6.1 Ventilation and Ventilation settings
Several studies have reported that air conditioning systems can reduce exposure to pollutant concentrations (Chan et al. 2002a; Qiu et al. 2017; Kolluru and Patra 2020). For example, Chan et al. (2002) and Kolluru and Patra (2020) found lower exposure concentrations in air-conditioned buses compared to non-airconditioned ones, while Kongtip et al. (2012) reported that non-air-conditioned bus drivers had significantly higher exposure to PM2.5 than A/C bus drivers. Similar results were observed in a study by Wu et al. (2013), which showed that using air conditioning in taxis reduced exposure levels by 83%, and Saksena et al. (2008) found that turning on the air conditioner in cars reduced PM10 levels by 62% but had no effect on CO levels. Tran et al. (2021) also reported that cars and taxis with controlled ventilation had the lowest exposure concentrations.
It was observed that the exposure level on subways was frequently lower than that of road transport such as buses and taxis Chan et al. (2002). This may be because air conditioning in train carriages helps prevent the infiltration of pollutants within the train (Li et al. 2017b). In contrast, a study by Tran et al. (2021) found that applying ventilation methods of mixing outside air with the air in the carriage was a prime contributor to PM in the carriage. Thus, the mode of ventilation is critical in determining exposure to pollutants.
While air conditioning has been found to reduce exposure to pollutants, some authors have observed elevated levels of CO2 caused by exhalation of commuters and insufficient air circulation/ventilation (Cheng et al. 2012). In fact, Kongtip et al. (2012) found the average CO2 concentrations on the four routes were found to be higher in A/C buses, with an average of 1274.32 ± 245.47 ppm, compared to non-A/C buses, with an average of 463 ± 42.27 ppm. The key finding from this section is that air conditioning can help reduce exposure to pollutants. However, the mode of ventilation is crucial in determining exposure levels, with controlled ventilation being the most effective. It is also essential to note that air conditioning can lead to increased CO2 levels in some cases, highlighting the need for proper ventilation.
6.2 Travelling Route/Condition
Several findings from these studies suggest that commuters' exposure to PM varies significantly across different transport modes, routes, and stops. A study by Qiu & Cao (2020) investigated the exposure of commuters to PM in road transport modes, and their findings indicated that commuters' exposure levels were lower in compressed natural gas (CNG) buses riding on arterial roads compared to the two-way ten-lane expressway. This can be attributed to the large amount of exhaust emissions that exacerbated the pollution level of the two-way ten-lane expressway. As (Manojkumar et al. 2021) reported, commuters passing through the traffic route were 1.1 to 3.9 more exposed to pollutants than those passing through the residential route and frequent pollutants elevation were often observed at traffic route near bus tops, major intersect and traffic signals. Similarly, Amouei Torkmahalleh et al. (2020) and Chan et al. (2002) found that high traffic conditions, emission from power plants and railway station and frequent stops by some bus commutes modes elevated the exposure to PM in the older area of the city. In contrast, Lin et al. (2022) observed that passengers taking buses passing through intracity areas are often exposed to a higher concentration of PM than those commuting through intercity ones, and exposure often becomes more elevated when approaching the bus station. This highlights the importance of considering the route and stops of a bus when assessing commuters' exposure to PM.
For train or subway transport modes, (Gong et al. 2019) found that commuters' exposure to PM was greatly influenced by whether the train was passing through an underground segment or an above-ground section of the train line. Their study showed that PM2.5 concentrations inside metro carriages increased by 58.3% as the metro was driven from underground to above-ground, mainly due to elevated PM2.5 on the road entering the carriage through the ventilation system. Lin et al. (2022) also observed that intracity trains often have higher elevated PM than intercity ones, and exposure often becomes higher when approaching train stations. Pedestrians' exposure to PM is also significant in areas with high vehicle emissions, such as intersections and bus stations. found that pedestrians' exposure to higher PM2.5 and PM10 may be influenced by the exhaust emitted near the ground level as well as by street-level PM (Qiu et al. 2017).
6.3 Seat Location
Recent research has investigated the impact of seat positioning in buses on individuals' exposure to air pollutants during their routine commutes. Qiu & Cao (2020) conducted a study to examine the effects of seat placement on air pollution levels encountered by passengers in CNG buses. According to the results, the region exhibiting the most significant accumulation of PM was situated at the back of the seat, succeeded by the rear compartment of the vehicle and the anterior portion of the bus. According to the authors, this phenomenon is attributable to the frequent use of the back doors as ingress points for new passengers, coupled with the capacious rear section of the bus which facilitates the infiltration of contaminants from the surrounding environment upon the opening of the doors. Kolluru and Patra (2020) conducted a study that investigated two distinct categories of buses, namely regular buses and AC buses. The rear seats of AC buses exhibited the highest concentrations of PM during the journeys. The proposition put forth by the authors posits that the heightened concentration observed in the rear seats can be attributed to multiple factors. Initially, it is noteworthy that the rear section of the bus generally accommodates a greater number of seats. As a result, the movement of commuters in this area instigates the re-suspension of dust particles present on the floor, window curtains, and seat covers. Additionally, the placement of the engines towards the back of the bus is a contributing factor to increased levels of PM in the vicinity of the rear seats. Crankcase emissions from the engine, which can be classified as a type of self-generated pollution, have the potential to infiltrate the interior of the vehicle and exacerbate the already heightened concentrations of PM.
In regular buses, PM10 concentrations were higher in the rear seats, while PM2.5 and PM1 concentrations were higher in the middle seats. The possible reason for the higher concentration of PM10 in the rear part is the resuspension of floor dust due to passengers' movement. In open window buses, the tailpipe is located between the axles towards the right side of the bus. The negative relative pressure during the bus's forward movement causes the exhaust leaving the tailpipe to be drawn back towards the surface of the bus. This phenomenon is more pronounced in the middle part of the bus, which might have increased the overall fine particle concentrations.
Chan et al. (2002a) conducted a study to investigate the influence of seat location on the level of exposure to air pollutants in double-decker buses. The research revealed a noteworthy dissimilarity in PM10 concentrations between the upper and lower levels of double-decker transportation means, whereby the upper deck was linked to a reduction of 18-25% in PM10 exposure for commuters. The findings of these research studies emphasise the significance of the seating position in relation to the level of air pollutants encountered during daily commutes. The results indicate that there is a potential elevated susceptibility to PM exposure for those who occupy the rear seats of compressed natural gas (CNG) buses or the lower level of double-decker buses. According to the research findings, it has been indicated that the utilisation of upper decks in double-decker buses can potentially serve as a more efficacious approach in mitigating the exposure to PM10 during daily commutes.
6.4 Type of Vehicle
The impact of the type of vehicle on the level of pollutants that commuters are exposed to during their routine travels has been determined to be highly significant. Qiu & Cao (2020) conducted a study to investigate the levels of PM in two distinct types of vehicles, namely the compressed natural gas (CNG) bus and the plug-in electric (PE) bus. The authors observed that the PE bus exhibited comparatively lower levels of PM than the CNG bus. The study revealed that the hermeticity of the polyethylene bus had a significant impact on protecting passengers from heightened levels of PM in the ambient air. On the contrary, it has been observed that the presence of open windows elevates the likelihood of commuters being exposed to pollutants. The efficacy of an air conditioning unit in filtering coarse particles and decreasing the level of exposure concentration was observed. In a comparable study, Gong et al. (2019) conducted a study aimed at assessing the effects of old and new trains on commuters' exposure to pollutants within the confines of the vehicle. According to the findings of the study, the mean concentration of PM2.5 within the older metro carriages was roughly three times greater than that observed in the newer metro carriages. The disparity was ascribed by the authors to the variances in the filtration techniques employed in each category of transportation. The filtration efficiency of wire mesh filters, which were utilised in the old metro trains, was lower in comparison to the fibrous air filters that are currently employed in the new metro trains. The results underscore the significance of taking into account the type of vehicle in evaluating the level of pollutants to which commuters are exposed. The study determined that the implementation of airtightness and filtration techniques plays a crucial role in mitigating the levels of exposure. According to Qiu and Cao's (2020) research, air conditioning has been found to be a viable method for decreasing exposure concentration.
6.5 Traveling Period
There is a limited amount of research that has been conducted on the effects of travel time, specifically during rush and regular hours, as well as morning, evening, and afternoon periods, on the exposure of pedestrians and commuters using different modes of transportation to PM concentration. Studies have consistently documented elevated levels of PM exposure during peak traffic periods, specifically during the late peak hours ranging from 18:00 to 19:00, and comparatively lower concentrations around midday, specifically between 12:00 to 13:00 or morning periods (08:15–09:30) (Chan et al. 2002b; Yan et al. 2015; Kumar and Gupta 2016; Al-Sareji et al. 2022). A study however, showed that exposure concentration in morning trips in traffic route were 1.8 times higher than afternoon trips and the exposure concentration in afternoon trips in residential route were higher than counterpart morning trips. Elevated levels of PM during peak traffic hours can be ascribed to augmented vehicular activity, resulting in a greater accumulation of contaminants in the atmosphere. In addition, it should be noted that the choice of transportation mode can have an impact on the level of exposure to PM. Specifically, individuals who commute by foot, train, bus, or taxi tend to experience elevated concentrations of PM during periods of heavy traffic (Al-sareji et al. 2022; Chan, Lau, Zou, et al. 2002; Yan et al. 2015). In contrast, Li et al. (2006) found no statistically significant distinction in the concentration of PM2.5 and PM1 during regular and rush hours within the train. However, there was a significant difference in the concentration of PM10, with higher exposure levels during regular periods.
Furthermore, research has conducted comparisons of PM exposure levels between weekdays and weekends. According to Chau et al. (2002), the levels of exposure during weekends were greater than those during weekdays. This was attributed to the fact that individuals tend to spend a larger proportion of their time in restaurants and pubs during weekends, as opposed to offices and schools during weekdays. The elevated levels of PM exposure observed during weekends underscore the significance of accounting for non-vehicular sources of PM, including indoor activities, in exposure evaluations.
6.6 Passenger Density
The significance of passenger density in determining the level of pollutant exposure during a typical commute has been established by various scholars through their respective research studies (Li et al. 2006; Cheng et al. 2012; Zhang et al. 2021). As per the findings of (Li et al. 2006), there is a substantial rise in the levels of CO2 and PM10 during peak hours, which can be ascribed to the influence of passenger volume. The discovery underscores the significance of regulating passenger volume during peak periods in order to mitigate the risk of exposure to detrimental pollutants. Zhang et al. (2021) have also documented a significant decrease in underground, walking, and car modes of transportation, amounting to 2.2, 2.6, and 5.9 times, respectively, during and after the Covid-19 lockdown. The considerable decrease can be ascribed to the substantial decline in the count of passengers resulting from the COVID-19 lockdown. Furthermore, (Cheng et al. 2012) and Kolluru and Patra (2020) have reported that the motion of passengers within the train results in the re-suspension of dust, thereby elevating the levels of PM concentration within the train.
6.7Poor Maintenance
Chan et al. stood out from the existing literature on vehicular pollutants by emphasizing the significance of poor maintenance in contributing to higher levels of pollutants in taxi cabs (Chan et al. 2002b). Through their research, they discovered that the levels of pollutants in taxis were significantly higher than in private cars due to leakages stemming from poorly maintained vehicle engines and exhaust systems, which allowed pollutants to infiltrate the vehicle cabin. The findings of their study highlight the importance of regular maintenance of vehicles, particularly for those in the transportation industry, to reduce the impact of pollutants on public health and the environment. The results of their study emphasize the need for ongoing efforts to raise awareness of the importance of vehicle maintenance, both in the private and commercial sectors, to ensure that vehicles operate efficiently and minimize their impact on the environment.
6.8 Engine Type
A team of researchers led by Wong et al. conducted an experiment in to evaluate the impact of different bus engine types on the in-cabin air quality for commuters (Wong et al. 2011). This study was significant as it went beyond the scope of previous research and investigated the effect of Euro II, Euro III, and Euro IV engines. The study revealed that exposure to CO did not differ significantly across the different engine types, but the engine types had a significant impact on the in-cabin concentration of PM10 particles. In general, buses with Euro IV engines provided a better in-cabin environment for commuters than those with Euro II and Euro III engines, which exhibited higher levels of PM10 particles. Based on their findings, the authors of the study recommended the implementation of air filtration upgrades and routine filter cleaning schedules as effective measures to improve the air quality in bus cabins (Wong et al. 2011).
6.9 Meteorological Parameters
The studies conducted by Li et al. (2017b), Onat & Stakeeva (2013), and Wu et al. (2013) underscore the importance of considering wind speed and wind direction when assessing exposure to pollutants during commutes. According to Li et al. (2017b), the mean exposure to PM2.5 in transport microenvironments during the winter was 2-4 times higher than during the summer. This finding was attributed to the higher ambient PM2.5 levels during the winter, which were the result of seasonal differences in prevailing wind direction. Onat & Stakeeva (2013) also observed that wind speed had a significant impact on in-vehicle and pedestrian exposure to PM2.5. Specifically, they found that exposure levels increased at wind speeds below 2 m/s and decreased as wind speeds increased to beyond 2 m/s during windy weather conditions. Similarly, Wu et al. (2013) highlighted the importance of wind speed and wind direction in determining total exposure to PM2.5. They noted that lower wind speeds were associated with higher levels of exposure to PM2.5 in all modes of transportation, except for taxi commutes. Conversely, as wind speeds increased, PM2.5 exposure levels decreased.
In terms of wind direction, Wu et al. (2013) found that higher levels of PM2.5 exposure were found in north-westerly and south-easterly wind sectors, both in-cabin and on the road. Pollution episodes were more likely to occur when north-westerly winds prevailed, as this was marked by weak regional flow and calm conditions, which were directly affected by emissions from sources in the northwest PRD, particularly agricultural activities in rural regions.
6.10 Temperature and Relative Humidity
Wu et al. (2013) and Onat & Stakeeva (2013) conducted a study that examined the influence of temperature and relative humidity on PM concentration in different transportation modes. Onat & Stakeeva (2013) reported that a weak correlation was observed between temperature and PM2.5 concentrations across various modes of transportation. Specifically, the correlation coefficients for bus, metro-bus, and car were 0.41, 0.075, and 0.10, respectively. The study revealed a negative correlation between temperature and particle concentration, indicating that an increase in temperature resulted in a decrease in particle concentration. The study found a slight positive association between PM2.5 levels and relative humidity, with the exception of the car transportation mode. Wu et al. (2013) reported analogous results, demonstrating an inverse association between temperature and PM concentration in the modes of transportation involving buses and taxis. The positive correlation between PM2.5 concentration and relative humidity, albeit weak, can be utilised to implement measures aimed at regulating humidity levels in vehicles. This, in turn, can lead to a reduction in the concentration of PM. The studies underscore the significance of comprehending meteorological variables that influence air quality and devising strategies to alleviate the detrimental consequences of air contamination.