In an article published on May 25, 2022, in the journal Microplastics and Nanoplastics, Scott Coffin from California State Water Resources Control Board, Sacramento, CA, USA, and 12 well-known scientists in the microplastics field, report the findings of a virtual expert workshop where participants discussed the hazards posed by microplastics in drinking water. The aim of the assessment was “to evaluate the feasibility and confidence in deriving a human health-based threshold value to inform the development of the State of California’s monitoring and management strategy for microplastics in drinking water.” California adopted a definition of “microplastics in drinking water” in June 2020. As part of the strategy development, a safe limit of microplastics in drinking water shall be derived and monitoring of microplastics in drinking water was planned for 2022.
First, the scientists identified peer-reviewed studies published through June 1, 2021, relevant for deriving a human health threshold for microplastics in drinking water, using the ProQuest database, Google Scholar, and PubMed. In vivo mammalian studies were evaluated for quality and reliability in a tiered approach. Tier 1 was performed using the screening and prioritization tool and analyzing a set of 14 quality criteria covering particle characterization, study design, and usability in risk assessment. After prioritizations, in Tier 2 eight experts evaluated the reliability of various specific health outcome endpoints.
In total 66 mammalian toxicity studies were identified covering 31 in vivo oral exposure studies and 41 in vitro mammalian cell line studies. Tier 1 screening evaluation reduced the number to 12 oral exposure in vivo studies that were “potentially fit-for-purpose” and applied to Tier 2 expert evaluation. Seven of the 12 prioritized studies focused on microplastic impacts on female and male reproductive systems and five on other physiological effects. Furthermore, over 80% of the studies only investigated the effect of one microplastic polymer type and particles in sizes between 0.040 and 20 µm.
Overall, no threshold for regulatory use could be developed due to the high uncertainties, e.g., data gaps in exposure and mechanistic understanding of microplastics toxicity. However, the authors presented “a framework for developing guidance values for MPs in drinking water and tested the relative sensitivity of the framework with existing (limited) data.” Based on the framework, they deduced a “non-regulatory health-based screening level value, which contains a significant level of uncertainty.” The value can be used to determine the volume that needs to be sampled to estimate human microplastic exposure via drinking water. They recommend California’s water authorities use a volume of 1,000 l in their monitoring activities.
Coffin et al. further provide monitoring and research recommendations to allow a more precise and confident calculation of a human-health-based threshold level for microplastics in drinking water.
Leah M. Thornton Hampton from Southern California Coastal Water Research Project, Costa Mesa, CA, USA, and co-authors evaluated the knowledge gaps limiting Coffin et al. in developing a microplastic threshold more closely and provided research recommendations to address them. In their perspective article published on July 2, 2022, in the journal Microplastics and Nanoplastics, they group the recommendations into four categories to discuss them in-depth. “1) i´Improved particle selection and characterization for toxicity testing; 2) experimental designs that allow for establishing dose-response curves; 3) the connection of microplastics to established or novel adverse outcome pathways (AOPs); and 4) a clearer understanding of exposure.” According to the authors, only by closing these gaps, the potential impacts of microplastics on human health and aquatic ecosystems could be understood and effective and efficient risk assessments and management strategies for microplastics could be developed.
In a mini-review article published on June 28, 2022, in the journal Frontiers in Nutrition, Elena Molina and Sara Benedé from the Spanish National Research Council, Madrid, Spain, also emphasized the knowledge gaps that prevent a human health risk assessment of microplastics and nanoplastics. The authors differentiated between three types of hazards plastic particles can have (i) physical hazards related to their particle characteristics such as size and shape, (ii), chemical hazards related to the additives they contain as well as the pollutants and antibiotics they absorb and transport, and (iii) biological hazards related to pathogens they may carry. Given the different types, hazard characterization could be “very extensive and complex.”
After shortly outlining the human health consequences of micro- and nanoplastics, they specifically focus on the role they may play in food allergies, which has not yet been studied. Since nano- and microplastics are present in food and beverages, they are taken up orally together with the food in the gastrointestinal tract. The authors describe the potential mechanism of how the taken-up nanoplastics could mediate food allergy sensitization and dysbiosis. For instance, the particle itself could damage the intestinal mucosa increasing permeability, leading to intestinal inflammation as well as oxidative stress, and could change the composition of gut microbiota. Furthermore, “nanoplastics could favor the intestinal absorption of allergens”, as well as cause “intestinal dysbiosis, which could promote sensitization to food allergens, contributing to the exponential increase in food allergies observed in recent years.” The authors emphasized that further research is needed that investigates the presence of micro- and nanoplastics in food, the particles` absorption in organisms, and whether they “could amplify the risk of allergic sensitization to food proteins.”
Coffin, S. et al (2022). “Development and application of a health‑based framework for informing regulatory action in relation to exposure of microplastic particles in California drinking water.” Microplastics and Nanoplastics. DOI: 10.1186/s43591-022-00030-6
Molina, E. and Sara Benedé (2022). “Is There Evidence of Health Risks From Exposure to Micro- and Nanoplastics in Foods?” Frontiers in Nutrition. DOI: 10.3389/fnut.2022.910094
Thornton Hampton, L. M. et al (2022). “Research recommendations to better understand the potential health impacts of microplastics to humans and aquatic ecosystems.” Microplastics and Nanoplastics. DOI: 1 0.1186/s43591-022-00038-y
This article was originally published by Lisa Zimmermann at the Food Packaging Forum.