Breathing plastics in Metro Manila, Philippines: presence of suspended atmospheric microplastics in ambient air

Microplastics (< 5 mm) have lately been identified in the atmosphere of urban, suburban, and even distant places far from plastic particle areas, suggesting the possibility of long-distance atmospheric transport of microplastics. However, the occurrence, fate, transmission, and effects of these suspended atmospheric microplastics (SAMPs) are all currently unknown in the Philippines. This study investigated the presence of suspected microplastic in the atmosphere of sixteen cities and one municipality of Metro Manila, Philippines. Sampling was conducted using a respirable dust sampler mounted with a Whatman GF/C filter paper at an intake flow rate of 1.4 L/min with Whatman GF/C filter paper. Results reveal that all seventeen sampling areas have the presence of SAMPs. A total of 155 SAMPs were found and confirmed in Metro Manila, with the highest concentration in Muntinlupa City and Mandaluyong City (0.023 SAMP/NCM). Fourteen SAMP types were identified across the sampling areas, ⁓ 74% with polyester. This study is the first record of the presence of microplastics suspended in the ambient air in the Philippines. It is estimated that an adult person in Metro Manila has the potential to inhale (5–8 per minute, normal minute ventilation) about 1 SAMP if exposed for about 99.0 to 132 h. Further studies should be done to evaluate the fate and health effects of these SAMPs in Metro Manila’s setting.


Introduction
Plastics are considered one of human's significant discoveries. It is tightly knitted to human civilization as it plays a significant role in our everyday lives. It is heavily utilized from clothing to coatings and transport vehicles to cleaning products, and the list goes on. However, as plastics have become abundant and sufficient to meet our demands, the disadvantages have become more visible. Most plastics research has, thus far, focused on the marine environment; however, other environmental compartments are becoming increasingly important (Acot et al. 2022;Gaboy et al. 2022;Sajorne et al. 2021;Bank and Hansson 2019;Horton and Dixon 2017).
Microplastics are synthetic solid particles or polymer matrices with regular or irregular shapes and sizes ranging from 1 to 5 mm, of either primary or secondary manufacturing origin, and are insoluble in water (Frias and Nash 2018). Microplastics are currently considered an emerging part of air pollution due to their inhalation in combination with other contaminants (Rochman et al. 2019). Because of its environmental persistence and potential health effects, microplastic pollution has recently received much attention from the scientific community and the authorities. Several scientific studies conducted in recent decades have revealed the presence of microplastics in soil, freshwater, and oceans Navarro et al. 2022;Sajorne et al. 2022;Wu et al. 2019). While aquatic plastic pollution is well proven, knowledge of MP pollutants in the atmosphere is still limited. Because of their small size and low density, microplastics can be transferred into the atmosphere and are easily transported by wind (Allen et al. 2019). According to a recent study by Liu et al. (2019a, b, c) and Dris et al. (2015), microplastics have been detected in the atmosphere using particulate matter measurements and wet and dry deposition (Brahney et al. 2020). Long et al. (2022) estimated in their atmospheric transport modeling that 1.4% of the total microplastics emitted into the atmosphere in Asia are deposited into the oceans via atmospheric transport, which is about 4.2 Gg per year. The major sources of atmospheric microplastics studied by Brahney et al. (2021) in the western USA are from road and braking emissions (84%), soil emissions coming from agriculture and landfill (5%), dust near urban centers (0.4%), and sea spray (11%).
Unlike other ecosystems, microplastics in the air can be directly and continuously inhaled into the human body, posing a health risk (Gasperi et al. 2018). The concentrations of non-fibrous microplastics and fibers in the atmospheric fallout in Dongguan city, China ranged from 175 to 313 particles/m 2 /day (Cai et al. 2017). Suspended atmospheric microplastics are widely distributed in Shanghai, with an estimated 21 microplastic particles inhaled by an adult in the city daily (Liu et al. 2019a, b, c). In Paris, France, Dris et al. (2015) reported atmospheric fallout with an average of 118 particles/m 2 /day, of which more than 90% are fibers. In Hamburg, Germany, Klein and Fischer (2019) reported high deposition (275 particles/m 2 /day) of microplastics, which are mostly composed of fragments (> 90%). With this evidence, the adverse impacts of microplastics are still not as visible compared to macroplastics.
Microplastics in the ambient air may enter the human system by breathing, among other potential entry routes. Upon inhalation at a significant level, it may cause distress to the respiratory system, cytotoxic and inflammatory effects, and autoimmune diseases (Dehghani et al. 2017;Dong et al. 2019;Rezaei et al. 2019). Since the human lung has a very thin tissue barrier smaller than 1 μm, it can allow the passage of nanoparticles, eventually exposing the bloodstream and other human body parts to small-sized microplastics (Lehner et al. 2019).
The National Capital Region, or Metro Manila, has been a critical region of the Philippines as consumption and income have been concentrated (Lambino 2010). The rapid urbanization of this region has made collateral issues on environmental integrity, especially air quality. Metro Manila ranked 62nd out of 92 cities monitored for air quality with a high yearly average concentration of PM 2.5 (IQAir 2020). The region's high levels of particulate matter have been attributed to motor vehicles (Simpas and Cruz 2014). Furthermore, microplastics in the ambient air in the said metropolitan area, including their putative health effects, are yet to be determined and studied. Hence, this study aims to assess the prevalence and characteristics of atmospheric microplastics in the ambient air of Metro Manila, Philippines. To the best of our knowledge, this study is the first record of the presence of microplastics suspended in the ambient air in the Philippines.

Background of the sampling area
Metro Manila, officially the National Capital Region, is an economic and governmental center of the Philippines. The region is situated in the southwestern part of Luzon island and is mainly composed of 16 cities and one municipality, namely, Las Piñas, Makati, Muntinlupa, Parañaque, Pasay, Pateros, Taguig, Quezon, Marikina, San Juan, Pasig, Mandaluyong, Manila, Caloocan, Malabon, Navotas, and Valenzuela ( Fig. 1).

Air sampling
Atmospheric microplastic sampling was conducted from December 16 to 31, 2021. One-time sampling was conducted in each city and municipality near the Department of Environment and Natural Resources' stationary air quality monitoring stations (Table 1, Fig. 1). Suspended atmospheric microplastic particulates were sampled using a respirable dust sampler with an intake flow rate of 1.4 L/min for 12 h, from 6:00 AM to 6:00 PM. This intake rate gauge of the sampler has been constantly monitored every 30 min to ensure that the intake rate is maintained. In the event of precipitation, air sampling will be delayed until the following day. The respirable dust sampler was placed in an open space near the road. The intake of the sampler was positioned 1.5 m from the ground to simulate the average human height. A GF/C Whatman filter paper was used to filter out the atmospheric microplastics from the suctioned air, with its intake positioned 1.5 m up from the ground. These filters were pre-examined microscopically to check the presence of any microplastic contaminants.
No standards exist for sampling atmospheric microplastics (Zhang et al. 2020). Though Liu et al. (2019a, b, c) conducted a 1-h sampling totaling 6 m 3 of air filtered, this study performed a 12-h sampling using the respirable dust sampler. Liu et al. (2019a, b, c) recommended and required in the study of SAMP quantification that a minimum of 70 m 3 of the air shall be filtered using an active sampling method. In this study, a total of 864 m 3 of air was sampled per sampling station. This study refers to microplastics in the atmosphere as suspended atmospheric microplastics (SAMP) to stress that these are readily suspended in the air at the time of sampling. The concentration of these MPs is expressed in units of SAMP per normal cubic meter of air sampled (SAMP/ NCM), where NCM signifies that the gasses are in "normal" conditions or at 20 °C and 1 atm.

Microplastic presence and sorting
After sampling, the filters were brought to a cleaned and controlled location to prevent contamination of filter papers while subjected to a light microscope. Wearing a non-woven coverall suit, visual sorting was done for microplastics collected on the filters (Stolte et al. 2015) using a binocular stereomicroscope mounted with a camera. Each sorted microplastic was individually embedded in a glass slide and labeled.
Each microplastic in the glass slide was photographed and classified using a compound microscope. The sample was then described based on its shape (fiber, fragments, films, or granules) and color (Liu et al. 2019a, b, c). Furthermore, the maximum dimension of each microplastic collected was measured using a compound microscope mounted with Moticam BTX8. The maximum dimension (MD) of microplastic was then classified based on sizes: class 1 (MD ≤ 100 µm), class 2 (100 < MD ≤ 250 µm), class 3 (250 < MD ≤ 500 µm), class 4 (500 < MD ≤ 1000 µm), and class 5 (1000 < MD ≤ 5000 µm) (Abbasi et al. 2018). Blank samples were made throughout the analysis process to estimate the presence of contamination.

Validation of microplastics
All sorted and suspected microplastics were placed individually in a clean glass slide and subjected to Shimadzu Fourier-transform infrared spectroscopy (FTIR) in a commercial laboratory. The spectra of each collected microplastic were then compared to the library of polymer spectra.

Abundance and distribution
A total of 155 suspended atmospheric microplastics (SAMPs) were found and confirmed among the samples collected in Metro Manila. The presence of SAMPs has been confirmed in all 17 sampling stations in Metro Manila (Table 1). The total number of observed SAMPs in each sampling station ranged from 1 to 19, with an average of 9.11 ± 5.46 SAMPs. The cities with the largest number of SAMPs were Mandaluyong City and Muntinlupa City (n = 19). On the other hand, Malabon City has the distinction of having the smallest number of observed SAMPs (n = 1)  (Table 1). No contamination was observed from the blank samples found during the sorting and analysis process. Most numbers of SAMPs were found in the northern, central, and southernmost cities of Metro Manila, specifically in Caloocan (n = 15), San Juan (n = 14), Mandaluyong (n = 19),and Muntinlupa (19). In between these high recorded locations, sampling areas with less than 5 SAMPs were detected in places like Las Piñas (n = 5), Paranaque (n = 5), Pasig (n = 3) in the north, and Quezon City (n = 2) and Malabon (n = 1).

Concentrations of SAMPs
Quantifying the concentration of SAMPs based on the total volume of air sampled, the highest concentration was observed in Muntinlupa city and Mandaluyong city (0.023 SAMP/normal cubic meter, NCM) ( Table 1). This high concentration was followed by Caloocan city (0.018 SAMP/ NCM), and San Juan city (0.017 SAMP/NCM) is not far behind. The lowest concentration of SAMPs was observed in Quezon City (0.002 SAMP/NCM) and Malabon city (0.001 SAMP/NCM). On average, the concentration of SAMPs in Metro Manila is 0.021 ± 0.006 SAMP/NCM.

Shape
A total of four shapes were recorded to be present in the atmosphere of the study sites (Fig. 2). Fibrous microplastic dominated the number of SAMPs (n = 179), accounting for 88% of all SAMPs collected. Fragment (n = 19) and film (n = 15) were also recorded and contributed 6 and 5% to the total SAMPs, respectively. The granule (n = 1) is the lowest recorded shape, comprising only 1% of the SAMPs collected, as presented in Fig. 3.
Fiber-shaped SAMPs were recorded to be the most abundant SAMP in all sampling stations (see Fig. 3). The composition of the shape varies per sampling area. Fragmented microplastics were present in only six cities, namely, Valenzuela, San Juan, Navotas, Marikina, Manila, and Mandaluyong City. Film shapes were also recorded in 8 sampling sites: Valenzuela, Quezon City, Paranaque, Navotas, Marikina, Makati, Las Pinas, and Caloocan City. However, the granulated MP was only found in Pasig City.

Color
A total of nine colors were identified during the microplastic identification of all sampling sites. Black (n = 75), blue (n = 49), and brown (n = 34) were, by far, the most common colors identified, which contributed 39, 25, and 14 respectively (Fig. 4). The fewest colors seen were grey (n = 5), yellow (n = 5), and white (n = 2), which made up only 2, 2, and 1%, respectively. Figure 4 shows that black SAMPs were dominant and present in 14 of 17 sampling sites. These sites were Valenzuela, Taguig, San Juan, Pateros, Pasay, Paranaque, Navotas, Muntinlupa, Marikina, Manila, Mandaluyong, Makati, Las Piñas, and Caloocan. These 14 cities were Fig. 2 Types of atmospheric microplastics based on shape: A granules, B fiber, C fragment, and D film also found to have more than two types of SAMPs identified. In Quezon and Pasig, on the other hand, only one or two colors were noted. Dris et al. (2015) pointed out that people tended to overestimate the amount of blue and red microplastics because they were easier to spot than other particles.

Size
The sizes of the microplastics were grouped into five following the study of Abbasi et al. (2018). dimension greater than 1000 μm but less than or equal to 5000 μm (n = 115), contributing 57% of the total SAMPs, as shown in Fig. 5. About 27% of the total microplastic recorded was under class 4, with a maximum dimension greater than 500 μm but less than 1000 μm or equal to 1000 μm (n = 26). Class 3 (250 < MD ≤ 500 μm) (n = 13) and class 2 (100 < MD ≤ 250 μm) (n = 4) were also recorded. However, class 1 was not detected in this particular study.
Class 5 was generally the most common class size in all sampling sites. However, it was not recorded to be present in Valenzuela, which only has class 3 and class 4 sizes (Fig. 5). Notably, only the city of Las Pinas has been documented to have 4 class sizes present in its atmosphere. Malabon (n = 1) only has class 5 as its size.

Polymer classification of SAMPs
A total of 15 different types of atmospheric microplastics were tested, including acrylonitrile butadiene styrene, epoxy, polyamide, polybutylene terephthalate, polycaprolactone, polyester, polyoxymethylene, polyphenyl sulfone, polypropylene, polystyrene, polyvinyl fluoride, polyvinylidene chloride, polyvinylidene fluoride, and silicone. Polyester is the most abundant of these types of SAMPs (n = 114), accounting for approximately 73.53% of the total number of SAMPs collected across the various sampling sites (Fig. 6). Lower-abundance SAMPs (1%) are primarily formed of acrylonitrile butadiene styrene, epoxy, polybutylene terephthalate, polycaprolactone, polyoxymethylene, polyphenyl sulfone, polyvinylidene fluoride, and silicone. Figure 6 depicts the relative abundance of SAMPs in the different sampling stations. All sampling stations showed a high quantity of polyester compared to other types of SAMPs. An average of 3 types of SAMPs was observed across different sampling stations. Mandaluyong has the highest number of SAMPs collected; this sampling station also ranked the highest in the number of the kinds of SAMPs recorded (n = 6). Though Muntinlupa City ranked the same as Mandaluyong with the highest registered number of SAMPs, three types of SAMPs were only recorded. Four types of SAMPs were observed in Manila City, Marikina City, San Juan City, and Valenzuela City.
Polyester is the most prevailing type of SAMPs and has been observed in all sampling stations. This is followed by polyethylene terephthalate and polyamide, observed in 10 and 6 sampling stations, respectively. Figure 7 shows the concentration of SAMPs in different sampling stations. In terms of concentration of SAMPs in Metro Manila, polyester is the most concentrated (0.13 SAMP/NCM) among the 14 types, followed by polyethylene terephthalate (0.02 SAMP/NCM) and polyamide (0.007 SAMP/NCM). The rest of the AMP types have less than 0.002 SAMP/NCM concentration.

Discussion
The emergence of microplastics in the air has been objectionably unknown, drawing interest in the scientific community. Most studies have concentrated on the abundance B A of atmospheric microplastics, specifically those considered fallout or settled on roads and public surfaces. Furthermore, studies on suspended microplastics have received less attention than studies on fallout or settled microplastics. This is because of the more complicated sampling method.
Several studies have been conducted in recent years showing the presence of microplastics in water, soil, and oceans Arcadio et al. 2022;Sajorne et al. 2022;Wu et al. 2019). Several methods of estimating the volume of atmospheric microplastic particles are broadly categorized as active and passive sampling or collection (Zhang et al. 2020). The majority of AMP studies conducted, according to Zhang et al. (2020), use passive collection methods such as in the studies conducted by Allen et al. (2019), Cai et al. (2017), Dris et al. (2017), and Klein and Fischer (2019). On the other hand, active collection methods for AMP studies include rain samplers, fallout collectors (Allen et al. 2019), and particulate samplers (Dris et al. 2017(Dris et al. , 2018Liu et al. 2019a, b, c;Abbasi et al. 2018). The particulate or pump samplers have helped quantify the concentration of atmospheric microplastics (Hayward et al. 2010), especially particles that are immediately suspended. In this study, we validated the presence of suspended atmospheric particulates in Metro Manila's ambient air. This study is the first record of the presence of microplastics suspended in the ambient air in the Philippines. The most common microplastic in Metro Manila is fiber, which has the highest reported shape detected in microplastic studies worldwide (Ebere et al. 2019;Zhou et al. 2017;Covernton et al. 2019).
Black-colored SAMPs are the most abundant in Metro Manila, which is in good agreement with the study conducted by Liu et al. (2019a, b, c) and Ambrosini et al. (2019). Though there were very few discussions and studies conducted explaining the dominance of black-colored MPs in the atmosphere, nonetheless, this is due to the exposure of microplastics in different environmental conditions that may lead to fragmentation and discoloration. Liu et al. (2019a, b, c) found that microplastics in Shanghai, China can form due to the exposure of larger plastic items to strain and fatigue, which induce mechanical abrasion on the surface, or UV light, which alters its chemical properties. Moreover, the black-colored SAMPs are plausibly due to the presence of absorbed ultrafine soot in the extracted MPs. This finding is in line with the finding reported by Cruz et al. (2019), where most of the particles collected with sizes 0.056-18 μm are greatly composed of black carbon or ultrafine soot. Though the most common color of SAMPs observed is black, microplastics have been discovered in various colors, including black-red, orange, yellow, brown, tan, off-white, white, grey, blue, and green (Bergmann et al. 2019;Rochman et al. 2019). A plastic particle's color cannot simply be utilized to determine its type or origin. Color information can be skewed since brighter colors are easier to identify during visual assessment (Rochman et al. 2019). Color analysis is helpful for the first visual assessment of microplastics in atmospheric samples, but spectral or chemical identification of these SAMPs is thought to be more critical (Zhang et al. 2020). This is because most microplastic particles are small and often weathered a lot.
The most common type of plastic polymer in Metro Manila is polyester and fiber-shaped. The presence of this has been similarly documented in the atmosphere of Paris, France (Dris et al. 2017); Dongguan, China (Cai et al. 2017); Yantai, China (Zhou et al. 2017), Shanghai, China (Liu et al. 2019a, b, c), and Surabaya, Indonesia (Syafei et al. 2019). The general microplastic pollution has been attributed to synthetic textiles (De Falco et al. 2019;Acharya et al. 2021). Microfibers are made when clothes break down over time. These tiny fibers can easily float in the air because they are so small.
In atmospheric microplastic investigation and research, particle size is essential. SAMPs extracted in this study were recorded as predominantly class 5 (1000 < MD ≤ 5000 µm), which is the same as SAMPs extracted and removed in Surabaya, Indonesia and Paris (Dris et al. 2015;Asrin and Dipareza 2019). Numerous studies show that the predominant sizes of SAMPs fall under class 3 and class 4 Cai et al. 2017;Allen et al. 2019;Klein and Fischer 2019;Liu et al. 2019a, b, c;Bergmann et al. 2019).
There are several methods of studying atmospheric microplastics, such as using volume samplers, the atmospheric deposition method (Klein and Fischer 2019), and others. Table 2 shows the SAMPs studies using the active method: particulate/pump sampling from 2019 to this present study. Dris et al. (2017) in Paris, France reported 0.30-1.50 particles/m 3 of SAMPs, whereas in Asaluyeh County, Iran, Abbasi et al. (2018) reported 23.60-23.93 particles/m 3 of SAMPs. The highest recorded SAMP concentrations were in Shanghai, China (Liu et al. 2019a, b, c) and in Surabaya, Indonesia (Syafei et al. 2019), with recorded 72-144 and 55.93-174.97 particles/m 3 of SAMPs, respectively. Comparing the SAMPs conducted in December 2021 to studies conducted using particulate or pump sampling method in Paris, France (Dris et al. 2017), Asaluyeh County, Iran (Abbasi et al. 2018), and Shanghai, China (Liu et al. 2019a, b, c), the concentration is lower. Several explanations have been presented by Liu et al. (2019a, b, c) for the differences, such as population density and the dilution effect of sea air. With the proximity of Metro Manila to the sea, the SAMP concentrations have been potentially diluted.
Several studies on the presence of microplastics in biological samples such as fish guts (Paler et al. 2021;Karbalaei et al. 2019) and clams (Bonifacio et al. 2022;Su et al. 2018) have been conducted. The quantification of MPs in these biological samples has been directly linked to the ingestion of organisms from their environment (Su et al. 2018). Fewer studies are being conducted worldwide on microplastic inhalation due to the presence of suspended atmospheric microplastics (Kannan and Vimalkumar 2021). Normal minute ventilation, or the amount of air a person would take in a minute, is roughly 5-8 L per minute (Levitan 2015). Given the average SAMP concentration in Metro Manila of 021 ± 0.006 SAMPs/NCM, a person without a mask is estimated to inhale about 0.00756-0.0101 SAMPs/h. This would also mean that a person exposed to Metro Manila's ambient air for 99.0-131.57 h will have a chance to inhale 1 SAMP or equivalent to inhalation of ~ 76 SAMPs/year. Amato-Lourenço et al. (2021) validated the presence of microplastics in human lung tissues obtained at autopsies. Their study observed about 33 polymeric particles and 4 fiber samples in 13 to 20 samples. These microplastics' potential respiratory health risks to animals and humans have not been thoroughly investigated. Inhaling these microplastics has been linked to pneumonia and other respiratory diseases, but no concrete evidence has been found. The fate and health effects of these SAMPs would remain uncertain in Metro Manila's setting. Only suppositions can be made on the sources of these SAMPs based on their physical and chemical characteristics.

Conclusions and recommendations
Microplastics have become ubiquitous in the environment, ranging from oceans to rivers to soil. Only recently have these microplastics in ambient air been discovered, with only a few studies focusing on their abundance and density. This is the first study in the Philippines to show that suspended atmospheric microplastics are present in all cities and the lone municipality of Metro Manila. Most of the SAMPs collected are fibers with diameters greater than 1000 but less than 5000 µm. The chemical characterization of the collected SAMPs is mostly polyester, implying that most of the SAMPs are derived from textiles. Due to the limited sample sizes at each city and one-time sampling, this study can only confirm the presence of microplastics in the air of Metro Manila. This study was conducted during the month of December, which is a dry season in Metro Manila. Given these limitations, future research to validate this work and determine the seasonal fluctuation, deposition rate, and resuspension rate of microplastics are recommended. The formation of these microplastics and the mechanism by which they are suspended in the atmosphere are not fully understood. The fate and consequences of these SAMPs are unclear, particularly in the Philippine context. Future research on atmospheric microplastics should concentrate on the sources and generation of microplastics, the mechanism of suspension, the temporal variation, the effect of environmental factors, and the total time that these MPs are suspended in the atmosphere. Because these microplastics will not degrade naturally in the environment, the rate of accumulation and their fate in the environment should be investigated. Finally, it is essential to understand the potential health impacts and environmental risks of atmospheric microplastics in Metro Manila.