Analysis of various transport modes to evaluate personal exposure to PM_2.5 pollution in Delhi

Highlights

• On-road PM_2.5 exposures in six transport microenvironments ae measured in Delhi.

• Travelling in auto rickshaws and walking leads to higher exposure.

• Travelling in AC-cars and the metro had the lowest overall exposure.

• PM_2.5 mass inhaled/km is 28.2 and 5.3 times for walking and rickshaw compared to that for a metro.

Abstract

Access to detailed comparisons of the air quality variations encountered when commuting through a city offers the urban traveller more informed choice on how to minimise personal exposure to inhalable pollutants. In this study we report on an experiment designed to compare atmospheric contaminants, in this case, PM_2.5 inhaled during rickshaw, bus, metro, non-air-conditioned car, air-conditioned (AC) car and walking journeys through the city of Delhi, India. The data collection was carried out using a portable TSI SidePak Aerosol Monitor AM520, during February 2018. The results demonstrate that rickshaws (266 ± 159 μg/m³) and walking (259 ± 102 μg/m³) modes were exposed to significantly higher mean PM_2.5 levels, whereas AC cars (89 ± 30 μg/m³) and the metro (72 ± 11 μg/m³) had the lowest overall exposure rates. Buses (113 ± 14 μg/m³) and non-AC cars (149 ± 13 μg/m³) had average levels of exposure, but open windows and local factors caused surges in PM_2.5 for both transport modes. Closed air-conditioned transport modes were shown to be the best modes for avoiding high concentrations of PM_2.5, however other factors (e.g. time of the day, window open or closed in the vehicles) affected exposure levels significantly. Overall, the highest total respiratory deposition doses (RDDs) values were estimated as 84.7 ± 33.4 μg/km, 15.8 ± 9.5 μg/km and 9.7 ± 0.9 μg/km for walking, rickshaw and non-AC car transported mode of journey, respectively. Unless strong pollution control measures are taken, the high exposure to PM_2.5 levels will continue causing serious short-term and long-term health concerns for the Delhi residents. Implementing integrated and intelligent transport systems and educating commuters on ways to reduce exposure levels and impacts on commuter's health are required.

Introduction

Approximately 58% of districts in India recorded ambient particulate matter PM_2.5 (particulates with aerodynamic diameter ≤ 2.5 μm) pollution above the National Ambient Air Quality Standard (NAAQS) and 99% above the WHO guidelines in 2015 (Chowdhury et al., 2019). According to the recent Global Burden of Disease study, ambient PM_2.5 pollution in India was responsible for more than 673 thousand deaths in 2017 (Stanaway et al., 2018), although the newly developed Global Exposure Mortality Model (GEMM) reported much higher PM_2.5-attributed deaths in India (2.219 million in 2015) (Burnett et al., 2018). According to the WHO Global Ambient Air Quality Database of PM_2.5 pollution levels in more than 1600 cities in the world in 2018, 13 Indian cities are among the 20 most polluted, with Delhi being the 6th most polluted city (annual average of 143 μg/m³) (World Health Organization, 2018). In winter, the annual average PM_2.5 concentration in 2018, reported by four air quality monitoring stations (Anand Vihar, Punjabi Bagh, RK Puram and Okhla) located across the city, was above 300 μg/m³, which is approximately 5 times higher than the Indian NAAQS of 60 μg/m³, and 30 times higher than the WHO guideline of 25 μg/m³ (Nandi, 2018). Traditionally health risk analysis was conducted by assuming that the total population is exposed to the same average PM_2.5 concentration in city-level or gridded level (10 km × 10 km or 1 km × 1 km) (Maji, 2020), although personal exposure monitoring campaigns in a city have indicated high space-time variation (Menon and Nagendra, 2018).

Epidemiological studies have linked exposure to PM_2.5 with various causes of premature mortality and morbidity (Bowe et al., 2019; Fu et al., 2019; Antonsen et al., 2020; Chen et al., 2017). Health risk studies assume equivalent toxicity for all chemical species in PM_2.5, but there is considerable evidence that the chemical composition, and sources of PM_2.5 influence its health effects much more, e.g. traffic-related PM_2.5, as vehicular exhausted PM_2.5 contain a high percentage of black carbon which has much more effects on human health (Matz et al., 2019; Costa et al., 2017; Jerrett et al., 2009; Monrad et al., 2017; Bowatte et al., 2017). In on-road microenvironments, due to the proximity of tailpipe emissions, exposure to traffic-related PM_2.5 concentration is higher than those in off-road locations (Chen et al., 2020). The travel-related exposure to on-road PM_2.5 pollution has been quantified by several studies for different microenvironments, classified as travel modes, ventilation status type of travel routes, and meteorological conditions. Table 1 summarizes some of the key past studies in various settings from across the world, analysing on-road exposure to PM_2.5 pollution. The range of concentrations in the table refers to the reported average values among all the microenvironments. There are only a few studies from India looking at exposure in three-wheeled auto-rickshaws (Apte et al., 2011; Goel et al., 2015). On-road high PM_2.5 concentration in vehicles are also observed in Indonesia (87–119 μg/m³), Turkey (30.6–120.4 μg/m³), and China (54.5–71.6 μg/m³), and the lowest values are from cleaner high-income settings in the USA (12–35 μg/m³), Europe (7.3–13.9 μg/m³) and Canada (8.6–71.9 μg/m³) (Table 1).

The cities in India differ significantly from the cities in developed countries represented in Table 1. For instance, ambient PM_2.5 concentrations in Indian cities are 4–8 times higher than most high-income settings (Stanaway et al., 2018), and the traffic condition in metropolitan Indian cities is worsening daily due to increasing levels of vehicle ownership and a higher number of old vehicles (Transport Department Government of NCT of Delhi., 2018). The rickshaw and bus are one of the most common forms of public transport in Delhi, providing low-cost mobility and connecting travellers to mass transit. The rickshaw and bus sector provides a livelihood for some of India's poorest citizens and is easily available means of public transport in most of the cities (Choudhary and Gokhale, 2016). Relatively few studies have investigated on-road exposures to PM_2.5 pollution, particularly whilst travelling on these modes, in developing-world megacities such as Delhi, where older vehicles are more common and high levels of congestion and travel times lead to higher personal exposure to PM_2.5 concentrations.

The objectives of this study are (a) to assess the on-road exposure to PM_2.5 in various travel modes, measured using an optical PM monitor, and (b) to estimate the total respiratory deposition doses (RDDs) of PM_2.5 in microenvironments in Delhi (more details in supplement material). The modes studied include auto-rickshaw (three-wheelers), bus, metro, non-air-conditioned (non-AC) car, air-conditioned (AC) car and walking.