The World Health Organization (WHO) has published a new report that provides, for the first time, a unified definition of what it means when a disease is spread through the air. The WHO worked with four major global health agencies in consultation with dozens of individual experts to compile the report, which should help avoid a situation like we had at the start of the COVID-19 pandemic, with fraught arguments over terminology.
What we mean when we say “airborne”
To most people, labeling a disease “airborne” would simply mean that you can catch it through the air. But this seemingly straightforward concept has been the subject of a lot of discussion and debate over the years, not least in the early months of the pandemic. The problem stems from the fact that there are a couple of different ways that diseases can spread through the air.
In the past, a was made between two types of airborne particles. “Respiratory droplets” generally referred to the larger particles that are expelled when an infected person coughs, breaths, sneezes, or even just talks. These particles may be breathed in if you have close contact with an infected person, or can fall onto and contaminate surfaces. “Aerosols”, on the other hand, were understood to be smaller – under 5 micrometers in size – and thus stay suspended in the air for longer, traveling greater distances.
When COVID arrived, there was a race to understand the main ways in which the disease was spread, so that public health advice could be targeted effectively. Recommendations like the 2-meter or 6-feet rule for social distancing, and advice about handwashing and sanitizing, make a lot of sense when you’re talking about a disease spread mainly via droplets. But if COVID had the potential to linger as aerosols, ventilation, air filtration, and mask-wearing might be more sensible priorities.
What followed, as recalled by WHO Chief Scientist Dr Jeremy Farrar in a recent , was a highly charged debate on whether to categorize COVID-19 as truly “airborne” under the old definitions.
In July 2020, the WHO that COVID could spread via droplets. But that was only after a group of experts in aerosol science publicly complained that the agency’s unwillingness to explore the possibility of aerosol transmission was putting people at unnecessary risk.
The next few years saw bitter disagreements around and continued emphasis on handwashing, arguments over are best, and grassroots campaigns to provide DIY air filters – known as – to schools and businesses, although about the utility of home air purification devices.
New definitions
Almost four years on, it’s now clear that COVID-19 particles can spread for than many initially realized, fitting the traditional definition for aerosol transmission.
To avoid the same level of confusion and potential public health risks next time around, the WHO began a far-reaching consultation in 2021, leading to the new report. It recommends that all respiratory droplets, regardless of size, be hereafter referred to as “infectious respiratory particles” or IRPs. A disease may now be said to spread “through the air” when its main mode of transmission relies on IRPs.
One of can then be applied.
Airborne transmission or inhalation is when IRPs travel any distance and are breathed in by another person. This is affected by factors like air temperature, humidity, and ventilation.Direct deposition is when IRPs expelled by an infected person end up directly on the exposed mouth, nose, or eyes of a nearby person and can enter their body.
So from now on, there’ll be no nitpicking about the size of the particles. It’s hoped that this change will bring “airborne” more into line with similar terms that are widely understood by the public, like “waterborne” and “bloodborne”.
In the press conference, Farrar described this current point as “base camp” and explained that the next crucial steps will be for all the bodies involved in the consultation to move forward as one, conducting research that will help us better respond to airborne pathogens in light of this new, shared understanding.
You can read the full report .