1 Introduction
The cumulative weight of man-made objects has now exceeded the total weight of all living organisms on Earth
[1]. As one of the typical materials, plastic is a synthetic or semi-synthetic material mainly composed of polymers. Due to its high plasticity, plastic can be molded, extruded, or pressed into various shapes. Additionally, because of its light weight, durability, flexibility, and low production costs, it has become the most widely used artificial material in both production and daily life
[2].
1.1 The Significance of Plastics in Contemporary Human Society and Production
Since its inception, the plastic-related industry has developed rapidly, with a significant increase in market share
[3]. By 2019, global plastic production had reached 400 million tons
[4] (
Figure 1a). Moreover, with economic development and rising consumption levels, there is a trend for further growth in plastic production. According to data provided by PlasticsEurope, as of 2022, the scale of the European plastic raw materials and plastic products industry had increased by about 20% compared to 2015
[5]. It is projected that by 2050, the annual global plastic production will exceed 1.1 billion tons.
Traditional plastics come in a wide variety, with common types including polyethylene (Polyethylene, PE), polypropylene (Polypropylene, PP), polyvinyl chloride (Polyvinyl chloride, PVC), polystyrene (Polystyrene, PS), polyurethane (Polyurethane, PUR), and polyethylene terephthalate (Polyethylene terephthalate, PET). Due to their varying properties, these plastics are used in a broad range of applications. By usage, they can be categorized into fiber and non-fiber types, which differ significantly in application fields and production processes. In 2019, the top three non-fiber plastic productions by volume were PE (28%), PP (19%), and PVC (13%). The shares of PET, PS, and PUR were all below 10%
[5] (Figure 1b); polyester fibers accounted for 70% of the total fiber plastic production, with PET making a significant contribution
[6]. Approximately 42% of non-fiber plastics are used in packaging, making it the largest consumer sector, with PE, PP, and PET being the primary materials. PET is favored due to its advantages such as light weight, transparency, impact resistance, and chemical corrosion resistance, which effectively protect the quality and enhance the safety of liquid products
[7-8]. The construction and building industry is the second-largest consumer sector, using 69% of total PVC production, accounting for 19% of non-fiber plastics
[9] (Table 1). PVC pipes, due to their strong corrosion resistance, easy installation, and long service life, have become the preferred material for water supply and drainage systems in the construction sector
[10].
The plastic industry plays a significant role in driving the global economy. Plastic production and processing companies provide a large number of job opportunities, covering multiple links and industries from raw material production to product manufacturing, and then to recycling and disposal. Moreover, plastic products are inexpensive and have superior performance, widely used in various products, which boosts production demand, improves production efficiency, and reduces manufacturing costs. Due to the broad application fields and enormous economic impact of plastics, they have become indispensable in modern industry and daily life.
1.2 Ecological and Environmental Risks of Traditional Non-Biodegradable Plastics During Their Application Process
Plastic products can cause pollution at various stages including production, use, and recycling, which may lead to significant global environmental issues
[11]. Due to an incomplete recycling system, high recycling costs, and insufficient public environmental awareness, a large amount of waste plastic has not been effectively recycled
[12-13]. According to the aggregated statistics of China's plastic products industry, the total output in 2023 was 74.885 million tons, while the amount of waste plastic recovered was only 19 million tons
[14]. Globally, only about 18% of plastic waste is recycled, 24% is incinerated, and the remaining 58% is landfilled or ends up in the natural environment
[6]. For example, in the United States, the landfill rate for waste plastic exceeds 75%, accounting for 19% of all municipal solid waste. At the current growth rate, it is estimated that by 2050, the cumulative amount of plastic waste that will be landfilled or enter the natural environment globally will reach nearly 120 million tons
[15].
Plastics are difficult to degrade in the natural environment, typically requiring hundreds of years or even longer to fully decompose. During this period, they continuously release harmful substances, threatening the safety of ecosystems
[12,16]. Among these, microplastic (Microplastics, MPs) pollution has become a focal issue in the fields of ecological environment and public health in recent years. MPs refer to plastic particles, fibers, and fragments with a particle size <5 mm, which enter the environment through the decomposition of large plastic products, industrial discharges, and personal care product emissions, etc.
[17]. MPs not only have negative impacts on ecosystems but also pose potential threats to human health
[18-19]. Recent studies show that MPs entering the human body can accumulate in arteries, potentially increasing the risk of heart disease, stroke, and other diseases
[20].
1.3 Current Production, Application, And Potential Risks Of Emerging Biodegradable Plastics
In the face of major environmental and health issues such as "white pollution" and microplastic (MPs) invasion, the idea of using biodegradable plastics (BPs) to replace traditional plastics to alleviate or even solve plastic pollution has been widely promoted
[21]. Theoretically, BPs can be decomposed into harmless substances (such as water, carbon dioxide, and biomass) relatively quickly under the action of microorganisms in the natural environment, thereby reducing the pollution caused by discharged plastics
[22].
The main raw materials for BPs include lignin, cellulose, starch, and bioethanol, which are renewable resources, reducing dependence on petroleum resources, and in turn promoting resource recycling and sustainable use. The main types of BPs include biobased polylactic acid (PLA), polyhydroxyalkanoate (PHA), and starch blends, as well as petroleum-based poly(butylene adipate-co-terephthalate) (PBAT), polycaprolactone (PCL), and polybutylene succinate (PBS)
[5]. Among these, BPs such as PLA, PHA, and starch blends accounted for 53.8% (approximately 1.14 million tons) of the global biobased plastics production in 2023
[23] (
Figure 2). BPs have been widely applied in multiple key areas. In the food packaging sector, BPs can be made into straws, disposable tableware, paper cups, etc. In agriculture, the primary applications of BPs include mulch films, compost bags, and slow-release carriers for pesticides. In the field of medical materials and devices, BPs (such as PLA) are extensively used to produce surgical sutures, artificial bones, artificial skin, medical stents, orthopedic needles, stents, and wound dressings due to their excellent biocompatibility, thereby advancing medical technology and enhancing the safety of treatment processes. It is projected that by 2028, the global production of BPs will reach approximately 4.61 million tons
[24].
In 2020, the National Development and Reform Commission and the Ministry of Ecology and Environment issued the "Opinions on Further Strengthening the Control of Plastic Pollution," further banning and restricting the production, sale, and use of plastic products, and promoting alternative products such as degradable plastic bags. This has ushered in new opportunities for the development of degradable materials. In recent years, policies such as the "14th Five-Year Plan for Green Industrial Development," the "14th Five-Year Plan for Raw Materials Industry Development," and the "Three-Year Action Plan for Accelerating the Innovative Development of Non-Grain Biobased Materials" have been frequently introduced, strongly supporting the high-quality development of biodegradable materials. By 2027, the scale of China's degradable materials industry is expected to reach 32.41 billion yuan, a year-on-year increase of 19.5%
[25].
However, there is still considerable controversy in academia over whether BPs are truly green, environmentally friendly, and safe. As the use of BPs becomes increasingly frequent, the potential risks posed by the biodegradable microplastics (BMPs) generated from their degradation and the oligomers released have become more severe
[19]. The BPs discharged into the natural environment do not degrade as quickly as designed. For instance, BPs in aquatic ecosystems may not be completely degraded in a short period, potentially producing more BMPs than petroleum-based plastics, thereby increasing potential hazards
[26]. Furthermore, some BMPs might even serve as a carbon source, disrupting the normal metabolic activities of certain microbial communities and disturbing ecological balance
[23]. Therefore, simply promoting the widespread use of BPs cannot entirely eliminate the pollution caused by traditional plastics, especially the environmental and health risks brought about by BMPs. However, research on the potential hazards of discarded BPs and the resulting BMPs is still in its infancy, particularly regarding their migration and transformation behaviors in the environment and their potential ecological and health effects. Compared to traditional non-biodegradable plastics, BPs, with their stronger degradability, are more likely to break down into BMPs. Yet, there is a lack of in-depth studies on the spatiotemporal distribution patterns of BMPs in different environmental media and the content of BMPs in various animals and plants. These questions seriously hinder the identification of the hazards of BPs and their derivatives, BMPs, and the development of preventive measures.
BPs, as alternatives to traditional plastics, have gained increasing policy support and have made certain progress in production and application. However, the environmental and health risks they pose still require further research and resolution to ensure that they truly achieve the goals of being environmentally friendly and sustainable in practical applications. Therefore, this review will delve into the potential environmental and health risks of BPs, particularly focusing on the analytical methods for BMPs and their presence in the environment. It will also analyze their exposure pathways and potential health effects, and look ahead to the future application trends and challenges of BPs, providing theoretical and technical support for the promotion and risk mitigation of BPs.
2 Degradable Plastics: Analytical Methods And Environmental Presence
BPs can be degraded into water, carbon dioxide, methane, and mineralized inorganic salts by bacteria, fungi, and algae in the environment. However, this process is influenced by environmental conditions such as temperature and humidity, and the activity of microorganisms. Inappropriate environmental conditions can significantly inhibit the degradation efficiency of BPs, thereby extending their lifecycle in the natural environment
[27]. BPs in the natural environment lead to the production of BMPs, and because the degradation efficiency of BPs is more easily affected, BPs may produce more BMPs than the MPs produced by hard-to-degrade plastics within the same time frame
[28]. Therefore, the environmental presence and ecological risks of BMPs warrant further attention. Studying the environmental behavior of BMPs requires reliable detection and analysis methods. Currently, the separation and characterization of BMPs often follow traditional MP analysis methods. Some of these methods, which involve strong chemical reactions suitable for hard-to-degrade plastics, might damage the structure of BPs, affecting the characterization conclusions. Thus, a balance must be achieved between extraction efficiency and plastic damage
[29]. This section mainly introduces the commonly used analytical methods for BMPs, the similarities and differences with traditional MPs, and the environmental presence of BPs.
2.1 Bmps Analytical Method
The analysis of non-degradable MPs in environmental samples generally includes two parts: sample preparation and detection analysis
[30]. Through sample pretreatment, MPs are separated and purified from environmental media such as the atmosphere, water, soil, and organisms, and then different instruments are used to characterize and quantify their mass concentration or particle size distribution, etc. (Figure 3).
2.1.1 Sample Preparation Method
For environmental samples with simpler compositions, direct physical methods such as visual sorting, filtration, and sieving can be used to separate MPs. However, these methods are time-consuming, difficult to distinguish MPs from other materials, and have certain requirements for particle size. Additionally, issues like filter membrane contamination may lead to false positive or false negative results
[31-32]. Currently, the commonly used sample preparation technique is density separation combined with organic digestion
[33], which can remove inorganic and organic matrix components from complex samples such as sediments and biological tissues. The reagents used in organic digestion include acids, bases, oxidants, and enzymes. HNO
3 has a destruction rate of 94% to 98% for organic matter, but Avio et al.
[34] found that it can cause partial dissolution of some polymers; Foekema et al.
[35] observed that when 10 mol/L KOH solution was added, organic matter was completely destroyed after 2 to 3 weeks, and most polymers are resistant to KOH; H
2O
2 is an effective oxidant, and using 30% H
2O
2 for 7 days can extract 70% of MPs
[36]; enzymes can be used to remove organic matter and some biological tissues, and the reaction conditions are relatively mild. Cole et al.
[37] degraded 97% of organic substances using protease K, although the cost is higher. The principle of density separation is that the density of plastics (0.8 to 1.6 g·cm
-3) is much lower than that of sediments (2.7 g·cm
-3), which is used to separate MPs from undigested inorganic substances. The method involves mixing high-concentration or saturated salt solutions with the sample at an appropriate volume ratio and separating the supernatant containing MPs at suitable time intervals
[38]. Since the density of MPs is affected by the concentration of additives, polymer type, and adsorbed substances, the selection of the salt solution for density separation needs to match. The National Oceanic and Atmospheric Administration of the United States recommends using a NaCl solution (1.2 g·cm
-3) as the flotation liquid
[39], which is widely used due to its economic reliability, but it is not suitable for high polymers such as PVC and PET. To overcome this limitation, high-density salts such as NaI (1.8 g·cm
-3) have been tested and used. Additionally, ZnCl
2, ZnBr
2, NaBr, 96% ethanol, and rapeseed oil can effectively separate certain polymers
[40]. Besides density separation, field-flow fractionation (such as asymmetric flow field-flow fractionation (AF4)), hydrodynamic chromatography (such as high-temperature gel permeation chromatography (HT-GPC)), size exclusion chromatography (SEC), electrostatic separation, and magnetic extraction can also achieve efficient separation of MPs
[33].
Currently, there are few separation and purification methods developed for BMPs. Most studies have introduced physical screening, organic digestion, and density separation methods. However, the physicochemical properties of BPs differ from those of traditional plastics, requiring a balance between digestion efficiency, plastic damage, experimental time, and cost. Optimization of related sample preparation methods is needed (Table 2). Yu et al. [29] used a multi-criteria decision-making (MCDM) method to comprehensively evaluate and optimize the extraction schemes for BMPs in five aquatic species. They selected enzymatic hydrolysis time, hydrolysis efficiency, mass loss, cost, polymer integrity, and size change as evaluation indicators. The optimal digestion method obtained was pepsin + H2O2, with a digestion efficiency as high as 99.56%. In terms of separation methods, NaI was found to be more suitable than olive oil for separating BMPs from digested residues. The study also found that, unlike traditional plastics, HNO3 and KOH caused significant mass loss (65%~87%) and noticeable size changes in degradable polymers, while enzymes and oxidants led to lower mass loss. Therefore, strong acids and bases should be avoided when digesting BMPs. For the density separation method used to separate BMPs from solid sample matrices, Berit et al. [41] conducted a series of experiments to investigate the effectiveness of four salt solutions (H2O, NaCl, SHMP, NaBr) in separating traditional MPs and BMPs from solid sample matrices (sand, artificial soil, and compost). It was found that the recovery rate of traditional MPs in each solution and matrix was higher than that of BMPs, and an increase in solution density would lead to an increase in the recovery rate of BMPs. The highest recovery rate of BMPs was achieved in NaBr solution, ranging from 17% to 85%. Since the study identified MPs visually, it was analyzed that the low recovery rate of BMPs mainly depended on the low visibility of the polymers during the sample processing. Subsequent optimization of BMP separation methods should reduce reliance on visibility. Additionally, the selection of salt solutions should also consider the type of polymer; for example, saturated ZnCl2 solution can dissolve cellulose, which may interfere with the extraction of BMPs containing cellulose.
2.1.2 Detection Of Bmps
Analytical techniques are crucial for the identification and quantification of MPs. Currently, the detection of MPs mainly relies on methods based on physical morphology and chemical properties
[43], which can be categorized into spectroscopy, mass spectrometry, etc.
[44]. The selection of specific analytical methods primarily depends on the research objectives and needs, and is also constrained by the physicochemical properties of MPs, such as particle size, shape, polymer type, hydrophobicity, and surface charge
[45]. The most commonly used microscopic spectroscopic methods for detecting MPs are Fourier Transform Infrared Spectroscopy (FTIR) and Raman spectroscopy
[46]. Currently, the detection of BMPs often references traditional MP analysis methods; for example, Yu et al.
[29] characterized five types of BMPs in aquatic economic species tissues using Raman spectroscopy: PBS, PCL, PHB, PLA, and starch-based plastics. However, these methods have cumbersome pretreatment processes and cannot accurately quantify. The quantification of MPs in environmental samples typically relies on thermal analysis methods, such as Thermal Extraction Desorption-Gas Chromatography/Mass Spectrometry (TED-GC/MS), Pyrolysis-Gas Chromatography/Mass Spectrometry (Pyr-GC/MS), and Differential Scanning Calorimetry; or through chemical methods such as Inductively Coupled Plasma Mass Spectrometry and chemical extraction combined with Liquid Chromatography and UV detection. Due to the limitations of infrared spectroscopy, Raman spectroscopy, and thermal analysis methods
[47], and because the detection methods developed for petroleum-based MPs may not be applicable to BMPs, related research is relatively scarce. The main quantitative detection techniques for BMPs in environmental samples that have been reported include Pyr-GC/MS, TED-GC/MS, and Liquid Chromatography-Tandem Mass Spectrometry (LC-MS/MS) (Table 2).
Pyr-GC/MS is an emerging technology for the qualitative and quantitative analysis of plastics in environmental samples. It has no restrictions on the size and shape of plastic particles, non-polymer materials do not interfere with plastic identification, and it can simultaneously identify and quantify multiple polymers. This technique requires a small sample amount, and the sample preparation process is simple, without the need to separate individual particles, thus improving analytical efficiency and reducing errors caused by density and optical separation, making it a cost-effective analytical method
[48]. This method has been effectively applied to the detection of MPs in various complex samples, including sediments, freshwater organic matter, and seawater. Pyr-GC/MS can also be used for BMPs in environmental samples, such as for analyzing PLA films, where it identified and quantified PLA thermal degradation products
[49]. Additionally, Okoffo et al.
[26] combined pressurized liquid extraction with Pyr-GC/MS to target the identification and quantification of five BMPs (PLA, PHA, PBS, PCL, and PBAT) in environmental samples, with limits of quantification (LOQ) ranging from 20 to 50 μg/g. However, Pyr-GC/MS is a destructive method that cannot provide the particle size distribution and shape of BMPs, and its LOQ is relatively high, mainly used for qualitative identification of polymers, with limited application in quantitative analysis.
For quantitative issues, Wang et al.
[50] proposed a new method for the quantitative analysis of PLA-MPs using alkaline-assisted thermal depolymerization combined with LC-MS/MS. This method, after removing background monomer compounds, efficiently depolymerizes PLA-MPs into lactic acid and infers the content of PLA-MPs by detecting the concentration of the product, lactic acid, with LC-MS/MS. The limit of quantification is 18.7 ng/g, and the spiked recovery rate is 93%. Compared to Pyr-GC/MS, the in-situ depolymerization and LC-MS/MS analytical methods have higher sensitivity but can only detect a single type of polymer. Additionally, this method also does not require the separation of MPs from the sample, ensuring the recovery rate of the samples.
TED-GC/MS rapidly identifies and quantifies MPs in different environmental samples by detecting polymer-specific thermal decomposition products (marker compounds). Compared to conventional MP detection techniques, the advantage is that it does not require time-consuming sample preparation steps, and the analysis only takes 2-3 hours
[51]. A recent study found for the first time when using TED-GC/MS to analyze PBAT and PLA that TED-GC/MS can accurately detect non-degradable polymer standards, but some degradable polymer markers can only be clearly identified at high concentrations
[52]. For example, in organic-rich activated sludge samples, two different marker compounds of PLA and PBAT were both interfered with, resulting in their average detection limits being 1.41-7.18 μg and 0.84-20.46 μg, respectively. The two degradable polyesters are significantly affected by the environmental matrix, possibly due to co-elution with the degradation products of the environmental matrix and potential interactions during the pyrolysis process. Therefore, compared to Pyr-GC/MS and LC-MS/MS, this method is not yet suitable for the quantitative detection of BMPs in complex organic systems.
In summary, spectroscopy is simple to operate and cost-effective, providing information on chemical structures and suitable for rapid, preliminary qualitative analysis. However, its sensitivity and resolution are limited, making it difficult to detect small particles such as nanoplastics. Mass spectrometry is often used for high-sensitivity quantitative analysis and can simultaneously analyze multiple MPs and their degradation products in a sample, making it suitable for the detection of complex samples. However, this method usually requires the destruction of the sample and involves more complex data analysis. Therefore, the choice of method should be based on specific needs, such as the nature of the sample, the required detection accuracy, time, and cost. It is evident that the current techniques for identifying and analyzing BMPs are not yet perfect. The primary challenge is how to effectively extract BMPs. In the future, methods for extracting BMPs from different types of samples (such as water, soil, air, and biological samples) should be developed, with an aim to improve extraction efficiency and recovery rates, overcoming the technical difficulties of the current methods, which are complex and have poor recovery rates. Additionally, efforts should be made to further enhance the accuracy and sensitivity of qualitative and quantitative detection of BMPs. In the future, a combination of instruments such as electron microscopes, particle analyzers, Raman spectroscopy, and mass spectrometry could be utilized to improve the accuracy of BMP identification and achieve high-throughput detection of different types of BMPs. Ultimately, the goal is to develop an advanced and highly integrated set of technical methods to meet the current demands for precise and efficient identification and quantification of BMPs.
2.2 Environmental Occurrence Of Bmps
According to the type of polymer and environmental conditions, BMPs formed by BPs may persist in the natural environment for decades. On one hand, the biodegradation process under natural conditions is slow; on the other hand, BMPs particles are mainly composed of microcrystals, and the crystal structure is not easily accessible to aging media, leading to a lower biodegradation rate, and thus they can exist for a long time
[53]. These BMPs typically originate from agricultural applications and the use of daily products, such as mulch films, packaging materials, food service products, cosmetics, and textiles, which bring pollution to ecosystems. Based on the limited research on the concentration of BMPs in the current environment, these materials are not uncommon in aquatic environments, with PLA, PHA, and PBAT being identified in clean water, wastewater, sewage sludge, and marine sediment samples
[54], among which PLA has the highest content, with a concentration range of 90~180 μg/g (Table 3).
Compared to traditional plastics, the degradation of BPs is more easily influenced by various factors, and their degradation rate may be accelerated, thus producing a large amount of BMPs
[55]. Lambert and Wagner
[56] studied the MPs released from four types of traditional plastic materials (PP, PE, PS, PET) and biodegradable PLA plastic under 30 ℃ UV irradiation. They found that after 112 days, an increase in the number of MPs was detected in all types of plastics, with PS plastic lids and PLA cups showing the highest increase in micron-sized particle release. Wei et al.
[53] compared the degradation processes of biodegradable PBAT and traditional LDPE in different aquatic environments, finding that PBAT produced more MPs than LDPE. This is because, in addition to microbial decomposition, UV degradation, oxidation, and erosion also collectively promoted the formation of BMPs. Weinstein et al.
[57] compared the degradability of biodegradable PLA with traditional PET, HDPE, and PS products in a salt marsh. After 4 weeks of natural weathering, MPs and biofilm were detected in all types of plastics. This indicates that under natural conditions, there is not much difference between BPs and traditional plastics in terms of degradation and MP production. Therefore, the degradability of BPs does not eliminate BMPs but rather has a greater potential for BMP accumulation.
In addition, due to the degradability of BPs, the impact of BMPs on the ecological environment should be considered as both physical and biochemical inputs
[58]. For example, BMPs in the soil may serve as a carbon source for microorganisms, affecting the composition and function of soil microorganisms. The release of degradation byproducts from BMPs can also have more far-reaching effects. Currently, research on the environmental behavior of BMPs is not comprehensive, and more experimental data are needed in the future to further investigate the environmental presence and ecological risks of BPs.
3 Advances In The Study Of Potential Environmental And Health Risks Of Biodegradable Plastics
Given the ubiquity of MPs, which have posed serious threats to the global ecological environment and human health, BPs are regarded as a potential solution to the environmental crisis caused by traditional plastics
[59]. During their use, BPs still inevitably impact the environment, and their ecotoxicity and health risks have attracted widespread attention in academia. Contrary to expectations, complete degradation of BPs requires specific conditions (such as high temperature, high humidity, and oxygen-rich environments). Therefore, under natural conditions, the degradation time of BPs is much longer than anticipated, and this process can generate a large number of BMPs and even biodegradable nanoplastics (Bio-Nanoplastics, BNPs). BMPs and BNPs, along with the toxic chemicals they carry or adsorb, can pose unpredictable risks to the environment and organisms
[60]. Additionally, compared to traditional plastics, the smaller particles produced by the faster breakdown of BPs, due to their increased reactivity and mobility, may have more profound impacts on ecosystems and humans
[61-62]. Therefore, this section will explore the toxic effects and potential mechanisms of BPs on various levels of organisms in the ecosystem, covering from primary producers to humans (Figure 4).
3.1 Potential Ecological And Environmental Risks Of Biodegradable Plastics
A stable food chain is a key factor in maintaining the balance of species numbers in an ecosystem. Primary producers, as the starting point of the food chain, play a crucial role in food supply and ecosystem functions. Research has found that BMPs and BNPs in the environment will have direct or indirect effects on the growth, physiology, and development processes of aquatic or terrestrial primary producers
[23] (Table 4).
In aquatic environments, the impact of BMPs on aquatic primary producers is mainly manifested as inhibition of algal growth, damage to cellular structures, and metabolic disorders
[23]. Studies have found that within a wide concentration range (10-1000 mg/L), PLA-MPs and PBS-MPs inhibit the growth of microalgae, and the severity of the inhibition is positively correlated with the exposure concentration and duration. Higher concentrations of BMPs promote the production of chlorophyll and extracellular polymers in microalgae, which may be a defensive mechanism induced by stress
[63]. In another study, Microcystis exposed to 10-200 mg/L of PLA-MPs exhibited oxidative damage and changes in cell morphology. With prolonged exposure, the growth of Microcystis was initially inhibited but then promoted
[64]. Therefore, BMPs can act as both a potential inorganic carbon source that promotes algal growth and as particulate matter that hinders algal growth, playing a dual role. Cyanobacteria suffer varying degrees of toxic damage when exposed to submicron and nanoscale PCL (polycaprolactone) and its oligomers. When exposed to different fractions of PCL-MPs or their combinations, both organisms showed excessive reactive oxygen species (ROS) production, changes in intracellular pH, and altered metabolic activities. However, alterations in membrane potential and morphological damage were observed only when exposed to a combination of PCL-NPs and PCL oligomers
[65]. Similarly, Yoshinaga et al.
[66] exposed brown algae to PCL-MPs particles or oligomers and found that damage to the plasma membrane and energy metabolism disorders were observed only in the oligomer treatment group. Thus, different sizes and polymerization degrees of BMPs significantly affect their toxicity levels.
From plankton to large marine animals, all can ingest MPs, which can lead to physical blockages and chemical damage
[62]. This ingestion not only reduces their feeding activities and disrupts the balance of intestinal microflora but also impairs their reproduction and growth. The mortality rate of Daphnia magna significantly increased when exposed to 5 mg/L PLA-MPs, and long-term exposure to concentrations of 1 and 5 mg/L PLA-MPs led to a reduction in offspring, changes in sex ratios, and an increase in malformed embryos
[67]. Viel et al.
[68] focused on the impact of PBS, PBSA, PCL, PHB, and PLA on marine invertebrates. They found that three types of BMPs affected embryonic development; PHB-MPs and PLA-MPs disrupted the first mitotic division of eggs and caused delayed development in other eggs, while PCL-MPs resulted in embryo malformations. Additionally, de Oliveira et al.
[69] used zebrafish experiments to demonstrate the accumulation of PLA-MPs in larvae, which was associated with altered locomotion and exploratory activities, as well as the inhibition of acetylcholinesterase function. Exposure to 100 mg/L PGA-MPs and PLA-MPs led to developmental delays in zebrafish 24 hours post-fertilization, reduced survival rates, affected sleep patterns, and induced anxiety-like behaviors
[70]. After combined exposure to PLA-MPs and PBAT-MPs, zebrafish exhibited shallow water behavior, anxiety-like responses, and avoidance behaviors. BMPs may cause behavioral abnormalities in zebrafish by triggering brain immune dysregulation, leading to neurotoxicity
[71]. Long-term ingestion of PLA-MPs significantly affected the social behavior of juvenile sea bass, characterized by decreased mobility, reduced internal clustering distance, and weakened active predatory responses
[72]. BPs also affect the visual function of zebrafish, with exposure to PLA-, PBS-, PHA-, and PBAT-MPs causing changes in early visual development-related genes in zebrafish larvae
[73]. Furthermore, compared to traditional plastics, some aquatic organisms, such as zebrafish and barramundi, showed a stronger preference for ingesting PLA-MPs
[74], and intestinal damage is another significant effect of BMPs. Duan et al.
[75] reported that after 90 days of exposure to PLA- or PET-MPs, adult zebrafish had higher residual amounts of PLA-MPs than PET-MPs. Although the degradation products of PLA can be absorbed by zebrafish as a carbon source, PLA-MPs cause more significant histological damage to fish intestines. Bao et al.
[76] similarly found that compared to traditional plastic PVC-MPs, tilapia that ingested PLA-MPs showed more pronounced intestinal tissue edema and significant dysbiosis of the gut microbiota.
For terrestrial environments, agricultural plastic mulch films are a significant application scenario for BPs, and thus the impact of BPs on crops and terrestrial fauna is worth noting. On one hand, BMPs adversely affect primary producers by altering soil properties and ecological functions
[77]. For example, BMPs can influence the nitrogen cycle, such as nitrogen fixation and anaerobic denitrification, mediated by soil microbial communities, their functions, and diversity, thereby affecting crop growth. Some residual agricultural mulches significantly reduce expressions related to carbon and nitrogen cycling in the soil, impacting soil fertility
[78]. On the other hand, BMPs may have direct toxic effects on plants. BMPs might cause mechanical damage to plants, leading to the production of reactive oxygen species (ROS) in plant cells. The accumulation of these ROS could damage plant cell membranes and other cellular structures, triggering a series of oxidative stress responses
[79]. Yang and Gao
[80] reported that MPs induced oxidative stress in rice stems and roots, with the genes encoding ammonium and nitrate transporters in the roots of the PBAT-exposed group being downregulated during the vegetative stage and upregulated during the reproductive stage. BMPs significantly inhibited the net photosynthetic rate during the vegetative period and reduced the expression of light response-related genes. Additionally, studies have focused on the impact of BMPs on seed germination and early growth of crops. For instance, compared to PP-MPs, PLA-MPs and PHB-MPs in the soil have a more pronounced inhibitory effect on the early root growth of sorghum, watercress, and mustard
[81]. Biodegradable mulches, similar to PS and PE mulches, also lead to decreased seed germination rates, reduced leaf area, and lower plant height
[82-83].
Similar to aquatic environments, BMPs and BNPs in soil environments may spread widely through the food chain. Compared to plants and microorganisms, animals, as the main consumers in terrestrial ecosystems, ingest more MPs either directly or through the food chain (by consuming food containing MPs and NPs), thus facing greater risks
[84]. An increasing amount of recent ecotoxicological evidence indicates that BMPs have toxic effects no less than those of traditional MPs, and their safety still needs further evaluation (
Table 4).
Earthworms are a common type of soil invertebrates, often used to evaluate the ecotoxicity of environmental pollutants. Ding et al.
[85] studied the effects of PLA-, PC-, and PE-MPs on the bio-toxicity, mortality, avoidance behavior, and reproductive responses of earthworms, finding that the concentration of BMPs had a greater impact than the type of plastic. Zhao et al.
[86] investigated the differential responses of earthworms to PLA-, PVC-, and PE-MPs in soil environments, revealing that the ecotoxicity of BMPs was time-dependent. After 28 days of exposure at a concentration of 50 g/kg, phenomena such as mucosal vacuolization, longitudinal muscle disorder, and granular lipid deposition appeared in the epidermis of earthworms. This study also emphasized that BMPs have similar toxicity to traditional MPs. Another study using fruit flies as a model explored the internalization and related impacts of PLA-NPs. They found that PLA-NPs were enriched through enzymatic vesicles, crossed the intestinal membrane, and were ultimately internalized by enterocytes, with their exposure triggering intestinal damage, oxidative stress, DNA damage, and inflammatory responses
[87]. Additionally, in a dragonfly nymph model, PLA-MPs showed greater toxicity than traditional plastics, causing redox imbalance and neurotoxic effects
[88].
3.2 Potential Health Risks Of Biodegradable Plastics
MPs are ubiquitous in the environment, capable of migrating between different environmental media, and can enter the human body through the food chain, direct inhalation, or skin contact
[89-90]. Based on food consumption estimates, each person may ingest 39,000 to 52,000 plastic particles annually
[91]. A large number of studies have detected MPs in human biological samples (including excreta, body fluids, and tissue organs), which has raised widespread health concerns
[92]. Similarly, with the widespread promotion and use of BPs, humans inevitably come into contact with BMPs and BNPs.
Despite many previous studies using lower organisms such as invertebrates and zebrafish to show that the toxic effects of BMPs are comparable to those of traditional plastics, the sensitivity and response patterns of these organisms are significantly different from those of terrestrial mammals, making it impossible to fully and accurately reflect the impact of BMPs on complex biological systems
[93]. To gain a more comprehensive understanding of the potential impact of BMPs on terrestrial mammals and human health, some recent studies have used mammals as models for investigation. Our previous work compared the toxicity differences of irregularly shaped PLA- and PVC-MPs on growing mice, showing that the biotoxicity of PLA-MPs is not inferior to that of PVC-MPs, and has a more significant impact on lipid metabolism and the digestive system. Specifically, both types of MPs led to increased oxidative stress levels in mice, changes in gut microbiota, and alterations in gene expression in the liver and colon. These findings raise questions about the biosafety of BPs and BMPs
[94]. In another study, male mice exposed to PLA-MPs for 10 consecutive days showed signs of liver damage and increased inflammation. This process was mainly due to PLA-MPs inhibiting the activity of CYP7A1 enzyme and activating the FGF-JNK/ERK signaling pathway, thereby disrupting normal bile acid metabolism
[95]. Zha et al.
[96] used multi-omics approaches to investigate the hepatotoxic effects of PLA-MPs and PLA-NPs from different exposure sources, finding that food-derived and airborne PLA-MPs induced hepatotoxicity by disrupting the "gut microbiota-liver" axis and the "airway microbiota-lung-liver" axis, respectively, leading to dysbiosis in the gut and lungs, metabolic changes in the gut, lungs, and serum, and alterations in the liver transcriptome.
As a type of degradable polyester, the transformation process of BPs in the intestine and the toxic effects caused by their degradation products have attracted attention. Wang et al.
[19] reported that PLA-MPs can be degraded by lipase in the gastrointestinal tract, producing an excess of PLA-NPs. These nanoparticles are formed by self-aggregation of oligomers driven by hydrophobic interactions, and after exposure, the oligomers and associated nanoparticles can bioaccumulate in different organs (including the liver, intestine, and brain), causing intestinal damage and inflammation. Liang et al.
[97] conducted a 28-day oral gavage study on mice and found that incomplete degradation of PLA-MPs in the gastrointestinal tract increases their bioavailability and toxicity, thereby exacerbating neurological damage. The mechanism of neurotoxicity induced by these MPs is similar to that observed in the pathology of Parkinson's disease, characterized by mitochondrial calcium overload in the midbrain region. In the past, the focus has been mainly on BPs themselves, but the above findings suggest that the degradation intermediates of BPs may also be potential contributors to polymer toxicity. Environmentally friendly biopolymers are typically composed of endogenous monomers, which can result in oligomers with the ability to interfere with the metabolism of organisms, leading to various toxic effects
[18]. Oligomers have complex structures and significant differences in physicochemical properties, with their types and numbers far exceeding those of the MPs themselves. Currently, our understanding of oligomers is just the tip of the iceberg. Therefore, future research should pay special attention to and conduct in-depth studies on the absorption, distribution, metabolism, excretion, and toxicity (ADMET) of BP oligomers.
Compared to mammalian models, in vitro cell models provide a rapid, cost-effective, and easily controllable method for multi-endpoint toxicity assessment and can to some extent simplify the exploration of toxicity mechanisms
[98]. These cell models not only help researchers quickly predict and assess the potential impact of these new materials on human health but also contribute to the development of relevant safety standards and regulatory policies (Table 5). Banaei et al.
[99] used the Caco-2 and HT29 colon adenocarcinoma cell lines to study the behavior of PLA-NPs released from tea bags in the intestinal epithelium, finding that these particles could be internalized and absorbed by intestinal cells. García-Rodríguez et al.
[100] used an in vitro bronchial epithelial model with the Calu-3 cell line and found that PLA-NPs were easily internalized by the barrier under air-liquid interface exposure conditions, leading to increased barrier permeability, increased mucus secretion, and significant genotoxicity and immune responses after long-term exposure. Additionally, as part of the innate immune system, macrophages play a crucial role in defending against foreign particles. Studies have shown that PLA-NPs lead to decreased phagocytic activity in macrophages and increased secretion of pro-inflammatory factors, such as TNF-α
[101]. The degradation intermediates of PLA, oligomers, are more pro-inflammatory than MPs themselves
[19]. In terms of neurotoxicity, Yoshinaga et al.
[66] found that, compared to PCL-MPs, their oligomers (tetramers) and monomers had stronger toxicity to PC12 cell-differentiated neurons, leading to decreased cell viability, impaired mitochondrial function, and changes in neuronal morphology. In another in vitro high-throughput screening study, methanol extracts of PCL activated five toxicity endpoints, including the pregnane X receptor (PXR/PXRE) and peroxisome proliferator-activated receptor γ (PPARγ/PPRE). The study suggests that this toxicity may be due to oligomers and nanoparticles released by PCL
[102]. Such multi-endpoint in vitro reporter bioassays combined with non-targeted chemical analysis have significant application value in assessing the safety of BPs, especially for screening key toxic components in plastic-related chemical mixtures. Researchers have used in vitro models, including yeast, luminescent bacteria
[103], fish cell lines
[104], and mammalian cell lines
[105], for high-throughput toxicity assessments of BP consumer product extracts. The results show that BPs exhibit significant toxicity, primarily caused by BMPs, BNPs, oligomers, and additives generated during their degradation. Previously, the focus was mainly on the effects of BMPs, but an increasing number of studies indicate differences in toxicity between BMPs and their degradation byproducts. Future research should further clarify the main sources and mechanisms of these "plastic toxicities."
Although there is currently direct evidence showing the distribution patterns and content levels of BMPs and BNPs in human tissues, recent epidemiological studies have indicated a close relationship between the content of MPs in the human body and cardiovascular diseases and intestinal inflammation. These findings are consistent with the results from animal and cell experiments. Unlike traditional plastics, BPs are more easily degraded in the body, producing smaller BNPs and oligomers, which can more readily enter the circulatory system through the gastrointestinal tract, penetrate biological barriers, and enter cells, potentially leading to more severe health risks such as intestinal inflammation. Additionally, during the degradation process, the additives contained within are more easily released, contributing to toxic effects on the body along with the polymers. On the other hand, the surfaces of BMPs and BNPs are rougher and contain abundant oxygen-containing functional groups and hydrogen bonds, suggesting that these characteristics make them more likely to bind with specific proteins in bodily fluids, forming a "protein corona." This will confer new biological properties to the nanoparticles, thereby affecting the interaction patterns, toxicity, and potential mechanisms of plastic particles with cells
[106].
Therefore, discarded BPs and their BMPs are not only emerging environmental issues but also significant public health concerns that require special attention and in-depth research on their impacts on the ecological environment and human health in the future. In particular, BMPs may have different mechanisms for disrupting organisms at various trophic levels. In addition to elucidating the environmental and health effects caused by BMPs, future research should also delve into the toxic mechanisms to provide new ideas and directions for the development of pollution control measures for BPs.
4 Conclusion and Prospects
Using BPs to replace traditional non-biodegradable plastics is one of the key measures to alleviate the problem of plastic pollution, and it has received high recognition and promotion at the national level in major countries worldwide. So far, an increasing number of industries both domestically and internationally have widely applied BPs, leading to some improvement in plastic pollution, including white pollution. Despite the broad prospects for BP applications, their practical use and disposal still pose significant ecological and health risks that cannot be ignored, especially the composite effects caused by BMPs, oligomers, and the release of toxic additives during degradation. The main environmental and health risk issues and challenges that urgently need to be addressed are: (1) how to help the public develop a correct understanding of the environmental behavior and potential hazards of BPs and BMPs; (2) how to establish efficient and simple methods for extracting BMPs from different environmental media and biological samples, and accurately identifying them with high throughput; (3) what are the health effects and potential mechanisms of long-term exposure to environmentally relevant doses of BMPs; (4) based on full consideration of the opinions of governments, industry, and the public, how to develop suitable management methods and standards for future BPs and BMPs. The specific issues are discussed as follows (Figure 5).
4.1 The Public Has Insufficient Awareness Of Bps And Bmps
The main reason why BPs have been widely promoted and used is that their core concept is degradability and environmental friendliness. When countries popularize and promote the use of BPs, they usually focus on highlighting their advantages while often neglecting the disadvantages and potential hazards of BPs. As a result, the public tends to believe that BPs can permanently solve the pollution and harm caused by traditional plastics. More seriously, this further deepens people's cognitive biases, making them firmly believe that BPs are environmentally friendly and can be discharged directly without any treatment. This leads to a large amount of BPs being discharged into the environment in an unregulated manner, thereby increasing the content of BMPs in the environment.
Therefore, in the future, before identifying, assessing, and addressing the potential risks of BPs and BMPs, it is first necessary to help people gain a comprehensive and in-depth understanding of the advantages and potential risks of BPs, especially the complete environmental behavior of discharged BPs. Only by correctly recognizing the potential risks of excessive BP emissions can we reduce the emission of BPs at the source and promote the true environmental friendliness of BPs.
4.2 Suitable Bmps Extraction And Detection Method Gaps
Similar to traditional refractory microplastics (MPs), biodegradable microplastics (BMPs) have been detected at various concentration levels and in different types in typical environmental media such as water, soil, and air. However, there is still a lack of reliable technologies to accurately identify the intrinsic environmental behavior patterns of BMPs. The current identification techniques for BMPs are derived from those used for traditional refractory MPs and have not yet formed a technical methodology system suitable for BMPs. In traditional pretreatment processes, the use of strong acids and bases may lead to an accelerated degradation of BMPs to some extent, thereby underestimating their actual presence in real environmental media and biological samples. On the other hand, BMPs are often mixtures, which directly leads to difficulties in obtaining standard testing samples. Therefore, compared to traditional MPs, the analytical detection process for BMPs is more complex. Based on existing technologies such as microscopic Raman spectroscopy, microscopic infrared spectroscopy, and chromatography-mass spectrometry, how to quickly and accurately qualitatively and quantitatively determine the types and contents of BMPs remains one of the main challenges in current and future BMP-related research.
4.3 The Long-Term Migration, Transformation Rules, and Health Risks of Bmps in the Body Need to Be Clarified
BMPs present in environmental media can be ingested by animals and humans through oral (drinking, eating) and respiratory routes. In addition, the increasing application of BPs in food processing and packaging indicates a rising trend in the variety and content of BMPs that may be inadvertently ingested in the future. However, to date, there is still limited understanding of the migration and transformation patterns of different types of BMPs within the body. Besides the need for improvement in sample pretreatment methods and in vivo qualitative and quantitative methods, the potential partial degradation of BMPs in the body further complicates their qualitative and quantitative analysis. Nevertheless, clarifying the absorption, distribution, metabolism, and excretion (ADME) patterns of BMPs in the body is a critical prerequisite for deeply interpreting their toxic effects and underlying mechanisms. Therefore, future research needs to further develop methods for investigating the migration and transformation patterns of BMPs in the body, as well as accurately identifying the ADME processes of BMPs with different properties.
More importantly, BMPs have non-negligible toxicity in the body. Compared to traditional MPs, BMPs, represented by PLA, can degrade into smaller-sized BMPs or even BNPs in the body, leading to more severe toxic effects. However, current research on the health effects of BMPs in the body is mainly focused on a few types of materials such as PLA and PBAT, with little toxicological assessment conducted on other widely produced and used BPs (such as starch-based and starch-PLA blend BMPs). Additionally, in the aforementioned in vivo and in vitro exposure experiments, there is a phenomenon of BMP exposure concentrations being higher than environmental background concentrations and exposure periods being relatively short, which does not align with the actual situation where people are exposed to low concentrations of BMPs over long periods. Therefore, future health risk assessments of BMPs in the body should prioritize those that are produced in large quantities and widely used, while also fully considering the actual environmental background concentrations and exposure durations, striving to accurately reflect and identify the long-term health hazards of typical BMPs in the body.
On the other hand, the toxicity mechanism of BMPs remains a black box. As typical particulate matter, the toxicity of MPs not only depends on their physical properties (particle size, shape, surface properties, degree of aging, etc.), but is also closely related to their chemical composition. Furthermore, different types of additives added during the production process of BMPs can be ingested along with the BMPs, further increasing the complexity of the toxicity mechanisms within the body. Therefore, future research needs to focus more on clarifying the toxicity mechanisms of different types of BMPs in the body, providing basic data for the management of BPs, and at the same time offering usable biomarkers for preventing the hazards of BMPs.
4.4 Bps And Bmps Management Methods And Governance Standards Are Lacking
The promotion of BPs has reached a global consensus. Therefore, in the future process of managing and handling BPs and BMPs, it is also necessary for government departments and scientists from various countries to strengthen communication and cooperation. When developing relevant management methods and governance standards, it is important to have a global perspective while also taking into account the local government policies and economic background. At the same time, the suggestions of local residents and businesses should be actively considered.
In summary, this paper provides a comprehensive review of the potential environmental health risks associated with BPs during their practical application, from multiple perspectives and levels. These include the application background and current status of BPs, the presence and analytical methods of BMPs generated during the degradation process in the environment, and their potential toxicity. Based on this, the paper further identifies the research gaps and suggests priority directions for future development, aiming to provide theoretical foundations and technical support for the large-scale promotion and use of BPs, as well as for reducing the corresponding environmental health risks.
National Natural Science Foundation of China(22276073)
National Natural Science Foundation of China(22376032)
Joint Fund of Colleges and Departments of Southeast University(2242024K40045)
Research Start-up Fund for New Faculty of Southeast University(RF1028623238)
Zhishan Young Scholar Fund of Southeast University by the Fundamental Research Funds for the Central Universities(2242024RCB0027)
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