Assessment and mitigation of toddlers' personal exposure to black carbon before and during the COVID-19 pandemic: A case study in Singapore

Abstract

Black carbon (BC), an important indicator of traffic-related air pollution (TRAP) in urban environments, is receiving increased attention because of its adverse health effects. Personal exposure (PE) of adults to BC has been widely studied, but little is known about the exposure of young children (toddlers) to BC in cities. We carried out a pilot study to investigate the integrated daily PE of toddlers to BC in a city-state with a high population density (Singapore). We studied the impact of urban traffic on the PE of toddlers to BC by comparing and contrasting on-road traffic flow (i.e., volume and composition) in Singapore in 2019 (before the COVID-19 pandemic) and in 2020 (during the COVID-19 pandemic). Our observations indicate that the daily BC exposure levels and inhaled doses increased by about 25% in 2020 (2.9 ± 0.3 μg m^-3 and 35.5 μg day^-1) compared to that in 2019 (2.3 ± 0.4 μg m^-3 and 28.5 μg day^-1 for exposure concentration and inhaled dose, respectively). The increased BC levels were associated with the increased traffic volume on both weekdays and weekends in 2020 compared to the same time period in 2019. Specifically, we observed an increase in the number of trucks as well as cars/taxis and motorcycles (private transport) and a decline in the number of buses (public transport) in 2020. The implementation of lockdown measures in 2020 resulted in significant changes in the time, place and duration of PE of toddlers to BC. The recorded daily time-activity patterns indicated that toddlers spent almost all the time in indoor environments during the measurement period in 2020. When we compared different ventilation options (natural ventilation (NV), air conditioning (AC), and portable air cleaner (PAC)) for mitigation of PE to BC in the home environment, we found a significant decrease (>30%) in daily BC exposure levels while using the PAC compared to the NV scenario. Our case study shows that the PE of toddlers to BC is of health concern in indoor environments in 2020 because of the migration of the increased TRAP into naturally ventilated residential homes and more time spent indoors than outdoors. Since toddlers' immune system is weak, technological intervention is necessary to protect their health against inhalation exposure to air pollutants.

Keywords: Black carbon; COVID-19; Mitigation; Personal exposure; Traffic.

Graphical abstract

Fig. 1

Overview of measurement timeline and safe distancing measures in Singapore.

Fig. 2

Diurnal cycle of PE to BC in 2019 and 2020 (days with natural ventilation, NV).

Fig. 3

Diurnal cycle of PE to BC in 2019 and 2020 (days with air conditioning, AC, and portable air cleaner, PAC).

Fig. 4

Representative time series of PE to BC concentration measured in: (a) 2019 and (b) 2020.

Fig. 5

Contributions of the home micro-environment to the BC exposure in the year 2019 and 2020 under different exposure scenarios. Different bars mean the average BC exposure in days with natural ventilation (NV), air conditioning (AC) and portable air cleaner (PAC) while the child was sleeping (SL).

Fig. 6

Traffic volume and composition during peak and non-peak traffic hours on weekdays (WD) and weekends (WK) in 2019 and 2020.