Microplastics in Drinking Water-WHO Report
Background Over the past few years, several studies have reported the presence of microplastics in treated tap and bottled water, raising questions and concerns about the impact that microplastics in drinking-water might have on human health. This report, which contains a summary of the evidence, key findings, recommendations and research needs, is the World Health Organization’s (WHO) first effort to examine the potential human health risks associated with exposure to microplastics in the environment. The focus of this report is on the potential human health impacts of exposure to microplastics through drinking-water. However, brief information on other routes of human exposure is included for context. This report was informed by literature reviews undertaken on the occurrence of microplastics in the water cycle, the potential health impacts from microplastic exposure and the removal of microplastics during wastewater and drinking-water treatment. Throughout the report, WHO experts examined the quality and relevance of the studies they found. The report was also informed by reviews conducted by several major entities; these are referenced throughout the report. As a category, microplastics encompass a wide range of materials with different chemical compositions, shapes, colours, sizes and densities. There is no scientifically-agreed definition of microplastics, although most definitions focus on composition and size.
Occurrence of microplastics in water
Microplastics are ubiquitous in the environment and have been detected in marine water, wastewater, fresh water, food, air and drinking-water, both bottled and tap water. Microplastics enter freshwater environments in a number of ways: primarily from surface run-off and wastewater effluent (both treated and untreated), but also from combined sewer overflows, industrial effluent, degraded plastic waste and atmospheric deposition. However, there are limited data to quantify the contribution of each the different inputs and their upstream sources. Further, the limited evidence indicates that some microplastics found in drinking-water may come from treatment and distribution systems for tap water and/or bottling of bottled water. A recent systematic review of the literature identified 50 studies detecting microplastics in fresh water, drinking-water or wastewater (Koelmans et al., 2019). The lack of viii Microplastics in drinking-water standard methods for sampling and analysing microplastics in the environment means that comparisons across studies are difficult. In addition, few studies were considered fully reliable. Nevertheless, some initial conclusions can be drawn. In fresh water, the frequency of microplastic particles by polymer type was consistent with plastic production volumes and plastic densities. A wide range of shapes and sizes were found. Only nine studies analysed microplastics in drinking-water, and fragments and fibres were the predominant shapes reported. The polymers most frequently detected were polyethylene terephthalate and polypropylene. For both freshwater and drinking-water studies, the smallest particles detected were often determined by the size of the mesh used in sampling, which varied significantly across studies. Particle counts ranged from around 0 to 103 particles/L in fresh water. In drinking-water, where smaller mesh sizes are typically applied, concentrations in individual samples ranged from 0 to 104 particles/L and mean values ranged from 10-3 to 103 particles/L. The smallest particle size detected was 1 µm, but this result is constrained by current methods. In most cases, freshwater studies targeted larger particles, using mesh sizes that were an order of magnitude larger than those used in drinking-water studies. Thus, direct comparisons between data from freshwater and drinking-water studies cannot be made.
Possible human health risks associated with microplastics in drinking-water
The human health risk from microplastics in drinking-water is a function of both hazard and exposure. Potential hazards associated with microplastics come in three forms: the particles themselves which present a physical hazard, chemicals (unbound monomers, additives, and sorbed chemicals from the environment), and microorganisms that may attach and colonize on microplastics, known as biofilms. Based on the limited evidence available, chemicals and microbial pathogens associated with microplastics in drinking-water pose a low concern for human health. Although there is insufficient information to draw firm conclusions on the toxicity of nanoparticles, no reliable information suggests it is a concern.
Particle toxicity is dependent on a range of physical properties, including size, surface area, shape and surface characteristics, as well as the chemical composition of the microplastic particle. The fate, transport and health impacts of microplastics following ingestion is not well studied and no epidemiological or human studies on ingested microplastics have been identified. However, microplastics greater than 150 µm are not likely to be absorbed in the human body and uptake of smaller particles is expected Executive summary ix to be limited. Absorption and distribution of very small microplastic particles including nanoplastics may be higher, however the database is extremely limited and findings demonstrating uptake in animal studies occurred under extremely high exposures that would not occur in drinking-water. The limited number of toxicology studies in rats and mice on ingested microplastics are of questionable reliability and relevance, with some impacts observed only at very high concentrations that would overwhelm biological clearance mechanisms and that therefore do not accurately reflect potential toxicities that could occur at lower levels of exposure. Based on this limited body of evidence, firm conclusions on the risk associated with ingestion of microplastic particles through drinking-water cannot yet be determined; however at this point, no data suggests overt health concerns associated with exposure to microplastic particles through drinking-water.
Polymerization reactions during plastic production do not generally proceed to full completion, resulting in a small proportion of monomers such as 1,3-butadiene, ethylene oxide and vinyl chloride, that can leach into the environment. Residual monomers may also arise as a result of biodegradation and weathering of plastics. However, the extent to which this occurs is uncertain. It is likely that unbound monomers resulting from these scenarios would leach into the environment, resulting in extremely small concentrations in drinking-water sources. Additives such as phthalate plasticizers and polybrominated diphenyl ether flame retardants are, for the most part, not covalently bound to the polymer and can more easily migrate into the environment. Migration can also be impacted by the molecular weight of additives, with small, low molecular weight molecules generally migrating at a faster rate than larger additives. Aging and weathering are likely to strongly influence migration, the overall impact of which is not well understood. However, relative to other emission routes of additives to the environment, it is anticipated that leaching from microplastic will be relatively small. If microplastics are ingested through drinking-water, the relative potential for the additives to leach from microplastics in the gastrointestinal tract is also poorly understood, with conflicting information reported in the limited number of available studies. It should be noted, however, that following the introduction of regulations limiting the use of many additives-of-concern from plastics, exposure is expected to become lower over time, although these substances can be present in older plastics which may degrade into microplastics in the environment. The hydrophobic nature of microplastic implies that they have the potential to accumulate hydrophobic persistent organic pollutants (POPs), such as polychlorinated biphenyls, polycyclic aromatic hydrocarbons and organochlorine pesticides. POPs indiscriminately sorb to organic carbon in the environment and therefore, the fraction of POPs sorbed x Microplastics in drinking-water to microplastics will be small relative to other environmental media such as sediment, algae and the lipid fraction of aquatic organisms. If microplastics are ingested through drinking-water, the relative potential for POPs to leach from microplastics is not well understood and will depend on a variety of factors, including the relative size of the particle, mass of chemical accumulated, relative level of contamination within the gut, and the gastrointestinal residence time of the particle. To assess potential health risks associated with exposure to chemicals associated with microplastics, WHO developed a conservative exposure scenario, assuming high exposure to microplastics combined with high exposure to chemicals and applied a margin of exposure (MOE) approach. Chemicals included in the assessment have been detected in microplastics, are of toxicological concern and have adequate or accepted toxicological point of departures to derive a MOE. MOEs were derived for each chemical by comparing the estimated chemical exposure for a very conservative exposure scenario to a level of exposure at which no or limited adverse effects were seen. A judgement of safety could then be based on the magnitude of this MOE. MOEs derived from the risk assessment were found to be adequately protective, indicating a low health concern for human exposure to chemicals through ingestion of drinkingwater, even in extreme exposure circumstances.
Biofilms in drinking-water are formed when microorganisms grow on drinking-water pipes and other surfaces. Although most microorganisms in biofilms are believed to be non-pathogenic, some biofilms can include free-living microorganisms and pathogens such as Pseudomonas aeruginosa, Legionella spp., and Naegleria fowleri. Biofilm-forming microorganisms attach faster to hydrophobic nonpolar surfaces, such as plastics, than to hydrophilic surfaces. Environmental conditions can also influence biofilm formation on plastics and microplastics. A limited number of occurrence studies in fresh water indicate the possibility that microplastics could enable the long-distance transport of pathogens and increase the transfer of antimicrobial resistant genes between microorganisms. However, there is no evidence to suggest a human health risk from microplastic-associated biofilms in drinking-water. The risk is considered far lower than the well-established risk posed by the high concentrations and diversity of pathogens in human and livestock waste in drinking-water sources. Further, the relative concentration of microplastics in fresh water is significantly lower than other particles that pathogens can adhere to in fresh water. For microplastics that are not removed during drinking-water treatment, the relative significance of microplastic-associated biofilms is still likely negligible due to the much larger surface area of drinking-water distribution systems and their subsequent ability to support more biofilms, compared to microplastics.
Treatment technologies for removing microplastics from water
Wastewater and drinking-water treatment systems—where they exist—are considered highly effective in removing particles with characteristics similar to those of microplastics. Properties relevant to removal in water treatment include size, density and surface charge. According to available data, wastewater treatment can effectively remove more than 90% of microplastics from wastewater, with the highest removals from tertiary treatment such as filtration. Although there are only limited data available on the efficacy of microplastic removal during drinking-water treatment, such treatment has proven effective in removing far more particles of smaller size and at far higher concentrations than those of microplastics. Conventional treatment, when optimized to produce treated water of low turbidity, can remove particles smaller than a micrometre through processes of coagulation, flocculation, sedimentation/flotation and filtration. Advanced treatment can remove smaller particles. For example, nanofiltration can remove particles >0.001 µm while ultrafiltration can remove particles >0.01µm. These facts combined with well-understood removal mechanisms point to the rational conclusion that water treatment processes can effectively remove microplastics. An important consideration is that wastewater and drinking-water treatment is not available nor optimized in many countries. Approximately 67% of the population in low- and middle-income countries lack access to sewage connections and about 20% of household wastewater collected in sewers does not undergo at least secondary treatment (UNICEF/WHO, 2019). In these places, microplastics may exist in greater concentrations in freshwater sources of drinking-water; however, the health risks associated with exposure to pathogens present in untreated or inadequately treated water will be far greater. By addressing the bigger problem of exposure to untreated water, communities can simultaneously address the smaller concern related to microplastics in surface water and other drinking-water supplies. Another factor to consider is how treatment waste is handled. Plastics are not usually destroyed, but rather transferred from one phase to another. For this reason, water treatment waste needs to be considered as a potential source of microplastics contamination in the environment. There are currently limited data available on how treatment wastes are handled and the impact they may have on the environment.
Managing plastic and microplastic pollution in the environment
Irrespective of whether there are any risks to human health from ingestion of microplastics in drinking-water, there is a need to improve management of plastics xii Microplastics in drinking-water and reduce plastic pollution to protect the environment and human well-being. Poorly managed plastic can contribute to sanitation-related risks and air pollution, and impact tourism and overall quality of life. If plastic emissions into the environment continue at current rates, there may be widespread risks associated with microplastics to aquatic ecosystems within a century (SAPEA, 2019), with potentially concurrent increases in human exposure. In response to concerns about the impact of plastic and microplastic pollution, public engagement and political commitment has increased. More than 60 countries are already taxing or banning single-use plastics, primarily plastic bags (UNEP, 2018). Strategies to reduce the number of plastics released into the environment are critical to the effort to minimize adverse impacts of discarded plastics. Where simple, low cost actions can be taken to make even a small difference to plastic inputs to the environment, it would be sensible to implement them. Actions could include improving recycling programmes, reducing littering, improving circular solutions, reducing the use of plastics where possible and decreasing waste inputs into the environment by industry. Care must be taken, however, when considering mitigation strategies so that addressing one problem does not simply result in the creation of a new one. This is particularly important in view of the limited data on sources of different sizes and types of microplastics, including the very small particles that are currently not well quantified. The benefits of plastic must also be considered before introducing policies and initiatives. For example, single-use syringes play an important role in preventing infections. Priority management actions should be “no regrets,” in that they confer multiple benefits and/or that they are cost-effective.
Routine monitoring of microplastics in drinking-water is not recommended at this time, as there is no evidence to indicate a human health concern. Concerns over microplastics in drinking-water should not divert resources of water suppliers and regulators from removing microbial pathogens, which remains the most significant risk to human health from drinking-water along with other chemical priorities. As part of water safety planning, water suppliers should ensure that control measures are effective and should optimize water treatment processes for particle removal and microbial safety, which will incidentally improve the removal of microplastic particles. However, for researchers, it would be appropriate to undertake targeted, welldesigned and quality-controlled investigative studies to better understand the sources and occurrence of microplastics in fresh water and drinking-water, the efficacy of different treatment processes and combinations of processes, and the significance Executive summary xiii of the potential return of microplastics to the environment from treatment waste streams including the application of sludge biosolids to agricultural land. Measures should also be taken to better manage plastics and reduce the use of plastics where possible, to minimize plastic and microplastic pollution despite the low human health risk posed by exposure to microplastics in drinking-water, as such actions can confer other benefits to the environment and human well-being. Research needs To better assess human health risks and inform management actions, a number of research gaps need to be filled. With respect to exposure, there is a need to better understand microplastics occurrence throughout the water supply chain, using qualityassured methods to determine the numbers, shapes, sizes, composition and sources of microplastics and to better characterize the effectiveness of water treatment. Research is also needed to better understand the significance of treatment-related waste streams as contributors of microplastics to the environment. With respect to potential health effects, quality-assured toxicological data are needed on the most common forms of plastic particles relevant for human health risk assessment. Further, a better understanding on the uptake and fate of microplastics and nanoplastics following ingestion is needed. Finally, given that humans can be exposed to microplastics through a variety of environmental media, including food and air, a better understanding of overall exposure to microplastics from the broader environment is needed.
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