In a viewpoint article published on February 9, 2018 in the peer-reviewed journal Environmental Science & Technology, Henriette Selck from the Roskilde University, Denmark, and Valery Forbes from the University of Minnesota, United States, argue that the “current [environmental] risk assessment frameworks misjudge risks of hydrophobic chemicals.”
Widely used hydrophobic, fat-soluble organic chemicals include, for example, petroleum, as well as some pesticides, solvents, and other industrial chemicals. REACH requires that contaminants are assessed based on their PBT properties (persistent, bioaccumulative, toxic), and accurately estimating these parameters is crucial to performing a reliable assessment of environmental risks. Historically, estimations for the aquatic risk assessment have been based on the assumption that living organisms take up only those chemical contaminants that are dissolved and present in the water column. Therefore, all empirical and modeling investigations have been focused on exposure via water, omitting other potential exposure sources such as sediment or diet.
However, the authors argue, this water-focused approach may lead to “inaccurate risk assessments” for hydrophobic organic chemicals, because these chemicals have low water solubility and high affinity for organic matter. In nature, concentrations of such chemicals in sediment are “magnitudes higher” than in the water column. Furthermore, it has been demonstrated that hydrophobic organic chemicals associated with particles or sediments may be taken up by living organisms, and “the importance of particle ingestion increases with hydrophobicity”, meaning that more fat-soluble chemicals are ingested with particles, rather than “filtered” from water through, e.g., gills. This implies that a focus on the accumulation of these chemicals from water alone “severely underestimates” bioaccumulation and trophic transfer (via food) of these chemicals.
On the other hand, both persistence and toxicity could be overestimated by the currently applied methodology, the authors further explain. This is because the toxicity of chemicals often depends on the uptake route, with lower toxicity often observed for the dietary compared to waterborne exposure. With regard to estimation of persistence, the authors point out that eukaryotes may play a substantial role in metabolization of hydrophobic organic chemicals. Therefore, the current focus on microbial degradation only should be extended to cover these interactions as well.
The authors conclude that the water-focused approach to risk assessment of hydrophobic organic chemicals may both overestimate and underestimate risk, and “these interacting sources of uncertainty may lead to less risk chemicals being inadvertently substituted for more dangerous chemicals.” To improve the current situation, environmental risk assessments performed for hydrophobic chemicals should additionally consider sediments as well as organisms that feed on them
Recently published studies underscore the importance of the arguments put forward by Selck and Forbes. For example, John Wilkinson and colleagues from the Kingston University London, UK, measured the presence of bisphenol A (CAS 80-05-9), bisphenol S (BPS, CAS 80-09-1) and several per- and polyfluorinated substances (PFAS) in sediments and benthic organisms of several UK rivers. They highlight that “accumulation at lower trophic levels [could be] a potential source [of exposure] for higher organisms” like fish and even humans.
Zhe Lu and colleagues from the Water Science & Technology Directorate, Canada, investigated the bioaccumulation, biomagnification, and spatial distribution of substituted diphenylamine antioxidants and benzotriazole UV stabilizers in the Great Lakes of North America. They found significant differences in the accumulation of these chemicals, both within the same species residing at different locations as well as between different species, implicating differences in the diet as a major contributing factor. They hypothesize that, as has also been suggested previously, a direct uptake of these industrial additives from the plastics ingested by seabirds could play a role.
Plastics and particularly microplastics and nanoplastics created by fragmentation of larger plastic items have become a ubiquitous pollutant in the environment, as Jia-Qian Jiang from the Glasgow Caledonian University, Glasgow, Scotland, UK summarizes in his review. Olubukola Alimi and colleagues from McGill University, Canada, also reviewed aggregation and deposition of microplastics and nanoplastics in aquatic environments, and highlighted their potential to enhance the transport of other pollutants in the environment due to contaminant sorption onto plastic debris.
Notably, however, in 2016 a review and modeling estimations put forward by Albert Koelmans from the Wageningen University, together with several other scientists, suggested that “microplastic ingestion is not likely to increase the exposure” of marine organisms to hydrophobic organic chemicals, because “overall the flux . . . bioaccumulated from natural prey overwhelms the flux from ingested microplastics from most habitats.” Importantly, the contribution of nanoplastics could not be assessed due to insufficient data on their true abundance.
Henriette Selck and Valery Forbes (2018). “Current risk assessment frameworks misjudge risks of hydrophobic chemicals.” Environmental Science & Technology (published February 9, 2018).
Wilkinson, J., et al. (2018). “Spatial (bio)accumulation of pharmaceuticals, illicit drugs, plasticisers, perfluorinated compounds and metabolites in river sediment, aquatic plants and benthic organisms.” Environmental Pollution 234: 864-875.
Lu, Z., et al. (2018). “Substituted diphenylamine antioxidants and benzotriazole UV stabilizers in aquatic organisms in the Great Lakes of North America: Terrestrial exposure and biodilution.” Environmental Science & Technology 52: 1280-1289.
Tanaka, K., et al. (2013). “Accumulation of plastic-derived chemicals in tissues of seabirds ingesting marine plastics.” Marine Pollution Bulletin 69, 219-222.
Jia-Qian Jiang (2018). “Occurrence of microplastics and its pollution in the environment: A review.” Sustainable Production and Consumption 13: 16-23.
Alimi, O., et al. (2018). “Microplastics and nanoplastics in aquatic environments: Aggregation, deposition, and enhanced contaminant transport.” Environmental Science & Technology (published December 21, 2017).
Koelmans, A., et al. (2016). “Microplastic as a vector for chemicals in the aquatic environment: Critical review and model-supported reinterpretation of empirical studies.” Environmental Science & Technology 50: 3315-3326.