Volatile organic compounds and polycyclic aromatic hydrocarbons exposure in asphalt paving: measurement by Proton Transfer Reaction Time-of-Flight Mass Spectrometry, Monte Carlo-based risk evaluation, and recommendations for risk reduction

Abstract

Research on worker exposure to volatile organic compounds (VOCs) during asphalt paving operations remains significantly limited, and regulatory frameworks governing such exposures are also insufficient. Previous studies have primarily focused on a limited number of major VOCs. However, this study employs high-resolution, high-performance Proton Transfer Reaction Time-of-Flight Mass Spectrometry (PTR-ToF-MS) to comprehensively evaluate exposure levels to 25 different VOCs. Additionally, Monte Carlo simulations were utilized to assess both non-carcinogenic and carcinogenic risks, thereby providing foundational data for future risk mitigation strategies. During asphalt paving operations, the concentrations of most VOCs increased by a factor of 2 to 10 compared to background levels. Nevertheless, none of the measured compounds exceeded 8-h time-weighted average occupational exposure limits; however, acrolein exceeded the ceiling threshold limit value established by the American Conference of Governmental Industrial Hygienists, indicating the need for peak-exposure control. The non-carcinogenic risk assessment revealed that the maximum Hazard Quotient (HQ) values for acetaldehyde, 1,3-butadiene, and acrolein exceeded 1, indicating potential adverse health effects. Furthermore, the Lifetime Cancer Risk (LCR) values for benzene and 1,3-butadiene surpassed established safety thresholds, confirming a significant increase in carcinogenic risk. Furthermore, this study proposes measures to reduce exposure to hazardous substances.

Introduction

Asphalt vapor emissions generated during asphalt paving operations consist of a complex mixture of volatile organic compounds (VOCs) and fine particulate matter, primarily resulting from the heating of bitumen. The International Agency for Research on Cancer (IARC) has classified asphalt vapor as a Group 2A carcinogen, indicating that it is probably carcinogenic to humans, with significant concerns regarding lung cancer risk. Studies have reported that workers chronically exposed to asphalt fumes exhibit higher lung cancer incidence rates compared to the general population (IARC). This elevated risk is attributed to the presence of hazardous compounds such as polycyclic aromatic hydrocarbons (PAHs), benzene, toluene, and xylene in asphalt fumes. For instance, a cohort study¹⁾ found a statistically significant increase in lung cancer incidence among asphalt workers, further reinforcing concerns about occupational exposure risks.

Despite these findings, research on the detailed chemical composition and health effects of asphalt vapor remains limited. Conventional occupational exposure monitoring methods, such as adsorption tube sampling followed by gas chromatography-mass spectrometry (GC-MS), primarily provide average exposure levels but fail to capture real-time fluctuations in VOC concentrations. This limitation is critical because worker exposure levels can vary substantially depending on different paving operation stages.

Proton Transfer Reaction Time-of-Flight Mass Spectrometry (PTR-ToF-MS) has emerged as a state-of-the-art analytical technology capable of detecting a wide range of VOCs in real time with high sensitivity and resolution. Compared to traditional methods, PTR-ToF-MS offers rapid response times, minimal sample preparation requirements, and the ability to detect low-concentration VOCs in complex environments. This technology has been successfully applied in various environmental and occupational health studies. For example, Lindinger et al.²⁾ utilized PTR-MS to measure VOC emissions in industrial settings, while Sahu et al.³⁾ employed PTR-ToF-MS for urban air pollution monitoring. In addition, Han et al.⁴⁾ applied this technique to assess VOC emissions from indoor building materials, demonstrating its efficacy in air-quality assessments, and Oh et al.⁵⁾ and Kim et al.⁶⁾ evaluated a variety of VOCs generated in cooking facilities as well as those emitted during the spraying of household chemical products. These studies indicate that PTR-ToF-MS is highly effective for monitoring airborne hazardous compounds.

However, despite the successful application of PTR-ToF-MS in various environmental research fields, its use in assessing VOC emissions from asphalt paving operations has not yet been explored. This research gap presents a critical opportunity to gain a deeper understanding of the dynamic exposure patterns and chemical composition of asphalt vapor emissions.

Meanwhile, Ramírez et al.⁷⁾ highlighted that previous studies on PAH-related health risks have predominantly focused on particulate-phase PAHs, whereas gas-phase PAHs can rapidly enter the bloodstream upon exposure, potentially posing a greater health risk. Their study found that the Benzo[a]pyrene Equivalent (BaP-eq) values of gas-phase PAHs accounted for 34% to 86% of the total, indicating that they cannot be overlooked.

Accordingly, this study aims to comprehensively assess VOC concentrations and compositions across various stages of asphalt paving operations by utilizing the real-time analytical capabilities of PTR-ToF-MS, including gas-phase PAHs. Through this approach, we seek to identify high-risk work stages and contribute to the development of tailored risk mitigation strategies.

While recent risk assessments of chemical exposures in industrial settings have largely relied on Control Banding techniques (for example, Kim et al.⁸⁾ applied the Chemical Hazard Risk Management (CHARM) tool, based on the UK’s COSHH Essentials, to evaluate benzene exposure in small-scale printing operations), the present study, in contrast, employs a Monte Carlo simulation-based probabilistic health risk assessment method to achieve more precise and quantitative insights.

This novel approach is expected not only to fill a critical research gap but also to contribute to the improvement of occupational health standards within the asphalt industry and enhancing worker safety. By providing detailed real-time data on VOC emissions across key phases of asphalt paving, this study seeks to establish a foundation for future research and regulatory advancements. The findings will contribute to the development of more effective exposure management strategies and serve as a crucial resource for policy decisions aimed at reducing occupational health risks associated with asphalt vapor exposure.

Materials and Methods

Study environment of asphalt paving operations

Road asphalt paving involves the application of asphalt concrete (a mixture of asphalt a semi-solid material refined from petroleum and aggregates such as gravel and sand) onto the pavement surface using an asphalt paver. The process is followed by compaction with a roller vehicle to ensure uniform distribution and achieve the required density and structural integrity.

In this study, continuous VOC monitoring was conducted in two representative environments of asphalt paving operations: the asphalt application phase and the subsequent compaction phase using a roller. The study site was located in a coastal small town in South Korea. Within a few kilometers of the worksite, there were no large industrial facilities known to emit VOCs, minimizing the influence of external sources. Field measurements were conducted in December, and on the day of monitoring the ambient temperature averaged 2.1°C (minimum −2°C, maximum 6.3°C).

Evaluation of VOC emissions during asphalt paving using PTR-ToF-MS

A total of 17 VOCs and 8 gaseous polycyclic aromatic hydrocarbons (PAHs) were measured in this study (Supplementary Table 1). Real-time monitoring of 25 gaseous compounds was performed using a vehicle equipped with a Proton Transfer Reaction Time-of-Flight Mass Spectrometer (PTR-ToF-MS). The monitoring vehicle moved at a constant speed approximately 3 meters from the asphalt paver during both the asphalt application and compaction phases (Fig. 1).

Fig. 1.

Measurement of volatile organic compounds (VOCs) exposure during asphalt paving operations using Proton Transfer Reaction Time-of-Flight Mass Spectrometry (PTR-ToF-MS). The PTR-ToF-MS is mounted inside the vehicle on the left, and the sampling tube is positioned at the worker’s breathing zone to capture real-time data while moving alongside the operation.

To closely simulate the actual inhalation exposure of workers positioned near the asphalt paver, the research team manually held the PTR-ToF-MS sampling tube and moved alongside the workers. However, individual workers were positioned beside, behind, or directly in front of the asphalt paver’s container (where asphalt is mixed and discharged onto the road). Consequently, the study was inherently limited in capturing the full range of exposure levels among different workers, and the measured values should be interpreted as indicative of typical exposures rather than precise individual exposures.

Background VOC concentrations were recorded for approximately 15 min before paving commenced. This was followed by VOC monitoring during 35 min of asphalt paving and an additional 10 min of rolling compaction. While the background measurements did not include emissions directly from the asphalt paving process, it is possible that trace amounts of diesel engine emissions were detected due to the presence of various diesel-powered vehicles at the worksite, including the research vehicle carrying the PTR-ToF-MS, dump trucks, and rollers.

The airborne concentrations of VOCs and PAHs emitted during asphalt paving operations were compared with internationally recognized occupational exposure limits, including the Occupational Safety and Health Administration Permissible Exposure Limits (OSHA PEL), the National Institute for Occupational Safety and Health Recommended Exposure Limits (NIOSH REL), the American Conference of Governmental Industrial Hygienists Threshold Limit Values (ACGIH TLV), and the California Division of Occupational Safety and Health PEL (Cal/OSHA PEL). The evaluation focused on substances with high exposure risk.

The PTR-ToF-MS provides results in parts per billion (ppb). To facilitate comparisons with other studies that report results in mass concentration units (µg/m³), the following conversion formula was applied:

$$\mathrm{C}\hspace{0.17em}(\mathrm{\mu g}/{\mathrm{m}}^3)\hspace{0.17em}=\hspace{0.17em}\mathrm{C}\hspace{0.17em}\left(\mathrm{ppb}\right)\hspace{0.17em}\times \frac{\mathrm{MW}}{24.45}$$

where C is the concentration, MW is the molecular weight of the compound, and 24.45 is the molar volume of an ideal gas at standard temperature and pressure (STP, 25°C and 1 atm).

Calibration and detection limits of PTR-ToF-MS

The Proton Transfer Reaction Time-of-Flight Mass Spectrometer (PTR-ToF-MS) allows for absolute quantification of analytes, even in the absence of standard reagents or calibration gases. In this study, calibration experiments were conducted using the TO-14A Aromatics Mix (RESTEK, 14 components) standard gas to verify linearity and instrument stability for selected compounds, including benzene, styrene, toluene, and trimethylbenzene. The calibration curves exhibited excellent linearity, with all R² values exceeding 0.99 (Supplementary Fig. 1).

The analytical detection limits were determined using the standard deviation of the baseline noise in zero gas measurements, multiplied by a factor of three (Supplementary Table 1).

Health risk assessment

Real-time concentrations of VOCs emitted during asphalt paving operations were measured using PTR-ToF-MS. Based on these data, a probabilistic health risk assessment was conducted using the Monte Carlo simulation method⁹⁾.

Following a review of the toxicological properties of the detected VOCs, non-carcinogenic risk assessment was performed for seven VOCs with available toxicity data. Moreover, carcinogenic risk assessment was conducted for three VOCs classified as Group 1 or 2A carcinogens by the International Agency for Research on Cancer (IARC)^{10, 11)}.

Since inhalation is the primary exposure route for VOCs in asphalt paving operations, the risk assessment focused specifically on this pathway. The evaluation was conducted under four different exposure scenarios: background exposure for 2 h, asphalt paving operation for 4 h, compaction with a roller for 2 h, and total daily exposure for 8 h.

Although field monitoring was conducted for 15 min of background exposure, 35 min of asphalt paving, and 10 min of roller compaction, the risk assessment was performed by scaling these periods to a typical 8-h workday (2 h of background activities, e.g., equipment setup and preparation; 4 h of asphalt paving operations; and 2 h of roller compaction) based on worker interviews.

Both non-carcinogenic and carcinogenic risk assessments were based on the Average Daily Dose (ADD), calculated using the following equation:

$$\mathrm{ADD}\hspace{0.17em}(\mathrm{\mu g}/\mathrm{kg}/\mathrm{day}) = \frac{\mathrm{C}\hspace{0.17em}(\mathrm{\mu g}/{\mathrm{m}}^3)\hspace{0.17em}\times \hspace{0.17em}\mathrm{IR} ({\mathrm{m}}^3/\mathrm{day})\hspace{0.17em}\times \hspace{0.17em}\mathrm{ET}\hspace{0.17em}(\mathrm{hour}/\mathrm{day}) \times \mathrm{EF}(\mathrm{day}/\mathrm{year}) \times \mathrm{ED}\hspace{0.17em}\left(\mathrm{year}\right)}{\mathrm{AT}\left(\mathrm{year}\right) \times 365\hspace{0.17em}(\mathrm{day}/\mathrm{year}) \times 24\hspace{0.17em}(\mathrm{hour}/\mathrm{day}) \times \mathrm{BW}\hspace{0.17em}\left(\mathrm{kg}\right)}$$

In this equation, C (µg/m³) represents the concentration of the chemical compound measured using PTR-ToF-MS, IR (m³/day) is the inhalation rate, ET (h/day) refers to the exposure duration, EF (d/year) represents the exposure frequency, ED (yr) is the exposure duration, AT (yr) denotes the averaging time, and BW (kg) indicates body weight.

The values and distributions of the parameters used in the simulation, as well as the reference concentrations (RfC) and inhalation cancer slope factors (CSF) for the target VOCs, are presented in Table 1.

Table 1. Exposure parameters and values used in the probabilistic risk assessment.

Parameter	Definition (unit)	Value	Basis	Distribution type
C	Chemical concentration (µg/m3)	Tables 1	Sampling	Lognormal
IR	Inhalation rate (m3/day)	20 m3/day	US EPA, 199715)	Fixed value
ET	Exposure time (h/day)	2 for background	Interview	Fixed value
		4 for pavement work
		2 for rolling operation
EF	Exposure frequency (d/year)	200	Interview	Fixed value
ED	Exposure duration (yr)	40 for cancer risk	Interview	Fixed value
AT	Averaging time (yr)	40 for non-cancer risk	US EPA	Fixed value
		70 for cancer risk
BW	Body weight (kg)	70	Interview	Fixed value
RfC	Reference concentration (µg/m3)	Benzene 30	EPA IRIS	Fixed value
		Toluene 5,000	EPA IRIS
		Ethylbenzene/xylene 100	EPA IRIS
		Styrene 1,000	EPA IRIS
		Acrolein 20	EPA IRIS
		1,3-Butadiene 2	EPA IRIS
		Acetaldehyde 9	EPA IRIS
CSF	Inhalation cancer slope factor (µg/kg/day)−1	Benzene 2.73×10−5	EPA IRIS	Fixed value
		Styrene 5.7×10−7	OEHHA
		1,3-Butadiene 3×10−5	EPA IRIS

US EPA: United States Environmental Protection Agency; IRIS: Integrated Risk Information System; OEHHA: Office of Environmental Health Hazard Assessment.

Table 2 presents both the arithmetic mean (AM) and geometric mean (GM) to characterize the data. However, the probabilistic risk assessment was then conducted by empirical sampling of the raw data in order to capture the full distribution¹²⁾.

Table 2. VOCs concentration by process in asphalt paving operations.

Hazardous VOCs	BAG	Pavement work							Roller operation
	AM	AM	SD	GM	GSD	Min	Max	Ratio*	AM	SD	GM	GSD	Min	Max	Ratio*
Formaldehyde	3.2	6.8	7.1	5.4	1.8	1.9	80.3	2.1	3.6	1.0	3.5	1.3	2.0	11.0	1.1
Acetaldehyde	13.5	70.6	142.8	36.9	2.7	7.3	1,727.1	5.2	18.3	8.9	16.4	1.6	6.8	59.9	1.4
1,3-Butadiene	2.4	24.8	36.9	14.8	2.6	1.4	420.1	10.4	7.0	2.8	6.6	1.4	3.1	29.1	2.9
Acrolein	3.6	34.9	48.1	21.1	2.6	1.6	429.5	9.7	12.0	6.4	11.1	1.4	5.8	70.0	3.3
Acetone/Propanal	13.0	31.0	66.2	16.2	2.6	3.3	816.9	2.4	7.6	3.4	7.0	1.5	3.4	22.6	0.6
Trimethylamine	1.3	1.8	2.4	1.4	1.8	0.3	30.4	1.4	0.9	0.2	0.9	1.3	0.4	1.7	0.7
Acrylamide	0.3	0.8	0.7	0.6	1.8	0.1	7.2	2.2	0.4	0.1	0.4	1.3	0.2	1.4	1.2
Butanal/MEK	1.8	7.8	16.2	4.3	2.5	1.0	209.2	4.3	2.1	0.8	2.0	1.4	0.7	5.4	1.1
(dimethyl) nitrosamine	1.0	1.1	0.6	1.1	1.5	0.4	7.5	1.1	0.6	0.1	0.6	1.3	0.3	1.1	0.6
Benzene	0.7	2.3	4.4	1.3	2.4	0.3	53.3	3.5	0.9	0.5	0.8	1.5	0.3	6.2	1.3
Diacetyl	0.7	3.4	6.6	1.9	2.4	0.4	85.8	4.5	0.9	0.3	0.9	1.4	0.4	2.1	1.2
Toluene	1.3	6.2	12.2	3.3	2.7	0.5	127.5	4.9	1.9	1.7	1.6	1.7	0.6	18.1	1.5
Phenol	0.5	4.5	5.1	3.0	2.4	0.2	40.3	9.4	1.8	0.9	1.6	1.5	0.7	8.3	3.7
Styrene	0.2	1.7	1.6	1.2	2.4	0.1	10.3	10.9	0.9	0.3	0.8	1.4	0.3	2.5	5.6
Eethylbenzene/Xylene	1.5	11.4	27.6	5.1	2.9	0.7	352.6	7.8	3.1	3.1	2.6	1.7	1.1	37.2	2.1
M-cresol	0.5	3.2	3.2	2.4	2.2	0.3	23.9	6.4	1.4	0.5	1.3	1.4	0.6	4.5	2.8
1,3,5-trimethylbenzene	0.8	11.1	23.2	4.9	3.2	0.4	241.6	13.4	2.9	4.0	2.0	2.2	0.6	48.8	3.5

*Ratio=Pavement (or Roller) / background (BAG). The number of samples is as follows: BAG: 871, Pavement work: 2,038, Roller operation: 525.

Unit: ppb. The analysis mode of the PTR-ToF-MS used in this study does not distinguish between compounds with identical molecular weights. VOCs: volatile organic compounds; GM: geometric mean; GSD: geometric standard deviation; MEK: methyl ethyl ketone.

For the non-carcinogenic risk assessment, the Hazard Quotient (HQ) was used. An HQ value exceeding 1 suggests that exposure to a specific compound may lead to non-carcinogenic health effects¹³⁾.

$$\mathrm{HQ}\hspace{0.17em}= \frac{\mathrm{ADD}\hspace{0.17em}(\mathrm{\mu g}/\mathrm{kg}/\mathrm{day})}{\mathrm{RfD}\hspace{0.17em}(\mathrm{\mu g}/\mathrm{kg}/\mathrm{day})\hspace{0.17em}\times \hspace{0.17em}\mathrm{BW}\hspace{0.17em}\left(\mathrm{kg}\right)}$$

where RfD (µg/kg/day) represents the reference dose, indicating the acceptable exposure level for VOCs. The RfD is calculated using the following equation:

$$\mathrm{RfD}\hspace{0.17em}(\mathrm{\mu g}/\mathrm{kg}/\mathrm{day})\hspace{0.17em}= \frac{\mathrm{RfC}\hspace{0.17em}(\mathrm{\mu g}/{\mathrm{m}}^3)\hspace{0.17em}\times \hspace{0.17em}\mathrm{IR}\hspace{0.17em}({\mathrm{m}}^3/\mathrm{day})}{\hspace{0.17em}\mathrm{BW}\hspace{0.17em}\left(\mathrm{kg}\right)}$$

For the carcinogenic risk assessment, the Lifetime Cancer Risk (LCR) was utilized. An LCR value exceeding 10⁻⁶(i.e., a risk of more than one additional cancer case per million people) indicates a potential concern for carcinogenic effects¹⁴⁾.

LCR = ADD (µg/kg/day) × CSF (µg/kg/day)^-1

where CSF represents the inhalation cancer slope factor, which quantifies the unit risk of cancer incidence per unit exposure.

Results and Discussion

Measurement results of volatile organic compounds (VOCs)

Table 2 presents the arithmetic mean, standard deviation, geometric mean, geometric standard deviation, minimum, and maximum concentrations of 17 VOCs measured under three different work conditions (background, paving, and roller operations). Additionally, it shows the ratio of arithmetic mean concentrations during paving and roller operations compared to background levels.

Formaldehyde had an average concentration of 3.2 ppb under background conditions, which increased to 6.8 ppb during paving operations, approximately twice the background level. This concentration was about half of the NIOSH TWA REL (16 ppb) (Supplementary Table 2).

The average benzene concentration under background conditions was 0.7 ppb, which increased to 2.3 ppb during paving operations, approximately 3.5 times higher than the background level. This concentration accounted for about 12% of the ACGIH TLV (20 ppb), with peak concentrations reaching 53.3 ppb. However, during roller operations, the benzene concentration decreased to 0.9 ppb, which was slightly above the background level (Fig. 2).

Fig. 2.

Time-resolved benzene concentrations during asphalt paving processes: frequent occurrence of peak concentrations during paving operations. The yellow line represents the background condition, the orange line indicates the paving operation, and the red line corresponds to the roller operation.

Acrolein showed a significant increase from a background level of 3.6 ppb to 34.9 ppb during paving operations, representing a tenfold increase. The maximum recorded concentration reached 429.5 ppb, which exceeded the ACGIH Ceiling TLV (50 ppb) by nearly tenfold (Supplementary Table 2). During the 35-min monitoring period for paving operations, acrolein concentrations exceeded 50 ppb for 14.6% of the time. Given that workers typically perform paving tasks for approximately four hours per day, this finding suggests that they could be exposed to concentrations above the ACGIH Ceiling TLV for roughly 35 min each workday.

Other compounds, including acetaldehyde, 1,3-butadiene, styrene, ethylbenzene/xylene, and 1,3,5-trimethylbenzene, also exhibited a five- to tenfold increase in concentration during paving operations compared to background levels (Table 2, Supplementary Fig. 2).

Benzene is a well-known carcinogen, and its concentration spikes significantly during asphalt mixing and pouring, necessitating strict exposure control measures. Similarly, acrolein, a strong respiratory irritant, poses potential acute exposure risks.

According to Chong et al.¹⁶⁾ (GC/MS method), the mean concentrations of benzene, toluene, and ethylbenzene during asphalt paving were 2.9 ppb, 3.9 ppb, and 5.7 ppb, respectively, which closely match the results of the present study. Total VOC concentrations (28 compounds) ranged from 64.9 to 327.4 ppb during the asphalt hopper stage, 1.2 to 262 ppb during the paving stage, and 1.2 to 14.9 ppb during the compaction stage, showing higher levels in the hopper and paving stages, consistent with our findings.

A 2001 NIOSH study¹⁷⁾ reported benzene levels below the quantitation limit (10 ppb) and TVOC concentrations of 0.24–74 mg/m³ during conventional asphalt paving. However, when crumb-rubber-modified asphalt binder was used, benzene and TVOC concentrations increased to 19–770 ppb and 0.38–224 mg/m³ (0.093–54.8 ppm, assuming a representative molecular weight of 100 g/mol), respectively.

In addition, Cui et al.¹⁸⁾ measured TVOCs with a PID (Photo-Ionization Detector) and reported higher exposures for pavement workers than for equipment operators, with average TVOC concentrations of 16.03–50.79 mg/m³ (3.96–12.56 ppm). The VOC composition was dominated by ethylbenzene (21%), m-xylene (20%), toluene (16%), p-xylene (10%), o-xylene (6%), and benzene (4%). The higher VOC levels reported by Cui et al. compared to those in our study and others may be attributed to winter sampling temperatures (−2°C to 6.3°C) versus summer conditions (21°C to 32°C in Cui et al.), differences in field conditions such as asphalt binder content and paving temperature, and potential PID sensor variability due to sample composition, humidity, temperature, and calibration methods^19,20,21).

Measurement results of PAHs

Table 3 presents the arithmetic mean, standard deviation, geometric mean, geometric standard deviation, minimum, and maximum concentrations of eight gaseous polycyclic aromatic hydrocarbons (PAHs) measured under three different work conditions. In addition, it shows the ratio of arithmetic mean concentrations during paving and roller operations compared to background levels.

Table 3. Gas-phase PAHs concentration by process in asphalt paving operations.

PAHs	BAG	Pavement work							Roller operation
	AM	AM	SD	GM	GSD	Min	Max	Ratio*	AM	SD	GM	GSD	Min	Max	Ratio*
Naphthalene	0.2	1.7	2.2	1.1	2.3	0.1	22.4	7.4	0.6	0.2	0.5	1.4	0.2	1.7	2.5
Acenaphthylene	0.1	0.7	0.6	0.5	2.2	0.0	4.4	6.3	0.2	0.1	0.2	1.4	0.0	0.7	2.3
Biphenyl/Acenaphthene	0.1	0.9	1.1	0.6	2.4	0.0	8.5	9.2	0.3	0.1	0.3	1.3	0.1	0.6	2.8
Fluorene	0.1	0.5	0.4	0.4	2.2	0.0	3.0	6.3	0.2	0.1	0.2	1.4	0.1	0.6	2.7
Anthracene/Phenanthrene	0.1	0.7	0.6	0.5	2.2	0.0	3.9	7.6	0.4	0.2	0.3	1.5	0.1	1.2	4.0
Dibenzothiophene	0.1	0.7	0.4	0.5	2.2	0.0	2.3	6.8	0.4	0.1	0.4	1.2	0.2	0.6	3.7
4H-cyclopenta[def]phenanthrene	0.1	1.3	0.6	1.1	2.4	0.0	2.9	15.0	0.9	0.2	0.9	1.2	0.5	1.5	10.4
Pyrene/Fluoranthene	0.2	0.6	0.2	0.6	1.7	0.1	1.3	3.0	0.6	0.1	0.6	1.2	0.3	0.9	3.0

*Ratio=Pavement (or Roller) / background (BAG). The number of samples is as follows: BAG: 871, pavement work: 2,038, roller operation: 525.

Unit: ppb. The analysis mode of the PTR-ToF-MS used in this study does not distinguish between compounds with identical molecular weights. PAHs: polycyclic aromatic hydrocarbons; GM: geometric mean; GSD: geometric standard deviation.

All eight PAHs, including naphthalene, exhibited increased concentrations during asphalt paving compared to background levels. Notably, 4H-cyclopenta[def]phenanthrene showed a 15-fold increase. However, most measured PAHs were either at very low concentrations relative to occupational exposure limits or lacked established exposure guidelines (Supplementary Table 2).

The sum of the 8-h TWA concentrations for the eight PAHs was 4.2 µg/m³, with naphthalene contributing the highest concentration at 1.3 µg/m³ (31%), followed by biphenyl/acenaphthene at 0.62 µg/m³.

A study by Germin-Aizac et al.²²⁾ reported gaseous PAH concentrations ranging from 0.18 to 1 µg/m³ during asphalt paving, which were lower than those observed in this study. The authors noted that gaseous PAH levels were higher than particulate PAH levels, and that PAH concentrations varied based on paving temperature, binder (bitumen) content, road type, and the inclusion of Recycled Asphalt Pavement (RAP).

Non-carcinogenic health risk assessment

The non-carcinogenic health risk assessment was conducted using the Hazard Quotient (HQ), where an HQ value exceeding 1 indicates a potential non-carcinogenic health risk. The evaluation targeted seven VOCs, including acetaldehyde, 1,3-butadiene, acrolein, benzene, toluene, styrene, and ethylbenzene/xylene.

The average HQ values for acetaldehyde, 1,3-butadiene, and acrolein were 0.2243, 0.3878, and 0.0622, respectively, suggesting a low overall risk. However, the maximum HQ values for these compounds were 6.2152, 9.2905, and 2.3640, respectively, indicating that some individuals may experience significant non-carcinogenic health effects (Table 4). The primary exposure source was asphalt paving, where VOC concentrations surged dramatically.

Table 4. Non-carcinogenic and carcinogenic risks of VOCs in asphalt work for process steps.

Chemicals		Non-carcinogenic risks				Carcinogenic risks
		Min	Mean	95th	Max	Min	Mean	95th	Max
Formaldehyde	Background	0.001494	0.004693	0.007006	0.014397	0.000326	0.000879	0.001307	0.002359
	Pavement	0.001670	0.018721	0.041880	0.160855	0.000354	0.003518	0.007747	0.036420
	Rolling	0.002235	0.005180	0.007355	0.011939	0.000401	0.000978	0.001406	0.002152
	Total	0.003038	0.029456	0.062021	0.240438	0.000637	0.005579	0.011918	0.042262
Acetaldehyde	Background	0.003277	0.018801	0.034851	0.079541
	Pavement	0.001124	0.177358	0.556758	5.548204
	Rolling	0.004972	0.026460	0.050155	0.141967
	Total	0.004050	0.224361	0.696647	6.215298
1,3-Butadiene	Background	0.002750	0.015471	0.028993	0.066645	1.79E-07	9.32E-07	1.73E-06	4.64E-06
	Pavement	0.003921	0.310280	0.957708	6.309419	2.12E-07	1.82E-05	5.72E-05	3.10E-04
	Rolling	0.011283	0.045443	0.072976	0.158974	7.71E-07	2.73E-06	4.43E-06	7.72E-06
	Total	0.001664	0.387875	1.316770	9.290530	1.18E-07	2.34E-05	7.89E-05	0.001167
Acrolein	Background	0.000108	0.002129	0.005559	0.027393
	Pavement	0.000529	0.043491	0.134871	0.915787
	Rolling	0.001724	0.007704	0.013238	0.034591
	Total	0.000274	0.062260	0.217321	2.364002
Benzene	Background	0.000022	0.000280	0.000611	0.002078	1.92E-08	2.25E-07	4.85E-07	2.00E-06
	Pavement	2.8E-05	0.001658	0.004808	0.028686	2.30E-08	1.39E-06	3.92E-06	4.28E-05
	Rolling	5.68E-05	0.000361	0.000658	0.002273	4.51E-08	2.97E-07	5.37E-07	1.28E-06
	Total	8.4E-05	0.002381	0.006740	0.032373	6.89E-08	2.00E-06	5.67E-06	3.57E-05
Toluene	Background	0.000000	0.000003	0.000008	0.000044
	Pavement	3.66E-07	2.79E-05	8.77E-05	0.000499
	Rolling	6.66E-07	4.79E-06	1E-05	2.56E-05
	Total	6.38E-07	3.68E-05	0.000119	0.000856
Styrene	Background	0.000000	0.000002	0.000005	0.000020	1.10E-10	1.20E-09	2.71E-09	9.66E-09
	Pavement	1.41E-06	4.71E-05	0.000135	0.000748	4.31E-10	2.63E-08	7.44E-08	6.83E-07
	Rolling	2.1E-06	1.18E-05	1.96E-05	4.5E-05	1.65E-09	6.68E-09	1.11E-08	2.67E-08
	Total	5.2E-07	7.16E-05	0.000250	0.004005	1.67E-10	3.94E-08	1.38E-07	1.85E-06
Ethylbenzene/	Background	0.000007	0.000186	0.000482	0.002379
Xylene	Pavement	2.65E-05	0.002347	0.007462	0.053962
	Rolling	4.45E-05	0.000388	0.000818	0.002331
	Total	2.57E-05	0.003206	0.010964	0.110327

While the non-carcinogenic risks for benzene, toluene, styrene, and ethylbenzene/xylene were generally low, benzene and styrene were further considered in the carcinogenic risk assessment due to their known carcinogenic properties.

Carcinogenic health risk assessment

The carcinogenic health risk assessment was conducted using the Lifetime Cancer Risk (LCR) metric. An LCR exceeding 10⁻⁶ (i.e., one additional cancer case per million people) was considered indicative of carcinogenic health risk. The assessment targeted benzene, styrene, and 1,3-butadiene.

The average LCR for benzene during background and roller operations remained below the risk threshold. However, during paving operations (4 h/day) and total daily exposure (8 h/day), the average LCR values were 1.39E-06 and 2.00E-06, respectively, exceeding the threshold by approximately 1.4-fold and 2-fold. Furthermore, the 95th percentile values indicated that the cancer risk increased by 3.9-fold and 5.6-fold, respectively. These findings suggest that benzene exposure during the paving process is a primary contributor to carcinogenic risk.

Styrene also exhibited an increasing cancer risk trend during paving operations, similar to benzene, but its overall carcinogenic risk was lower. The maximum LCR value was 1.85E-06, slightly exceeding the threshold, indicating that some individuals may be at low-level carcinogenic risk.

1,3-Butadiene followed a similar pattern to benzene. The average LCR value exceeded the threshold, and the maximum value reached 0.001167, significantly exceeding the acceptable limit. These results suggest that some individuals may face considerable carcinogenic health risks due to 1,3-butadiene exposure. Notably, paving operations were the main source of exposure, as 1,3-butadiene concentrations increased sharply during this phase.

Given these findings, effective strategies to reduce VOC exposure—particularly during asphalt paving operations are urgently needed.

Although risk assessments on asphalt paving operations are relatively rare, Cui et al.¹⁸⁾ also reported that some asphalt paving workers exceeded the benzene-related cancer risk threshold (0.01%). Their analysis identified work duration and job location as the most influential variables affecting carcinogenic VOC exposure.

To mitigate these health risks, Cui et al.¹⁸⁾ suggested that using Warm Mix Asphalt (WMA) and foamed asphalt could reduce asphalt temperature, thereby significantly decreasing VOC emissions. Additional strategies included minimizing asphalt agitation and dumping to limit VOC volatilization and applying high-pressure water mist to suppress VOC dispersion.

Although not specific to asphalt paving, Khoshakhlagh et al.²³⁾ conducted a carcinogenic and non-carcinogenic risk assessment of 1,3-butadiene and styrene exposure in carpet manufacturing. They reported average worker exposure levels of 0.017 ppm for 1,3-butadiene and 2.84 ppm for styrene. The average carcinogenic risk values were 5.13 × 10⁻³ and 1.44 × 10⁻³, respectively, far exceeding the USEPA threshold (10⁻⁶). The average non-carcinogenic risk values also exceeded the acceptable limit, at 8.50 for 1,3-butadiene and 5.13 for styrene.

In the present study, the measured concentrations of 1,3-butadiene and styrene were 0.025 ppm and 0.002 ppm, respectively. The average carcinogenic risk for 1,3-butadiene was 2.34E-05, exceeding the threshold, whereas styrene concentrations were significantly lower than those in Khoshakhlagh et al.²³⁾, resulting in a much lower carcinogenic risk (3.94E-08).

For non-carcinogenic risks, only 1,3-butadiene exceeded the acceptable limit at the 95th percentile (HQ=1.3) and at its maximum value (HQ=9.3).

Strategies for reducing health risks

Among the three work phases, VOC concentrations and carcinogenic risks were highest during asphalt paving. Thus, efforts should focus on reducing VOC emissions and minimizing worker inhalation exposure.

To achieve this, the present study reviewed mitigation strategies proposed by Cui et al. and also identified additional measures, including reducing exposure to diesel exhaust (IARC Group 1 carcinogen), minimizing crystalline silica exposure, and requiring workers to wear combination dust and gas masks (Table 5).

Table 5. Proposed strategies for reducing VOC and particulate hazardous substance exposure during asphalt paving operations.

Mitigation measures	Reduction methods	Description	References
Reduction of VOC Emissions	Minimizing asphalt temperature, mixing, and dumping; applying high-pressure water spraying	· Utilizing Warm Mix Asphalt (WMA) and foamed asphalt, which generate lower VOC emissions, helps reduce VOC volatilization and dispersion. · Applying water mist using high-pressure sprayers prevents VOC diffusion into the work environment.	18)
Reduction of worker exposure	Optimization of work direction	Conducting asphalt paving in the same direction as the wind helps minimize workers’ exposure to VOCs.
Reducing individual worker exposure time	Implementation of a job rotation system	To prevent prolonged exposure to VOCs, a shift rotation system should be introduced, ensuring that individual exposure durations are minimized.
Reduction of VOC emissions	Lowering paving temperature and adjusting binder ratio	· Reducing the temperature of asphalt mixtures can decrease the evaporation of hazardous substances. · Using eco-friendly materials with low VOC content (low-VOC materials) can help minimize the release of harmful substances during operations.	22)
Reduction of diesel exhaust exposure	Engine idling control and exhaust reduction equipment	· Large diesel-powered vehicles at paving sites, such as asphalt pavers, macadam rollers, tire rollers, and dump trucks, continuously emit diesel exhaust, which disperses into the surrounding environment. · Diesel-powered vehicles should turn off their engines when waiting on-site to reduce emissions. · Vehicles should be equipped with exhaust control devices to minimize particulate emissions.	Proposed by the authors.
Reduction of crystalline silica exposure	Implementing wet processes during asphalt milling, removal, and aggregate placement	· Crystalline silica can become airborne when aggregates in asphalt are crushed, and exposure may occur during new aggregate placement, leveling, and compaction. · Wet methods should be used during asphalt milling, removal, and subsequent aggregate placement to minimize silica dust.	Proposed by the authors.
Wearing multi-purpose masks for both particulate and gas hazards		Workers are simultaneously exposed to hazardous VOCs and particulate matter, necessitating the use of multi-purpose respiratory masks that provide both dust and gas protection.	Proposed by the authors.

VOC: volatile organic compound.

Asphalt paving operations continuously generate diesel exhaust due to the presence of multiple heavy-duty vehicles. The elemental carbon (EC) concentrations, a proxy for diesel emissions, measured among three paving workers were 6 µg/m³, 5 µg/m³, and 3 µg/m³, while the EC concentrations for macadam roller operators and tire roller operators were both 3 µg/m³.

Although these values were below the recommended Cal/EPA exposure limit (20 µg/m³), they were still significantly higher than typical ambient levels (1 µg/m³).

Crystalline silica exposure was detected in only one of the three paving workers, at a concentration of 2 µg/m³. However, this was likely due to the absence of key silica-emitting processes, such as asphalt milling, removal, and aggregate placement, during the study period (unpublished data). Furthermore, according to a NIOSH study on crystalline silica exposure among paving workers during asphalt road paving operations with an active milling machine, the geometric mean concentrations of respirable crystalline silica varied by site. At Site A, the concentration was 0.006 mg/m³, while at Site B, it was 0.009 mg/m³, which is approximately one-third of the ACGIH TWA TLV (25 µg/m³). However, the maximum concentration reached 0.024 mg/m³, which was comparable to the TLV level²⁴⁾. Diesel engine exhaust particulates are classified as an IARC Group 1 carcinogen and are also a major ultrafine particulate matter (PM) pollutant, with growing concerns about their impact on human health²⁵⁾.

These findings emphasize the urgent need for enhanced occupational exposure control measures in asphalt paving operations, particularly to reduce VOC exposure during paving activities and to control exposure to diesel engine exhaust particulates and respirable crystalline silica dust in the surrounding environment.

Moreover, since asphalt paving is an outdoor operation, seasonal variations necessitate protective measures to prevent heat-related health issues in hot conditions and cold stress in low temperatures. In hot environments, workers should wear moisture-absorbing and breathable work clothing, UV-protective sunglasses to shield against direct sunlight, and apply sunscreen for skin protection. In cold environments, appropriate protective clothing, including insulated jackets, thermal hats, cold-resistant footwear, and insulated gloves, is required to prevent cold-related health risks.

Study limitations

VOCs emitted during asphalt paving operations were measured using a PTR-ToF-MS mounted on a mobile vehicle, which moved alongside the asphalt paver. However, a key limitation of this approach is that it does not fully account for variations in worker exposure levels based on individual work positions and tasks.

Workers who operate in close proximity to the asphalt paver or perform manual tasks such as spreading or shoveling asphalt mix may experience higher VOC exposure levels than those reported in this study. Conversely, workers positioned at a greater distance from the paver are likely exposed to lower VOC concentrations than those observed.

Despite this limitation, efforts were made to closely represent actual worker exposure levels by positioning the PTR-ToF-MS sampling tube near workers and moving alongside them. As a result, the measured concentrations are considered to closely approximate the average exposure levels of asphalt paving workers.

Conclusion

Research on worker exposure to VOCs during asphalt paving operations remains highly limited, and protective regulations or standards for such exposures are insufficient. While previous studies have primarily focused on a few key compounds, this study utilized high-resolution, high-performance PTR-ToF-MS to comprehensively assess 25 target compounds (17 VOCs and 8 gas-phase PAHs). Furthermore, Monte Carlo simulations were applied to evaluate both non-carcinogenic and carcinogenic risks, leading to recommendations for future risk mitigation strategies.

During paving operations, benzene, acrolein, and most other VOCs increased by a factor of 2 to 10 compared to background levels. None of the measured compounds exceeded workplace 8-h TWA OELs; however, acrolein exceeded the ACGIH ceiling TLV, highlighting the need to manage short-term peak exposures in specific work scenarios.

In the non-carcinogenic risk assessment, the maximum HQ values for acetaldehyde, 1,3-butadiene, and acrolein exceeded 1, indicating potential adverse health effects. Additionally, the LCR values for benzene and 1,3-butadiene surpassed established safety thresholds, demonstrating a significant increase in carcinogenic risk. Since this study measured VOC concentrations near the asphalt paving site, and considering that work positions and tasks vary among workers, some workers may experience higher actual exposure levels than those reported.

Therefore, engineering controls such as reducing bitumen content and lowering paving temperatures, as well as administrative measures such as job rotation during peak exposure periods, should be implemented to reduce VOC exposure during asphalt paving operations. Additionally, proper personal protective equipment (PPE) capable of filtering both particulate and gaseous pollutants should be used, and improvements in work practices should be considered.

Furthermore, many PAHs lack established TLVs, underscoring the need for additional research and the development of occupational exposure limits.

Based on these findings, we emphasize the necessity of regular work environment monitoring and further research to enhance worker protection in asphalt paving operations.

Conflict of Interest

The authors declare that they have no conflicts of interest.