Particulate-Bound Polycyclic Aromatic Hydrocarbons and Heavy Metals in Indoor Air Collected from Religious Places for Human Health Risk Assessment

Abstract

Particulate matter (PM) has been associated with various health issues. However, the most hazardous constituents of fine particles remain unclear, particularly in Asia where the chemical compositions are highly diverse and understudied. This study investigated the concentration and health risks of particulate-bound polycyclic aromatic hydrocarbons (PAHs) and heavy metals in the indoor air of religious spaces in Bangkok, Thailand. Air samples were collected from four religious sites during periods of high activity using a six-stage NanoSampler to capture particle sizes ranging from <0.1 to >10 µm. Chemical analyses were conducted using gas chromatography-mass spectrometry (GC-MS/MS) for PAHs and inductively coupled plasma-mass spectrometry (ICP-MS) for heavy metals. The results revealed significantly elevated concentrations of PM2.5, PAHs (notably benzo[a]anthracene (BaA), chrysene (CHR), and fluoranthene (FLU)), and heavy metals (particularly Mn, Ni, and Cu). Health risk assessments indicated that both the incremental lifetime cancer risk (ILCR) and hazard quotient (HQ) values for several pollutants exceeded the U.S. EPA safety thresholds, suggesting serious cancer and non-cancer health risks for workers exposed to these environments over prolonged periods. This study highlights incense burning as a dominant source of toxic indoor air pollutants and underscores the urgent need for mitigation strategies to reduce occupational exposure in religious buildings.

1. Introduction

Indoor air pollution significantly compromises the air quality, with prolonged exposure to contaminants posing substantial health risks [1]. Fine particulate matter can infiltrate the respiratory system, leading to infections, inflammation, and increased mortality from respiratory and cardiovascular diseases [2,3].

In Asian countries, religious activities constitute a major source of indoor particulate matter (PM) pollution [4]. The combustion of incense in enclosed worship spaces generates multiple hazardous pollutants including volatile organic compounds (VOCs), carbon and nitrogen oxides, various hydrocarbons, heavy metals (HMs), and polycyclic aromatic hydrocarbons (PAHs) [5]. These emissions become significantly more concentrated due to the characteristic combination of high occupant density and inadequate ventilation typically found in religious buildings [6].

In Thailand, a study of indoor pollutants in religious spaces in Chiang Mai revealed that incense burning during festivals significantly increased the PM_2.5 (625 ± 147 μg/m³, 8-h) and PAHs (90 ± 41 ng/m³), exceeding the EU safety limits by 32-fold. PM_2.5 strongly correlated with carcinogenic PAHs (r = 0.618), confirming incense as the dominant pollution source. The toxicity equivalent concentration (TEQ) values indicate that the inhalation of these PAHs is strongly associated with potential human health risks [7]. A biomarker study demonstrated that Thai temple workers faced severe carcinogen exposure from incense smoke, with benzene levels (45.9 µg/m³) and 8-hydroxy-2′-deoxyguanosine (8-OHdG) doubling the control values (p < 0.001) while showing significantly impaired DNA repair capacity, which indicates a substantial cancer risk [8].

Beyond PAHs, a study at Tzu Yun Yen Temple, Taiwan, revealed that incense burning generated particulate matter dominated by fine particles, with metallic composition analysis showing iron (Fe), zinc (Zn), and chromium (Cr) as the predominant elements in both the PM_2.5 and PM_2.5–10 fractions [9]. A comparative study of incense combustion identified transition metals and heavy metals, including Fe, Zn, Cu, and Mn, emitted smoke and residual ash, with concentrations varying by incense composition. These fine particulate-bound PAHs and heavy metals can rapidly enter the human body, posing significant health risks and by-products that are also known to trigger cellular-level pro-inflammatory responses, which may contribute to adverse health effects [10]. Furthermore, chronic exposure to incense-derived metals like Cr and Ni is associated with carcinogenic effects, while Pb and Cd exposure may cause neurological and renal damage in temple workers and nearby communities. Toxic heavy metals (Pb, Cd, As, Hg) are able to bioaccumulate in aquatic ecosystems when improperly disposed, posing risks to both marine life and humans through the food chain [11].

Recent statistics from Thailand’s Buddhist Monastery Department revealed approximately 450 active temples nationwide, with nearly 18,000 monks, novices, and religious workers residing in Bangkok’s sacred spaces [12]. As temple worship continues to serve as a cornerstone of spiritual and cultural life for both local devotees and international visitors, critical gaps persist in understanding the health implications of airborne pollutants generated within these environments. This study aimed to investigate the characteristics of particulate-bound PAHs and heavy metals, whose PM counts range across the size spectrum from ultrafine (PM0.1) to coarse (PM10), within the indoor air of religious spaces located in Bangkok, evaluate the relationship between human exposure to indoor air pollutants in religious spaces, and assess the associated health risks for workers in Bangkok, Thailand.

2. Materials and Methods

2.1. Sampling Sites

Bangkok, the capital of Thailand, has experienced an urban growth process dominated by extension and densification. Because of the city’s popularity and convenient transportation, Bangkok’s inner-city area is densely populated by workers and workplaces and has become an urban community. Many temples and shrines in the community are surrounded by different environments such as roadsides, riversides, midtown landscapes, and congestion.

All four sampling sites were conducted at closed or almost closed buildings of temples or religious spaces in the inner city of Bangkok, where religious activities and incense burning are still active [13]. Based on our observations, during Buddhist holy days, daily building occupancy across sites 1–4 vary between 300 and 1500 individuals.

According to the traditional practices found in Southeast Asia, people usually go to the temple on days marked Buddhist Holy Days and visit temples and shrines on weekends or significant days. Therefore, indoor air samples were collected on significant religious holidays or weekends as high-activity days at four religious spaces from February to May 2024, defined as site 1 to site 4.

2.2. Sampling of Particulate Matter

The instrument was installed in the incense burning zone at a similar distance from the burning spot at each site. We operated during the summer season in Thailand and collected indoor air for 8 h according to the building’s operating hours using a six-stage NanoSampler air sampling device from Kanazawa University, Japan. Particulate matter was collected on quartz fiber filters, 2500QAT-UP 55 mm (Shima Trading Company, Tokyo, Japan) and separated in bins <0.1, 0.1–0.5, 0.5–1, 1–2.5, 2.5–10, and >10 µm in size with an inlet flow rate of 30.0 L/min. All filters were baked in a muffle furnace under 500 °C and kept in a dry desiccator cabinet for 48 h before weighing. The filters were weighed before and after sampling using an XPR Micro-Analytical Balance (Mettler Toledo, Bangkok, Thailand). After sampling, 24 samples (6 samples/site) were collected from all sampling sites. The filters were kept in a dry desiccator cabinet for 48 h and then stored in a refrigerator at −5 °C for further analysis. The collected filter samples were analyzed for particulate matter content using the gravimetric method, which determines the particle concentration based on weight. Subsequently, the extracts were processed to isolate PAHs for analysis via GC-MS/MS and heavy metals for analysis with ICP-MS.

2.3. Quantitative Analysis of PAHs

The detailed procedures for sample extraction and analysis were developed from a previous study [14]. Half-cut portions of the filters were extracted using an organic solvent mixture of n-hexane and dichloromethane in a 1:1 volume ratio and then concentrated using an Eppendorf Concentrator. The extraction process was repeated twice, and samples were stored in a freezer at −80 °C. Prior to chemical analysis, the samples underwent re-constitution by adding 150 µL of the organic solvent. The reconstituted solution was then transferred into amber vials for subsequent chemical analysis.

The concentration of PAHs in the prepared sample solutions was analyzed using a gas chromatography tandem mass spectrometry (GC-MS/MS) device, model GCMS-TQ8050 NX. The analysis was conducted by comparing the results against a calibration curve generated from 16 standard PAH compounds (TraceCERT^® 2000 µg/mL, Merck Ltd., Bangkok, Thailand): naphthalene (NAP), acenaphthylene (ACY), acenaphthene (ACE), fluorene (FLU), phenanthrene (PHE), anthracene (ANT), fluoranthene (FLT), pyrene (PYR), benzo[a]anthracene (BaA), chrysene (CHR), benzo[b]fluoranthene (BbF), benzo[k]fluoranthene (BkF), benzo[a]pyrene (BaP), benzo[g,h,i]perylene (B[ghi]P), indeno [1,2,3-c,d]pyrene (IND), and dibenzo[a,h]antracene (DahA).

2.4. Quantitative Analysis of Heavy Metals

The detailed procedures for the sample extraction and analysis were developed from a previous study [15,16]. One-third-cut portions of the filters were digested using 65% SupraPure grade nitric acid (National Institute of Metrology, Bangkok, Thailand) in a digestion vessel. The vessel was sealed and placed in a microwave digestion system, operated at 175 °C for 45 min. After digestion, each clear yellow solution was transferred into a Teflon beaker and heated on a hot plate to evaporate the acid. Then, the volume was adjusted using Type I water (ultra-pure water). The solution was stored in a refrigerator at 5 °C for subsequent chemical analysis.

The concentration of heavy metals in the prepared sample solutions was analyzed using inductively coupled plasma-mass spectrometry (ICP-MS), specifically a Thermo Scientific iCAP™ RQ ICP-MS (Thermo Fisher Scientific Inc., Waltham, MA, USA) device. The analysis was conducted by comparing the results against the calibration curves of lithium (Li), chromium (Cr), manganese (Mn), cobalt (Co), nickel (Ni), beryllium (Be), vanadium (V), copper (Cu), arsenic (As), selenium (Se), cadmium (Cd), and lead (Pb) generated from a mixed standard solution (Thermo Fisher Scientific, Waltham, MA, USA), under the conditions of kinetic energy discrimination (KED).

2.5. Health Risk Assessment

2.5.1. Particulate Matter Health Assessment

The concentration of indoor air pollutants to which workers are exposed (C_exposure) within religious spaces was calculated to assess the health risks associated with exposure to particulate-bound PAHs and heavy metals in the indoor air, using a sampling time of 8 h for each religious place:

$${\mathrm{C}}_{\mathrm{e}\mathrm{x}\mathrm{p}\mathrm{o}\mathrm{s}\mathrm{u}\mathrm{r}\mathrm{e}}=C_{indoor air}{\displaystyle \frac{ ET\times EF\times ED}{AT}}$$

(1)

where C_{indoor air} is the concentration of indoor air pollutants in the religious building (mg/m³), ET is the exposure time (hour/day), EF is the exposure frequency (day/year), ED is the exposure duration (year), which was 25, and AT is the average exposure time, which was 8 h [17].

2.5.2. PAHs Health Assessment

The PAH cancer and health risk values were assessed according to the Guidelines for Characterizing Risk and Hazard. The incremental lifetime cancer risk (ILCR) of inhaling carcinogenic PAHs was calculated using the carcinogenic risk equation and compared against the standard ILCR threshold to assess the health risks associated with human exposure to carcinogenic PAHs through inhaling the indoor air of religious spaces:

$${\mathrm{I}\mathrm{L}\mathrm{C}\mathrm{R}}_{\mathrm{P}\mathrm{A}\mathrm{H}\mathrm{s}}={\displaystyle \frac{C\times IR\times ET\times EF\times ED\times csf}{BW\times AT}}$$

(2)

where C is the PAH concentration in fine particulate matter collected from indoor air, IR is the inhalation rate, which was 30 m³/day, ET is the exposure time (hour/day), EF is the exposure frequency (day/year), csf is the cancer slope factor (mg/kg.day), ED is the exposure duration (year), which was 25, and AT is the average exposure time, which was 70 × 365 for carcinogenic PAHs. The reference csf values for BaA and CHR were 0.39 and 0.039, respectively [17].

The hazard quotient (HQ) for inhaling non-carcinogenic PAHs was calculated and compared against the standard HQ threshold to assess the health risks associated with human exposure to non-carcinogenic PAHs through inhaling the indoor air of religious spaces:

$${\mathrm{H}\mathrm{Q}}_{\mathrm{P}\mathrm{A}\mathrm{H}\mathrm{s}}={\displaystyle \frac{ADD}{RfD}}$$

(3)

where ADD is the average daily dose and RfD is the reference dose (mg/kg.day). The reference RfD values for NAP, FLU, and PHE were 0.000857, 0.04, and 0.03, respectively [17].

2.5.3. Heavy Metal Health Assessment

The heavy metal cancer and health risk values were assessed according to the Guidelines for Characterizing Risk and Hazard. The ILCR of inhaling carcinogenic heavy metals was calculated using the carcinogenic risk equation and compared against the standard ILCR threshold to assess the health risks associated with human exposure to carcinogenic heavy metals through inhaling the indoor air of religious spaces:

$${\mathrm{I}\mathrm{L}\mathrm{C}\mathrm{R}}_{\mathrm{H}\mathrm{e}\mathrm{a}\mathrm{v}\mathrm{y} \mathrm{m}\mathrm{e}\mathrm{t}\mathrm{a}\mathrm{l}\mathrm{s}}=LADD\times csf$$

(4)

where LADD is the lifetime average daily dose and csf is the cancer slope factor (mg/kg.day). The reference csf values for Mn and Cu were 1.43 × 10⁻⁵ and 4.02 × 10⁻², respectively [17].

The HQ for inhaling non-carcinogenic heavy metals was calculated and compared against the standard HQ threshold to assess the health risks associated with human exposure to non-carcinogenic heavy metals through inhaling the indoor air of religious spaces:

$${\mathrm{H}\mathrm{Q}}_{\mathrm{H}\mathrm{e}\mathrm{a}\mathrm{v}\mathrm{y} \mathrm{m}\mathrm{e}\mathrm{t}\mathrm{a}\mathrm{l}\mathrm{s}}={\displaystyle \frac{ADD}{RfD}}$$

(5)

where ADD is the average daily dose and RfD is the reference dose (mg/kg.day). The reference RfD values for Ni, Pb, Cd, As, and Cr were 8.4 × 10⁻², 4.2 × 10⁻², 6.3, 15.1, and 42, respectively [17].

The implications, units, and reference values in Equations (1)–(5) are listed in Table S1. The considered exposure scenario assumes a breathing rate of 30 m³/day with workers exposed to PAHs inside the religious building for 8 h daily over a 25-year period [17]. All components were calculated from the concentrations of PAH and HM. Worker health data including exposure duration and symptoms were collected via standardized questionnaires and analyzed using EPA risk assessment models to calculate the cancer risk and hazard quotients, as shown in Table S2.

An ILCR between 10⁻⁶ and 10⁻⁴ indicates potential risk, whereas a value greater than 10⁻⁴ indicates high potential risk. If the HQ is less than 1, there are no significant health concerns. However, if the HQ is equal to or greater than 1, potential adverse health effects may arise [17].

2.6. Quality Assurance and Control

All chemicals used were analytical grade reagents. We employed ultrapure water (18.2 MΩ·cm resistivity) produced using a Millipore Milli-Q purification system (Bedford, MA, USA). Prior to use, all glassware underwent a 24-h acid wash in 10% (v/v) nitric acid followed by thorough rinsing with deionized water. Quantification was performed using multi-element standards. Method detection limits (MDLs) were established following the IUPAC guidelines, calculated as three times the standard deviation (3σ) of ten procedural blanks [18]. Field blanks were processed identically to the samples and analyzed routinely. All reported values were blank-corrected. Approximately 10% of samples were analyzed in replicate to ensure method precision.

2.7. Statistical Analysis

All statistical analyses were performed using IBM SPSS Statistics (Version 29). Elements for which >30% of the samples were below the limit of detection were not considered in the discussion. Mean comparisons of PAH and heavy metal concentrations were conducted using a t-test, with the statistical significance set to p < 0.05.

3. Results

3.1. Particle Mass Concentration

The total mass concentration of particulate matter (PM) in the indoor air measured at four different sites ranged from 397 to 1125 µg/m³ (Figure 1). Pictures of the dust samples collected on each filter layer from site 1 to site 4 are provided in Table S3. Site 2 yielded the highest total PM concentration, exceeding 1000 µg/m³, with notable amounts of PM > 10 and PM 2.5–10. Site 1 had the second-highest total concentration, with around 600 µg/m³, predominantly consisting of PM > 10 and PM 2.5–10. Sites 3 and 4 yielded significantly lower concentrations, each around 400 µg/m³, with PM 2.5–10 and PM 1–2.5 comprising the largest fractions. PM ≤ 0.1 consistently represented the smallest proportion across all sites. When comparing the total mass concentration between each site, it was found that the mass concentration at site 2 increased by 53.6%, depending on the number of visitors and incense sticks burned at the site. We observed that higher visitor numbers coincided with increased incense use (up to 30 sticks/person), directly contributing to the measured PM peaks. Furthermore, the scanning mobility particle sizer (SMPS) observations indicated that the indoor PM was consistently higher than the outdoor PM. This result suggests that the PM in the indoor air was influenced by incense burning [19].

3.2. PAH Concentration

The particulate matter PAH concentrations shown in

Table 1. PAH concentration (µg/m³) of each sampling site presented as the mean ± standard deviation and the range of concentrations (minimum–maximum).

PAHs	PAH Concentrations (µg/m3)
	Site 1 a (n = 6)	Site 2 b (n = 6)	Site 3 c (n = 6)	Site 4 d (n = 6)
NAP	19.03 ± 1.46 (17.50–20.42)	60.04 ± 1.93 (58.83–62.27)	ND	ND
FLU	41.47 ± 3.14 (38.65–44.85)	46.28 ± 3.41 (42.34–48.3)	22.93 ± 8.08 (17.36–32.2)	17.90 ± 4.62 (14.24–23.09)
FLT	ND	125.93 ± 34.61 (97.86–164.60)	ND	ND
PHE	ND	331.29 ± 46.98 (295.90–384.59)	ND	ND
BaA	ND	851.79 ± 188.49 (636.05–984.56)	ND	ND
CHR	ND	290.88 ± 49.06 (235.19–327.75)	ND	ND
PYR	ND	118.53 ± 36.40 (78.83–150.32)	ND	ND
Total	60.5 ± 4.60	1824.74 ± 360.88	22.93 ± 8.08	17.90 ± 4.62

^{a, b, c, d} = significant differences (p < 0.05) among groups of sampling periods. Abbreviations: NAP (naphthalene), FLU (fluorene), FLT (fluoranthene), PHE (phenanthrene), BaA (benzo[a]anthracene), CHR (chrysene), and PYR (pyrene).

revealed that certain PAHs were detected in each religious building. The total concentrations of PAHs in religious spaces on high activity days exhibited the highest total PAH concentration at site 2, which was 1824.74 µg/m³, followed by site 1, with a PAH concentration of 60.5 µg/m³. On the other hand, FLU was found at site 3 and site 4, with respective concentrations of 22.93 and 17.90 µg/m³. The detection limits of the PAHs found in this study ranged from 4.30 to 8.83 µg/m³.

This study identified the percentage contribution of various PAHs bound to PM. PYR was found to be the predominant compound, accounting for 47% of the total PAH composition. This PAH was followed by BaA at 18%, CHR at 16%, FLT at 7%, FLU at 6%, PHE at 3%, and NAP at 3%. The other PAHs with detection limits lower than 4.30 µg/m³ were not detected. When compared with the same site with increased incense burning, a greater variety of PAHs was detected. However, BaA remained the most predominant compound, as shown in Figure 2.

We calculated the isomeric ratios of some PAHs to study the sources of PAHs, as shown in

Table 2. Isomeric ratios of PAHs.

Isomeric Ratios	Ratio Values of PAHs Within Site 2	Reference Values	Sources	References
FLT/(FLT+PYR)	0.52	<0.40	Petroleum input	[20]
		0.40–0.50	Fossil fuel combustion
		>0.50	Grass, wood, and coal combustion
BaA/(BaA+CHR)	0.75	<0.20	Petroleum	[21]
		0.20–0.35	Petroleum or combustion
		>0.35	Combustion
ΣLMW/ΣHMW	0.32	<1	Pyrogenic including incomplete combustion	[22]
			of fossil fuels or wood
		>1	Petrogenic sources including spilled oil or petroleum products
ΣCOMB/ΣPAHs	1.26	0.3	Petrogenic	[23]
		0.7	Pyrogenic

Abbreviations: FLT (fluoranthene), BaA (benzo[a]anthracene), CHR (chrysene), PYR (pyrene), LMW (low molecular weight), HMW (high molecular weight), and COMB (PAHs emitted from combustion).

. The ratio values for high activity days can be interpreted to determine the exact origin. The FLT/(FLT+PYR) and BaA/(BaA+CHR) ratios identified the sources of natural material combustion as well as the PAHs with low and high molecular weight (LMW and HMW, respectively) and PAHs emitted from combustion (COMB). The ΣLMW/ΣHMW and ΣCOMB/ΣPAHs indicated that the PAHs originated from a pyrogenic process.

3.3. Heavy Metal Concentration

The analysis revealed that particulate matter from site 2 had the highest heavy metal concentrations (

Table 3. HM concentration (µg/m³) of each sampling site presented as the mean ± standard deviation and the range of concentrations (minimum–maximum).

Element	Metal Concentrations (µg/m3)
	Site 1 a (n = 6)	Site 2 b (n = 6)	Site 3 c (n = 6)	Site 4 d (n = 6)
Li	0.116 ± 0.016 (0.098–0.140)	0.965 ± 0.212 (0.739–1.341)	0.314 ± 0.053 (0.279–0.358)	0.355 ± 0.033 (0.313–0.430)
V	0.407 ± 0.049 (0.353–0.474)	0.382 ± 0.040 (0.355–0.460)	0.247 ± 0.004 (0.243–0.253)	0.412 ± 0.033 (0.375–0.465)
Cr	2.147 ± 0.112 (2.054–2.320)	ND	0.382 ± 0.021 (0.345–0.403)	1.928 ± 0.088 (1.756–2.005)
Mn	4.536 ± 0.270 (4.077–4.768)	17.081 ± 0.696 (16.345–18.340)	0.880 ± 0.080 (0.757–0.967)	4.513 ± 0.295 (4.120–4.806)
Co	0.736 ± 0.085 (0.636–0.857)	ND	0.316 ± 0.021 (0.295–0.337)	0.258 ± 0.044 (0.194–0.295)
Ni	3.377 ± 0.196 (3.056–3.578)	8.204 ± 0.280 (7.946–8.635)	0.509 ± 0.159 (0.307–0.684)	10.272 ± 1.672 (8.354–12.464)
Cu	ND	5.854 ± 1.263 (3.475–6.874)	1.337 ± 0.248 (1.078–1.754)	5.346 ± 1.164 (3.982–6.865)
As	0.366 ± 0.041 (0.300–0.398)	0.717 ± 0.080 (0.578–0.799)	0.161 ± 0.032 (0.108–0.190)	0.289 ± 0.054 (0.206–0.367)
Cd	0.059 ± 0.016 (0.037–0.075)	0.156 ± 0.038 (0.097–0.178)	ND	0.273 ± 0.067 (0.195–0.356)
Pb	4.086 ± 0.235 (3.793–4.268)	3.999 ± 0.572 (3.450–4.934)	2.504 ± 0.494 (1.934–3.020)	5.289 ± 0.971 (3.407–5.930)
Total	15.830 ± 1.019	37.358 ± 3.182	6.650 ± 1.093	28.935 ± 4.422

^{a, b, c, d} = significant differences (p < 0.05) among groups of sampling periods. Abbreviations: Li (lithium), V (vanadium), Cr (chromium), Mn (manganese), Co (cobalt), Ni (nickel), Cu (copper), As (arsenic), Cd (cadmium), and Pb (lead).

). Site 2 had the highest total concentration (37.358 µg/m³), followed by site 4 (28.935 µg/m³), site 1 (15.83 µg/m³), and site 3 (6.65 µg/m³). Among the individual metals, copper (Cu) presented notably high levels at site 2 (5.854 µg/m³) and site 4 (5.346 µg/m³). Nickel (Ni) and manganese (Mn) were particularly abundant at site 2, with concentrations of 8.204 µg/m³ and 17.081 µg/m³, respectively. The chromium (Cr) concentration was highest at site 4 (1.928 µg/m³) and below the limit of detection at site 2. Lead (Pb) had relatively elevated concentrations at all sampling sites, with the highest observed at site 1 (4.086 µg/m³). Other metals such as lithium (Li), vanadium (V), cobalt (Co), arsenic (As), and cadmium (Cd) were detected at comparatively lower concentrations across all sites. The types of heavy metals present in fine particulate matter differed among the religious buildings, potentially due to the use of incense sticks from various manufacturers with distinct chemical formulations and metal contents. Higher amounts of heavy metals were associated with activity, particularly in smaller PM fractions (PM < 0.1 μm and PM 0.5–1 μm), which are more respirable and potentially more hazardous.

3.4. Particle Matter Health Assessment

We collected the personal information values for health risk assessments among the 37 workers, as listed in Table S3. The health risks associated with human exposure to fine particulate matter within religious spaces were assessed with the average C_exposure values for each religious place as follows: site 1 (2731.29 mg/m³), site 2 (18,274.94 mg/m³), site 3 (2083.51 mg/m³), and site 4 (5560.52 mg/m³).

In addition, the average C_exposure values that the workers were exposed to in this study exceeded the air quality guidelines set by the World Health Organization (WHO). Specifically, the WHO stipulates that humans should not be exposed to PM10 concentrations exceeding 45 µg/m³ for prolonged periods (WHO, 2021) [24].

3.5. PAH Health Assessment

The ILCR values for inhaling BaA and CHR were also calculated, as shown in Figure 3a, with average values of 1.86 × 10⁻¹ and 6.91 × 10⁻³, respectively. Both values exceeded the limitations for ILCR according to the USEPA guideline (10⁻⁴), indicating that BaA and CHR detected in religious spaces pose a significant cancer risk to workers with prolonged exposure. Although the average ILCR-BaA was found to be higher than ILCR-CHR, it can be inferred that inhaling BaA carries a higher cancer risk than inhaling CHR.

The HQ values for inhaling NAP, FLU, and PHE were calculated next, as shown in Figure 3b, with average values of 435.20, 9.23, and 39.20, respectively. These average values also exceeded the limitations of HQ according to the USEPA guidelines (1), indicating that exposure to NAP, FLU, and PHE through inhalation can lead to adverse health effects.

3.6. Heavy Metal Health Assessment

The ILCR values for inhaling Ni, Pb, Cr, As, and Cd were calculated, as shown in Figure 4a. The results showed that the average ILCR for As was the highest at 9.08 × 10⁻³, while the average ILCR for Pb was the lowest at 2.38 × 10⁻⁴. We further found that the average ILCR values for inhaling five heavy metals exceeded the limitations of ILCR according to the USEPA guidelines of 10⁻⁴, indicating that the Ni, Pb, Cr, As, and Cd detected in religious spaces pose a significant cancer risk to workers with prolonged exposure.

The HQ values for inhaling Mn and Cu were calculated, as shown in Figure 4b, with average values of 3.18 × 10⁴ and 5.25, respectively. When compared with the limitations of HQ according to the USEPA guidelines, these average values exceeded 1, indicating that exposure to Mn and Cu through inhalation could lead to adverse health effects.

4. Discussion

This study focused on investigating the chemical components in PM, from coarse particles down to the finest ultrafine PM, within an urban area at a crowded site in central Bangkok. Thailand’s Department of Health has established indoor air quality guidelines for public buildings (50 μg/m³ PM_2.5 for 24-h exposure), however, our study identified significant limitations in addressing ritual-specific pollutants. The measured PM_2.5 concentrations in temples surpassed both Thailand’s indoor guideline by 7–14.4-fold and the WHO’s air quality guideline (AQG) (75 μg/m³) by 4.7–9.6-fold [24,25]. These extreme pollution levels with similar patterns were observed during Chinese New Year events in Chiang Mai and Hong Kong [7,26,27], revealing that current Thai standards, while progressive for conventional indoor spaces, remain inadequate for religious environments where PAHs and heavy metals predominate.

The concentrations of NAP, FLU, FLT, PHE, BaA, CHR, and PYR were found to be different in Chiang Mai and Taiwan, with BbF, BaP, InD, and BPER identified as the predominant PAHs within religious spaces [7]. Chemical analysis identified BaA as the dominant congener (47% of total PAHs), followed by CHY and PHE, which are the primary products of incomplete combustion processes. FLU was the only PAH found in fine particulate matter from all four religious spaces in this study. The significant levels of FLU were likely due to the presence of FLU in dyes, pigments, and pesticides, which are common components of the materials used in incense production [28,29]. An examination of materials used in the incense manufacturing industry in Thailand revealed that most incense sticks are composed of wood and powdered dyes [30]. The higher proportions of dominant PAHs found in this study likely reflect Thailand’s unique incense formulations, which incorporate pigment-rich materials and aromatic woods burned under distinct temple ventilation conditions and mechanical airflow systems common in the compared studies. Along with the impact of PAHs from incense material, heavy metals showed significant links to respiratory issues after human exposure.

In addition to PAHs, heavy metals posed significant risks. The heavy metal profile aligns with the results of the heavy metal analysis in religious spaces in Taiwan and Kuwait [31,32]. Characterization from a study on materials used in incense production at an incense factory in Roi Et, Thailand, revealed the presence of heavy metal contaminants in dyes and incense sticks and found the most abundant elements to be Mn and Pb [33].

The average C_exposure in this study indicates that workers were exposed to high levels of particulate matter from incense burning. These elevated levels led to an increase in the inhalation doses of pollutants among workers, resulting in higher exposure levels within their bodies [34]. The average ILCR was higher than the average ILCR values reported for workers in shrines in Chiang Mai and Taiwan, which was higher than the risks associated with exposure to road dust in urban areas [7,35,36,37]. The average HQ was higher than the HQ values derived from PAHs associated with common household activities [38]. In contrast, the indoor air of religious spaces in Taiwan yielded HQ values for non-carcinogenic heavy metals that were below the standard HQ threshold [35].

However, this study’s findings were constrained by limited access to high-exposure zones, seasonal fluctuations in temple activities, and intermittent monitoring opportunities during religious events. Therefore, in future studies, we recommend increasing the number of sampling locations and extending the duration of air sampling to ensure a more comprehensive assessment of health risks associated with inhaling PAHs and other heavy metals.

5. Conclusions

This study examined the characteristics of PAHs and heavy metals in fine particulate matter derived from incense burning within the indoor air of religious spaces located in Bangkok, Thailand. The analysis detected the presence of particulate-bound PAHs and heavy metals across PM < 0.1 and PM > 10; primarily focusing on PM_2.5, at four religious sites including NAP, FLU, FLT, PHE, BaA, CHR, and PYR as well as various heavy metals. The calculated PAH concentration ratios indicated that incomplete combustion and pyrogenic process were the primary contributors. Health risk assessments associated with exposure to particulate-bound PAHs and heavy metals revealed that workers face elevated health risks, with high ILCR and HQ values. These risks include increased cancer risk and other adverse health effects resulting from the inhalation of indoor air pollutants in religious spaces.