Critical Review pubs.acs.org/est
Potential Health Impact of Environmentally Released Micro- and Nanoplastics in the Human Food Production Chain: Experiences from Nanotoxicology Hans Bouwmeester,* Peter C. H. Hollman, and Ruud J. B. Peters RIKILT Wageningen University and Research Center, P.O. Box 230, Akkermaalsbos 2, 6700 AE, Wageningen, The Netherlands ABSTRACT: High concentrations of plastic debris have been observed in the oceans. Much of the recent concern has focused on microplastics in the marine environment. Recent studies of the size distribution of the plastic debris suggested that continued fragmenting of microplastics into nanosized particles may occur. In this review we assess the current literature on the occurrence of environmentally released micro- and nanoplastics in the human food production chain and their potential health impact. The currently used analytical techniques introduce a great bias in the knowledge, since they are only able to detect plastic particles well above the nanorange. We discuss the potential use of the very sensitive analytical techniques that have been developed for the detection and quantification of engineered nanoparticles. We recognize three possible toxic effects of plastic particles: first due to the plastic particles themselves, second to the release of persistent organic pollutant adsorbed to the plastics, and third to the leaching of additives of the plastics. The limited data on microplastics in foods do not predict adverse effect of these pollutants or additives. Potential toxic effects of microplastic particles will be confined to the gut. The potential human toxicity of nanoplastics is poorly studied. Based on our experiences in nanotoxicology we prioritized future research questions.
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are in the low micron range.10,20,21 It is questionable whether nanosized plastics can be detected with the currently used analytical methods. If nanoplastics are present in the human food production chain, potential health risks should be carefully evaluated, similar to engineered nanoparticles.22,23 This review summarizes current knowledge on the presence of micro- and, if available, nanosized plastics in the environment in relation to their potential occurrence in the human food production chain. In order to allow for an assessment of potential adverse effects on human health, current detection and characterization methods are discussed, together with indications of potential toxicity of nano- as well as microplastics. Our recent experience from nanotoxicological studies will be applied to try to advance this field of micro- and nanoplastics. Directions for future research priorities by merging these separated research domains will be proposed.
INTRODUCTION The plastic industry expanded yearly by 8.7% from 1950 to 2012, resulting in an approximate worldwide production of 288 million tonnes of plastics in 2012.1 These high production volumes, its intense use and rapid disposal are leading to an accumulation of plastic debris all over our planet. High concentrations of plastic debris have been observed in the oceans, especially in the subtropical ocean gyres,23 in (very rural) lakes4−6 and even in distinct (arctic) regions.3,7,8 Part of the plastic debris is highly visible on beaches. The discovery of the so-called plastic soup in the middle of the Pacific and Atlantic Ocean generated concern that these plastics could choke and starve (through accumulation in stomachs) wildlife. In addition, concern was raised that the plastic debris could carry a wide variety of nonindigenous and potentially harmful organisms around the planet2,3,7,8 Much of the recent concern, however, has focused on microplastics formed by fragmentation of this floating plastic debris.9 Microplastics are dominated by plastic particles smaller than 1 cm in diameter10 or ∼20 μm diameter fibrous materials.11 These plastic particles are known to absorb persistent organic pollutants12−14 and may contain up to 4% of their weight as additives such as plasticizers.15 Microplastics have been reported as carriers of these organic contaminants in marine and freshwater environments.3,16−19 Recent studies suggest that millimeter-sized particles can be nanofragmented to even smaller particles, the so-called nanoplastics.2 This poses analytical challenges as the lower size limits of the most frequently used sampling and detection techniques © 2015 American Chemical Society
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PRODUCTION, USE, AND ENVIRONMENTAL FATE OF PLASTICS Plastic production has strongly increased during the last 50 years, in 2012 world production reached a new record of 288 million tonnes (Figure 1).1,3,24 Use of plastic materials in North America and Western Europe reached about 100 kg per capita per year in Received: Revised: Accepted: Published: 8932
March 3, 2015 May 20, 2015 July 1, 2015 July 1, 2015 DOI: 10.1021/acs.est.5b01090 Environ. Sci. Technol. 2015, 49, 8932−8947
Critical Review
Environmental Science & Technology
away from civilization. Computer model simulations, based on satellite-tracked floats since the early 1990s, suggest that the plastic litter may remain in the gyres for many years.3 Plastic Particles in the Terrestrial Environment. Direct release of micro (nano) meter-sized plastic particles into the environment is getting an important source of plastic occurrence in the environment. The Nanotechnology Consumer Products Inventory of the Woodrow Wilson Institute now contains 12 consumer products containing polymer nanoparticles.27 Microplastics with a modal size of 5 mm in diameters) on the sea floor, sea surface and shorelines has been summarized by Lambert and colleagues (2014).26 Plastic debris is transported by ocean currents and will tend to accumulate in a limited number of subtropical convergence zones or gyres.2 Therefore, it may turn up thousands of miles
Figure 2. Polyethylene microplastics in a sample of toothpaste. The plastic particles have a shape that suggest they are produced by grinding larger particles. The small black dots are pores of the filter that was used to collect the samples, the scale bar is 20 μm.
also found in cleaning agents (scrubbers),11 whereas textile fibers are shed from clothes during washing.31 These fragments are transported to sewer systems but waste treatment plants are currently not able to remove these materials because they escape the filtering process.31 In addition, not all sewage water will pass through a sewer treatment plant on its way to rivers and oceans. An estimation of the per capita use of microplastics in personal care products in the U.S. population is about 1 g per year.32 However, this is very likely an underestimation because microplastics probably are mainly derived from washing-clothes, rather than from cleaning agents or personal care products. This is suggested by the proportion of polyester and acrylic fibers in microplastics in sewage effluents which points to a predominant textile origin.31 Lastly, microplastics may be transferred to the atmosphere by the wind via, for example, drying of clothes, contaminated sewage sludge employed as fertilizer, and disintegration of agricultural polyethylene foils.33 This demonstrates that atmospheric transport has to be considered as a route of microplastic contamination. The composition of plastics highly depends on their intended use, the major components of plastics are synthetic polymers. Plastic films for packaging materials (plastic bags, plastic sheets) mainly consist of low-density polyethylene. These products are easiest to escape into the environment as wind-blown debris, and are likely the major component of terrestrial plastic litter.7 PET (polyethylene terephthalate) is the major component of plastic bottles. Textile fibers have a high content of polyester, and will additionally contain acrylic polymers.31 Polyethylene is by far the 8933
DOI: 10.1021/acs.est.5b01090 Environ. Sci. Technol. 2015, 49, 8932−8947
Critical Review
Environmental Science & Technology Table 1. Plastics Production in the U.S. in 2012.35 type of polymer
generation (thousand tons)
% total production
low density polyethylene (LDPE) polypropylene (PP) high density polyethylene (HDPE) polyethylene terephthalate (PET) polystyrene (PS) polyvinyl chloride (PVC) polyactide (PLA) other total
7350 7190 5530 4520 2240 870 50 4000 31 750
23.1 22.6 17.4 14.2 7.1 2.7 0.2 12.6
presence as environmental contaminant, concerns have been raised regarding some of the nonpolymeric compounds of microplastics. For an evaluation of their environmental and health impact, two types of processes are distinguished: (1) leaching of chemicals to the ecosystem from the particles, which has been described for phthalates but also for styrene monomers.3,17−19,36−38 and (2) the possibility that persistent toxic contaminants adsorb on the surface of microplastics over time.3 Persistent organic pollutants (POPs) present in surface waters and sediments, adsorb to organic phases such as particulate and dissolved organic matter, sediments and synthetic polymers3,12−14,16−19 (also see Lambert and colleagues26). Contaminants well-known to adsorb to such particles include polychlorinated biphenyls (PCBs), polyaromatic hydrocarbons (PAHs), organochlorine pesticides (e.g., DDT, HCH), together with many other persistent organic pollutants.3,17 These contaminants generally are hydrophobic and therefore have an affinity for microplastics which is orders of magnitude higher than that for water. In addition, the small particle size (hence high surface to volume ratio) of microplastics strongly increases the amount adsorbed per gram plastic. As a consequence, microplastics efficiently extract and concentrate contaminants, a phenomenon widely used in analytical chemistry as solid phase extraction (SPE), and illustrated in a number of laboratory studies.39,40,13 Adsorption of contaminants does results in a wide distribution of contaminated microplastics (Figure 3).
largest produced synthetic polymer, and comprises more than 40% of the plastics produced (Table 1), this is reflected by the composition of microplastic debris where the top three of polymer types reported are polyethylene, polypropylene and polystyrene.10 Polymers and residual monomers are not the only constituents of plastics. Many other chemicals, named additives, are combined with the polymers in order to shape the desired physical properties of the plastics. These include plasticizers (phthalates), inert fillers, brominated flame retardants, bisphenol analogues, surfactants, additives to prevent oxidation and to enhance resistance to UV radiation and high temperatures, pigments, dispergents, lubricants, antistatics, nanoparticles or nanofibers, biocides, and fragrances (Table 2).15,26 These additives may have Table 2. Leachable Chemicals from Plastics.17,36 type of additive
function
Phthalates monomethyl phthalate (MMP) dimethyl phthalate (DMP) diethylhexyl phthalate (DEHP) butylbenzyl phthalate (BBzP) monobutyl phthalate (MBP) dibutyl phthalate (DBP)
Plasticizer
Alkylphenols trisnonylphenol phosphites (TNP) nonylphenol (NP) octylphenol (OP)
Plastizer/Stabilizer
Bisphenol A (BPA)
Monomer/Additive
Organotin Compounds mono- en dialkyltin carboxylates tin mercaptans tin sulfides
Stabilizer
Polybrominated Diphenyl Ethers (PBDEs) tetrabromobisphenol A (TBBPA)
Flame Retardant
Figure 3. Concentrations of contaminants in microplastics in the North Pacific Central Gyre (solid diamonds) and the Japanese coast of the Pacific Ocean (open circles). PCBs: Polychlorinated biphenyls; DDE: p,p’-dichlorodiphenyl dichloroethene); PAHs: polyaromatic hydrocarbons); PBDEs: polybrominated diphenyl ethers; NP: nonylphenol; OP: octylphenol; BP: bisphenol.17
Fragmenting into Microsized Plastics. Although most plastics that enter the marine environment are buoyant and float on the sea surface, there are numerous reports of sunken plastic debris of all kinds that have settled to the sea floor at all depths from intertidal to abyssal environments. Through prolonged exposure to UV light and physical abrasion, plastic debris will eventually fragment into smaller bits and pieces.6,7,9,11,26,41 Especially on shorelines, photo degradation will make plastic brittle and abrasion through wave action will enhance fragmentation. It has been suggested that the sea floor should be considered as the ultimate sink for plastic debris.42 The number of reports on the occurrence of microplastics (defined as 0.2−2 mm) has exploded in the last couple of years, and demonstrate the relevance of this fragmentation process. Studies focused on the occurrence (potential accumulation) at the sea surface (the neustonic habitat) and sediment. Only some
a high impact on the environment because of their large production volumes and the known or suspected toxicity of many of these compounds. Approximately 4% of the weight of plastics are additives.15 About half of these additives are plasticizers.34 The additives in plastics have been shown to leach out during the life cycle of the product18 and toxicity of leaching to aquatic life has been demonstrated.19 Nonpolymeric Chemicals (Additives) And Adhering Contaminants of Microplastics. Apart from their physical 8934
DOI: 10.1021/acs.est.5b01090 Environ. Sci. Technol. 2015, 49, 8932−8947
Critical Review
Environmental Science & Technology Table 3. Overview of Microplastic Occurrence Studies Showing Mean Number of Particles and Size Indications region
sample
counts
size class
Tamar Estuary
water
0.028/m3
