Characteristics of indoor and outdoor Polycyclic Aromatic Hydrocarbons (PAHs) pollution in TSP in rural Northeast China: A case study of heating and non-heating periods

Abstract

Approximately 91% of the world’s population lives in an air-polluted environment, and environmental pollution has become a widespread concern. Urban indoor and outdoor air pollution has been fully researched and effective control measures have been proposed. However, the issue of air pollution in rural areas has not been explored in depth. Compared to urban air pollution, the rural air pollution problem is more complex and urgent. Due to climatic factors and economic conditions in rural Northeast China, most households use solid fuels such as biomass straw and coal as domestic energy during the heating period, which will cause serious pollution problems of Total Suspended Particulate (TSP) and Polycyclic Aromatic Hydrocarbons (PAHs). To investigate the pollution characteristics of PAHs in indoor and outdoor TSP in rural Northeast China during the heating and non-heating periods, a medium-sized particulate matter collector 1108A was used to collect TSP for 7 days, and GC-MS was used to detect PAHs. The results showed that indoor TSP and PAHs pollution levels were the highest during the heating period. PAHs source analysis by Diagnostic Ratio (DR) and Principal Component Analysis (PCA) indicated that the main sources were biomass and coal combustion, vehicle emissions, and domestic waste incineration. According to the results of carcinogenic risk model calculations, there is a potential carcinogenic risk to the population in the Northeast rural living area. This study reflects the pollution characteristics and sources of indoor and outdoor TSP and PAHs in rural Northeast China during heating and non-heating periods, and provides a reference for further prevention and control of air pollution in rural areas, which is conducive to improving the living environment and improving human health.

Introduction

According to the Seventh Census of China [1] and the data published by the Ministry of Housing and Construction [2], the rural population accounts for 36.11% of the total population, and the housing area accounts for 40.98% of the country. Rural people and buildings are an integral part of the study of architecture and human health, and the living environment and physical health of rural people cannot be ignored. The combination of cold and long winters, living conditions, and solid fuel burning in the rural northeast of China has led to poor quality of the living environment for farmers, making it more likely to induce several diseases [3]. Therefore, it is of great significance to study the rural environment in the Northeast China.

At present, particulate matter is the primary pollutant in the air of rural Northeast China [4], and particulate matter entering the human body through breathing poses a serious health risk, increasing the risk of respiratory disease [5, 6], cardiovascular disease [7, 8] and premature death [9, 10]. Particulate matter carries a type of volatile organic compound (VOCs)-Polycyclic Aromatic Hydrocarbons (PAHs), which are toxic, persistent, and bio-accumulative [11]. Long-term or excessive exposure to PAHs in humans can lead to health disorders such as cancer, reproductive problems, and genetic mutations [12]. Therefore, the environmental health problems caused by PAHs have attracted a lot of attention. Scientific research has found that indoor pollution in industrial cities is closely linked to outdoor pollution, with extreme outdoor pollution leading to serious indoor pollution. Studies have shown that outdoor pollution is the main contributor to indoor particulate matter through convection and infiltration [13, 14]. The sources of pollution from PAHs in cities are diverse, with the atmosphere mainly consisting of emissions from the power generation industry, which are in turn exchanged with coal and oil combustion [15], and vehicle emissions [16]. The interior is mainly used for activities such as smoking [17], cooking [18], and incense burning [19]. However, in recent years, TSP and PAHs pollution in rural areas is more severe than that in cities, and there are differences in the sources of pollution from urban areas [20]. Rural buildings are older and have a less airtight envelope, so outdoor pollution has a higher tendency to interfere with indoor pollution. However, indoor sources of pollution in rural areas, especially during the heating period, contribute more to indoor pollution and can also influence outdoor pollution. The main differences between rural sources of PAHs and urban sources are the combustion of biomass and waste incineration. Global data statistics have shown that more than 3 billion people worldwide still use biofuels to meet their energy needs [21], and indoor and outdoor air pollution caused by their combustion has become a major health risk factor [22]. Several studies in China and abroad have reported indoor and outdoor contamination with TSP and PAHs in rural areas. The indoor TSP concentrations were 697 ± 297 μg/m³ and 470 ± 256 μg/m³ during the heating and non-heating periods in Shanxi rural areas, and the TSP concentrations during the heating season were about 1.5 times higher than those during the non-heating season [23]. The PAH concentrations were 118.25 ng/m³ in outdoor TSP during the heating period in rural Beijing, which was considerably higher than that before heating [24]. in Henan, PAHs concentrations in PM₁₀ during the heating period (754.1 ± 1220 ng/m³) were about 2.2 times higher than those during the non-heating period (342.8 ± 463.2 ng/m³) in coal-fired rooms [25]. Some foreign studies have also shown that pollution from burning solid fuels during the heating period is very serious. The PAH concentrations in PM₁₀ in Tehran’s rural areas were 31.8 ng/m³ and 26.6 ng/m³ in the cold season and warm season, respectively [26]. Nepal TSP and PAHs were significantly different between the non-heating and heating periods in the Kathmandu Valley. TSP concentrations ranged from 50 to 150 μg/m³ and 400 to 800 μg/m³, and PAHs concentrations ranged from 0 to 100 ng/m³ and 200 to 500 ng/m³, respectively [27]. Rural southern Germany, which uses biomass such as wood as a domestic fuel, also exhibits more pollution during the heating period [28].

Research on particulate matter and PAHs in urban areas of Northeast China is well established [29–31], but little research has been reported on particulate matter and PAHs in rural areas of Northeast China. In this study, PAHs in TSP were analyzed by the continuous sampling of indoor and outdoor TSP for 7 days each during the heating and non-heating periods in a farming household in Northeast China. The objectives of this study were:

1. To analyze the distribution of indoor and outdoor TSP concentrations during heating and non-heating periods.

2.  To explore the pollution characteristics and main pollution sources of indoor and outdoor PAHs during heating and non-heating periods.

3.  To calculate the carcinogenic risk of PAHs to the population by the health risk assessment model from the US EPA.

This study fills a gap in the research on Northeast China and provides a scientific basis for remediating environmental pollution and improving human health in rural Northeast China.

Materials and method

More than 200 PAHs have been identified, and in the 1980s, the U.S. Environmental Protection Agency (US EPA) listed 16 PAHs as priority pollutants for control in the environment [32, 33]. 2 rings: Naphthalene (Nap). 3 rings:Acenaphthene (Ace), Acenaphthylene (Acy), Anthracene (Ant), Fluorene (Flu), and Phenanthrene (Phe). 4 rings:Benzo(a)anthracene (BaA), Chrysene (Chr), Fluoranthene (Fla), and Pyrene (Pyr). 5 rings:Benzo(a)pyrene (Bap), Benzo(b)fluoranthene (BbF), Benzo(k)fluoranthene (BkF), and Dibenzo(a,h)anthracene (DahA). 6 rings:Benzo(ghi)perylene (BghiP) and Indeno (1,2,3-cd) pyrene (IcdP).

Sites and sampling

In this study, the sampling site was selected from an ordinary farming household in Shuangyang District, Changchun City, Jilin Province, China. The sampling site is adjacent to the provincial highway and is located on the east side of the highway (Fig. 1). There is no industry in the vicinity of the sampling site, only three small grain processing plants. The village where the sampling site is located is surrounded by fields, 85% of which are corn fields and 15% are rice fields. The natural situation of the sampling object is the residential population of 3 people, a house construction area of 85m². The sampling time was divided into two parts, the non-heating period (2021.09.05 ~ 2021.09.11) and the heating period (2021.11.05 ~ 2021.11.11), continuously sampling for 7 days, 24 h per day. Sampling was performed with a 1108A particulate sampler (Qingdao Zhongte, China) with a flow rate of 100 L/min with Swedish Munktell MK360 grade 90 mm quartz filter membranes. One filter membrane was collected each day indoors and outdoors. The indoor and outdoor TSP sampling was performed at the same time. Because the sampling equipment makes a certain amount of noise, in order not to disturb people’s rest, the indoor monitoring point was set up in the living room. According to the “Indoor Air Quality Standard” (GB/T 18883–2002) [34], the monitoring point is 1 m away from windows and walls and 1.5 m away from the floor, which is the same height as the human breathing zone. The outdoor sampling point is set in the courtyard to prevent interference from the sampling building, and the sampler is placed 5 m from the sampling building. The common motor vehicles in the village were cars and diesel vehicles. The farmers lived a normal life during the sampling period with no special requirements.

Fig. 1.

Sampling position

Extraction and purification

Take 1/2 of the collected filter membrane and cut it into a 30 ml brown glass bottle. Add 15 ml of extractant (acetone: n-hexane = 1:1) and place in a sonicator at 25 °C for 30 min. Then add 15 ml of extractant and continue the ultrasonic extraction for 30 min. Transfer the sonicated solution into a pear-shaped flask and spin evaporate to 1–2 ml with a rotary evaporator. The solution after spin evaporation was transferred into the activated SPE column, and the liquid flowed into the new pear-shaped bottle through the SPE column. Rinse the pear-shaped vial with 10 ml of lysis solution (dichloromethane: hexane = 3:7) and transfer the rinse solution into a new pear-shaped vial as well. And spin evaporates to 0.5–1 ml, nitrogen blowing, and then fix the volume to 1 ml with hexane to be measured.

Chromatographic conditions

Control parameters: electron bombardment source (EI) was ion source 70 eV, electron multiplier voltage (EMV) was 1678 eV, the column was 122-5532UI (30.00 m × 250 μm × 0.25 μm), carrier gas was high purity helium, the column flow rate was 1.0 ml/min, pre-column pressure was 8.2317 psi, inlet temperature was 250 °C, transfer line temperature was 280 °C, ion source temperature was 230 °C, injection volume 1 μL, no split injection.

Ramp-up procedure: start at 60 °C, maintain for 2 min, then ramp up to 140 °C at 30 °C/min, then to 300 °C at 8 °C/min, and retain for 10 min. Scan mode: selective ion scan (SIM). The characterization of the sample was mainly based on the retention time of the characteristic ions and chromatography.

Quality assurance and Quality control

The quartz filter membranes were baked in a muffle furnace at 600 °C for 4 h before sampling, and then placed in a constant temperature and humidity chamber (T = 25 °C, R = 50%) for 48 h. The membranes were weighed three times with an electronic balance (accuracy 0.1 mg) to take the average value A₀ and placed in an aluminum foil sealed bag for use. After sampling, the membrane must still be dried for 48 h to a constant weight and weighed three times to take the average value of A₁. The weighed membrane is placed in an aluminum foil sealed bag and stored in an ultra-low temperature refrigerator at −40 °C for measurement. Before sampling, the particulate sampler 1108A must be calibrated by power. After the calibration is completed, the sampling airflow is stabilized and sampling starts.

In order to ensure the reproducibility and accuracy of the experimental process, blank labeled experiments were also analyzed in the experimental process to verify whether the experimental method would meet the recovery requirements. The stability of the instrument operation was verified through parallel sample experiments. In order to avoid excessive deviation in the peak location of PAHs in the sample testing process, standard samples were regularly used to correct the baseline position and further blank samples were analyzed to judge whether the reagent was contaminated. The quality control of experiments was conducted as the internal standard method using 5 representative substances that were selected as specified in HJ 646–2013 [35]. The recovery rate of the indicator Naphthalene-D8, Acenaphthene-D10, Philippines-D10, Chrysene-D12, Pyrene-D12 were 65.6 ± 20.1%, 61.5 ± 19.1%, 83.3 ± 17.1%, 95.6 ± 12.1%, 97.6 ± 10.1%, respectively. After completing the above steps, the 16 kinds of priority-controlled PAHs in samples were analyzed by GC-MS.

Health risks assessment

Among the 16 PAHs, BaP is one of the most toxic PAHs [36], but there are differences in the toxicity of PAHs with different ring numbers. Therefore the toxicity equivalent factor (TEF) of benzo[a]pyrene (Bap) is often used as a reference to calculate the TEF of other monomeric PAHs and to calculate the total toxic equivalent content (TEQs) relative to BaP, calculated as follows [37]:

$${\mathit{TEQ}}_s=\sum _{i=1}^{16}\left(C_i,\times ,{\mathit{TEF}}_i\right)$$ 1

where, TEQs - total toxic equivalent content, μg/kg; C_i - content of monomeric PAHs, μg/m³; TEF_i - toxicity of monomeric PAHs Equivalent factor; TEF is taken as shown in Table 1 [38].

Table 1.

Toxicity equivalence factors for monomeric PAH

Monomer PAH	TEF	Monomer PAH	TEF	Monomer PAH	TEF	Monomer PAH	TEF
Nap	0.001	Flu	0.001	Fla	0.001	BkF	0.1
Ace	0.001	Phe	0.001	Pyr	0.001	DahA	1
Acy	0.001	BaA	0.1	Bap	1	BghiP	0.01
Ant	0.01	Chr	0.01	BbF	0.1	IcdP	0.1

PAHs pose a carcinogenic risk to humans through three main routes: hand-to-mouth ingestion (ILCR_ing), inhalation (ILCR_inh), and dermal exposure (ILCR_der). Equations (2) ~ (5) represent the carcinogenic risk of PAHs to adults or children through the above three pathways, respectively [39, 40]. The meanings and values of various parameters are shown in Table 2.

$${\mathit{ILCR}}_{\mathit{ing}}=\frac{TE{Q}_s\times {\mathit{CSF}}_{\mathit{ing}}\times \sqrt[3]{BW/70}\times {\mathit{IR}}_{\mathit{ing}}\times EF\times ED}{BW\times AT\times {10}^6}$$ 2

$${\mathit{ILCR}}_{\mathit{inh}}=\frac{TE{Q}_s\times {\mathit{CSF}}_{\mathit{inh}}\times \sqrt[3]{BW/70}\times {\mathit{IR}}_{\mathit{inh}}\times EF\times ED}{BW\times AT\times PEF}$$ 3

$${\mathit{ILCR}}_{\mathit{der}}=\frac{TE{Q}_s\times {\mathit{CSF}}_{\mathit{der}}\times \sqrt[3]{BW/70}\times SA\times SL\times ABS\times EF\times ED}{BW\times AT\times {10}^6}$$ 4

$$TILCR={\mathit{ILCR}}_{\mathit{ing}}+{\mathit{ILCR}}_{\mathit{inh}}+{\mathit{ILCR}}_{\mathit{der}}$$ 5

Table 2.

Meaning and values of parameters

Symbol	Meanings	Unit	Child	Adult	Reference
CSFing	Carcinogenic slope coefficient of hand-to-mouth ingestion	(kg·d)/mg	7.3		[41]
CSFinh	Carcinogenic slope coefficient of respiratory intake	(kg·d)/mg	3.85		[41]
CSFder	Carcinogenic slope coefficient of skin intake	(kg·d)/mg	25		[41]
EF	Exposure frequency	d/a	180		[42]
ED	Exposure duration	a	6	24	[42]
AT	Average exposure time	d	25,550		[43]
IRing	Dust ingestion rate	mg/d	200	100	[42]
IRinh	Inhalation rate	m3/d	5	20	[44]
BW	Body weight	kg	16	62	[44]
PEF	Particle emission factor	m3/kg	1.32 × 109		[45]
SA	Dermal exposure area	cm2	1600	4350	[42]
SL	Skin adhesion	mg/(cm2·d)	0.2	0.07	[46]
ABS	Dermal adsorption fraction	–	0.13		[46]

When ILCR or TILCR is <10⁻⁶, it means that there is no carcinogenic risk or the carcinogenic risk is negligible. When 10⁻⁴ < ILCR or TILCR<10⁻⁶, indicating a potential carcinogenic risk. When ILCR >10⁻⁴, it indicates a higher risk of carcinogenesis.

Results and Discussions

Indoor and outdoor TSP concentration analysis

The mass concentrations of indoor and outdoor TSP during the sampling period are shown in Fig. 2. The indoor and outdoor TSP concentrations showed the same trend during both the non-heating and heating periods. In the non-heating period, the outdoor TSP concentrations ranged from 13.89 to 40.63 μg/m³ (Average: 21.68 μg/m³) and the indoor variations were 86.80 ~ 184.03 μg/m³ (Average: 126.00 μg/m³). In the heating period, outdoor TSP concentrations varied 86.81 ~ 152.78 μg/m³ (Average: 115.58 μg/m³) and indoor variations 395.83 ~ 496.53 μg/m³ (Average: 432.54 μg/m³). According to the Ambient Air Quality Standards (GB 3095–2012) [47] and Indoor Air Quality Standards (GB 18883–2002) [34], the secondary concentration limit of outdoor TSP is 300 μg/m³ and the indoor PM₁₀ concentration limit is 150 μg/m³ (Because there is no relevant concentration limit for TSP in the standard, indoor particulate matter is dominated by PM₁₀ [48], so the PM₁₀ concentration limit is used instead of the TSP concentration limit). In the non-heating and heating periods, the exceedance rate of outdoor TSP was 0, and the exceedance rate of indoor TSP was 28.57% and 100%. Indoor particulate pollution is very serious in the Northeast.

Fig. 2.

Indoor and outdoor TSP mass concentrations during the sampling period

In terms of indoor TSP concentration, the average value of the heating period is about 3.43 times higher than that of the non-heating period. Most farmers use straw as a domestic fuel for heating and cooking indoors during the heating period. The handling of straw will cause the flow of particulate matter, and the burning straw will also increase the concentration of particulate matter. In the outdoor TSP concentration, the heating period is about 5.3 times higher than the non-heating period on average. Flue gas from indoor straw burning during the heating period is discharged outdoors through the chimney, resulting in a localized increase in TSP concentrations in the villages where the sampling sites are located. In non-heating periods, when the sampling sites are in agrarian conditions, the number of motor vehicles in use increases, and tailpipe emissions increase, also increasing TSP.

Indoor and outdoor PAHs concentration analysis

The indoor and outdoor concentrations of 16 monomeric PAHs during the sampling period are shown in Fig. 3. The total PAHs concentration and average concentration in different periods are shown in Table 3.

Fig. 3.

Average concentration of 16 monomeric PAHs

Table 3.

PAHS concentration in this study

Periods	Locations	Max concentration/(ng·m−3)	Min concentration/(ng·m−3)	Average concentration/(ng·m−3)
Heating period	Indoor	99.14	74.22	84.55
	Outdoor	86.18	68.77	78.96
Non-heating period	Indoor	45.26	34.41	39.66
	Outdoor	56.06	43.04	48.55

Indoor PAHs are approximately 2.13 times higher during the heating period than those during the non-heating period. Outdoor PAHs are about 1.63 times higher during the heating period than that in the non-heating period. By comparing the rural indoor and outdoor PAHs pollution in different locations (Table 4), the indoor pollution levels are lower than that of the areas listed in the reference. The outdoor pollution levels were similar to Henan (64 ± 6 ng/m³，89 ± 22 ng/m³) [25] and Sichuan (72 ± 42 ng/m³) [49]. However, it is lower than that of Hebei (1389 ng/m³) [50], Jiangsu (439 ± 25 ng/m³) [51], and Shaanxi (189 ± 115 ng/m³) [52]. It is possible that the rural areas of the Northeast are relatively underdeveloped and not as economically developed as the above-mentioned areas. In addition, different energy mixes and differences in climatic conditions can also play a role. In comparison with the study of Li Na et al. [53], it was found that PAHs pollution was much higher in rural areas than in urban areas, even though they belonged to the same jurisdiction as Changchun city.

Table 4.

Indoor and outdoor PAHs concentrations in different rural/urban areas in China from previous studies

Locations	Season	Indoor concentration/(ng·m−3)	Outdoor concentration /(ng·m−3)	Reference
Henan	Autumn	150 ± 105	64 ± 6	[25]
Henan	Winter	222 ± 144	89 ± 22	[25]
Sichuan	Winter	133 ± 108	72 ± 42	[49]
Hebei	Winter	3288	1389	[50]
Jiangsu	Autumn	546 ± 95	439 ± 25	[51]
Shaanxi	Winter	211 ± 120	189 ± 115	[52]
Changchun (Urban)	Winter	1. 567	–	[53]

The ring number distribution of 16 PAHs was further analyzed (Fig. 4). During both the non-heating and heating periods, the highest levels of 5 rings PAHs were found indoors and outdoors, followed by 3 rings and 4 rings, and the least by 2 rings. The 2–3 rings are collectively referred to as the low ring (LR), the 4 rings as the middle ring (MR), and the 5–6 rings as the high ring (HR). The trend of ring weight contribution is consistent across the subplots shown in Fig. 4. They are presented as (a) HR (66.58%) > LR (19.04%) > HR(14.38%); (b) HR (51.49%) > LR (27.19%) > HR(21.32%)；(c) HR (44.65%) > LR (29.82%) > HR(25.53%)；(d) HR (45.15%) > LR (29.40%) > HR (25.45%). A comparison with Bai et al.’s study [37] of indoor PAHs in office buildings in Changchun revealed that the ring number distribution was mainly 4 rings, while the present study was mainly 5 rings. This may be due to the different sources of pollution and building structures of sampling sites in rural and urban areas.

Fig. 4.

Distribution of PAHs rings

PAHs in TSP source analysis during heating and non-heating periods

Diagnostic Ratios (DR) of PAHs

Diagnostic Ratios (DR) have been used as tracers to distinguish between various source characteristics of PAHs e.g., exhaust emissions from diesel and gasoline vehicles [54, 55], biomass and wood burning [56], and fossil fuel combustion such as coal, etc. Table 5 shows the reported characteristic diagnostic rates for PAH from different sources and the diagnostic ratios of this study.

Table 5.

PAHs diagnostic ratios used as a source indicator

PAH ratios	This study				Diagnostic ratios		References
	IN*	ON*	IH*	OH*	Value range	Indication source
Ant/(Ant+Phe)	0.60	0.57	0.54	0.54	<0.1	Petroleum sources	[60]
					>0.1	Combustion source
IcdP/(IcdP+BghiP)	0.76	0.75	0.68	0.70	<0.2	Petrogenic	[56]
					0.2 ~ 0.5	Liquid fossil fuel (automotive and crude oil) combustion
					>0.5	Grass, wood, coal combustion
Flu/(Flu+Pyr)	0.33	0.29	0.24	0.24	<0.4	Petroleum combustion	[61]
					0.4 ~ 0.5	Petroleum and its refined products combustion
					>0.5	Wood, biomass, and coal combustion
BaP/BghiP	2.67	2.07	1.86	1.98	<0.6	Traffic emissions	[62]
					>0.6	Nontraffic emissions
BaA/(BaA + Chr)	0.58	0.54	0.52	0.53	0.2–0.35	Coal combustion	[26]
					>0.35	Vehicular emissions
					>1	Diesel combustion

*IN Indoor during the non-heating period; ON Outdoor during the non-heating period; IH Indoor during the heating period; OH Outdoor during the heating period

The source analysis of indoor and outdoor PAHs during the non-heating and heating periods by DR showed that the PAHs pollution sources were combustion sources and vehicular emissions. Combined with the actual situation in rural Northeast China, the combustion sources were mainly the combustion of biomass, wood, coal, and domestic waste. Vehicular emissions are mainly gasoline combustion exhaust emissions from domestic cars, motorcycles and diesel exhaust emissions from agricultural tractors. Related studies that investigated the source of contamination of PAHs used the diagnostic ratio method, and the results obtained were consistent with the present study. Kumar et al. used DR to demonstrate that the main sources of atmospheric PAHs in rural eastern India are heat sources and vehicle emissions [57]. Chen et al. showed from the ratio of IcdP/ (IcdP + BghiP) and Flu/ (Flu + Pyr) that PAHs from rural areas are mainly associated with emissions from biomass, coal, and oil combustion [58]. Kalisa et al. showed that coal combustion and domestic wood combustion are the main sources of PAHs in rural areas of Japan and New Zealand [59]. DR cannot clearly understand the primary and secondary sources of PAHs contamination and their contributions, PCA is used to accurately dissect the sources of contamination as following.

Principal Component Analysis (PCA) of PAHs

Principal Component Analysis (PCA) is used as a tool to distinguish major air pollution sources and to assign individual tracers to each component [16]. The principle is to convert the original set of variables into a smaller set of linear combinations that account for most of the variance in the original set. The main function is to reduce the number of variables while retaining as much of the original information as possible so that variables with similar characteristics can be grouped into factors [63]. Source groupings were determined using PCA with maximum variance rotation and retaining eigenvalues >1 for the principal components of the complete PAH concentration dataset [64]. The factor analysis in this study was conducted using the statistical analysis software SPSS 26.

In this study, all factors with eigenvalues greater than 1 were extracted and rotated using the Kaiser normalized maximum variance method, and the three factors with higher loadings were selected to explain the variance of the data to determine the source of contamination by PAHs. The correlation and principal component results of the 16 PAHs during the sampling period are shown in Fig.5.

Fig. 5.

PAHs correlation and PCA results. a Indoor during non-heating period. b Outdoor during non-heating period. c Indoor during heating period. d Outdoor during heating period

The results of the principal components of indoor PAHs in the non-heating period showed that factor 1 was DahA (contribution: 45.13%), which was a high ring PAH. HR is generally derived from emissions from internal combustion engines [65], so it can be considered as a motor vehicle exhaust emission. Rural houses in the Northeast are generally older, and windows and other envelope structures are poorly airtight. Factor 2 is Pyr (contribution: 38.38%), which is identified as an identifier of wood combustion [66] and thus can be considered as a wood combustion source. Factor 3 is Ace (contribution: 10.50%), which is a low ring PAHs. It is usually considered to be produced by burning coal [67] or domestic waste [68]. Because there is no coal burning during the non-heating period, there is a garbage disposal site near the sampling site, which will focus on burning domestic garbage, and a small amount of garbage is occasionally burned indoors, so it can be considered domestic garbage burning.

Factor 1 of the principal component results for outdoor PAHs during the non-heating period was Acy and Nap (r = 0.792) (43.86% contribution), which are low ring PAHs and can be considered as waste incineration. Factor 2 is DahA and Bghip (r = 0.992) (contribution: 36.86%), which are high ring PAHs and can be considered as motor vehicle exhaust emissions. During this sampling period, the northeastern countryside will have an autumn harvest with more use of agricultural diesel vehicles, which can further be identified as diesel exhaust emissions. Factor 3 is Flu (contribution: 10.73%), which is usually considered to be coal and biomass combustion [69]. During the autumn harvest in the northeast, farmers will burn some of the straw in the field and then transport the remaining straw back home, so it can be considered a source of straw burning.

The principal component result factor 1 of indoor PAHs during the heating period is Flu (contribution: 51.14%), which is generated by coal and biomass combustion. Straw (corn stalks and rice stalks) are mainly used as fuel for heating and cooking in rural areas in the Northeast and are therefore recognized as a biomass burning source. Factor 2 is Bap and BbK (r = 0.953) (contribution of 24.28%), which are usually considered motor vehicle indicator sources [70]. Factor 3 is BaA (17.41% contribution), which is usually used as an indicator source for coal combustion [71]. About 10% of the farm households in the villages where the sampling sites were located underwent house renovation to remove the original pots and pans and build small coal-fired boilers for heating. Smoke from coal combustion was emitted into the atmosphere, and pollution was caused by penetration through doors and windows into the interior of the sampling site.

The results of the principal components of outdoor PAHs during the heating period, factor 1 is Ant, Phe, and Fla. (r = 0.964, r = 0.969) (contribution: 57.84%), which belong to the same low ring PAHs and can be considered as coal [72] and domestic waste combustion. Factor 2 is Nap (contribution: 20.40%), which also belongs to low ring PAHs and is similarly considered as domestic waste combustion. Factor 3 is DahA (contribution: 12.98%), which can be identified as motor vehicle exhaust emissions.

Some studies have used PCA to determine the sources and contribution of PAHs in rural areas, which are close to the results of this study. Sources of indoor PAHs include cooking fuels and traffic emissions in the rural Niger Delta of Nigeria [73]. López-Ayala showed that PAHs pollution in Mexican rural atmospheric TSP was mainly caused by diesel and gasoline vehicles, industrial furnaces, coal, and wood combustion [74]. Hu et al. used PCA to identify the main sources of PAHs in rural Guiyang as coal-fired emissions (52.5%), traffic gasoline (21.4%), and other miscellaneous sources (14.2%) [75]. Chen et al. reported the sources of pollution from PAHs in rural Nepal. Biomass combustion (50.6%) and vehicle emissions (30.4%) were the top two sources of PAHs, followed by coal combustion (11.6%) and air-soil exchange (7.4%) [76]. Based on the results of this study and the above studies, it can be found that the main sources of indoor and outdoor PAHs pollution in rural areas are combustion sources (combustion of biomass, wood, and coal), traffic emission sources (exhaust emissions from diesel and gasoline vehicles) and other sources (natural sources, soil sources, etc.).

Health risk assessment of indoor and outdoor PAHs during heating and non-heating periods

According to the health risk model provided by the U.S. Environmental Protection Agency (US EPA), the carcinogenic risk of PAHs to humans was assessed. The carcinogenic risk of monomeric PAH in adults and children under the three carcinogenic routes of intake was calculated according to Eqs. (2) ~ (5) (Fig. 6).

Fig. 6.

ILCR for adults and children with indoor and outdoor PAHs during the non-heating and heating periods

ILCR of outdoor PAHs for adults (3.21 × 10⁻⁵) was about 1.11 times higher than that for children (2.88 × 10⁻⁵) during the non-heating period; ILCR of indoor PAHs for adults (3.43 × 10⁻⁵) was about 1.14 times higher than that for children (3.07 × 10⁻⁵). ILCR of PAHs for adults outdoors during the heating period (4.91 × 10⁻⁵) was about 1.12 times that of children (4.40 × 10⁻⁵); ILCR of PAHs for adults indoors (5.29 × 10⁻⁵) was about 1.11 times that of children (4.75 × 10⁻⁵). The TILCR were all between 10⁻⁶ and 10⁻⁴, indicating that the current concentrations of PAHs in the environment posed some carcinogenic risk to the population. The total cancer risk is higher during the heating period than during the non-heating period. The source analysis results show that the main source in the non-heating period is motor vehicle exhaust, which is less often due to combustion. Biomass, coal combustion and motor vehicle emissions work together in a superposition to make the heating period more high-risk.

The carcinogenic risk of every PAH to adults and children is mainly due to hand-oral and dermal ingestion, while the carcinogenic risk by inhalation route is negligible (Table 6). Several related studies are consistent with the pattern presented in this study [31, 77, 78]. The cancer risk values for all three exposure routes for adults were, from largest to smallest, ILCR_der > ILCR_ing > ILCR_inh, while the risk values for children were ILCR_ing > ILCR_der > ILCR_inh. Children have higher exposure through their hands and mouths than adults. Studies by Li et al. [79] and Wei et al. [80] have proven this view. This is because children may rely more on hand-to-mouth contacts, such as eating with their hands and sucking on their fingers. Adults have a larger respiratory volume and skin surface area than children, resulting in a higher risk of carcinogenesis through the respiratory route compared to dermal routes. Among the 16 PAHs, five monomeric PAHs, BaP, BbF, BkF, DahA, and Icdp, had ILCR >10⁻⁶, indicating a potential carcinogenic risk. The ILCR of BaP is higher than 10⁻⁵, and special attention should be paid to the protection against BaP. These five PAHs are all classified as high ring PAHs by ring number, and their main sources are fossil fuels and incomplete combustion of internal combustion engines. It is even more important to promote the use of clean energy and new energy vehicles in the Northeast to reduce the serious pollution caused by the burning of biomass and coal as well as motor vehicle exhaust, control the emission of PAHs, and reduce the health hazards to human beings.

Table 6.

Three pathways of cancer risk with indoor and outdoor PAHs during the non-heating and heating periods

Periods	Locations	Adults				Children
		ILCRing	ILCRinh	ILCRder	TILCR	ILCRing	ILCRinh	ILCRder	TILCR
Non-heating period	Indoor	1.45 × 10−5	1.16 × 10−9	1.97 × 10−5	3.43 × 10−5	1.79 × 10−5	1.79 × 10−10	1.28 × 10−5	3.07 × 10−5
	Outdoor	1.36 × 10−5	1.09 × 10−9	1.85 × 10−5	3.21 × 10−5	1.68 × 10−5	1.68 × 10−10	1.20 × 10−5	2.88 × 10−5
Heating period	Indoor	2.25 × 10−5	1.80 × 10−9	3.05 × 10−5	5.29 × 10−5	2.77 × 10−5	2.77 × 10−10	1.97 × 10−5	4.75 × 10−5
	Outdoor	2.08 × 10−5	1.66 × 10−9	2.82 × 10−5	4.91 × 10−5	2.57 × 10−5	2.57 × 10−10	1.83 × 10−5	4.40 × 10−5

Conclusions

In this study, TSP was sampled continuously during heating and non-heating periods in rural northeastern China, from which PAHs were extracted for detection by GC-MS, the sources were resolved by DR and PCA, and health risks were assessed using a lifetime carcinogenic risk model. The results showed that indoor TSP concentrations were severely exceeded and the pollution levels were higher than those outdoors. Indoor and outdoor PAHs were about 2.13 and 1.63 times higher during the heating period than that of the non-heating period. PAHs rings analysis showed the highest level of 5 rings PAHs, followed by 3 rings and 4 rings, and the least 2 rings. According to DR and PCA, the main sources of indoor and outdoor PAHs were biomass and coal combustion, vehicle emissions, and domestic waste incineration. During the heating period, combustion sources dominated and were mixed with motor vehicle emission sources, and during the non-heating period, vehicle emissions dominated and were influenced to a small extent by outdoor biomass combustion. The incineration of domestic waste affects the emissions of low ring PAHs and care should be taken not to neglect them. Using carcinogenic risk models, the current residential environment is assessed to have potential carcinogenic risks to the population. BaP, BbF, BkF, DahA, and IcdP are the five monomeric PAHs with high-risk contributions, which should be protected with focused attention.

Limitations

There are some limitations to this study. Firstly, this study sampled before and during the heating period. The sampling period was only 14 days. There is no year-round monitoring and analysis of rural Northeast China, and there will be remaining factors that will have some effect on PAHs. For example, many fireworks [81, 82] and indoor smoking [83] during the Chinese New Year in rural Northeast China may increase the concentration of PAHs. Secondly, only adults and children were distinguished in the calculation of the carcinogenic risk assessment, and no further systematic division of the population was made. The population structure of rural areas in Northeast China is different from that of urban areas, with young and middle-aged people going out to labor, while the remaining population structure is dominated by the elderly [84], and more attention should be paid to the health effects on the elderly. The above-mentioned will provide a scientific basis for our future work to focus on the health of rural people and improve rural air quality. We strive to create a healthy and sustainable living environment for rural residents.

Author’s contribution

Chunhui Li: Conceptualization, Methodology, Formal analysis, Investigation, Writing-original draft, Visualization. Li Bai: Conceptualization, Resources, Writing-original draft, Supervision, Funding acquisition. Han Wang: Formal analysis, Investigation, Visualization. Guangming Li: Investigation, Visualization. Yongbo Cui: Investigation, Visualization.

Funding

This study was financially supported by the National Natural Science Foundation of China (NSFC) (51508224) and the National Key Research and Development Program of China (2017YFC0702700).

Data Availability

Not applicable.

Declarations

Ethical Approval and Consent to participate

Not applicable.

Consent to participate

Not applicable.

Consent for publication

All authors have read and agreed to published this version of the manuscript.

Competing interests

The authors declare no competing interests.