II. DESPITE THE METHODOLOGICAL CHALLENGES IN DETECTING AND CHARACTERISING PLASTICS AND THE RISKS THEY POSE TO HUMAN HEALTH, THE WARNING SIGNS ARE MULTIPLYING
A. ANALYSING PARTICULATE PLASTICS AND THE ASSOCIATED HEALTH RISKS IS FRAUGHT WITH METHODOLOGICAL DIFFICULTIES
· The methodological challenges involved in characterising and quantifying particulate plastics
These are related to the wide variety of compositions, sizes and shapes of plastics.
Analyses of mineral water and tap water produce results that vary considerably from one study to another. However, work carried out by the French standardisation organization Afnor has produced a standard for characterising microplastics in water, setting an international benchmark that makes it easier to compare results.
Detecting and quantifying particulate plastic in human samples is fraught with similar problems. A study on the amount of microplastics ingested by humans caused a stir in 201914(*), estimating it at 5 grams per week, the equivalent of a credit card. Since then, several studies have shown considerably lower amounts, without reaching a consensus. In 2022, a scientific study15(*) estimated that it would take 23,000 years to ingest the equivalent of a credit card. Another study16(*) estimated plastic ingestion at 4 micrograms per week, a million times less. A very recent study17(*) conducted in 109 countries, both industrialised and developing, showed high exposure of 500 milligrams per day in South-East Asian countries, mainly as a result of seafood consumption.
Analytical methods and processes need to be improved to avoid contaminating the samples being analysed, for example by the widespread use of plastic objects in laboratories. The proliferation of formulations for plastic materials also exacerbates the challenges of analytical work.
Given the difficulties involved in understanding plastics, the quantity of microplastics and, especially, nanoplastics in the environment is certainly underestimated.
· Nanoplastics detection remains in the early stages
A literature review in 2023 highlighted the presence of nanoplastics in certain foods, such as tea18(*) and rice19(*).
Similarly, a study20(*) - yet to be confirmed - has concluded that plastic water bottles contain 250,000 particles per litre, 90% of which are nanoplastics.
However, these particles' small size and the diversity of the environments in which they are found pose a real methodological challenge in detecting and quantifying them. For example, there is currently no technique for detecting nanoparticles in the lungs. However, researchers are interested in nanoplastics because they are likely to cross the intestinal barrier or the epithelium and enter the bloodstream to reach secondary organs.
· The limits to laboratory models
Most studies are carried out on commercial particles, which are spherical and essentially made of polystyrene, which does not reflect what is actually found in the environment.
And yet plastic particles' toxicity depends on their physico-chemical characteristics and shape. Fibres of a certain length can disrupt phagocytosis, since macrophages are unable to ingest them fully when they are too long. This can cause persistent inflammation.
The doses used in the laboratory are often very high, and the long-term effects have been relatively under-researched, in particular because cohorts have not been established. Furthermore, studies are often carried out on healthy people, when they should be extended to those at risk. For example, patients suffering from chronic inflammatory bowel disease - Crohn's disease or ulcerative colitis - were found to have more microplastics in their faeces than healthy volunteers.
Beyond these methodological limitations, the warning signs of the risks that particulate plastics pose to human health are multiplying.
* 14 In 2019, the WWF warned of the quantity of plastic ingested by humans, estimated at 5 grams per week - the equivalent of a credit card. In 2021, these results were confirmed in a study by Kala Senathirajah et al. « Estimation of the mass of microplastics ingested. A pivotal first step toward human health risk assessment. », Journal of hazardous Materials, volume 404, Part B, 15 February 2021.
* 15 Martin Pletz, « Ingested microplastics: Do humans eat one credit card per week? », Journal of Hazardous Material Letters, Volume 3, November 2022.
* 16 Nur Hazimah Mohamed Nor et al., « Lifetime accumulation of microplastic in children and adults », Environmental Science and Technology, 2021, 55, 8.
* 17 Xiang Zhao, Fengqi You, « Microplastic human diatery uptake from 1990 to 2018 grew across 109 major developing and industrialized countries but can be halved by plastic debris removal », Environmental Science and Technology, 2024, 58, 20.
* 18 Laura M. Hernandez, Elvis Genbo Xu, Hans C. E. Larsson, Rui Tahara, Vimal B. Maisuria, and Nathalie Tufenkji, « Plastic teabags release billions of microparticles and nanoparticles into tea », Environmental Science & Technology, 25 September 2019, Vol 53 (21), 12300-12310.
* 19 Pinal S. Bhavsar, Mandeep B. Solanki, Yasuhito Shimada, Sumit B. Kamble, Shashikant P. Patole, Govind B Kolekar, Anil H. Gore, « Microplastic contamination in indian rice: A comprehensive characterization and health risk assessment », Journal of Hazardous Materials, Volume 480, 5 December 2024, 136208.
* 20 Naixin Qian et al., « Rapid single-particle chemical of nanoplastics by SRS microscopy », Proceedings of the National Academy of Sciences, 16 January 2024.