Environmental impacts of engineered nanomaterials—Our current understanding

The fields of nanoscience and nanotechnology have recently been advancing rapidly, increasing the possibilities for product applications. As of 2013, the Nanotechnology Consumer Products Inventory (CPI) lists 1814 products incorporating nanomaterials from 622 companies in 32 countries [1]. The nanotechnology industry is expected to continue its upward growth trajectory, reaching US $75.8 billion by 2020 [2]. As the industry continues to grow, research is lagging behind on the potential effects to the environment and human health [2]. In 2010, we conducted our first study to identify the knowledge gaps in our understanding of the environmental impact of engineered nanomaterials [3]. Large gaps were identified that would prevent sound decision-making by government or industry. In the 5 yr since that study, the scientific community around the globe has taken on the challenge to provide us with a better understanding of the ecotoxicity of engineered nanomaterials. In this letter we report how the field of study has evolved and if the gap areas identified in our previous study [3] have been filled. The literature from January 2010 to December 2015 was searched using Web of Science for studies describing the ecotoxicological assessment of engineered nanomaterials, yielding 2451 articles. Toxicology studies on mammals or mammalian cell lines were excluded from the search. Compared with 370 studies found in 2010, the number represents an increase of over 600% in the number of studies reported. Bacteria remain the most commonly chosen organisms to study the toxicity of engineered nanomaterials (36% of published studies; Table 1). Fungi, algae, and protozoa complete the microbial panel, with 22% studies using them as the target organisms. Altogether, in the period analyzed, 57.2% of the studies testing the ecotoxicologic properties of nanoparticles employed microbial cultures. This is a 15% increase from the number observed in 2010 [3]. Fish, plants, and microbial communities were the next most widely studied targets, in decreasing order, by 15%, 9%, and 8% of studies, respectively. The literature is scarce for reports on toxicity against daphnids and shrimp (4%), molluscs (1%), worms (1%), and frogs (0.4%; Table 1). No reports were available in 2010 on the toxicity of engineered nanomaterials against agriculturally important pollinators (e.g., honeybees) or birds [3], nor were such reports found in the present review. Analysis of the literature based on the engineered nanomaterials tested showed that silver was most widely studied nanomaterial (30%), followed by ZnO (14%) and TiO2 (14%) nanomaterials (Table 1). This is in sharp contrast to the results observed in 2010, where TiO2 was the most widely studied nanomaterial followed by C60 [3]. Widely produced nanoparticles such as SiO2, gold, carbon nanotubes, quantum dots, and transition metal nanomaterials form only a small fraction of the literature. The data suggest that even after years of research and resource allocation, a wide gap exists in our understanding of the ecotoxicity of engineered nanomaterials. It is imperative that the industry and the scientific community immediately widen their choices of nanoparticles and target organisms. We need to know any direct and/or indirect risks posed by the nanoparticles to all the biotic components of the ecosystem before long-term or permanent harm is done. Surprisingly, even for the target organisms that have been extensively studied, there is still no consensus within the scientific community as to whether any of the engineered nanomaterials are toxic. It is our opinion that the primary reason for this lack of consensus is the wide gap between the research setup used to study toxicity and real-world scenarios. In most of the literature, researchers have studied toxicity using nanomaterials obtained from the industry as-is or synthesized and purified in the laboratory, an environment in which the toxicity studied is simplified, and the target organism is exposed to a single dose of nanomaterials. None of these conditions hold true in the practical world. A target organism in an environment will be exposed to periodic release of nanomaterials in conjunction with other contaminants from the primary sources. The released nanomaterials will be exposed to a wide range of organic and inorganic compounds before reaching the target organism. The situation is further compounded by the fact that researchers use experimental protocols developed to study other environmental contaminants without consideration given to the properties of nanomaterials. Only when we have conclusive evidence of the risk posed by nanoparticles will appropriate environmental risk management strategies and policies be developed. Until then, the precautionary principle of risk management will limit our ability to exploit the potential benefits of engineered nanomaterials.