commentary Focusing the research efforts

Nanomaterials have unique physical and chemical properties that are useful for various consumer and industrial applications, but these very same properties may give rise to unique biological reactivity. This has led to mounting concerns over the safety of nanomaterials, and pressure to control the potential risks. The toxicological properties of metal, metal oxide nanoparticles, quantum dots, fullerenes, carbon nanotubes, nanoclays and polymers have been investigated in many laboratories (see the Organisation for Economic Development and Co-operation list of nanomaterials prioritized for testing1). Because of the huge diversity and limited amount of nanomaterials available for testing, many of these tests are done in vitro to limit costs and the use of animals. This has resulted in numerous publications on the cytotoxic activity of different nanomaterials, ranging from pristine nanoparticles to complex industrial formulations, using different cell types and doses. Yet, progress is slow, and there are at present no standard methods for testing2. Here, we systematically reviewed published studies that report in vitro cytotoxicity of silica nanoparticles (SNPs) — a material that is widely used and studied by many, including us3 — to show the gaps in knowledge and the need to better focus our research efforts. We searched PubMed and the reference lists of retrieved publications for papers published in English and looked at records up until August 2011. We excluded papers that used SNPs > 100 nm, mesoporous SNPs, in vivo studies, exclusive genotoxicity or reprotoxicity investigations and toxicology studies for medical applications, and came up with 38 papers eligible for analysis4–41. Key data were extracted and summarized in two tables: Supplementary Table S1 contains the physical and chemical characteristics of the nanoparticles tested, and Supplementary Table S2 contains the experimental conditions and general conclusions. We wanted to know whether these 38 papers could answer some of the basic questions that should precede a hazard assessment. We postulated that to convincingly provide an answer, at least two independent studies using different cell types and different SNPs over a relevant range of doses should show consistent results. Some of the most striking outcomes are highlighted below.