Inhalation Transmissibility Framework

Purpose. This document specifies the Inhalation Transmissibility Framework (ITF), a structured evidence-based methodology for assessing whether, and by which pathway, a pathogen transmits through inhalation. It is the authoritative reference for the framework’s question set, evidence handling, scoring mechanics and classification logic, and it documents the assessment software that implements them. The problem addressed. The COVID-19 pandemic demonstrated that the scientific community lacked a reproducible method for determining transmission route. Consensus on airborne transmission of SARS-CoV-2 remained contested well into 2021 despite substantial evidence, with consequences for ventilation policy, respiratory protection and public messaging. Existing approaches ask whether transmission occurred rather than how, apply evidence standards that few real-world studies can meet, and do not systematically evaluate the biological prerequisites that determine which routes are possible at all. A failure mode the framework is built against. Where a transmission route has not been studied, or has been studied by methods incapable of detecting it, frameworks that represent evidence as a single signed quantity return the same result as they would for a route studied and excluded. Absence of evidence is thereby reported as evidence of absence. This is not a hypothetical concern: it is the mechanism by which airborne transmission of SARS-CoV-2 was for a period treated as unproven, and by the same standard the airborne transmission of measles and tuberculosis would also fail to qualify. The ITF is constructed so that this substitution cannot occur silently. Architecture. The framework comprises seven questions in two groups. Three scored biological prerequisites (Q1 respiratory tract replication, Q2 survival in airborne particles, Q3 receptor distribution) establish mechanistic capability. Two unscored anatomical context sub-questions (Q1.1, Q3.1) record where replication and receptors are located, informing interpretation without contributing to any determination. Three transmission pathway questions (Q4A natural, Q4B procedure-associated, Q4C environmental aerosolisation) establish what has been demonstrated epidemiologically. Evidence is graded within four domains weighted by relevance to human inhalation transmission.