The International Organization for Standardization (ISO) recently published ISO/TR 23463:2022, “Nanotechnologies — Characterization of carbon nanotube and carbon nanofibre aerosols to be used in inhalation toxicity tests.” The standard reviews characterization of carbon nanotube (CNT) and carbon nanofiber (CNF) aerosols for inhalation exposure studies. According to the standard, the framework for material characterization for inhalation studies consists of:
- Characterization of as-produced (pristine) or supplied material;
- Characterization of administered material;
- Characterization of material following administration; and
- Human exposure characterization.
The standard focuses on the first two characterization needs, which include physicochemical properties (e.g., size, size distribution, aggregation/agglomeration, and shape) and measurement of concentration (e.g., mass, number, surface area, and volume). The standard states that these parameters can be measured by direct (on-line) or indirect (off-line) methods and each technique needs specific sampling procedures. The standard notes that the limited technologies in the generation and characterization of nanofibers make it difficult to perform inhalation toxicity studies, however, “although the inhalation exposure to CNT and CNF is highly likely in the workplace, and research facilities, where they are in use.” The standard provides the current status of CNT and CNF aerosol characterization used in the inhalation toxicity tests, as well as the physicochemical properties of CNTs and CNFs and their relationship with toxicity end points. ISO notes that the standard neither provides guidance on aerosol characterization for other carbon nanomaterials, nor provides guidance for characterization of CNT and CNF aerosols in the workplace or ambient air.