The World Health Organization (WHO) has released a new report based on analysis of current research related to microplastics in drinking-water.

Background Information:

Microplastics encompass a wide range of materials composed of different substances, with different densities, chemical compositions, shapes and sizes.

There is no scientifically-agreed definition of microplastics, although they are frequently defined as plastic particles < 5 mm in length. However, this is a rather arbitrary definition and is of limited value in the context of drinking-water since particles at the upper end of the size range are unlikely to be found in treated drinking-water. A subset of microplastics < 1 µm in length are often referred to as nanoplastics.

Microplastics may enter drinking-water sources in a number of ways:

• from surface run-off (e.g. after a rain event), to
• wastewater effluent (both treated and untreated),
• combined sewer overflows,
• industrial effluent,
• degraded plastic waste and
• atmospheric deposition.

Surface run-off and wastewater effluent are recognized as the two main sources, but better data are required to quantify the sources and associate them with more specific plastic waste streams. Plastic bottles and caps that are used in bottled water may also be sources of microplastics in drinking-water.

In freshwater studies, reported microplastic particle counts ranged from around 0 to 1000 particles/L. Only nine studies were identified that measured microplastics in drinking-water; these studies reported particle counts in individual samples from 0 to 10 000 particles/L and mean values from 10^-3 to 1000 particles/L. A comparison of the data between fresh water and drinking-water studies should not be made because in most cases freshwater studies targeted larger particles, using filter sizes that were an order of magnitude larger than those used in drinking-water studies.

Particles may cause impacts in the body, depending on a range of physicochemical properties of the particle, including size, surface area and shape. However, the fate, transport and health impacts of microplastics following ingestion are not well studied, with no human studies on ingested microplastics.

Although plastic polymers are generally considered to be of low toxicity, plastics and microplastics can contain unbound monomers and additives. Hydrophobic chemicals in the environment, including persistent organic pollutants, may also sorb to the plastic particle.

Biofilms in drinking-water are formed when microorganisms grow on drinking-water distribution systems and other surfaces. Most microorganisms that are part of biofilms are non-pathogenic. However, some biofilms can include pathogens such as Pseudomonas aeruginosa, Legionella spp., non-tuberculosis Mycobacterium spp. and Naegleria fowleri.

The health risk from microplastics in drinking-water is a function of both hazard (potential to cause adverse effects) and exposure (dose). The same substance can have different effects at different doses, which depends on how much of the substance a person is exposed to and may also depend on the route by which the exposure occurs, e.g. ingestion, inhalation or injection. 

Microbial pathogens represent the most significant public health threat in drinking-water. A significant source of faecal contamination in drinking-water is inadequately or untreated wastewater. About 20% of wastewater collected in sewers does not undergo at least secondary treatment and an even higher proportion of people lack access to sewage connections or other appropriate systems for collecting and treating wastewater. Therefore, although wastewater effluent is recognized as a key source of microplastic pollution in freshwater, pathogens and other chemicals associated with the lack of effective sewage treatment are of greater concern. By addressing the bigger problem of exposure to faecally contaminated water, communities can simultaneously address the smaller concern related to microplastics.

Wastewater and drinking-water treatment systems—where they exist and are optimized—are considered highly effective in removing particles of similar characteristics and sizes as microplastics. 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. Drinking-water 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. Advanced treatment can remove even smaller particles; for example, nanofiltration can remove particles >0.001 µm while ultrafiltration can remove particles >0.01 µm.

Key Messages:

Microplastics are ubiquitous in the environment and have been detected in a broad range of concentrations in marine water, wastewater, fresh water, food, air and drinking-water, both bottled and tap water.

The data on the occurrence of microplastics in drinking-water are limited at present, with few fully reliable studies using different methods and tools to sample and analyse microplastic particles.

The potential hazards associated with microplastics come in three forms:

• physical particles,
• chemicals and
• microbial pathogens as part of biofilms.

Based on the limited evidence available, chemicals and biofilms 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 related to the physical hazard of plastic particles, particularly for the nano size particles, no reliable information suggests it is a concern.

Limited evidence suggests that key sources of microplastic pollution in fresh water sources are terrestrial run-off and wastewater effluent. However, optimized wastewater (and drinking-water) treatment can effectively remove most microplastics from the effluent.

For the significant proportion of the population that is not covered by adequate sewage treatment, microbial pathogens and other chemicals will be a greater human health concern than microplastics.

Recommendations:

Water suppliers and regulators should continue to prioritize removing microbial pathogens and chemicals from drinking-water that are known significant risks to human health.

As part of water safety planning, water suppliers should ensure that control measures are effective, including optimizing water treatment processes for particle removal and microbial safety, which will incidentally improve the removal of microplastic particles. Routine monitoring of microplastics in drinking-water is not necessary at this time.

To better assess the human health risks and inform management actions, researchers should undertake targeted, well-designed and quality-controlled investigative studies to better understand the occurrence of microplastics in

• the water cycle and in drinking-water throughout the water supply chain,
• the sources of microplastic pollution and
• the uptake, fate and health effects of microplastics under relevant exposure scenarios.

Irrespective of any human health risks posed by exposure to microplastics in drinking-water, measures should be taken by policy makers and the public to better manage plastics and reduce the use of plastics where possible, to minimize plastics released into the environment because these actions can confer other benefits to the environment and human well-being.

Useful Links:

Link to the WHO news release:

https://www.who.int/news-room/detail/22-08-2019-who-calls-for-more-research-into-microplastics-and-a-crackdown-on-plastic-pollution

Link to the WHO Report:

https://www.who.int/water_sanitation_health/publications/microplastics-in-drinking-water/en/