Debate on droplet spread up in the air

Queensland University of Technology aerosol scientist Lidia Morawska. Picture: Lyndon Mechielsen

From early on in the pandemic in Australia and around the world, debate has raged as to whether Covid-19 spreads via the airborne or droplet route. At the heart of the feud between Australia’s top infection control experts and those labelled “miasma theorists” – who emphasised airborne spread of respiratory infections – were scientific misrepresentations that have permeated public health guidelines and messaging for decades.

In April 2020, the World Health Organisation tweeted: “FACT: #Covid-19 is NOT airborne”. In an about face, the WHO in April acknowledged airborne transmission as a source of SARS-CoV-2 spread, but the organisation remains wedded to the idea that respiratory droplets – that fall to the ground after an ­infected person coughs or sneezes – are the dominant form of transmission.

Australian infection control guidelines, both commonwealth and state, also continue to state that droplets are the primary cause of virus transmission.

“These dogmas, once they’re adopted particularly by people that don’t understand the science, are very hard to shift,” says the president of the Australian Medical Association (WA), Andrew Miller, one of the most strident critics of the Infection Control Expert Group which advises the Australian Health Protection Principle Committee and formulates national guidelines.

The “droplet dogma” of which Dr Miller speaks is based on a method of classification of the size of respiratory particles that has recently been proved to be a misnomer. Superspreader events have been documented in thousands of indoor venues around the world, notoriously at a choir practice in Skagit County, Washington. Yet airborne particles that are almost certainly responsible for such superspreader events have been subjected over the course of decades to a false method of measurement that classifies anything more than five microns as a droplet. Aerosol scientists have challenged this definition, arguing that airborne particles could be as large as 300 microns in a “continuum” of size in which the smallest particles can remain suspended in poorly ventilated air for many hours.

“Although it’s been recognised within a smaller group of aerosol scientists for a number of years, it has become particularly evident that the concepts of droplet and aerosol that we have used up until now are probably incorrect,” says research scientist Euan Tovey, who leads the Allergen Research Laboratories at the Woolcock Institute of Medical Research. “It’s now understood that larger particles formerly said to be droplets actually behave like aerosols. In other words, they travel larger distances, they don’t fall immediately to the ground, they can go around corners, they can go out a door and into another door. They stay airborne long enough to transmit disease. And so the concept of droplet and aerosol needs to be reconsidered.”

Establishing exactly how what were essentially aerosols came to be described as droplets is not straightforward. Linsey Marr, an engineering professor at Virginia Tech who also studies infectious diseases, has been instrumental in challenging what’s become known as the droplet-aerosol ­dichotomy. With the assistance of fellow researchers, Professor Marr traced the source of WHO and CDC definitions of droplets and aerosols, discovering that influential scientists – most notably Alexander Langmuir, the first Chief of Epidemiologic Services at the US Communicable Disease Center until the 1970s – had misrepresented the droplet theory proposed in the 1930s by Harvard engineer Wilfred Firth Wells. Wells and his wife Mildred had investigated the transmission of tuberculosis, establishing that it was an airborne pathogen. Langmuir had claimed in a speech to public health workers in 1962 that the airborne particles that lodge deep in the lungs and cause tuberculosis were smaller than five microns. In fact, Wells’ threshold for what constitutes a droplet rather than an aerosol was 100 microns. The idea that airborne particles are those smaller than five microns has dominated public health policy ever since.

Australia’s infection control experts may soon be forced to update their guidance that droplets are the predominant source of SARS-CoV-2 spread in light of the fact that many of what the ICEG labels as droplets are aerosols. Both ICEG and NSW Clinical Excellence Commission guidelines state five microns as the delineation of aerosol versus droplet.

Lidia Morawska, director of the International Laboratory for Air Quality and Health at QUT, says there are indications Australia is “ready for a change”. “The first important step is to acknowledge the significance of airborne transmission as an important ­aspect of the transmission of respiratory infections,” she says.

The Australian Government Safety and Quality Commission for Healthcare has already moved to prescribe airborne precautions. Whether the infection control guidelines are subject to similar review is an issue under active review.