In April 2024, the World Health Organization (WHO) released a Global Technical Consultation Report on Proposed Terminology for Pathogens That Transmit Through the Air, proposing a significant change to how airborne transmission is described in public health. The new framework abandons the traditional size-based terms “aerosols” and “droplets” and instead introduces the umbrella descriptor “transmission through the air,” encompassing two categories: “airborne transmission/inhalation” and “direct deposition”.
The goal, WHO explained, was to reduce confusion stemming from inconsistent usage of terms during the COVID-19 pandemic. The proposed terminology reflects the understanding that infectious respiratory particles (IRPs) span a continuum of sizes and are not easily categorized by discrete thresholds. However, the changes have prompted strong reactions from experts in aerosol science, infection control, and public health, who caution that the new framework may oversimplify complex transmission dynamics and introduce new ambiguities into both scientific communication and policy.
Key Changes in WHO’s Terminology
The WHO report eliminates the longstanding “droplet vs. aerosol” dichotomy, instead proposing:
- Transmission through the air: A general category akin to “waterborne” or “bloodborne.”
- Airborne transmission/inhalation: IRPs that enter the respiratory tract through breathing.
- Direct deposition: IRPs that settle directly onto the mucous membranes of another person.
The report states that this terminology avoids misleading implications tied to specific particle sizes and distances, arguing that the physics of respiratory emissions involves complex “puff clouds” with a range of particle sizes.
Scientific Pushback: Are We Losing Necessary Precision?
Many scientists have welcomed the WHO’s effort to address outdated and overly simplistic models of transmission. However, others caution that abandoning established terminology may introduce more confusion than it resolves.
In an editorial published in Aerosol Science and Technology, a multidisciplinary group of experts—including aerosol scientists and infectious disease physicians—argued that while the false dichotomy between droplets and aerosols should be retired, it should not be replaced with an equally arbitrary divide between “direct deposition” and “airborne transmission.” They warn that the new terms do not reflect the continuum of particle behavior and offer little practical guidance for infection control strategies.
Similarly, a recent Lancet correspondence emphasized that scientific precision relies on domain-specific terminology, and “homogenizing” language for cross-disciplinary use risks eroding the very granularity required for effective research and public health response.
Concerns About Infection Control and Policy Implications
The implications of WHO’s language shift go beyond academic semantics. Infection prevention measures—including the use of respirators, room ventilation, and personal protective equipment (PPE)—are often determined based on the presumed route of transmission.
“To have these new terms be taken up by various institutions and scholars, as the authors of the report propose, will be complex and unrealistic, requiring extensive retraining and the rewriting of hundreds of documents. Pragmatically, a new set of terms will add to existing confusion, not reduce it.”
Critics argue that by eliminating references to particle size, the new terminology weakens the link between the physical behavior of pathogens and appropriate control measures. For example, distinguishing between a particle that remains airborne for hours versus one that settles quickly has direct relevance to when N95 masks or airborne isolation rooms are needed.
Furthermore, the WHO report explicitly states that considerations of “availability, access, and affordability” helped shape the terminology—a move that some experts argue conflates scientific definition with policy pragmatism. As aerosol chemist Jose-Luis Jimenez put it, “If we allow the availability of PPE to influence the definition of how a disease is transmitted, we risk never being able to protect vulnerable populations adequately”.
Relevance to Public Health Security and the National Interest
Clear and consistent terminology is essential not just for researchers and clinicians, but for public health authorities communicating with the general population. Confusion over airborne transmission during the COVID-19 pandemic delayed critical interventions such as improving ventilation and widespread mask use, contributing to preventable illness and death.
For the general public and policymakers alike, terminology influences perception of risk and expectations around appropriate protections. If the new WHO language is adopted without sufficient education and clarity, it may lead to inconsistent responses during future outbreaks—posing risks to both health security and public trust. Countries must be able to translate scientific understanding into operational decisions swiftly, and terminology plays a foundational role in that process.
Looking Forward: Progress or New Complications?
The WHO acknowledges that the report is just a first step in a broader consultation process. Yet many researchers stress that further refinement is needed. Some advocate for recalibrating, rather than replacing, the term aerosol—suggesting that a term with widespread recognition and rich scientific meaning should not be abandoned. Others argue for an “Airborne Infection Control Index” or similar tool that could align transmission risk with specific control strategies in a more practical, adaptable way.
Ultimately, while the WHO’s effort reflects progress in aligning global terminology with modern science, the debate underscores an enduring truth: words matter. How we define disease transmission influences how we study it, communicate it, and respond to it. The challenge now is ensuring that efforts to unify language do not come at the cost of clarity, scientific rigor, or public safety.
Sources and Further Reading
World Health Organization. Global Technical Consultation Report on Proposed Terminology for Pathogens That Transmit Through the Air. April 18, 2024.
Reid JP, Ferro AR, Finn A, et al. WHO report removes the aerosol/droplet dichotomy but does not move us forward in infection control strategies. Aerosol Science and Technology. 20 Aug 2024.
Greenhalgh T, MacIntyre CR, Ungrin M, et al. Airborne pathogens: controlling words won’t control transmission. The Lancet. 11 May 2024.
Lee BU. New aerosol terminology in the transmission of pathogens. The Lancet Microbe. 3 July 2025.
Molteni M. WHO expands which pathogens can be transmitted through the air. STAT News. 18 April 2024.