Investigation into Beijing commuters’ exposure to ultrafine particles in four transportation modes: bus, car, bicycle and subway

Abstract

Traffic-related air pollution contributes considerably to commuters' daily air pollution exposure. Ultrafine particles (UFP) is one of the most commonly measured traffic-related air pollutant during commuting. There are very limited studies on commuting exposure to air pollution in China. To fill the knowledge gap, we conducted the first study that compares commuters' UFP exposure among four modes in a Chinese city. In this study, we measured particle number concentration (PNC) levels of 63 trips in four transportation modes: bus, car, bicycle and subway in 2015 on a typical commuting route between urban and suburban areas in Beijing. We found that buses experienced the highest PNC (geometric mean (SD): 35,066 (10,856) particles cm⁻³), followed by bicycles (geometric mean (SD): 28,277 (7942) particles cm⁻³), then cars (geometric mean (SD): 16,302 (5156) particles cm⁻³), and finally the subway (geometric mean (SD): 13,245 (2936) particles cm⁻³). Within the subway mode, we detected the highest PNC at station entrances, followed by the transfer tunnel, while cabin areas showed the lowest levels. For the cabin areas of the subway mode, PNC concentrations were higher when the trains were overground than underground. Measures are needed to reduce commuters’ exposures, particularly those traveling by bus and bicycle. Switching the bus transportation system from diesel to cleaner power, such as compressed natural gas or electricity, may help.

Introduction

Air quality in urban areas is affected mainly by road traffic emissions(Heydari et al., 2020), which have an established adverse effect on populations’ health(Kasuga, 1989). Particles from vehicle exhaust, resuspension of road dust and tyre and brake wear, such as fine particulate matter (PM_2.5), ultrafine particles (UFP, PM_0.1), as well as toxic gases such as carbon monoxide (CO), nitrogen oxides (NO_x), and volatile organic compounds (VOCs), emitted from the combustion of fossil fuel(Kittelson, 1998), have been demonstrated to be associated with various adverse health outcomes, including but not limited to cardiac and respiratory morbidity and mortality(Analitis et al., 2006; Nayebare et al., 2019), cancer(Borm et al., 2004), adverse effects on fetal growth(Dejmek et al., 2000) and male fertility(Xue and Zhang, 2018).

Ultrafine particles are defined as particles with diameters less than 100 nm, typically constituting more than 90% of particle number count (PNC) in areas exposed to traffic emissions(Morawska et al., 2008). One of the major sources of UFP in urban areas is vehicle emissions, formed through gas-to-particle conversion or incomplete fuel combustion(Amec, 2011; Morawska et al., 2008; Shi et al., 1999). Compared to their larger-size counterparts, UFP cannot be filtered out by the nose or bronchioles, and are capable of penetrating deep and depositing efficiently into the lung, entering blood circulation, and translocating to various target organs, such as the heart, liver, and neural system, within hours, thus posing greater risks to human health(Geiser et al., 2005; Genc et al., 2012; Kreyling et al., 2006; Kumar et al., 2014). Translocated UFP lead to oxidative stress and inflammatory responses in target tissues, directly harming these organs(Leikauf et al., 2020; Schraufnagel, 2020). UFP in the lung is also capable of causing damages to distal organs through lung inflammation and subsequent release of proinflammatory cytokines(Schraufnagel, 2020). UFP are significantly associated with mortality(Wichmann et al., 2000), lung injury(Leikauf et al., 2020) and increased burden on patients with respiratory diseases(Dick et al., 2003), cardiovascular diseases(Genc et al., 2012), cognitive dysfunction(Cory-Slechta et al., 2018), diabetes(Bai et al., 2018), and cancer(Goldberg et al, 2017, 2018;; Ohlwein et al., 2019).

Commute duration and proximity to emission source can affect the level of exposure, and thus the associated health risk, for commuters(Cole-Hunter et al, 2012, 2013;; Gee and Raper, 1999). The level of pollutant exposures in the traffic micro-environment is highly elevated compared to elsewhere(Shi et al., 1999). Thus, even short durations of commuting can pose great threats to health due to the high level of air pollution in traffic and daily repeated exposures during commuting, especially in rush hours(Kaur et al., 2007). Mode of transport, as well as the commuting route and type of vehicles, plays an influential role in the exposure levels of particulate matters(Zuurbier et al., 2010). Several studies have been carried out to compare the exposure levels of UFP among travel modes and vehicles in developed countries(Knibbs et al., 2011), including Europe(de Nazelle et al., 2017; Moreno et al., 2015; Onat et al., 2019; van Nunen et al., 2020), United States(Ham et al., 2017), Canada(Apparicio et al., 2018). However, these results are inconsistent. In Asia, several studies have focused on the relationship between transportation mode and PM_2.5(Huang et al., 2012; Qiu et al., 2017; Shen and Gao, 2019; Wang et al., 2017; Zheng and Qiu, 2020); however, few studies have compared the UFP concentration among different travel modes.

The relationship between travel modes, UFP, and health in Asian cities has not been sufficiently investigated. Our approach was to measure UFP on a route between a suburban destination and an urban destination in the Beijing metropolitan area in the present study. We repeated measurements on the same route with four distinct travel modes (bus, car, bicycle and subway) to capture UFP exposure during commuting. For the subway mode, as it involved different micro-environments (station, underground, transfer tunnel, overground and ambient environment), a comparison within this travel mode was made to reveal actual exposures of different micro-environments.