Why health experts are abandoning the old distinction between aerosols and respiratory droplets

Health · Published 3 October 2026 Swipe more like this →

In short: For decades, public health policy relied on a rigid size cutoff to classify how respiratory pathogens travel. New consensus among global health agencies reveals this binary is scientifically flawed. Pathogens exist on a continuous spectrum of particle sizes, and understanding this is critical to controlling future outbreaks and pandemics.

The World Health Organization, alongside major public health agencies from the US, Europe, Africa, and China, has officially updated its terminology to address long-standing confusion in scientific communication. Following a multi-year consultation process from 2021 to 2023, experts agreed to move away from the traditional dichotomy of 'aerosols' and 'droplets.' Previously, a 5-micrometer cutoff was often used to distinguish between these categories, but researchers now recognize that infectious respiratory particles (IRPs) form a continuous spectrum of sizes. Their behavior—how long they stay suspended and how far they travel—depends on environmental factors like humidity, temperature, and airflow rather than a fixed size threshold.

Under the new framework, 'through the air' transmission is the umbrella term for pathogens traveling via IRPs. This is further divided into two specific descriptors: 'airborne transmission,' where particles are inhaled by another person, and 'direct deposition,' where particles land directly on the eyes, nose, or mouth of someone nearby. This shift is not merely semantic; it addresses the challenges faced during the COVID-19 pandemic, where inconsistent terminology hindered public health responses and policy guidance. By establishing a shared language, agencies hope to better coordinate research agendas and public health interventions for pathogens like influenza, tuberculosis, and SARS-CoV-2.

The complexity of airborne transmission remains a significant research frontier. While some diseases like tuberculosis are obligate airborne, others like influenza are opportunistic. Environmental conditions, such as poor ventilation in crowded indoor spaces, significantly enhance transmission risk. Even in extreme environments like the International Space Station, where microgravity causes virus-laden particles to remain suspended longer, researchers are using tools like the COVID Airborne Risk Assessment to model infection probabilities. These studies highlight that while interventions like HEPA filtration and masks are effective, the underlying physics of how we expel and inhale pathogens is far more nuanced than previously assumed.

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