Nowadays it is well known that air pollution supposes a major threat to human health (GBD 2013 Risk Factor Collaborators, 2015). The pollutants emitted by combustion sources, such as traffic emissions (Figure 1), are suspected to be particularly harmful (HEI Panel on the Health Effects of Traffic-Related Air Pollution, 2010). Black Carbon (BC) and ultrafine particles (UFP, particles with an aerodynamic diameter <100nm) are much better tracers of traffic emissions than particulate matter of <10µm (PM10) or <2.5µm (PM2.5; the latter two, being regulated by the UE legislation). In fact, epidemiological studies report a better association of short-term health effects with BC than with PM2.5 or PM10 (WHO, 2012). Due to their small size, UFP may affect human health more strongly than larger-size particles (Chen et al., 2016).
Commuters are particularly exposed to traffic-related air pollutants owing to their proximity to the source (Figure 2). Both BC and UFP concentrations decrease exponentially downwind away from the road/highway (Zhu et al., 2006). Therefore, exposure assessment during commuting deserves special attention. The miniaturisation of air pollution monitors (for further information, see http://www.greenhub.cat/when-and-where-are-we-more-exposed-to-air-pollution-the-personal-exposure-assessments/) has allowed the proliferation of personal measurements studies in different transport microenvironments over the few last years. These studies show how commuters come in contact with highly variable and with short-time extreme peak concentrations of atmospheric pollutants. These result in the contribution of commuting time (which represents around 6% of the daily time) to 12-23% of the daily exposure to BC (Dons et al., 2011; Rivas et al., 2016).
The exposure during commuting is highly conditioned by the mode of transport. In which one will you get the lower exposure? This is a very difficult and tricky question, because it might depend on your specific city and its public transport system (e.g. the fleet of the buses, the type of underground trains, etc), the ventilation conditions (if you are in a vehicle, are the windows open?) but also on which pollutant you want to assess. For example, the highest PM2.5 and PM10 concentrations are expected to be found in the underground system (Adams et al., 2001; Martins et al., 2016; Rivas et al., 2017) while, on the other hand, the exposure to BC and UFP concentrations is expected to be higher in the road modes (such as bus and car modes) but also for people who is cycling and walking. Moreno et al. (2015) indicated the following hierarchy regarding the results on UFP concentration from different studies: urban background < underground < tram < walking in a suburban main road < walking and cycling in the city centre < bus. On the other hand, PM10 and PM2.5 concentrations in the underground can be double (or even more times) of those found in the bus or in the car. These results are in accordance to the source of each pollutant, as well as the dispersion conditions. There are no combustion sources in the underground, and therefore we do not expect to find very high levels of ultrafine particles. The high level of PM10 and PM2.5 particles that we found in the underground systems is explained by the emission of ferruginous particles from the mechanical abrasion between rails, wheels and brakes, as well as for the hindered dispersion of these particles in such an enclosed space (Figure 3).
Figure 3. Commuters in the Barcelona underground. Source: www.elpais.com (Albert Garcia).
The ventilation condition is also an important parameter to take into account when assessing concentrations and might be behind the different results in different studies. For instance, concentrations of PM2.5 were lower in buses than in cars in Barcelona (De Nazelle et al., 2012), but the opposite was observed in London (Adams et al., 2001; Rivas et al., 2017). Closing or opening the windows, and having or not the recirculation option on might lead to very different concentrations inside the vehicle (car, bus, train carriage, etc).
Further research is needed in order to determine which is the safest travel mode in terms of exposure to air pollutants and which are the health effects of being exposed to air pollutants during commuting. First, as we have been discussing, there are important differences in the concentrations observed in the different modes of transport. Second, the actual dose a person receives is dependent on how active the transportation is: those who walk or cycle (active transportation) have a higher physical activity, which is translated into higher breathing rates. Therefore they would get higher doses than a person travelling by car/bus (low physical activity) if they were exposed to the same pollutant concentration (a short note here: the benefits of the physical activity overweight the negative aspects of a higher dose of air pollutants in active transportation; Mueller et al., 2015). Finally, each pollutant has a different degree of toxicity. Even if you are exposed to higher PM2.5 levels in the underground, the chemical composition of these particles differ from the PM you are exposed next to a busy road.
The use of the public transportation system, cycling or walking is encouraged for commutes within the city (especially the active transportation, because of the benefit of the physical activity). The lowest the amount of private cars on the roads, the lowest the emissions of traffic-related pollutants would be, as it would be the exposure of urban population (and particularly commuters) to these emissions. Moreover, the use of private car leads to the highest emission per/person, which is actually unfair for those people who choose (or have) to use public transportation or walk/cycle for their travels.
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