Overview
ISO/TR 21624:2020 - Nanotechnologies: Considerations for in vitro studies of airborne nano‑objects and their aggregates and agglomerates (NOAA) collates current knowledge on systems and biological models used to assess inhalation-relevant exposures of airborne nano‑objects in vitro. The technical report focuses on exposure and assessment systems that simulate inhalation toxicology study design, comparing air‑liquid interface (ALI) and submerged approaches, and summarizing considerations for particle generation, characterization, dosing and choice of cell systems.
Key topics and technical considerations
- Modes of exposure
- Direct exposure (aerosol delivered to cells under submerged, intermittently submerged or ALI conditions) and indirect exposure (collection of aerosols and subsequent suspension).
- ALI systems are preferred for inhalation-relevant testing because they better reproduce airway cell phenotype and particle deposition.
- Characterization of NOAA
- Importance of monitoring particle size, aggregation/agglomeration state, life‑cycle transformations and how aerosol generation can alter particle properties.
- Choice of cell systems
- Guidance on mono‑culture, co‑culture and three‑dimensional models that reflect epithelial differentiation, mucus production and barrier functions.
- Dose selection and metrics
- Considerations for relevant concentrations, deposited dose assessment and use of in silico tools for dosimetry (e.g., to relate administered dose to deposition).
- Practical limitations
- Effects of suspending media on agglomeration, sedimentation and dissolution under submerged conditions; potential for misleading toxicity outcomes if in vitro conditions diverge from inhalation realities.
- Adverse Outcome Pathways (AOPs)
- Annex A discusses applying AOPs to design mechanistic, in vitro‑based approaches.
Practical applications
ISO/TR 21624:2020 provides a structured, pragmatic reference for:
- Designing inhalation‑relevant in vitro studies of nanoparticles and NOAA
- Selecting exposure chambers and ALI platforms for nanosafety testing
- Choosing biologically relevant cell models (mono‑ and co‑cultures, 3D) and dose metrics
- Interpreting differences between submerged and ALI exposures for regulatory or risk assessment contexts
Who should use this standard
- Nanotoxicology researchers and academic laboratories
- Industrial R&D and nanosafety teams developing inhalation exposure testing
- Respiratory toxicologists and in vitro assay developers
- Regulatory scientists and risk assessors seeking harmonized approaches for in vitro inhalation studies
- Instrument and exposure system manufacturers
Related standards
- ISO/TR 19601 (overview of aerosol generation and in vivo testing) - referenced in the report
- ISO 15900 and ISO/TS 80004 series - referenced for terminology and definitions
This technical report is a guidance/overview document (not a prescriptive protocol) aimed at harmonizing approaches for in vitro assessment of airborne nano‑objects and improving the physiological relevance of inhalation toxicity testing. Keywords: ISO/TR 21624:2020, NOAA, nanotechnologies, in vitro inhalation, air‑liquid interface, aerosol exposure, nanoparticle dosimetry, nanosafety.