For decades, plastic pollution has been recognized as an environmental problem, but its urgency has only been acknowledged more recently. Current discussions address not only the visible, but also this invisible form of plastic pollution, the microplastic particles. The physical hazards of large plastic items are obvious and well-documented, e.g., for marine animals, but the potential hazards of microplastics for humans and the environment are much more difficult to identify. Quantification and characterization of microplastics also present unique challenges, making it difficult to assess their risk for humans and the environment. In this article we address open questions arising from the links between microplastics, food packaging and human health. More detailed information is available in the FPF dossier article (LINK) about microplastics.
Definition, types, and sources of microplastics
Microplastics are polymeric particles forming a highly heterogenous group. They differ in size, shape, surface characteristics, source, material type, and chemical composition. Typically, microplastic particles are categorized according to their size. A pragmatic definition describes all plastic particles that are smaller than 5 mm as microplastics  while a more recent definition differentiates between microplastics (range: 1-1000 µm) and nanoplastics (range: 1-1000 nm) . Exemptions are typically made for fibers which are still considered as microplastics if only the length exceeds the upper size limit of 5mm .
Depending on their source, microplastics can be grouped into primary and secondary microplastics. Intentionally manufactured microplastic particles are considered primary (for use, e.g., in cosmetics and industrial abrasives) whereas all degradation products of larger plastic items belong to the group of secondary microplastics including, e.g., fibers released from synthetic textiles, degradation products of plastic litter, and abrasion products from car tires.
Properties and analysis of microplastics
The properties of microplastics are determined by many factors, such as the chemical composition, particle size, particle shape, crystallinity, density, and surface chemistry. It is challenging to analyze all these characteristics in a sample containing many different plastic particles [4, 5]. In addition, the properties of microplastics can change over their life cycle and these changes may even affect each other (e.g., further physical degradation leads to a higher particle concentration in a sample, chemical migration results in altered stability which in turn changes physical degradation processes). At the moment, no internationally standardized analytical methods are available so far, which makes the comparison of individual studies difficult.
In order to analyze microplastic particles, it is necessary to avoid contamination in all process steps. For liquid samples, nets or filter membranes are typically used to retain microplastics. Biological samples are either investigated by analyzing the complete animal (e.g., mussels), the content of the gastrointestinal tract, or specific tissues . Solid environmental samples (e.g., soil, sand) are taken with suitable tools and commonly suspended in a saturated salt solution allowing a separation of microplastic particles based on their density. Many different chemical and enzymatic processes are applied to further remove organic and inorganic material [7, 8].
The analytical techniques applied to identify, characterize, and quantify microplastics can be broadly grouped into visual, spectroscopic, and chromatographic methods. Visual analysis under a stereomicroscope gives direct results but does not allow a comprehensive characterization of a sample . For a more precise analysis, optical vibrational spectroscopic methods can be applied to differentiate plastics from other particles and to identify the polymer types. Fourier transform infrared (FTIR) and Raman spectroscopy are common tools that do not destroy the samples and allow the analysis of particles down to 10-20 µm and 1 µm, respectively [7, 10]. Different chromatographic methods can provide information about the polymer type, additives, and the molecular mass distribution [7, 11]. However, these methods require rather big particles which are destroyed during analysis.
Human exposure and health hazards
Microplastic particles have been measured all over the planet, both in the abiotic environment and in biota. The presence of microplastics within the food chain and in processed human food contributes to oral exposure. Most available data on microplastics in the environment are on marine ecosystems, but also freshwater and terrestrial ecosystems and the atmosphere are contaminated . Microplastic particles have been measured in aquatic invertebrates, fish, seabirds, and marine mammals . Some of these animals are part of the human diet and their consumption may lead to exposure to microplastics. In addition, microplastic particles have been found in drinking water, beer, milk, honey, salt, and sugar . Food packaging and processing play a role in the contamination of food by microplastics as indicated in a few studies [15-21]. Ingestion of food and beverages contributes to human exposure to microplastics, and oral uptake via household dust and inhalation of airborne particles are further sources. However, data on the intake and presence of microplastics in humans are too scarce to allow human exposure calculations .
The effects of exposure to microplastics on human health are largely unknown, but toxicity pathways were proposed based on data from animal experiments. Microplastic particles can potentially translocate into the circulatory system and different tissues. In addition, they may result in oxidative stress, cytotoxicity, inflammation, or immune reactions, serve as carriers for harmful chemicals and microorganisms, cause lesions in the respiratory system, and disturb the gut barrier and the microbiome [22-25]. Plastic particles in the nanometer range are of particular concern as they are predicted to penetrate biological tissues more easily . Such results provide important indications of potential adverse effects, but they are not directly applicable for risk assessment of microplastics due to methodological limitations .
The use of certain primary microplastics in specified personal care products such as rinse-off cosmetics, soap, and toothpaste was banned in many countries all over the world (e.g., the U.S., Canada, New Zealand, Taiwan, South Korea, France ). In January 2019, the European Chemicals Agency (ECHA) proposed to restrict intentionally added microplastics in many consumer and professional products . While these restrictions only have a direct impact on primary microplastics, other regulatory and legal frameworks may also indirectly affect the abundance of secondary microplastics. In 2019, two reports reviewed implemented European legislation as well as ongoing policy measures that could help to prevent and attenuate microplastics in air, soil, and water. In addition, the EU plastics strategy and the European action plan for the Circular Economy both address plastic pollution, which could indirectly affect the formation of secondary microplastics.
Microplastic pollution is ubiquitous and persistent, and it is likely to increase in the future as plastic production is predicted to grow, and the formation of secondary microplastic particles occurs with a time lag . However, knowledge about the effects of microplastics on human health and the environment is limited, and standardized methods addressing the occurrence and effects of microplastics are missing. Therefore, intense discussions are currently taking place on how to assess the risks of microplastics . In any case, there is a broad consensus that specific measures should be taken to limit the future increase of microplastic particles in the environment and biota, for example, through addressing the use of plastic and preventing environmental plastic pollution.
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