Occupational hazard to on-road air pollution within passenger transport micro-environments: evidence from traffic congested areas in Colombo, Sri Lanka

Abstract

Background

Traffic-related on-road air pollution is a major contributor to outdoor air pollution globally, particularly in developing countries most often due to outdated vehicle technology, and weaker emission control regulations. Drivers providing passenger transport services are constantly at higher risk. This study investigated the exposure to air pollutants of transport microenvironments (TME) of such vehicles most popular in heavy-traffic congested areas in the district of Colombo, Sri Lanka.

Methods

A cross-sectional study was conducted in the TME of three vehicular types: three-wheeled taxi, non-AC bus and AC car. Randomly selected vehicles made 288 trips (96 by each vehicle type) of more than one hour on four pre-arranged weekdays on typical sunny days, on the same road with minimal off-road air pollution. Ambient and in-vehicular particulate matter (PM) concentrations were measured using real time monitors, while being driven under the same conditions.

Results

The average ambient PM_2.5 concentration was 115.5 μg/m³ (SD = 6.2), representing fine particles in the ambient air. As a fraction of the total PM₁₀ concentration (PM_2.5/PM₁₀), it was 0.83. In comparison, this fraction was 0.85 within three-wheeled taxis, 0.83 within non–AC buses and 0.93 within AC cars. PM₁₀ and PM_2.5 median concentrations were 456.3 μg/m³ (SD = 47.2) and 386.2 μg/m³ (SD = 26.7) within three-wheeled taxis, 292.1 μg/m³ (SD = 36.3) and 239 μg/m³ (SD = 20) within non–AC buses and 63.5 μg/m³ (SD = 7) and 59.2 μg/m³ (SD = 6) within AC cars, respectively. PM ratio (in-vehicular/ambient) was three-fold within three-wheeled taxis; two-fold within non–AC buses; and two times lower within AC cars (p < 0.001). When comparing the PM ratios between vehicles, it was seven times higher inside a three-wheeled taxi than in an AC car.

Conclusions

Compared to other TMEs, drivers of three-wheeled taxis were most vulnerable to on-road air pollution. Redesigning such vehicles for safer cabin environment is recommended.

Background

Air pollution, which is defined as ‘contamination of the environment by any physical, chemical or biological agent that modifies the natural characteristics of the atmosphere’ [1], is considered the single most important environmental health problem in the world today. It is typically classified as ‘indoor’ and ‘outdoor’ air pollution [2].

Outdoor air pollution arises from various natural and man-made sources found in the built-environment [3], and is categorised accordingly, into traffic-related, industrial and agricultural [4]. Of these, traffic-related air pollution stands out as a major contributor to the overall ambient air quality [5]. It encompasses emissions from vehicles and affects both commuters on the road (on-road air pollution) and individuals living, work-in or attending school near major roads (near-road air pollution) [6]. Research has shown that on-road traffic-related air pollution has a greater detrimental impact on the health and wellbeing of commuters compared to other sources [7]. Studies have further revealed that the pollutant levels inside a vehicle, i.e. within the transport microenvironment (TME), can be 2–10 times higher than the on-road concentrations [8]. However, the exact exposure levels to air pollutants depend on the specific microenvironments that individuals inhabit. In this regard, different microenvironments used for passenger transport, such as personal automobiles, public transport modes and hired vehicles, have been frequently researched into, to ascertain the most adverse in-cabin air pollutant concentrations [9, 10].

Especially in Asian cities, the overall traffic contributes to 20–40% of the ambient air pollution, with much higher levels in highly traffic congested streets [9]. This heightened exposure poses considerable health risks, particularly for individuals who spend a considerable amount of time on road during peak traffic hours, such as the drivers of vehicles providing passenger transport services. Consequently, such exposure has been recognised as a serious occupational hazard.

Notably, the TMEs of open non-air conditioned (AC) vehicles tend to have higher pollution levels compared to the TMEs of enclosed AC vehicles [10]. There may be several other actors, such as the vehicle clearance, speed, time spent in traffic and driving habits contributing to this difference. However, there is limited research comparing the air pollutants within different passenger TMEs, making it an under-explored area.

Sri Lanka, a South Asian country, is currently facing a growing problem of on-road air pollution owing to high population density, rapid vehicular growth, inadequate traffic management systems and unplanned urbanization [11]. A significant decline in air quality has been noted over the years, particularly in densely populated districts like in the commercial capital, Colombo [12]. For passenger transport, non-AC buses are in operation in the public transport service, while AC cars and open-sided three-wheeled taxis, predominantly provide private taxi services. Three-wheeled taxis, also known as ‘tuk-tuks’, remarkably represent the second largest category of vehicles in Sri Lanka (1 062 447), with over a million in operation [13]. Although they provide an essential and cost-effective mode of transport, three-wheeled taxi drivers remain neglected even more than the public bus drivers on occupation health-related aspects in Sri Lanka. Hence, this study aimed to compare the exposure to air pollutants of TMEs of the vehicles popularly used for passenger transport services in relative to the ambient environment in heavy traffic-congested areas in Sri Lanka. This research addresses the occupational hazard of neglected occupational groups; and has broader implications for the entire society.

Methods

This was a cross-sectional study conducted in heavy traffic congested areas in Colombo, Sri Lanka (Table 1).

Table 1.

Characteristics of the transport microenvironments (TME) of passenger transport services considered for the study

Vehicle type	Details of the vehicles
Three-wheeled taxis	• Motorized [14]
	• Seating capacity less than 4
	• Non-air conditioned (open-windowed)
	• Fuel type—Petrol
Air-conditioned cars	• Curb weight 1150–1800 kg and shadow 6.5–8.5 m2
	• Seating capacity 5
	• Air-conditioned
	• Engine capacity 800 to1000 cc
	• Fuel type – Petrol
Non-airconditioned buses	• High floor buses with BS 3/4 engine
	• Seats – 49, 54, 58
	• Non-air conditioned (open-windowed)
	• Fuel type – Diesel

This district representing the most populous region in Sri Lanka [15] and reports the highest vehicle density with 70 min spent on average during travel per person per day [16]. It has a complex motorable road network, of which Class A (national highway roads connecting major provincial cities and major ports which allow 70 km/h which are further classified as AA, AB and AC roads) and Class B (major roads connecting centres of district administration such as urban cities with district secretariats and courts or feeds highways and expressways which allow maximum 60 km/h) roads cross many strategic junctions and commercial hubs, thus are subject to heavier traffic congestion compared to other road types.

The study included vehicle trips made by three types of vehicles (three-wheeled taxis, non-AC buses and AC cars) representing the most common TMEs used in passenger transport services in heavy traffic congested areas in Colombo District. Vehicles manufactured before 2010, those with broken exhaust pipes or with added smoke producing sources (i.e., cigarettes, incense sticks) that were likely to produce additional air pollution, and air-conditioned cars which did not use recycled air mode and moderate fan speed inside vehicles or had additional features to reduce air pollution (e.g., cabin filters) were not considered for the study.

The minimum number of vehicle trips required for the study was 96 from each passenger TME, in order to detect an average ratio between in-vehicle and ambient pollutant concentrations, based on the findings of a study in India [8, 17]; standard deviation (SD) of 0.05; Z value of 1.96; and precision of 0.01 [18]. This sample was obtained by randomly selecting 6 vehicles each, from a list of non-AC buses belonging to the National Trasport Board; and from two separate lists of AC cars and three-wheeled taxis obtained from a reputed taxi company (18 in total) that were routinely commuting on four one-way trips per day over four days (3 types of passenger TME × 6 vehicles per TME × 4 one-way trips per day × 4 days). All the vehicles were carrying more than one passenger as a service along the same road selected for the study. The selection of road was done purposively from among Class A and Class B roads of Colombo District that recorded an average vehicular speed of less than 25 miles per hour during peak hours on typical weekdays, as per the real-time traffic information obtained from google maps. This selection also considered the following features: a straight road with good road conditions extending over relatively flat land (measurements may get affected when done on uneven roads, road abrasion, winding roads, brake and tyre wear) [19]; at least 10–15 km long (measurements should be done at least for one hour as per ambient air quality standards) [9, 20, 21]; serves as one of the main entry points to Colombo City and crosses a considerable number of main junctions and cities, leading schools and business/commercial zones [22]; located inlands (measurements may get affected if close to the coastal belt owing to sea breeze) [23]; not overtly crossing major industrial zones (measurements affected by off-road air pollution); and not include a railway crossing (air pollutant levels abruptly increase due to re-suspension of particles) [24, 25].

The trips of all 18 vehicles took place along the road selected for the study, on four pre-arranged weekdays (excluding public holidays and school vacation) which forecasted a typical sunny day with no rain in the next 24 h. Each vehicle made two trips in the morning and another two in the evening per day. On each day, all the vehicles were requested to arrive at a pre-arranged starting point (between 7.00–7.15 am for morning trips and between 4.00–4.30 pm for evening trips) and to drive back once they reached the end point. In order to assess the exposure to air pollution during each journey, particulate matter (PM) monitors were fixed to each vehicle at the starting point and removed at the end point. The monitors were fixed immediately behind the driver’s head in three-wheeled taxis and buses, and on the dashboard of cars, so that the monitors were at the breathing level of the driver. The monitors were connected to the cars through a USB port, to buses through the USB output of CD player, and to the three-wheeled taxis directly through the battery via a lighter socket with a fuse and clips. Simultaneously, the on-road ambient air pollutant levels were measured using PM monitors fixed in the outside environment at the starting and end points.

During each vehicular trip, travel time including the traffic condition (towards and against traffic) was recorded. Also, the quantities of PM₁₀, PM_2.5 and PM₁ within each TME (in-vehicular) as well as in the environment (ambient) were assessed in μg/m³, based on the real time PM monitor data derived using Light Scattering Techniques. The PM monitors continuously recorded data at every 30 s and stored it in a data-logger, so that data could be retrieved at any time during the sampling period. These monitors have been developed and validated by the National Building Research Organization (NBRO), Sri Lanka and are proven to be valid and reliable [26]. To ensure accurate quantification, the monitors were calibrated against ꞵ-attenuated technique, the gold standard sampling technique recommended for PM testing. Reliability of the monitors was assessed in 10 randomly selected vehicles, by comparing the one-hour readings of two PM monitors fixed inside the same vehicle. Prior to data collection, the monitors were calibrated. The data were collected by a team of IT graduates trained by the technical staff of NBRO.

Data analysis

Data analysis was done using the Statistical Package for the Social Sciences (SPSS) version 22. The average ambient as well as in-vehicular air pollutant concentrations (PM₁₀, PM₁₀—PM_2.5, PM_2.5 and PM₁); PM fractions (PM₁₀-PM_2.5/PM_10, PM_2.5/PM₁₀ and PM₁/PM_2.5); PM ratios between ambient and in-vehicular environments (e.g. PM₁₀ in-vehicular/PM₁₀ ambient); and PM ratios between vehicle types (e.g. non-AC bus PM₁₀/AC car PM₁₀) were calculated. These values were compared between the three types of TME. Since the continuous PM data did not assume a normal distribution (p < 0.01 in Shapiro Wilk normality test), non-parametric Kruskal–Wallis Rank Sum Test was used for comparisons. Further, pair-wise comparisons were made using Mann–Whitney U test.

Results

A total of 288 vehicular trips (96 from each TME type) were completed. On average, the longest travel time was recorded for buses (mean = 77 min; SD = 14) and the shortest for three-wheeled taxis (mean = 43 min; SD = 8.11). These differences across the TME types were significant (p < 0.001).

Ambient air pollution

Particles less than or equal to 10 μm in aerodynamic diameter (PM₁₀) include both ‘coarse’ particles (PM₁₀-_2.5, with particle sizes between 2.5 and 10 μm) and ‘fine’ particles (PM_2.5, with particle sizes less than or equal to 2.5 μm). The latter group also has submicron (PM₁ with particle size less than 1 μm) particles. Coarse particles usually represent the dust in air, while fine particles represent the vehicular emissions.

In ambient air (Table 2), the average concentration of PM₁₀ was 139.1 μg/m³ (SD = 14.7), with a predominance of fine particles. As a fraction of the total particles less than 10 μm, fine particles (0.83) and coarse particles (0.17) highlighted a five-fold difference. Within the concentration of fine particles, submicron particles as a fraction were 0.65.

Table 2.

Distribution of the air pollutants in ambient environment

Ambient air pollutants	Mean (SD)	Median (Q3-Q1)
PM level (μg/m3)
PM10	139.1 (14.7)	140 (128–150)
PM10—PM2.5	25.0 (11.0)	24.3 (15–33)
PM2.5	115.5 (6.2)	115 (109–119)
PM1	74.6 (6.1)	72.9 (71–80)
PM fraction
(PM10—PM2.5)/PM10	0.17 (0.06)	0.16 (0.11–0.22)
PM2.5/PM10	0.83 (0.06)	0.8 (0.77–0.88)
PM1/PM2.5	0.65 (0.05)	0.64 (0.62–0.66)

Air pollutant levels within passenger TMEs

Figures 1, 2, 3 and 4 compares the air pollutant levels in the three passenger TMEs.

Fig. 1.

Air pollutant concentration within the transport microenvironments

Fig. 2.

PM fraction within the transport microenvironments (N = 96)

Fig. 3.

Distribution of the PM ratios of each passenger TME with ambient air

Fig. 4.

Comparison of the Ratios of Vehicular PM Between TMEs

Within the passenger TME, the highest median concentrations of coarse particles as well as fine particles and submicron particles were noted in three-wheeled taxis followed by non-AC buses (Fig. 1).

The lowest concentrations were seen for all three types of particles in the AC cars. In three-wheeled taxis, fine particles as a fraction of the total particles less than 10 μm (0.85) were much higher than that calculated for coarse particles (0.15), giving almost a six-fold difference (Fig. 2).

The corresponding difference in PM fractions of fine and coarse particles was five-fold (0.83/0.17) in non-AC buses and in AC cars, it was more than ten-fold (0.93/0.07). In contrast, submicron particles as a fraction of the fine particles were highest for AC cars ((0.79) compared to the same value obtained for non-AC buses and three-wheeled taxis (0.66).

Comparison of the PM levels relative to the ambient air pollutants in the three passenger TMEs

In-vehicular air pollutants of each passenger TME were compared as a ratio with ambient air pollutants (Fig. 3).

PM₁₀ level as a ratio of the ambient air pollutants was highest for three-wheeled taxis followed by non-AC buses and AC cars. It further reflected three-times higher vehicular concentration within three-wheeled taxis; two-times higher concentration within non-AC buses; and two-times lesser concentration within AC cars compared to its ambient air. These values differed significantly (H = 255.12; df = 2; p < 0.001) and in pairwise comparisons (p < 0.001). The corresponding PM_2.5 ratios as well as PM₁ ratios in relation to its ambient air reflected three-times higher concentration within three-wheeled taxis; two-times higher within non-AC buses; and two-times lesser within AC-cars. The three types of TMEs differed significantly from each other in relation to PM_2.5 ratios (H = 255.124; df = 2; p < 0.001) and PM_2.5 ratios (H = 255.027; df = 2; p < 0.001).

Comparison of the vehicular PM as a ratio between passenger TMEs

The highest PM concentration ratio was between three-wheeled taxis and AC-cars, while the lowest was between three-wheeled taxis and non-AC buses (p < 0.05) (Fig. 4).

Further, the PM ratios between three-wheeled taxi and AC car, and those between non-AC bus and AC car were decreasing in trend, as the PM size decreases.

Discussion

The study provides novel insights, to address the knowledge gaps in comparison of common passenger transport services in Asian countries, which are subject to traffic-related on-road air pollution as an occupational hazard, particularly in the context of Sri Lanka. It sheds light on occupational hazards associated with in-vehicular pollution in the three most popular passenger transport services. These findings provide important evidence for environmental hygiene policy development and management. Among these, three-wheeled taxis pose the highest risk for in-cabin air pollution, followed by non-AC public buses, compared to AC cars.

Ambient air pollution

The study reported an average PM₁₀ and PM_2.5 concentrations of 139.1 μg/m³ and 115.5 μg/m³, respectively. in ambient air during the sampling period, which highlights elevated levels compared to historical estimates for Colombo City (50 μg/m³ of PM₁₀ and 34 μg/m³ of PM_2.5 in 2000–2008) [27]. While these values cannot be directly compared to the 24-h Sri Lankan Air Quality Standard (100 μg/m³) owing to sampling limitations, they emphasize the potential for significant short-term exposure risks. This indicates an increasing trend in air pollution particularly in densely populated districts like Colombo in Sri Lanka, where annual ambient PM₁₀ levels consistently surpass the WHO, United States Environmental Protection Agency (USEPA) and Sri Lankan Guideline values [28]. In contrast, developed countries, which benefit from robust air quality regulations, effective emission control measures, advanced transportation systems, cleaner energy sources, and comprehensive urban planning, report lower PM₁₀ concentrations, such as 52.5–127.4 μg/m³ in Australia [29] and 61–107 μg/m³ in Netherlands [30]. PM_2.5 concentrations are also lower, ranging from 21.8 μg/m³ in Los Angeles, USA to 67–33 μg/m³ in Netherland [30]. With regards to other Asian countries, only a few, namely Taiwan, Malaysia and Hong Kong exhibit lower ambient air pollution levels similar to developed countries [31–33]. In contrast, Indian studies report PM₁₀ concentrations as high as 658.4 μg/m³ in Delhi [34]; 252.63–388.72 μg/m³ in Puducherry [24]; 382.3 μg/m³ in Lucknow City [35]; and 304 μg/m³ in Kolkata, which are well above the levels observed in our study. PM_2.5 concentration in India also exceeds those found in our study, ranging from 132–253 μg/m³ in Delhi [8] and 31–291 μg/m³ in Lucknow City [36] and in China of 132.5 µg/m³. Such variations within Asian countries may be attributed to differences in factors such as fuel quality, energy sources, industrial activities, population density, and the effectiveness of air pollution control policies.

Comparison of ambient air with in-vehicular air of different TME

The study revealed that air pollution was greater within the vehicles used for passenger transport service than outdoor, in terms of the absolute PM concentration and PM ratio. It was also shown that with regards to in-vehicular pollution, all air quality parameters were higher in the non-AC vehicles (three-wheeled taxis and public transport buses) compared to AC vehicles. Studies conducted in various regions of the world support our findings, demonstrating that open windows or non-AC vehicles lead to increased PM_2.5 and PM₁₀ concentrations [8, 19, 30, 37, 38].

Notably, three-wheeled taxis imparted the greatest risk for in-cabin air pollution. This could be that, owing to its smaller size, they travel closer to the ground and at the same level of the exhaust pipe of heavy vehicles in front. In the absence of any studies from Sri Lanka, the findings are compared with a study conducted in New Delhi, India known for its heavy traffic congestion. It reported a PM_2.5 ratio of 1.5 for three-wheeled taxis compared to 0.6 for AC cars [17]. Comparatively, our study reports even higher PM ratios for three-wheeled taxis, with PM₁₀ ratios at 3.27 compared to 0.45 for AC cars and 2.09 for non-AC buses. The corresponding PM_2.5 ratios were 3.36, 0.52 and 2.08 and PM₁ ratios were 3.44, 0.62 and 2.09, respectively. This highlights the elevated risk associated with three-wheeled taxis.

Compared to three wheeled taxis, AC cars exhibit the lowest levels of in-cabin air pollution, even less than in ambient air, likely due to the filtration effects of air conditioning systems. However, when considering PM_2.5/PM₁ fraction, which is more concerning for health effects, AC cars had the highest values. This may be attributed to air conditioning systems being more effective at filtering coarse particles than fine particles.

The differences noted in air pollutant values could be due to the accuracies of the equipment used for measurements. The study addressed this by using PM as the proxy indicator, which is shown to be more reflective of air quality [39] than other criterion air pollutants (PM, NO₂, SO₂, CO, O₃ and Pb) [40]. Also, having the monitors placed closer to ground level minimized the effect of larger particles (re-suspension). Further, confounders such as road, environmental and vehicle conditions were minimised by applying rigorous criteria when selecting the vehicles and road, limiting the study to sunny days and selecting the same road and times for measurements.

Our study provides vital information on the exposure of two highly neglected occupation groups (public bus and three-wheeled taxi drivers) highlighting their occupational hazard to air pollution in heavy traffic congested areas. Such studies are scarce in the region, with its burden greatly masked. The study findings are applicable to other countries of similar economic backgrounds, where non-AC vehicles are the main mode of public transport in urban cities [4].

Findings highlight major implications on the occupational hazard especially of three-wheeled taxi drivers in Sri Lanka, where over 110 000 vehicles entering Colombo are three-wheeled taxis amounting to 23% of the total vehicle population. This is in parallel with a dramatic growth of three-wheeled taxis as large as 107.6% from 2010 to 2018 in Sri Lanka. Mitigating air pollution however presents challenges. On one hand, drivers providing these services are usually of lower socio-economic class, and therefore solutions for mitigation may be difficult to apply with their current behavioural attitudes, and on the other, these services are extremely popular for low-income residents. Therefore, abrupt bans are impractical, considering the livelihoods of half a million individuals in Colombo District alone and the convenience offered in terms of money and time to commuters during peak hours. Instead, practical solutions such as transitioning of three-wheeled taxis to electric vehicles and establishing AC and solar-powered charging points at taxi parking stands to improve fuel efficiency are recommended. Future research should also focus on environmental interventions such as establishing urban vegetations and fountains to minimise the impact of air pollution.

Our study aimed to assess air pollution exposure across common transport microenvironments (TMEs), with a particular focus on three-wheeled taxis, a widely used mode of transport, primarily in developing countries. However, comparable studies are limited due to methodological constraints. Many existing studies on air pollution across TMEs do not include three-wheeled taxis as a distinct category [6, 37, 41–45]. Additionally, they often fail to conduct measurements under identical environmental conditions within the same time period, assess drivers'personal exposure levels, or measure the same air pollutants as in our study [8, 17, 46].

The study has some limitations. Since the data were collected mainly during typical sunny days, variability in weather conditions could influence pollutant levels, limiting the generalizability of the findings. Although we measured ambient PM levels, we were unable to conduct continuous 24-h monitoring, which limits our ability to directly compare our findings with the standard 24-h values in this context. Finally, the focus on vehicular emissions alone overlooks other potential sources of air pollution, such as industrial emissions, which may also contribute to overall air quality.

Conclusions

The average ambient PM₁₀ concentration was 139 μg/m³ (SD = 14), comprising four-times more fine particles than coarse particles. Compared to AC cars, three-wheeled taxis followed by non-AC buses showed the highest concentrations of fine as well as submicron particles. Also, fine particles as a fraction of the total PM₁₀ (PM fraction) was six-fold higher compared to coarse particles in three-wheeled taxis. The PM ratio (in-vehicular/ambient) was three times higher in three-wheeled taxis; two times higher in non-AC buses; and two times lesser in AC cars (p < 0.001). When comparing the PM ratios between vehicles, it was seven times higher inside a three-wheeled taxi than in an AC car. This evidence of non-AC TMEs (both three-wheeled taxis and public transport buses) being more vulnerable to in-vehicular air pollution, compared to AC passenger TME, underscores the importance of mitigating air pollution within non-AC TMEs, by designing such vehicles with a safer cabin environment and environmental interventions to minimise the impact of air pollution.

Abbreviations

AC

Air Condition

AQS

Air Quality Standards

CI

Confidence Interval

IARC

International Agency for Research on Cancer

IQR

Interquartile Range

NBRO

National Building Research Organization of Sri Lanka

PM

Particulate matter

PR

Prevalence Ratio

SD

Standard Deviation

SE

Standard Error

TME

Transport Micro-environment

US EPA

United States Environmental Protection Agency

WHO

World Health Organization

Authors’ contributions

All authors (Wittachchikoralalage Dona Chamila Niroshini Adikaram, Carukshi Arambepola) equally contributed to idea conception, design, data collection, statistical analyses, data interpretation and manuscript drafting. All authors reviewed the manuscript and approved the final manuscript for submission.

Funding

The Directorate of Non-Communicable Diseases and the National Health Development Fund of Ministry of Health, Sri Lanka were allocated funds for the procurement of air quality monitoring equipment from the National Building Research Organization (NBRO) and to facilitate financial support for research assistants.

Data availability

The data that support the findings of this study are available from the corresponding author, upon reasonable request.

Declarations

Ethics approval and consent to participate

The study was evaluated and approved by the Ethic Review Committee, faculty of Medicine, University of Colombo, Sri Lanka (Protocol No: EC-18–127). All methods were carried out in accordance with relevant guidelines and regulations. Informed written consent was obtained from all participants for study participation.

Consent for publication

Not applicable.

Competing interests

The authors declare no competing interests.