Daily commuting increases level of contaminants inhaled by urban community and it is influenced by mode and time of commuting. In this study, the commuters’ exposure to ambient particulate matter (PM2.5) and nitrogen dioxide (NO2) was assessed during three modes of travel in six different road stretches of Chennai. The mean distance of road stretches was 25 km and the exposure to pollutants was assessed during peak hours and off-peak hours. The average travel duration was in the range of 39 to 91 min in motorbike, 83 to 140 min in car and 110 to 161 min in bus. Though there was variation on exposure to concentration in modes of transportation, the maximum exposure concentration of PM2.5 was observed as 709 μg/m³ in bus and the minimum exposure concentration was 29 μg/m³ in closed car. Similarly, the maximum exposure concentration of NO2 was observed to be 312 μg/m³ in bus and the minimum exposure concentration was 21 μg/m³ in car. The concentration of elements in PM2.5 was in the order of Si > Na > Ca > Al ≥ K > S ≥ Cd, with Si and Cd concentration as 60% and < 1% of the PM2.5 concentration.

Carbon monoxide (CO) is a colorless and odorless gas, has more affinity to hemoglobin, which increases carboxyhaemoglobin content in the blood. The increase in carboxyhemoglobin decreases the oxygen carrying capacity of blood and causes carbon monoxide poisoning, resulting in headache, nausea, dizziness and in extreme cases death. The emission of carbon-monoxide from idling vehicles at traffic intersections emit 45-55% more CO, compared to transit vehicles in roundabouts [1-5]. The fuel consumption increased by about 150% when accessories like air-conditioner was used during idling [6]. The total quantity of fuel wasted at five intersections was about 389.68 L of diesel and 810.38 L of petrol per day and the estimated monetary loss due to idling was ranging from Rs.3crore to 5crore /annum [2, 7-10]. The in-vehicle air is an extension of outdoor air, which infiltrates through ventilation settings, cracks and vents [11-13]. The in-vehicle CO is affected by outside sources and it reaches peak values when the surrounding vehicles are idling [14]. Thus, there is a possibility of in-vehicle CO build-up at traffic intersections. Irrespective of the age of the car, there was a high likelihood for self-pollution [15]. The car cabin acts as a sink during A/C ON-mode, which results in increase of in-vehicle CO by accumulation [16]. The overview of studies that have been conducted on in-vehicle CO is presented in Table S1. This study has two parts, first part is the monitoring of ambient CO levels at a traffic intersection in Chennai along with fuel consumption during vehicle idling and the second part of the study includes effect of ambient CO on in-vehicle CO at the signalized traffic intersections and modeling of In-vehicle CO due to ambient CO using Principal Component Regression Analysis (PCRA). 2. Material and Methods 2.1. Monitoring of CO The ambient CO was monitored at a signalised traffic intersection of Vijayanagar, Velachery, Chennai which connects three main