Precise microplastic risk assessment requirements

In an article for the Institute of Water magazine Spring 2023 edition, WRc's Technical Director of Toxicology Dr Nabil Hajji discusses the need for further research into microplastic pollution and outlines the steps that must be followed to accurately determine the effects on human health.

Microplastic pollution has recently been recognised as a global crisis posing significant concern to the environment and public health due to their ubiquity, bioavailability, persistence, and potential physico-chemical toxicity on human health. Plastic pollution leakage into aquatic ecosystems has grown sharply in recent years and is projected to more than double by 2030. 40% of plastic is a single use that is persistent in the environment and when degraded over time, it is converted to microplastics (less than 5mm) and nanoplastics (less than 0.1mm) that can be ingested by animals and people.

The presence and toxicity of microplastics in seafood and the marine environment have been actively explored for the past decade but the information is scarce regarding the potential risks to human health associated with microplastics exposure. This has prompted the WHO and EU Commission to urge the scientific community to take greater steps to fill in the knowledge gaps.

Without an available risk assessment and associated data, this does limit the establishment of whether extensive regulatory actions are required for safeguarding public health and wellbeing worldwide. The UK Committee on Toxicity of Chemicals in Food, Consumer Products, and the Environment (COT) also echoes this concern. They recommend that research prioritises a risk assessment for microplastics by establishing standardised methods for the quantification of different microplastics in various food sources (including water), and gathering human-relevant information on absorption and accumulation, as well as profiling related toxicities.

All these authoritative bodies have highlighted an urgent need to evaluate the toxicity of microplastics to human tissues, explore the mechanisms of their cellular toxicity and to develop standardised tests for microplastics. Such information is essential to determine whether, and to what extent, microplastics pose a risk to human health, and to help in designing mitigation measures as necessary.

Why is an accurate risk assessment for micropollutants required?

Several thousand chemicals are associated with plastics. This includes distinct additives, plasticisers, pigments, antimicrobial agents, heat stabilisers, UV stabilisers, fillers, and flame retardants (i.e., polybrominated diphenyl ethers [PBDES]). 

New guidance and approaches for microplastic risk assessment that could establish the safe concentrations of microplastic chemicals in drinking water for human health is needed and not currently available.

Microplastics entry into human system

One of the major entry points of microplastic and nanoplastics into the human system is represented by the ingestion of contaminated food. Once ingested, it can have an adverse effect on the intestine, causing inflammation and potential association to the presence of inflammatory bowel diseases (IBD) and its presence in other organs including the liver, placenta, and brain, most likely being transported by the circulatory system.

Risk Assessment for microplastic requirements

Advanced knowledge of micropollutants in water systems

There are several plastic degradation factors, and the most important ones are UV, thermal and oxidative degradation. Those factors induce a very slow breaking down of the plastic into tiny microplastics pieces. 80% of plastic in the ocean originates on land. Understanding how plastic reaches oceans or other water bodies after it has been thrown away will clarify how it has degraded. It will also help us identify which materials degrade easily.

Microplastics Sampling

Filtering is the most common method of separating microplastics from water samples. WRc are currently evaluating exciting new innovations in this field, including vortex technologies such as hydrocyclones, which separate mixtures due to differences in gravity. The type of technology and filter pore size determine the size of microplastics that are captured.

Technology for Microplastics detection and characterisation

Fourier Transform Infrared spectrometer (FTIR) is a recommended method for microplastic characterisation. FTIR focuses on the differentiation and characterisation of individual chemical bands to identify the speciation of the specimen at a molecular level. FTIR is equipped with a database of many chemicals involved in the fabrication of plastics. Several materials completely absorb Infrared radiation, like water; consequently, it may be impossible to get a reliable result. Drying the filtered water sample is crucial for accurate microplastics materials characterisation.

Analytical skills in toxicological data gathering

Once the chemical associated with microplastics is discovered, its toxicological data is obtained from available public databases (i.e., WHO, ECHA, EPA and EFSA). It is important to know the toxicological study's reliability (i.e., Good Laboratory Practice [GLP]) to ensure that high-quality testing was performed. The major limitation of performing risk assessment is the lack of toxicological data available on public databases.

In silico toxicological analysis

The use of in silico Quantitative Structure Activity Relationship (QSAR) survey and Threshold of Toxicological Concern (TTC) for toxicity prediction will support the analysis of the toxicity of microplastics. The QSAR toolbox utilises a complex machine, learning algorithms to establish a relationship between chemical structure and the modelled endpoint to predict chemical toxicity.

Human tissue models

Cell toxicity models are fundamental for validating toxicity. Several models simulate human tissue and are relevant to produce a standardised risk assessment for chemicals, including microplastics chemicals.

Toxicity mechanisms for microplastics

Omics in toxicology provide a tool to characterise and quantify the molecular and biochemical changes in cells, tissues and organisms following exposure to chemicals and toxic substances. Therefore, Omics approaches (including genomics, proteomics and other -omics) are recommended when assessing the toxicological effect of chemicals associated with microplastics. The Omics output will offer much wider analysis and will clarify the major pathways involved in microplastics toxicity.

At WRc, research and evaluation are currently being carried out to identify the essential requirements for an accurate microplastics risk assessment. As there is no reliable microplastics risk assessment in existence in the world at present, WRc are working in collaboration with Queen Mary University London on a funded Innovate UK research project to fill this much needed gap. This is an essential step forward for microplastic methodology and technology worldwide.

Institute of Water magazine, Spring 2023