Considerations of Environmentally Relevant Test Conditions for Improved Evaluation of Ecological Hazards of Engineered Nanomaterials

Abstract

Engineered nanomaterials (ENMs) are increasingly entering the environment with uncertain consequences including potential ecological effects. Various research communities view differently whether ecotoxicological testing of ENMs should be conducted using environmentally relevant concentrations-where observing outcomes is difficult-versus higher ENM doses, where responses are observable. What exposure conditions are typically used in assessing ENM hazards to populations? What conditions are used to test ecosystem-scale hazards? What is known regarding actual ENMs in the environment, via measurements or modeling simulations? How should exposure conditions, ENM transformation, dose, and body burden be used in interpreting biological and computational findings for assessing risks? These questions were addressed in the context of this critical review. As a result, three main recommendations emerged. First, researchers should improve ecotoxicology of ENMs by choosing test end points, duration, and study conditions-including ENM test concentrations-that align with realistic exposure scenarios. Second, testing should proceed via tiers with iterative feedback that informs experiments at other levels of biological organization. Finally, environmental realism in ENM hazard assessments should involve greater coordination among ENM quantitative analysts, exposure modelers, and ecotoxicologists, across government, industry, and academia.

Figure 1

Conceptual environmental release scenarios for engineered nanomaterials (ENMs) across their life cycles. Potential release sites, clockwise from upper left, include: Primary ENM manufacturing; Landfill with solid waste including nano-enabled electronics, consumer goods, and permitted industrial waste; Secondary processing or goods manufacturing sites using ENMs; Consumer (household) use of ENM-enabled products; Agricultural ENM-enabled product use; Marine or freshwater ENM-enabled product, including coatings, use; Waste treatment with aqueous effluent and solids residuals that may contain ENMs or transformation products thereof. According to the legend, colored circles adjacent to each location indicate the highest expected relative nanomaterial (NM) concentration; the NM forms are as-produced (°1) or in products (°2) or in mixed waste streams (°3); release destinations include waste infrastructure and major environmental compartments (soil, water, air).

Figure 2

Interactive “tiers” of ENM ecological hazard assessment. Screening studies (left) are recommended starting points for ENM assessments, and are typically conducted under laboratory conditions, using microcosms, batch reactors, or microplates, depending on the experiment. Examples include assessing organismal to population growth effects, bioavailability (including to communities), ENM physical behavior, and trophic transfer. If screening assay results warrant, mesocosms (upper right box) may be initiated to simulate actual environmental conditions in longer-term experiments, to determine the potential for ENMs to impinge on ecosystems. Screening assay results may also motivate determining mechanisms (lower right box) of observed effects on cells or macromolecules and to characterize biochemical, physical, and chemical interactions of ENMs with biological receptors. Knowledge gained within each tier is used to refine the approaches in the other tiers, thereby improving the relevance of each activity. Results inform development of dynamic process-based mathematical models (curved lines linking across tiers) of biological effects.

Figure 3

Conceptual exposure and effects assessments in ENM environmental risk assessment. Exposure derives from far-field emissions, transport, and transformation processes leading to environmental accumulation (bottom “wedge” of ENM complexity and concentration) of either more complex mixtures, or of specific ENMs or transformation products. Top “wedges” depict that either ENM complexity or ENM concentration can increase or decrease along the path of far-field ENM transport. At biological receptors, adverse effects are predicated on near-field exposures. Homo- and heteroaggregation (of multiple particles, here depicted as two) are particle-specific phenomena that may prevent near-field exposure. Biotic responses can influence bioavailability, and thus near-field exposures. Direct effects to biota may manifest across all levels of biological organization (subcellular, to individual, population, community, and ecosystem); effects can also be indirect, e.g., from physical effects of ENMs on nutrient availability.