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Cyclists and pedestrians inhale more particles produced by road traffic than users of motorized transport | Press room

PFor the same 30-minute period, users of different types of transport when they move inhale more than twice the dose of soot carbon they inhale when they are at a place (home, work, other). © Unsplash

The measurements of individual exposure to pollution, which are generally taken at the place of residence, ignore two major parameters: the much greater exposure during journeys and the variations in air volumes, and consequently in the quantity of airborne pollutants, inhaled according to the physical activity of people during their travels. A team of scientists supervised by Basile Chaix, Inserm research director at the Pierre Louis Institute of Epidemiology and Public Health (Inserm/Sorbonne University), studied the impact of these parameters on the exposure to soot carbon, an airborne pollutant produced by road traffic. In works published in Environment International, the research team compares different modes of transport and shows that, although walking and cycling expose the user to a concentration of soot carbon much lower than that of motorized transport (public or private), the increase ventilation due to physical activity leads to the inhalation of greater quantities of this pollutant.

Soot carbon is considered one of the best markers of road traffic. It is notably generated by the incomplete combustion of fossil fuels and other molecules produced by road traffic. Previous studies have shown that exposure to soot carbon can cause chronic respiratory disease, neurological damage, and cardiovascular disease.

In general, individual exposure to air pollutants is defined by the quantity of these pollutants in the air estimated or measured at the place of residence. However, this method ignores the significant variations in exposure related to travel and activities. However, these are responsible for a significant part of daily exposure to airborne pollutants.

A number of epidemiological studies have suggested that users of motorized vehicles (private or public transport) are more exposed to air pollution than users of so-called “active” transport, even if cyclists ride near the road traffic. However, these studies do not take into account the role of minute ventilation (flow corresponding to the lung volume mobilized in 1 minute by breathing) which is specific to each individual and varies very widely depending on the activity carried out during the activities, and which therefore has an effect on the dose of pollutants inhaled by each individual.

Doctoral student Sanjeev Bista and Inserm research director Basile Chaix, at the Pierre Louis Institute of Epidemiology and Public Health (Inserm/Sorbonne University), therefore sought to quantify the carbon soot concentrations to which users are exposed. of different types of transport and the quantities of this air pollutant inhaled during their daily journeys. The data was collected in the Grand Paris metropolis between 2018 and 2020 as part of the MobiliSense study funded by the European Research Council.

The research team followed 283 participants for 6 days each. During their journeys (locations collected by GPS) and between two journeys (when they were at their place of residence or at work for example), a sensor worn over the shoulder by each participant made it possible to measure the concentration aerial carbon soot at their breathing zone (near the nose and mouth). The journeys have been segmented according to the different modes of transport used during a trip; in the end, almost 7,500 travel segments were analyzed. By taking into account the minute ventilation of each person in each segment of movement (estimated by means of an accelerometer measuring physical activity), the dose of soot carbon inhaled by the participants during each journey could thus be quantified.

The results of the analyzes show that for the same period of 30 minutes, the participants when they move inhale more than 2 times the dose of soot carbon that they inhale when they are at a place (residence, work, other).

Furthermore, so-called “active” modes of transport are associated with a lower average soot carbon concentration in the breathing zone than motorized transport (with an even lower exposure for walking than for cycling). Compared to walking, this is +2.20 μg carbon soot per m3 of air on average in public transport with a maximum of +3.08 μg/m3 in the metro – i.e. almost twice as much as during active transport – and +2.29 μg/m3 in private motor vehicles.

However, the ranking of modes with respect to exposure is largely reversed when considering the dose actually inhaled instead of the concentration measured in the breathing zone. Cycling is associated with the highest soot carbon inhalation (+0.41 µg for 30 minutes of travel compared to walking), whereas other modes of public transport (except the metro however ) involve less soot carbon inhalation (for example -0.94 µg for the tramway for a 30-minute journey compared to walking). Similarly, the use of a private motorized vehicle is associated with less carbon soot inhalation, while this mode is associated with a higher concentration compared to walking.

Thus, although less exposed in terms of soot carbon concentrations than users of motorized transport, pedestrians and cyclists inhale more of this pollutant for an equivalent journey time.

“The fact that the amount of soot carbon inhaled is greater in active modes of transport when the user is exposed to a lower airborne concentration than a user of motorized transport is explained by a much greater minute ventilation when the ‘we use active transport’, says Basile Chaix. Indeed, the volume of air absorbed increases with the intensity of physical activity. Variations in the latter from one mode of transport to another therefore have a decisive impact on the quantity of pollutants inhaled. Thus, if cyclists inhale more soot carbon, it is because cycling combines greater physical activity and proximity to road traffic than walking.

However, it is important to point out that the inhalation of air pollutants is only one element of the picture of benefits and risks associated with the different modes of transport, and that the other pieces of the puzzle, namely exposure, must also be considered. noise, stress in transport and physical activity carried out, for which the practice of walking and cycling is widely recommended concludes the researcher. Future studies by the team will explore the physiological response of study participants, in terms of blood pressure and lung function, to airborne pollutants in transport microenvironments.

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