Comparison of ultrafine particles exposure in different transportation modes in Dhaka city

Abstract

Ultrafine particles (UFP) are airborne particles with a size of less than 100 nm. Due to their smaller size and presence in the air in large numbers, they pose a serious health risk. This study measures and compares in-vehicle UFP concentrations in selected transportation modes in Dhaka city, Bangladesh. Transportation modes are: (i) public bus, (ii) private car with windows open, (iii) private car with windows closed and air recirculation off, (iv) private car with windows closed and air recirculation on, and (v) Rickshaw. UFP concentrations are measured as total particle number concentrations (PNC), a commonly used metric for UFP. About 10 days of repeated measurements were collected in each transportation mode on a selected route in different parts of the day (morning, mid-day, afternoon). Results indicate that (1) in-vehicle PNCs vary by transportation mode, higher in the public bus and private car with open windows and lower in the private car with closed windows and air recirculation on. The inter-modal variations are about a factor of two. (2) In all selected transportation modes, the in-vehicle PNCs are substantially higher than the urban background level; they are about 4-8 times higher in the public bus and private car with open windows and 2-4 times higher in the private car with closed windows and air recirculation on. (3) In-vehicle PNCs vary spatially and temporally. For all selected transportation modes, concentrations are typically higher in the morning than in mid-day and afternoon. Spatial variations (a factor of 2-4) are much larger than temporal variations (a factor of 1.2-1.8). (4) Substantially higher in-vehicle PNCs and their inter-modal variations have implications on PNC personal exposures. About 1-2 hours commuting time (4-8% of daily hours) contributes 20-35% of daily PNC exposures. These exposures are higher for public bus commuters and lower for private car commuters, specifically, those who commute with car windows closed and air recirculation on. (5) Inter-modal variations of in-vehicle PNCs are linked with exposure disparity among commuters’ income groups. The commuters in the lower-income group (< BDT 25K per month) have about 20% higher exposure than the higher income group (> BDT 75K per month). Results from this study could help to design appropriate mitigation measures for reducing personal exposure to UFP.