Peer-reviewed research on nanomaterials and their toxicology has grown nearly 600 percent since the year 2000, increasing almost exponentially across the 7-year period. As noted by Lubick (2008), the scholarly literature is dispersed across a wide range of disciplines and journals. Our search of SciFinder Scholar produced approximately 900 total articles in about 58 different journals. The journals with the greatest number of relevant articles had 18 articles at most and are spread across Chemistry, Biology, Physics and Engineering fields. This is consistent with the interdisciplinarity of nanotechnology, showing that if one is to stay current on the published literature, it requires maintaining a knowledge base from a variety of different sciences.
The journal articles were also analyzed by country of origin, using GIS plots of the institution at which the research was performed. As shown in Fig. 1, the United States leads in publications on nanotoxicology with almost 550 more publications. China is the second, followed by Germany, the UK, and Japan. This diversity of where the research is performed, along with the interdisciplinary nature of the nanotechnology field, can be one explanation for the variety of approaches and lack of standard methods for studying the toxicity of nanomaterials noted by Lubick (2008); Fischer and Chan (2007).
To characterize the approaches and techniques used to study the toxicology of each nanomaterial, we relied on the ICON EHS database, searching by method of study for each nanomaterial type. While there are differences in the number of studies for each nanomaterial, as shown in Fig. 2, we found that most research has relied on in vitro techniques across all specified material types. This confirms the conclusions of Fischer and Chan (2007), Hutchison (2008), and Lubick (2008), showing that there has been a lack of in vivo studies; even fewer focus on environmental impact and fate. The low number of studies on semiconductors and the absence of any environmental studies on these materials are particularly striking, especially as these materials are closer to commercialization in nano-electronics and currently available in the form of quantum dots for fluorescent tags in cell and mice studies.Footnote 5 This is not surprising, however, considering the far lower costs of in vitro research when compared to the cost of in vivo studies.
As Fischer and Chan (2007) point out, the toxicology of nanomaterial is likely to change with not only material type, but also the exposure route. The exposure pathway would also determine the dosage, and thus has implications in the toxicity of nanomaterials. To address the question about which exposure pathways are most researched, we again utilized the ICON EHS database to characterize the articles based on material type. Our results show that most research has not specified an exposure pathway (Fig. 3). This may be reflective of the relative infancy of the field, as most studies have been concerned with acute toxicity and not with the complex interactions that would take place in an actual (human) organism. It also points to a predominant concern for traditional dose-dependent mortality without sensitivity to multifaceted and/or chronic exposure hazards and morbidity. When specified, inhalation exposure has been the most researched pathway. This may signal that much work in the field is based on seminal research dealing with exposure to fine and ultrafine particles in the context of air pollution (Oberdörster et al. 2005).
Using ICON EHS database, we also characterized the articles by the cell/organism type that would be affected by the nanomaterials. As shown in Fig. 4, most research has utilized mammalian tissues or organisms. This shows a focus on how the nanomaterial would affect humans or human models (such as mice) and again reflects less overall attention to environmental toxicity. This is particularly noticeable in the low numbers of studies of nanomaterials in soil.
SciFinder Scholar was used to explore what research has been done on nanomaterial life-cycle and to characterize at what stage the toxicity of nanomaterials has been determined. We relied on the categorization of articles by the search engine and assigned categories to one of three stages: Basic Materials, Components or Parts, or Nano-Enabled Products. As shown in Fig. 5, most articles have been concerned with the toxicity of basic materials, rather than intermediate materials or components which incorporate nanoparticles, or nano-enabled consumer products. We found no evidence of toxicological research on the environmental fate of nano-enabled consumer products in the publicly available literature.