By Liya E. Yu
Associate Professor
Department of Civil & Environmental Engineering, College of Design & Engineering
NUS Environmental Research Institute (NERI)
National University of Singapore
Benzene, toluene, ethylbenzene and total xylene[1] (BTEX) are among the most closely monitored volatile organic compounds (VOCs) in the atmosphere. BTEX together are mainly emitted from human activities related to crude oil, including excavation, refinery, manufacturing of petroleum products (e.g., gasoline), vehicle emissions, etc. although they can be individually used for specific purposes. For example, toluene is often used as solvent for various industrial processes (e.g., degrease). BTEX concentrations are regulated worldwide due to concerns over their adverse impacts on occupational and public health. For public health, one of the most notable regulations is Acute Exposure Guideline Level (AEGL) developed by US Environmental Protection Agency[2]. AEGL comprises three levels, with level 1 specifying the most stringent (lowest threshold) exposure concentration at which individuals could experience discomfort and mild irritation, over five exposure durations of 10 min, 30 min, 60 min, 4 hours and 8 hours.
In this summary, the hourly ambient concentration of BTEX measured at nine monitoring stations at areas across Singapore between 2021 to 2025 were analysed to better understand how BTEX levels vary across different locations and across time. The median concentrations of BTEX are used for various discussion below unless specified otherwise. The analysis will look into the temporal and spatial trends distribution trends before discussing the potential BTEX trends in the future.
Temporal concentration trends
The annual median concentration of BTEX together significantly (p<0.05) decreased from 1.24 ppb in 2021 to 1.10 ppb in 2025. Analysing it further by individual compounds, ethylbenzene showed a similar general decreasing trend with the lowest concentration in 2025 (0.12 ppb). While benzene and toluene showed differential yearly trends with the highest concentration occurring in either 2021 or 2022, they exhibited the lowest concentrations in 2024 and 2025, respectively. Xylene concentrations were lowest in 2023, while relatively higher concentrations were observed in 2021 and 2024. Taking all five-year data together, the hourly concentration of individual BTEX in Singapore is significantly lower than the AEGL-1 threshold (Table 1) by at least 165 times. In comparison with published data from more than 15 research studies conducted since early 2000s at approximately 110 ambient air monitoring locations (away from direct emission sources) in East Asia, South Asia, Europe, and the United States, the mean hourly concentration of individual BTEX compounds in Singapore were comparable, and at times, lower than the levels published in the studies. This demonstrates effective regulation and control of BTEX emissions in Singapore, and is comparable to many cities worldwide.
Among individual BTEX compounds, toluene exhibits the highest concentration among BTEX. Its maximum concentration of 267 ppb is several orders of magnitude lower than the AEGL-1 (67 ppm, Table 1), which indicates that levels are not of immediate health concern. Nonetheless, its relative dominance among BTEX compounds is indicative of emissions from industrial and commercial activities (e.g., oil refinery, metal degreasing, pharmaceutical production, rubber manufacturing, painting, coating, varnishes, etc.). Although benzene is of greater toxicological concern than toluene, the ambient concentrations were also low and several orders of magnitude lower than its AEGL-1 60-min threshold (52ppm, Table 1).
Table 1. Hourly concentration of BTEX in Singapore during 2021–2025 against AEGL 1 60-min exposure.
| SN | VOC | AEGL-1 60-min exposure (ppm) 1 | Concentration (ppb) 2
(range, median, mean±standard deviation) |
|---|
| 1 | Benzene | 52 | BDL3–115.65, 0.16, 0.29 ± 0.59 |
| 2 | Toluene | 67 | BDL3–266.74, 0.53, 1.13 ± 2.04 |
| 3 | Ethylbenzene | 33 | BDL3–198.59, 0.13, 0.26 ± 0.76 |
| 4 | Xylene | 130 | BDL3–444.96, 0.28, 0.52 ± 1.17 |
1Acute Exposure Guideline Level (AEGL) 1: Notable discomfort, irritation, or certain asymptomatic non-sensory effects. However, the effects are not disabling and are transient and reversible upon cessation of exposure (US Environmental Protection Agency, USEPA); 1 ppm = 1000 ppb
2 Based on NEA monitoring stations at residential areas (in alphabetical order): Bishan-Ang Mo Kio, Changi-East Coast Park, Jurong Hill, Lower Seletar, Pulau Tekong, Pulau Ubin, Punggol, Queenstown, and Sentosa
3BDL: Below detection limit of analytical instruments
Looking at the individual BTEX monthly concentrations in Singapore across the monsoon seasons, which ranged from 0.09–1.10 ppb, it was found that that the concentrations were significantly (p<0.05) higher during the inter-monsoon months (typically April-May and October-November) where the wind conditions tend to be stagnant with lower wind speeds. The individual compounds consistently showed the highest concentration in November, followed by April and October, and the lowest concentration in February. The differential monthly concentrations could substantially depend on meteorological conditions (e.g., wind speed, rainfall and dilution effects), local emissions and transported impacts.
In terms of diurnal trends, there is an increase in BTEX concentrations during 0600–0700 hours and 1600–1700 hours, and reached the lowest concentrations at 1300–1400 hours. The increased BTEX could be attributed, in part, to heavier on-road vehicle emissions during the morning and afternoon rush hours, one of commonly known sources, although the influence of industrial emissions can also play a part. The lowest concentration in the early afternoon could be caused by more prominent outflux processes dominating over the domestic emissions and transported emissions. The outflux processes in the early afternoon comprise faster wind speed, higher planetary boundary layer (PBL)[3]. Faster wind speed and higher PBL typically allows for stronger atmospheric mixing and dilution, reducing the concentration of airborne pollutants (including BTEX) in the ambient air. Stronger photochemical degradation due to stronger sunlight, UV rays and hotter ambient temperatures in the early afternoon accelerate the decomposition of airborne pollutants, a natural self-cleansing mechanisms in the atmosphere, thereby reducing their concentrations.
Spatial concentration trends
Across the nine monitoring locations, the highest mean hourly total BTEX concentrations ranged from 0.38 ppb to 3.08 ppb. In general, locations closer to industrial activities or major emission sources exhibited relatively higher BTEX levels, whereas coastal and offshore sites showed lower concentrations. The spatial patterns were generally consistent across the individual BTEX species, with annual mean hourly concentrations ranging from 0.11–0.36 ppb for benzene, 0.15–1.46 ppb for toluene, 0.03–0.29 ppb for ethylbenzene, and 0.05–0.50 ppb for xylene.
Indicative source influences on individual sites
Having residential, commercial and industrial activities in close proximity, coupled with strong atmospheric mixing, brings the footprints of multiple sources to almost everywhere in Singapore. Indicative concentration ratios of toluene over benzene (T/B), xylene over benzene (X/B), and xylene over ethylbenzene (X/E) could suggest the dominant sources of influence and fresh vs. aged emissions of BTEX. For instance, the lower (T/B) ratio (1.3) at Pulau Tekong indicates that primary influence of vehicular exhaust emissions contributed to the BTEX concentrations, and reflects its relatively far distance from industrial activity.
Other monitoring sites with T/B ratios ranging from 1.8–5.1 suggest that the influence of BTEX were largely from toluene-rich emission sources. At Jurong, such sources could be associated with industrial activities, such as petrochemical industries and manufacturing industries for paint, coatings and varnishes, pharmaceuticals. Comparable T/B ratios were also observed at Lower Seletar and Bishan-Ang Mo Kio, where contributions from a combination of commercial activities, solvent use, paint application, and vehicular emissions may contribute to such ratios.
The X/B and X/E ratios indicate that almost all residential sites have mixed influences of fresh and aged emissions[4], emphasising the complex influences of diverse sources.
Potential concentration trends in future ambient BTEX
While further analyses of data at individual monitoring sites are recommended for better understanding of the predominant source emissions during the northeast, southwest and inter-monsoon seasons, the rising numbers of electric vehicles without tailpipe emissions is expected to reduce roadside BTEX concentrations when it increasingly replaces on-road vehicles powered by fossil fuels (e.g., gasoline or diesel).
In terms of industrial activities, depending on the manufacturing processes of transition fuels (e.g., natural gas) and future energy sources (e.g., hydrogen fuel) in the Southeast Asia, BTEX concentrations in our urban environment may vary. In particular, it is worthwhile to closely monitor toluene to assess whether the reduced tailpipe emissions from growing numbers of on-road EVs may offset its concentration profile over time.
[1] Xylene in BTEX represents the total xylene including three isomers of m-xylene, p-xylene and o-xylene (m,p,o-xylene).
[2] Accessible at https://www.epa.gov/aegl/access-acute-exposure-guideline-levels-aegls-values#chemicals
[3] Planetary boundary layer refers to the lowest portion of the atmosphere where the earth surface influences temperature, moisture, and wind with strong mixing
[4] In published research work, an X/B concentration ratio >1 indicates fresh emissions. An X/E concentration ratio ranging from 1.2–2.1 indicates aged emissions. Aged emissions can be featured with more transformation (e.g., degradation) in emitted VOCs.