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1. Introduction

Air pollution has become a mainstream global environmental pollution problem in recent decades [1,2]. Particulate matter (PM) is one of the major factors contributing to air quality deterioration, leading to adverse health effects on humans [3–6]. PM is an extremely complex mixture defined in many ways, including formation pathway, emission source, chemical composition, and PM size [5]. The formation pathway of PM involves the direct release by emission sources into the atmosphere or secondary formation via nucleation, vapour condensation, adsorption, and absorption of gaseous precursors, primary PM, or secondary PM [5]. In terms of the source, natural sources include dust, sea salt, living vegetation, volcanic activity, and forest fires, whereas anthropogenic sources mainly involve combustion processes, including stationary sources (domestic, industrial, and agricultural activities) and mobile sources (vehicles, aircraft, and shipping traffic exhaust) [7,8]. Regarding the chemical composition, PM contains many kinds of inorganic and organic compounds, including water-soluble ions, trace elements, crustal material, elemental carbon, and organic carbon, many of which are harmful to human health [9–13]. According to the size, PM is generally divided into inhalable coarse particles (PM with an aerodynamic diameter smaller than 10 μm, PM₁₀) and inhalable fine particles (PM with an aerodynamic diameter smaller than 2.5 μm, PM_2.5) [14]. However, other sizes of PM also can be collected in the atmosphere by using different types of air sampler, such as PM₁, PM₂, PM_2.1, and PM₄, which aerodynamic diameters are smaller than 1, 2, 2.1, and 4 μm, respectively [15–18].

PM is of concern not only to researchers but also to the general public. Numerous toxicological and epidemiological studies have proven the adverse links between exposure to PM and health effects [19–26]. The cancer risk resulting from PM exposure has also been demonstrated by the International Agency for Research on Cancer (IARC) [27]. According to the report by the World Health Organization (WHO), PM in outdoor air is responsible for approximately 4.5 million premature deaths every year, or close to 10% of the total deaths on a global scale [28]. Among these deaths, approximately 2 million deaths, which represent approximately 5% of the global total deaths, are due to damage to the lungs and respiratory system directly attributable to PM [28]. Moreover, WHO also reported that almost 3 billion people worldwide still rely on solid fuels for cooking and heating, leading to approximately 4 million people premature deaths due to household indoor air pollution [29], which is almost equal to the deaths caused by outdoor PM pollution.

To protect public health, relatively strict indoor and outdoor air quality standards have been prescribed by WHO. For indoor household fuel combustion, WHO has strongly recommended that the emission rate target of PM_2.5 should not exceed 0.23 mg/min under unvented conditions, and 0.80 mg/min under vented conditions [29]. For outdoor air, the annual average PM_2.5 and PM₁₀ concentrations cannot exceed 10 and 20 μg/m³, respectively, and 24-h average concentrations that cannot exceed 25 and 50 μg/m³, respectively [30]. Currently, outdoor PM regulation standards also have been implemented in many cities worldwide. Table 1 lists various PM_2.5 and PM₁₀ regulation standards set by the governments of several countries according to their national conditions.

Specifically, in North America, the 24-h and annual average PM_2.5 concentration standards in the United States of America (USA) are 35 μg/m³ and 12 μg/m³, respectively [3]. Mexico (12 μg/m³) and Canada (8.8 μg/m³) have also defined relatively low standards for the annual average PM_2.5 concentration [31,32]. In South America, Brazil determined the final standards of PM_2.5 and PM₁₀ in 2018, which 24-h and annual average are the same as those prescribed by WHO [33]. However, Chile had relatively higher standards of PM_2.5 (24-h: 50 μg/m³; annual: 20 μg/m³) than these above countries [34]. In Australia, the 24-h average PM_2.5 and PM₁₀ concentrations are the same as ones prescribed by the WHO, while the annual average of PM_2.5 (8 μg/m³) is lower and of PM₁₀ (25 μg/m³) is higher than the WHO-prescribed levels [35]. In the European Union (EU), Russia, and some Asian countries, the PM_2.5 and PM₁₀ regulation standards are mostly higher than those in the above countries. The annual average PM_2.5 concentration standards are 15 μg/m³ (Japan), 25 μg/m³ (EU, Russia, South Korea, and Mongolia), 35 μg/m³ (China), and 40 μg/m³ (India), respectively [36–38]. In South Africa, only the PM₁₀ regulation standard has been defined, which does not exceed 40 μg/m³ for the annual average concentration and 75 μg/m³ for the 24-h average concentration [39].

 Table 1. Regulation standards of PM_2.5 and PM₁₀ set by several governments.

In PM, polycyclic aromatic hydrocarbons (PAHs) are a class of persistent organic chemicals with at least two aromatic rings, mainly directly emitted as a result of the incomplete combustion of various organic materials, including both natural sources, such as forest fires and volcanic eruptions, and anthropogenic sources, such as the combustion of fossil fuels and biomass [40,41]. Several hundred PAHs have been detected worldwide, and the United States Environmental Protection Agency (US EPA) has classified 16 PAH species in a priority control pollutant list, which can be generally divided into low-molecular-weight PAHs (LMW PAHs, MW < 200 g/mol), medium-molecular-weight PAHs (MMW PAHs, 200 ≤ MW < 250 g/mol), and high-molecular-weight PAHs (HMW PAHs, MW ≥ 250 g/mol) these three categories. LMW PAHs exhibit a relatively high vapour pressure and easily occur in the gaseous phase, whereas HMW PAHs exhibit a much lower vapour pressure than that of LMW PAHs, and they mainly occur in the particle phase [42–47]. The vapour pressure of MMW PAHs is between those of LMW and HMW PAHs [47–50], suggesting that they may occur in both the gaseous and particle phases, and phase partitioning largely depends on factors such as meteorological conditions (their occurrence in the gaseous phase increases at a relatively high ambient temperature, and their occurrence in the particle phase increases at a relatively low ambient temperature) [48,51]. In contrast, a previous study has reported that the half-lives of PAHs range from a few hours to days, and the half-lives of MMW and HMW PAHs are longer than that of LMW PAHs [52], which indicates that particulate matter-bound (PM-bound) PAHs could be transported across long distances to other regions worldwide before attenuation [53–57].

PAHs are widely known for their carcinogenicity, mutagenicity and toxicity, and they pose a serious threat to human health [58,59]. A previous study has noted that with increasing MW, the carcinogenicity and acute toxicity of PAHs increase and decrease, respectively [60]. However, LMW and MMW PAHs can react with other gaseous air pollutants, such as ozone (O₃, which is a strong oxidizing agent that can damage human lung function, thus threatening human health [61]) and NO_x, to produce derivatives with a relatively low vapour pressure that more easily occur in the particle phase than their parent PAHs, and their mutagenicity and toxicity may be higher than those of the parent PAHs [62,63]. In addition to benzo[a]pyrene (BaP), which is classified in Group 1 (carcinogenic to humans), seven species are classified in Group 2A (probably carcinogenic to humans), and twenty-five species are classified in Group 2B (possibly carcinogenic to humans) [27,64,65]. Moreover, several emission sources in outdoor air, including coal combustion, coal tar pitch, coke production, diesel engine exhaust, tobacco smoke, and wood dust, are classified in Group 1 by the IARC [27], which may release many PAHs and derivatives. Due to these harmful effects on human health, it is necessary to clarify the concentrations, compositions, and major contributors of PAHs in the atmosphere.

2. Concentrations of Atmospheric PM-Bound PAHs

2.1. Personal Exposure to PM-Bound PAHs

Research data have indicated that several factors, including conditions of the body, exposure routes, and environmental conditions, can influence the mechanisms of PAHs that are absorbed or adsorbed by human bodies [70]. PAHs may affect human health via inhalation, ingestion, and dermal (skin) exposure, with inhalation being the main exposure pathway for PM-bound PAHs [70]. Exposure varies from person to person due to variations in various physical factors such as the breathing rate. Table 2 lists several studies of personal exposure to PM-bound PAHs in different cities, which have focused on different participants, including non-occupational and occupational exposure.

Table 2. Personal exposure concentration (average and/or range, ng/m³) of PM-bound PAHs in several studies.

Regarding non-occupational exposure, residents living in rural areas [71,72] generally inhale higher PM-bound PAH concentrations (4.2–655 ng/m³) than residents living in urban areas (0.4–11.9 ng/m³) [73,74]. This is because solid fuels such as coal and wood are still mainly used by rural residents for cooking and heating an many PAHs are emitted from these sources due to their low combustion efficiency, while urban residents mostly use clean fuels such as liquefied petroleum gas and natural gas. Thus, children living in Italy exhibited very low exposure levels and slight seasonal variation (0.65 and 0.63 ng/m³) [75], whereas children living in China exhibited relatively high exposure levels and a relatively large seasonal variation (27.31 and 58.18 ng/m³) [76]. A possible reason is the different urban type, whereby Rome (Italy) is a typical commercial city and Tianjin is one of the largest industrial centers of China, leading to the background concentration of PM-bound PAHs in Tianjin being higher than that in Rome [75,76]. In addition, the heating systems in Tianjin can lead to more PM-bound PAHs being released into the atmosphere in the winter [76]. Harbin is a large industrial city in northeast China, and it has been reported that the annual average PM-bound PAH exposure concentration in patients with chronic obstructive pulmonary disease is 186.85 ng/m³ [77], which is much higher than that seen in residents living in Hong Kong, Zhuhai, and Wuhan, China [73,74]. In Harbin, the annual temperature range can be up to 60°C and the heating period can account for half of the year, thus, coal combustion for heating is also a large factor, in addition to industrial coal combustion, leading to the annual average concentration of PM-bound PAHs being higher than other cities in South and Central China [77].

Regarding occupational exposure, drivers, office workers, and policemen, as indicated in Table 2, did not exhibit very high PM-bound PAH exposure concentrations (1.03–39.08 ng/m³) but the values differed among cities and revealed seasonal variations [15,16,78]. In addition to the low background concentrations of PM-bound PAHs in cities, a possible reason is that few direct emission sources were impacting the above professionals in space. However, both housewives and highway toll station workers exhibited relatively high exposure levels [79–81], which were several times higher than those for the professionals described above. The high exposure levels for housewives (116 and 310 ng/m³) mainly occurred due to the use of fuel for cooking, while those for highway toll station workers (319.90 ng/m³) largely occurred due to the inhalation of traffic-related PAHs, such as vehicle exhaust [79–81]. Traffic emissions are also a major source for exposure for newsagents because their workplaces are usually located near roads. However, the exposure levels of PM-bound PAHs for newsagents were much higher in Tehran (Iran) (5570 ng/m³) [82] than those measured for highway toll station workers in Tianjin (China) [81]. A possible reason could be that the pollution levels may be higher in Tehran than that in Tianjin, and the background PM-bound PAH concentrations may be very high. Moreover, studies have revealed very high exposure levels of PM-bound PAHs for seafarers and Chinese kitchen workers, ranging from approximately 760 to 12,108 ng/m³ [83,84]. The high exposure levels for the former occur due to ship engine emissions and their long residence times on ships, while those for the latter occurred due to factors such as the use of various fuels, food type, cooking location and methods. Also, many PM-bound PAHs contained in food can be ingested by the human body through eating [85,86].

The results found that background concentrations in different cities could affect the exposure levels of residents, and the indoor microenvironment could also influence the exposure level because most people spend a lot of time indoors. In addition, the characteristics of the PAH phase distribution determine that the PM-bound PAH concentration is higher during cold periods than that during warm periods. Moreover, people with various jobs exhibit different exposure levels, especially regarding occupational exposure, and workers are exposed to very high health risks in spaces with high PM-bound PAH concentrations. Overall, although the PM type and determined PAH number differed among the above studies, personal exposure to PM-bound PAH depends on many complex factors, including indoor microenvironments, outdoor environments, daily activities (such as jobs), and body conditions.

2.2. Indoor Concentrations of PM-Bound PAHs

Indoor air quality is crucial to human health because people spend more than 80% of their time indoors [73]. The indoor PM-bound PAHs in different microenvironments are emitted by many sources, including cooking, smoking, heating, stoves, chemical spraying, machines such as laser printers, and outdoor sources. Studies have noted that indoor air may even be worse than the outdoor air with the PM-bound PAHs [71,87]. Table 3 summarizes the indoor concentrations of PM-bound PAHs in different microenvironments in certain cities in recent years. Some studies have simultaneously examined personal exposure, as described in Section 2.1. [71,73,79,80,83,84].

Regarding residential indoor air, the PM-bound PAH concentrations in rural households (738 ± 321 ng/m³) were much higher than those in urban households (0.186–276 ng/m³), and the concentrations observed in northern Chinese cities (15–276 ng/³) were higher than those observed in southern Chinese cities (1.0–7.3 ng/m³) [71,73,79,80,88], similar to those of personal exposure (Section 2.1.). The PM-bound PAH concentrations in households in Jeddah (Saudi Arabia) (18.5 ng/m³) [89] were comparable to those in Bursa (Turkey) (22 ng/m³) [90], while in Madrid (Spain) [91] they were much lower (0.186 ng/m³) than in the above cities. In both the Jeddah and Bursa studies air samplers were placed in the living room of residences, while in Madrid, Spain, it was placed on the first floor of a residential building [89–91]. The different locations of air samplers could lead to different PM-bound PAH concentration measurements. On the other hand, a high concentration of PM-bound PAHs (318 ± 314 ng/m³) was observed in infants’ rooms in Harbin (China) in winter, was not only suggested a high health risk for infants, but also for people who staying in the room [92].

Regarding school indoor air, Table 3 does not indicate a large concentration difference in Wuhan (China), among university dormitories (31.3 ng/m³), laboratories (27.0 ng/m³), and offices (32.4 ng/m³) [93]. The annual average concentration differences observed in universities between dormitories and offices in Beijing (34.1 and 32.1 ng/m³) were also not large [88], and comparable to those seen in Wuhan [93]. However, the concentrations observed in university offices in Jeddah (12.7 ± 5.1 ng/m³) [89] were lower than those observed in university offices in Beijing, consistent with the household results. Moreover, the PAH concentrations observed at the laboratories and offices in Harbin (115 and 96.6 ng/m³) in winter [94] were approximately three times higher than those observed in Beijing and Wuhan [88,93]. In contrast, the PM-bound PAH concentrations in primary or secondary classrooms ranged from 0.45 to 29.83 ng/m³, and the concentrations were comparable across Beijing, Warsaw and Gliwice (Poland), and Porto (Portugal), but they were very low in São Paulo (Brazil) [17,95–97]. Different from other sampling cities, São Paulo is close to the Equator and its hot climate can greatly influence the gaseous/particles phase distribution of PAHs. Moreover, the shorter sampling period used in São Paulo than in the other cities also had an impact on the PM-bound PAHs results [96].

Regarding public indoor air, the lowest concentration level (2.39–7.4 ng/m³) was observed in shopping malls in Islamabad (Pakistan), bakeries in Bari (Italy) and hotels in Jeddah [89,98,99]. This may occur because these public places contained few PAH sources. The second-highest concentration level (39.58–155.11 ng/m³) was observed in hotels in Jinan (China) in public bars in Warri (Nigeria) and in office buildings in Changchun (China) [100–102]. The higher concentration of PM-bound PAHs in hotels in Jinan than that in hotels in Jeddah possibly occurred due to the local background urban concentrations, and is consistent with the results for Beijing and Jeddah [88,89]. The relatively high concentration in public bars may be the result of factors such as tobacco smoking, while in office buildings, this may occur due to emissions from machines such as printers [103]. The seasonal differences observed in office buildings may occur due to the gaseous/particles phase distribution of PAHs at different temperatures and various outdoor pollution sources via window opening or ventilator operation [102]. The highest concentration level (550–39,000 ng/m³) was observed in fire stations, ships, and Chinese kitchens [83,84,104]. Smoke originating from fires contains a large number of PM-bound PAHs, which could be adsorbed onto the helmets and clothes of firefighters and transported to fire stations [104]. The high concentrations observed in ships and Chinese kitchens determine the personal exposure levels of seafarers and Chinese kitchen workers, respectively, because of engine fuel combustion and cooking, respectively [83,84]. However, the lower concentration in kitchens than the personal exposure concentration for kitchen workers possibly occurred because the air sampler was likely not located close to the cooking bench for safety reasons (high temperature) [84].

Table 3. Indoor concentrations (average and/or range, ng/m³) of PM-bound PAHs in several studies.

In contrast to personal exposure studies, air samplers remain fixed indoors rather than being portable. Hence, the indoor concentrations of PM-bound PAHs are related to the type of space and sampler location. Different space uses to determine whether direct sources of PAH emissions occur, and different air sampler locations may determine whether more or less PAHs are collected. However, the air is exchanged between indoor and outdoor environments via opening windows or operating fans. Therefore, the air exchange speed and local outdoor concentration exert some considerable impacts on the indoor concentration. For example, if PM-bound PAHs are notably generated indoors and the air exchange speed is low, the indoor concentrations may be higher than the outdoor concentrations (such as in Chinese kitchens [84]). Although the PM type and determined PAH number differed among the above studies, the resulting impacts of PM-bound PAHs concentration levels were not large for the overall characteristics of different microenvironments, e.g., the concentrations were much higher in kitchens than those in classrooms, and higher in rural households than those in urban households.

2.3. Outdoor Concentrations of PM-Bound PAHs

The atmospheric behaviours of outdoor PM-bound PAHs are more complex because they are not only dependent on various direct emission sources including industrial and traffic emission [105], wood and biomass burning [59], but also many complex atmospheric physical and chemical factors, including interactions with other pollutants, photochemical degradation, and dry and wet deposition [106,107]. Due to the different conditions in countries worldwide, the concentrations of PM-bound PAHs also vary among regions. In this review, relevant reports (in English) were retrieved on outdoor PM-bound PAHs worldwide in recent years, and the available data are summarized in Table 4.

As indicated in Table 4, Auckland (New Zealand) attained the lowest annual average concentration of outdoor PM-bound PAHs (0.31 ± 0.19 ng/m³) [108] among all the cities/countries. Cities in the Americas also attained relatively low overall average concentrations levels of outdoor PM-bound PAHs, which ranged from 0.84 to 10.2 ng/m³ [67,109–113]. In Europe, the observations in A Coruña (Spain) (7.56 ng/m³) [114] exhibited a lower annual average concentration than that in cities in southern Spain (26.2 ng/m³) [115]. A seasonal variation was observed in Milan (Italy) (0.40 and 72.8 ng/m³) [116] and Wadowice (Poland) (10.5 and 80.6 ng/m³) [117] which was larger during cold periods than that during warm periods. During cold periods, the concentration of PM-bound PAHs in Nicosia (Cyprus) (1.62 ng/m³) [118] was the lowest among European cities. The concentrations observed in Brno (Czech Republic) (20.7 ng/m³) [119] were comparable to those observed in Zagreb (Croatia) (25.4 ng/m³) [120], whereas the concentrations in Sarajevo (Bosnia-Herzegovina) (64.8 ng/m³) [120] were much higher than those in Zagreb, even though the samples were collected during the same period. During warm periods, slight differences in the PM-bound PAH concentration were observed among Moscow (1.32–7.68 ng/m³), St. Petersburg (1.71–6.30 ng/m³), and Kazan (2.95–9.61 ng/m³) in Russia [121]. Although most countries in Europe as shown in Table 5 are developed countries, the overall concentration levels of outdoor PM-bound PAHs were relatively higher than those in the Americas. One reason was the different sampling periods that the concentrations were mostly annual average in the Americas, whereas were mostly seasonal average in Europe. In addition, different meteorological conditions in different periods can lead to the concentration differences of PM-bound PAHs. Moreover, the higher population density in Europe (~73/km²) than in the Americas (~21/km²), with more human activities including industrial and traffic emission also can increase PM-bound PAHs concentrations.

Table 4. Outdoor PM-bound PAHs concentrations (average and/or range, ng/m³) in some cities worldwide.

In Africa, the limited available studies involving short-term observations of outdoor PM-bound PAHs showed that the average concentrations in Algiers (Algeria) (7.47 ± 1.21 ng/m³) [122] and Pretoria (South Africa) (4.11 ng/m³) [123] were much lower than that in Kigali (Rwanda) (52.7 ng/m³) [124]. On the other hand, Ofori et al. [125] summarized the studies on the PAHs from 2005 to 2019 in Africa. Only 14 reports among 121 papers focused on the indoor and outdoor PM-bound PAHs. Of those, only four papers related to outdoor PM-bound PAHs in the last five years, in which air samples were collected in Rwanda (this report), Tunisia (urban area, 2.8 ± 3.4 ng/m³), Nigeria (industrial area, 73–143 ng/m³), and Egypt (rural: 323 ng/m³; suburban: 503 ng/m³; urban: 417 ng/m³) [125].

In Asia, China has the largest population globally. Since the severe haze event in 2013, researchers have paid special attention to the atmospheric environment. Yan et al. [126] reviewed 270 studies of PM-bound PAHs in 67 cities from 2001 to 2016, which covered seven typical regions, including North, Northwest, Northeast, East, Central, South, and Southwest China. It has been reported that the annual mean concentrations of PM-bound PAHs in these cities range from 3.35 to 910 ng/m³, and those in the northern regions are higher than those in the southern regions [126]. In this paper, Table 4 lists the concentrations of PM-bound PAHs in several major Chinese cities collected within 5 years. The results for these cities indicate average concentrations ranging from 1.36 to 1056 ng/m³ [18,73,127–136], and the seasonal and regional differences are consistent with the results reported by Yan et al. [126]. In other Asian cities, relatively low concentrations levels of PAHs have been observed in Japan [137–139], South Korea [140,141], Vietnam [142], Singapore [143], Malaysia [144], Thailand [145], Qatar [146], and Lebanon [147], ranging from 0.56 ng/m³ (Doha, Qatar) to 29.5 ng/m³ (Gwangju, South Korea). Relatively high PAH concentration levels have been observed in Mongolia [148], Pakistan [149], India [150–152], and Iran [153,154] ranging from 0.66 ng/m³ (Bushehr, Iran) to 773 ng/m³ (Ulaanbaatar, Mongolia), which countries are the most polluted areas in the world. In Mongolia, wood and biomass burning for cooking and heating were the largest emission sources of PM-bound PAHs in the winter [148], while in India and Iran, traffic emission was the main contributor for PM-bound PAHs through the years [151,153]. The air pollutants in Iran were also largely influenced by the air masses long-range transported from Iraq and other Middle East areas [155]. Moreover, due to the special location, frequent recirculation of air masses resulted in the increased residence time of PM in Iran and lead to high air pollutants levels [156].

According to the simultaneously obtained PM_2.5 and PM₁₀ data, as summarized in Table 4 [109,115,127,131,146,149,152], the concentrations of PM_2.5-bound PAHs accounted for approximately 65% ~ 95% of the total PM-bound PAHs, proving that PAHs mostly occur in PM with a small size, which is consistent with previous studies reporting that the adsorption of PAHs depends on the PM type [69]. Although high outdoor concentrations of PM-bound PAHs were observed in certain cities, they were not as high as the personal exposure concentrations for seafarers and kitchen workers nor the indoor concentrations observed in ships and Chinese kitchens [83,84]. This occurs because expansive outdoor spaces facilitate the diffusion and dilution of air pollutants. Moreover, outdoor air pollutants easily degrade or transform in the atmosphere due to various meteorological conditions and other reasons. Moreover, although there have been a large number of environmental observation studies on PM-bound PAHs over the past few decades, different countries and regions have focused on different PM-bound PAH research aspects. Some major cities have published many reports on various PAH species, while other cities have published no research reports on PAHs at all.

This entry is adapted from the peer-reviewed paper 10.3390/ijerph18042177