such commuters may actually be exposed to a higher inhaled dose of pollutants [8].

Some of our findings are consistent with this incomplete literature. For instance, we found that those walking to work had consistent statistically significant reductions in FEV₁ and FVC, which may be the result of the higher pollution exposure in this microenvironment. For instance, one study in urban Guangzhou, China, found that pedestrians were exposed to high concentrations of particulate material <10 μm in diameter, which exceeded the levels found in public transport [34]. Similarly, we found largely consistent significant reductions in lung function in those taking the bus as compared to cyclists, a finding which may again also be explained by elevated within-bus pollution as seen in some studies [13, 23]. The cause of reduced lung function in pedestrians compared to cyclists, however, is less apparent. One possibility is that walkers are somehow exposed to higher levels of certain pollutants than cyclists, which as discussed above, has been demonstrated in some studies. Given that cycling may require greater coordination and health status than walking, it is also conceivable that the higher lung function of cyclists as compared to walkers is the result of a confounding “healthy cyclist” effect. Finally, the apparently protective effect of private car use as compared to cyclists is not clearly consistent with the literature on pollution concentrations. Indeed, we found that cyclists had an elevated risk of airflow obstruction compared to those using a private car, although only when defined by GOLD, and not by LLN, criteria. One possibility, however, is that the increased minute ventilation required for cycling elevates the inhaled dose of pollutants so as to outweigh the lower pollutant concentrations seen by cyclists in some studies [8]. For instance, in one study that directly measured the respiratory parameters of study subjects, despite exposure to similar overall concentrations of pollutants, cyclists (as compared to car passengers) had much higher inhaled quantities of pollutants as a result of having a greater than four-fold higher minute ventilation [35]. Finally, it is important to emphasise that the significant differences that emerged in some analyses (but not others) for company car, taxi use and underground use were variable and highly inconsistent in sensitivity analysis. Particularly in light of the very small numbers of users of these vehicles in this study, we do not believe that any conclusions should be drawn for those three modes of transportation from this study.

We acknowledge that our study has some strengths and some weaknesses. It is a large-scale, population-based study with an objective outcome. We were able to control for multiple important potential confounders, including occupation. There were also proportionally few subjects excluded for missing data. The most important limitation of this study is its cross-sectional design, which limits our ability to draw causal inferences. Some of the findings, however, are unlikely to be explained by reverse causality. For instance, there is no obvious reason why individuals who developed reduced lung function or airflow obstruction would stop driving and start cycling (or walking) to work. Confounding by unmeasured variables is always an additional concern in observational studies. For instance, in this study, we were unable to adjust for family history of lung disease, as this information was not in our questionnaire. However, for this genetic factor to function as a confounder, it would have to be related to not only lung function but also to mode of commuting, which seems highly improbable. Another concern relates to the possibility of residual confounding by SES, which is known to be related to lung function [36]. Notably, despite substantial reductions in power, some of our results remained significant after restricting the analysis to individual SES categories (e.g. for pedestrians). Additionally, we would expect that residual SES confounding might have biased some (but not all) of our other findings towards the null (for instance, we found that cyclists had higher lung function than bus riders, despite being less educated). Another concern relates to the fact that the exposure was determined as the mode of commuting used to get to work as ascertained at the time the survey was performed; it therefore may not reflect modes of transportation previously used, as well as secular trends in ambient pollution exposure over the course of subjects' lives. It should be similarly emphasised that the differences we found may not necessarily be the result of the mode of commuting itself but of various other factors associated with that mode, such as characteristics of the route used. Finally, given differences in the transportation and urban milieu among cities, it would be important to repeat this analysis in different urban environments.

It is important to note that although our results were statistically significant, many of the effect estimates for FEV₁ and FVC % predicted were small, and would be of marginal clinical significance on the individual level. However, even relatively small shifts in the distribution of a risk factor for disease, like blood pressure, can result in substantial aggregate harm or benefit at the population level [37]. Likewise, the impact on pulmonary function of a nearly ubiquitous exposure like commuting may, in aggregate, be substantial.

Thus, considered together with other studies, this investigation may have important public health ramifications. Urbanisation is being accompanied by a tremendous trend towards motorisation in nations such as China and India [14, 15, 38]. In addition to increasing the risk of road injury [39], motorisation will probably be accompanied by increasing TRAP production. Ironically, in this study, those contributing the least to roadway pollution (e.g. pedestrians and perhaps cyclists) may in fact be those most affected by it, while those contributing the most (e.g. private car users) may, in fact, be protected. At the same time, however, it is important to note that regular physical activity has beneficial effects for health on its own. Therefore, from our perspective, the solution lies not in the further acceleration of trends towards motorisation but instead in the reduction of TRAP production itself, so as to ensure a safe commuting environment for all.