Microplastics and Their Health Effects
According to the World Health Organization
1. Human Exposure
Human exposure to NMP is widely recognized as occurring predominately through the diet or by inhalation (1, 4, 10, 11, 19, 83, 84). The possibility of human exposure to MP was raised by the observation of MP in seafood, such as mussels, intended for human consumption (85, 86). Other studies have demonstrated the occurrence of MP in food and drinking-water, food packaging and both indoor and outdoor air (1, 83, 87–94). Thus, MP occur in drinking-water, a variety of foods and beverages and air, although insufficient quantitative data are available for a full exposure assessment (95). This section summarizes the data available for assessing human exposure to NMP. A continuing challenge to characterizing and quantifying concentrations is, however, the lack of standard analytical methods for identifying NMP of varying polymer composition, size and shape in foods, beverages and air (2, 3, 11, 59, 60, 96–99). Recommendations for improving sampling and analysis from the previous WHO report (1) are shown in Box 2.
1.1 Occurrence in Drinking-Water
In the first WHO report (1), human exposure to MP in drinkingwater was summarized for both tap and bottled water. In a critical review of nine studies of MP in fresh and drinkingwater, the concentrations ranged from 0 to 104 particles/L (1). As mentioned above, characterization and quantification of MP in drinking-water are limited by the lack of standardized methods, which also limits comparison of studies, even by the same group (2, 60, 100–103). Koelmans et al. (2) proposed a scoring system for assessing the quality of studies on concentrations of MP in drinkingwater. The system is based on eight criteria developed for evaluating studies of MP in samples of biota (28) according to how samples are collected, handled and analysed. A score of 0, 1 or 2 is given to each criterion. The criteria are:
- description of the sampling method used,
- sample volume and number of samples,
- sample processing and storage,
- mitigation of contamination of samples during preparation and handling,
- use of clean air conditions in the laboratory
- use of negative controls to quantify and characterize laboratory contamination,
- use of positive controls to quantify rates of recovery of particles and
- analytical verification of polymers associated with particles isolated from samples.
1.2 Occurrence in Air
1.2.1 Particulate matter in air
Particulate matter (PM) in air is a complex mixture of particles from various sources, generated both naturally and by human activity (127, 128). NMP in air are thus one component of a heterogeneous mixture of particles. The concentrations can be characterized in various ways, most commonly as mass per volume but also as particle number counts or total surface area per volume of air. “Total suspended particulate concentration” was once a routine metric for monitoring PM in air, covering a broad particlesize range distribution of 0 to about 40 μm. For the purposes of human health risk assessment, respirable PM are often defined as particles with an aerodynamic diameter of < 2.5 μm (fine particles) (127). The “inhalable fraction” refers to coarse particles with an aerodynamic diameter > 2.5 μm, which are usually defined as the fraction between 2.5 and 10 μm, although references to the “inhalable fraction” can include sizes ≤ 100 μm (127–129). Aerodynamic diameter is used as a surrogate for particle size. Ultrafine particles have a mobility diameter < 0.1 μm and do not usually contribute significantly to the total mass of particles; however, when expressed as particle number counts, particles < 0.1 μm dominate the entire respirable size range. The contributions of different sizes of PM to airborne particle mass (or number) vary substantially, as do the emission factors, including gaseous precursors, physical characteristics and chemical composition. Studies of the contribution of particles due to tyre and road wear to PM ≤ 10 µm (PM10), for instance, indicated an average contribution of about 1.9%, with a range of 0.42–2.48%.
Particulate matter (PM) in air is a complex mixture of particles from various sources, generated both naturally and by human activity (127, 128). NMP in air are thus one component of a heterogeneous mixture of particles. The concentrations can be characterized in various ways, most commonly as mass per volume but also as particle number counts or total surface area per volume of air. “Total suspended particulate concentration” was once a routine metric for monitoring PM in air, covering a broad particlesize range distribution of 0 to about 40 μm. For the purposes of human health risk assessment, respirable PM are often defined as particles with an aerodynamic diameter of < 2.5 μm (fine particles) (127). The “inhalable fraction” refers to coarse particles with an aerodynamic diameter > 2.5 μm, which are usually defined as the fraction between 2.5 and 10 μm, although references to the “inhalable fraction” can include sizes ≤ 100 μm (127–129). Aerodynamic diameter is used as a surrogate for particle size. Ultrafine particles have a mobility diameter < 0.1 μm and do not usually contribute significantly to the total mass of particles; however, when expressed as particle number counts, particles < 0.1 μm dominate the entire respirable size range. The contributions of different sizes of PM to airborne particle mass (or number) vary substantially, as do the emission factors, including gaseous precursors, physical characteristics and chemical composition. Studies of the contribution of particles due to tyre and road wear to PM ≤ 10 µm (PM10), for instance, indicated an average contribution of about 1.9%, with a range of 0.42–2.48%.
1.3 Occurrence in Food
The presence of plastic debris ingested by marine organisms was reported in a number of studies conducted in the late 1960s and early 1970s (173–181) and now includes observations of a broad range of MP in marine, freshwater and other organisms (80, 182–192). MP may be ingested either directly from the water column or sediment or indirectly by consumption of lower trophic prey that have recently ingested MP (193). The evidence indicates that MP released into the environment can enter the food chain, with implications for human exposure through consumption of seafood and fish (7, 56, 85, 194–200). In a review of studies of ingestion of plastic debris, including MPs, by > 800 species and approximately 87 000 non-human organisms, the average concentration was four particles per individual (201). Depending on the study, the concentration of MP > 500 μm was measured in the stomach and intestines, with limited data on concentrations in the tissues of fish consumed by humans.
1.4. Ingestion
Particles that are ingested are considered to be available systemically only when they are absorbed by the intestinal epithelium, pass through the liver and are distributed via the bloodstream throughout the body. A number of physiological barriers significantly limit the absorption and systemic bioavailability of particles from the gastrointestinal tract, although local absorption may occur. As discussed in the previous WHO report (1), microplastics > 150 μm ingested from drinking-water are expected to pass through the gastrointestinal tract without being absorbed. A fundamentally important physiological barrier in the gastrointestinal tract is mucus, a selectively permeable hydrogel that acts as a physical barrier to particle diffusion across the epithelial tissues. The main structural component of the mucus layer is mucin, a highly glycosylated protein with oligosaccharide side-chains that include terminal sialic acid and sulfate residues, resulting in a net negative charge (382). The average pore size of the mesh-like structure formed by the interactions of mucins is 10–500 nm. The mucus layer significantly impedes the diffusion of small particles by interaction filtering (i.e., electronic and hydrophobic interactions) and can fully block the penetration of larger particles by both steric (i.e., size) and interaction filtering. The rate of passage of particles along the gastrointestinal tract is usually effective in ensuring that most ingested particles are excreted. Szentkuti (383) studied the rate of diffusion of carboxylated fluorescent latex NP of various sizes across the mucus layer to the enterocyte surface and found that 14-nm particles passed through the mucus layer within 2 min and 415-nm particles within 30 min; however, micrometresize particles did not diffuse through the mucus. Although the author observed permeation of both 14- and 415-nm particles, none of the particles was endocytosed by the enterocytes but appeared to move in the opposite direction with mucus.
Conclusion;
Although it is known that exposure to high concentrations of PM is associated with respiratory effects, limited quantification of NMP in air obviates a robust risk assessment. Thus, research to identify adverse effects that are intrinsic to NMP would provide guidance for an NMP-specific human health risk assessment. The available data do not allow firm conclusions on the risks to human health of inhalation or ingestion of NMP, but, as NMP are part of the PM mixture, the health impacts will not exceed those of PM.