Development of a base set of toxicity tests using ultrafine TiO₂ particles as a component of nanoparticle risk management

Abstract

The development of a risk management system for nanoscale or ultrafine particle-types requires a base set of hazard data. Assessing risk is a function of hazard and exposure data. Previously, we have suggested “parallel tracks” as a strategy for conducting nanoparticle research. On the one hand, mechanistic studies on “representative” nanoparticles could be supported by governmental agencies. Alternatively, with regard to commercial nanoparticles, the environmental, health and safety (EHS) framework would include a minimum base set of toxicity studies which should be supported by the companies that are developing nano-based products. The minimum base set could include the following criteria: substantial particle characterization, pulmonary toxicity studies, acute dermal toxicity and sensitization studies, acute oral and ocular toxicity studies, along with screening type genotoxicity, and aquatic toxicity studies.

We report here the toxicity results of a base set of hazard tests on a set of newly developed, well-characterized, ultrafine TiO₂ (uf-TiO₂) particle-types. In vivo pulmonary toxicity studies in rats demonstrated low inflammatory potential and lung tissue toxicity. Acute dermal irritation studies in rabbits and local lymph node assay results in mice indicated that uf-TiO₂ was not a skin irritant or dermal sensitizer. Acute oral toxicity studies demonstrated very low toxicity and uf-TiO₂ produced short-term and reversible ocular conjunctival redness in rabbits. Genotoxicity tests demonstrated that uf-TiO₂ was negative in both the bacterial reverse mutation test and in an in vitro mammalian chromosome aberration test with Chinese hamster ovary cells. The results of aquatic toxicity screening studies demonstrated that uf-TiO₂ exhibited low concern for aquatic hazard in unaerated, 48 h, static acute tests using the water flea, Daphnia magna; exhibited low concern for aquatic hazard in unaerated, 96 h, static acute tests using the rainbow trout, Oncorhynchus mykiss; and exhibited medium concern in a 72 h acute test using the green algae Pseudokirchneriella subcapitata.

To summarize the findings, the results of most of the studies demonstrated low hazard potential in mammals or aquatic species following acute exposures to the ultrafine TiO₂ particle-types tested in this program.

Introduction

The intent of a nanoparticle risk management framework is to implement a systematic process for identifying environmental health and safety (EHS) risks related to exposures to newly developed engineered nanomaterials. The development and forthcoming commercialization of many different engineered nanomaterial-types for uses in a variety of industrial, chemical, manufacturing, consumer, medical and diagnostic applications will present a challenge for companies and regulators to ensure the safety of products for workers and ultimately, consumers. Moreover, the development of effective and safe products containing nanomaterials should be a fundamental component of the product stewardship process. This is an integral component of the broad engagement process with stakeholders on EHS issues, contributing to the public awareness and confidence in products developed through nanoscale science and engineering.

An EHS framework is being developed to characterize the potential risks related to exposures to nanoscale or ultrafine particle-types. Of course, determination of risk is a product of both exposure and hazard assessments. However, in many cases the exposure potential cannot be ascertained, due in large part, to the current limitations of technology to measure nanoscale particle exposures in the workplace. Nonetheless, the risk management framework could include a minimum base set of toxicity (hazard) screening studies which provide a fundamental characterization of the potential hazards of the particle-types being investigated for human health and ecological effects. This base set process is modeled, in part, on the preparation of the SIDS (screening information data sets) used by OECD (Organisation for Economic Co-operation and Development) and US EPA (United States Environmental Protection Agency) for the investigation of HPV (High Production Volume) chemicals (http://www.epa.gov/chemrtk/pubs/general/sidsappb.htm http://www.epa.gov/chemrtk/pubs/general/sidsappb.pdf http://www.oecd.org/dataoecd/13/14/36045229.pdf).

This base set serves as a reference point for the type of screening information that should be addressed as a new product is being developed. The generation of a base set of information is important to sufficiently characterize the EHS impacts of nanoscale or ultrafine materials and to build the foundational data that is necessary to develop a risk assessment framework for nanomaterials.

The minimum base set is an evolving concept designed to characterize the hazards associated with exposures to nanomaterials, both in mammalian species as well as in ecological environments. Justification for these particular tests rests on the following criteria: (1) potential routes of exposures (i.e., pulmonary, dermal, oral and/or ocular); (2) screening for potential carcinogenic effects (mutation and chromosomal aberration assays); and (3) screening for potential toxic aquatic effects (exposures to rainbow trout, Daphnia, and algae). The base hazard set of tests is not meant to provide for an exhaustive assessment of toxicity, but is designed to facilitate a reasonable balance between an adequate toxicity characterization and a practical strategy for the development of new nanomaterials. Thus, the goal is to make the base set sufficiently robust in order to guide adequate risk-evaluation processes, in a manner commensurate with existing regulatory and voluntary standards.

This manuscript briefly describes the methodology and results of ten different toxicity studies conducted with newly developed ultrafine TiO₂ particle-types. Prior to the commencement of the studies, substantial physicochemical characterization of the test materials was carried out. Subsequently, hazard studies were implemented which included the following tests (see Table 1): a dose–response pulmonary bioassay study in rats with postexposure periods lasting through 3 months; a dermal irritation test in rabbits; a skin sensitization study in mice; an acute oral toxicity study in rats; and an eye irritation study in rabbits; two in vitro genotoxicity studies, i.e. a bacterial reverse mutation test and chromosomal aberration study; as well as three aquatic toxicity screening studies with rainbow trout, daphnia, and green algae. Thus, it is proposed that these studies form the basis for a minimum base set of hazard tests as a component of a nanoparticle risk management system.