Emission characteristics of carboxylates in PM_2.5 from incense burning with the effect of light on acetate

Highlights

• Carboxylate emissions from the burning of four brands of incense were studied.

• PM_2.5 mass emissions were inversely correlated with ash production.

• Acetate was the largest contributor of carboxylates at 87% of the total.

• Acetic acid was present in emissions at high levels for all four incense types.

Abstract

Incense burning produces potentially harmful particulate matter. In this study we investigated the emissions of PM_2.5 and gaseous acetic acid from four brands of traditional incense; Liao and Shang Lao Shan (SLS), sold in Taiwan, and Thai Yellow (Thai Y) and Thai Black (Thai B), sold in Thailand. Additionally, photochemical reactions of PM_2.5 carboxylates emitted from incense burning were studied via a simulated light experiment. The average PM_2.5 mass emission factor of each incense type was inversely correlated with the ash production of that incense. The Thailand incense carboxylate emissions were markedly higher than the Taiwan incense. Acetate accounted for 87.46% of total carboxylate emissions, with acetate emitted from the Thailand incense 1.26 times higher than from the Taiwan incense. Phthalate was detected in the PM_2.5, indicating the presence of plasticizer. Concentrations of PM_2.5 acetate, formate, pyruvate, glutarate, succinate, fumarate and tartarate were reduced in simulated light (51.5%–97.1% of those under dark), indicating that these seven types of carboxylate are easily photodegradable. In contrast, malonate, maleate, oxalate and phthalate concentrations in light were 1.17–1.84 times higher than in darkness, indicating photochemical reactions contribute to the formation of these species. The formation of the low-molecular weight dicarboxylates oxalate and malonate was most noticeable. Acetic acid, highly irritating to the respiratory system and skin, was present at high levels for all four incense types, as shown by the gaseous acetic acid/PM_2.5 acetate ratios of 1.03–3.61. Burning incense indoors can generate high concentrations of PM_2.5 acetate that increases the risks of respiratory and contact irritation, particularly when burning the Thailand incense. Moreover, burning incense in poorly ventilated, dimly lit indoor areas (e.g., temples and homes) can markedly increase the risk of irritation because the gaseous acetic acid is not degraded as it would be in light.

Introduction

Religious ceremonies, traditional festivals, and god worshiping in Taiwan and other Asian countries involve the use of firecrackers, fireworks, and incense. Incense burning and joss paper burning are common in these countries. These burning activities create emissions that can lead to highly concentrated localized air pollution during festival periods. Moreover, burning incense in poorly ventilated or confined spaces (e.g., temples and homes) directly affects air quality in those spaces and hence human health (Pope et al., 2002, Wang et al., 2012, Yang et al., 2013, Lyng et al., 2015, Mentese et al., 2015). Many studies have indicated that incense, cooking and scented candles are the main sources of indoor particles (See et al., 2007, Orecchio, 2011, See and Balasubramanian, 2011, Stabile et al., 2012). Habre et al. (2014) adopted a sulphur tracer method to detect sources of indoor PM_2.5 pollutants and found that the main pollutant sources were cooking, house cleaning, incense burning, and smoking. Because of the increasing incidence of asthma, the connections between indoor pollutants and asthma as well as other respiratory problems have received wider attention (MacLennan et al., 1977, Cohen et al., 2013). Incense is always burned in a smoldering, incomplete, slow, and lengthy fashion and produces incense smoke that contains gas and particulates. Burning incense indoors increases the concentration of indoor PM concentration. Kao and Lung (2000) found that burning incense in Buddhist temples generated high particulate concentrations, most of which was <2.5 μm diameter (PM_2.5) (Yang et al., 2007, Tsai et al., 2010, Kuo et al., 2015) that were markedly higher (10–20 times) than the indoor air quality standard in Taiwan (PM_2.5 < 35 μg m⁻³). Because incense commonly releases aromatic fragrances when burnt, and because of the central role incense burning plays in the religious lives of Asian peoples, the polluting aspect of incense burning is usually overlooked (Kuo et al., 2015).

Incense burning generates PM_2.5 with complex composition including black carbon, elemental carbon, organic carbon, metals (e.g., potassium, lead, nickel, manganese, titanium and vanadium), ions, and hazardous polycyclic aromatic hydrocarbons (PAHs) (Yang et al., 2007, Yang et al., 2012, Yang et al., 2013, Tsai et al., 2010, See and Balasubramanian, 2011; Habre et al., 2014). In addition to releasing pollutants such as carbon monoxide and oxides of nitrogen, incense burning generates the carcinogen formaldehyde (Wang et al., 2007, Cohen et al., 2013). See and Balasubramanian (2011) investigated six brands of incense and reported that the PM_2.5 emission factors were as low as 0.4 mg g⁻¹ for smokeless incense but as high as 44.5 mg g⁻¹ for the smoke-releasing brands. Lin et al. (2007) compared nine brands of incense sold in Taiwan and revealed that under identical burning conditions, different brands of incense exhibited similar emission rates (approximately 0.5 mg min⁻¹), and that a short burning time resulted in low particle emissions and a high amount of ash. Yang et al. (2012) improved the formula of an incense fragrance powder by increasing the content of calcium carbonate to decrease the emission of PAHs, thus creating a safe incense product. Dalibalta et al. (2015) analysed Arabian incense (Bakhour) and detected 42 potential carcinogens and at least 200 compounds known to be highly irritating to the eyes, skin, respiratory system, and digestion system. Kuo et al. (2015) investigated four brands of incense and detected high concentrations of allergy-causing substances, namely eugenol and isoeugenol, in the PM_2.5 emitted on burning. The particle size distributions in aerosols during incense burning vary according to burning conditions and environmental factors (e.g., retention time in the atmosphere and dilution level). Such aerosols mainly exhibit a particle size <2.5 μm, with some submicron in size (Kleeman et al., 1999, Yang et al., 2007).

Wang et al. (2007) noted that the aerosol concentrations in indoor environments in Beijing and Shanghai, China, were substantially higher than in outdoor environments because of smoking, incense burning, or poor ventilation. Specifically, formaldehyde, acetaldehyde and acetone were the main carbonyl groups in the indoor environments. Loupa et al. (2007) explored candle burning in churches and detected the presence of hydrochloric acid, nitric acid, formic acid, and acetic acid, which were generated through photochemical reactions. Tsai et al. (2010) revealed that among the PM_2.5 species generated through incense burning, anhydrosugars (1078.3–1169.8 μg g⁻¹ incense) were the most abundant species (46.7%–52.2% of the identified components in particulate matter), followed by inorganic salts (30.4%–31.8%), carboxylic acids (12.0%–17.1%), and sugar alcohols (4.44%–5.38%). These species can therefore serve as potential indicators of incense burning. Formate, acetate, succinate, glutarate and phthalate are the most abundant carboxylic acid species and can potentially harm human health.

Studies of emissions from incense burning indicate that being exposed to an incense-burning environment for extensive periods (e.g., temple workers) can induce chronic skin contact irritation, migraines, chemical respiratory allergy, and even cancers (Navasumrit et al., 2008). This potential public health problem thus merits further investigation. Incense burning generates carboxylates such as acetate and formate that are irritating to human skin and eyes. The concentrations at which these compounds become irritants are different for different individuals and hence limiting values are difficult to pinpoint. One person might be sensitive to incense odor and another might be uncomfortable due to high-concentration particulate matter. One study analysed the effect of air pollution, including acetic acid, on lung development from 10 to 18 years of age (Gauderman et al., 2004), finding that exposure and sensitivity to air pollution including PM and acid vapor in indoor environments adversely affects respiratory function and/or skin, but also noted that the interactions are extremely complicated, especially with chronic exposure. Bernstein et al. (2008) noted simply that excessive exposure to these irritant substances can have a negative effect on the respiratory system. Previous studies have yet to fully address how light conditions affect the composition of incense burning-related substances in indoor environments. Therefore, in this study we simulated indoor illumination scenarios to investigate the impact of light on PM_2.5 carboxylates generated from incense burning.