Rethinking Airborne Pathogens: WHO Proposes New Terminology for Disease Spread


The WHO suggests changing terminology to better understand pathogen transmission through the air, emphasizing N95 mask use and improved ventilation.

Coronavirus cell or COVID-19  (Adobe Stock 340164484 by phonlamaiphoto)

Coronavirus cell or COVID-19

(Adobe Stock 340164484 by phonlamaiphoto)

Recently, the World Health Organization (WHO) initiated a paradigm shift in preventing pathogens from spreading through the air. If that phrasing seems non-scientific and does not use the jargon of “airborne pathogens” or “aerosolized pathogens”, it is by design. It is advocated in a new WHO report regarding the terminology of “pathogens spread through the air.” A proposed overarching term similar to “waterborne” and “bloodborne.”

Segments of the medical community have maintained misconceptions regarding how pathogens spread through the air. Too many healthcare experts believe that an “airborne” pathogen will only spread under certain circumstances or unusual conditions, such as during an aerosolizing procedure.
I have observed similar misguided advice during a public health online meeting. A question was asked when one should wear an N95 mask instead of a surgical mask when treating patients with COVID-19. The answer was to wear N95 masks in high-risk settings, such as during procedures that produce aerosols. The correct answer should have been always to wear an N95 mask when exposed to patients with COVID-19 and that surgical masks have little place in preventing airborne transmission.
Health care settings have been the primary source of the spread of respiratory illnesses, and the response to stopping this spread can be suboptimal. This was exemplified by the 2003 SARS-CoV-1 outbreak in Toronto. A report of this outbreak, published by the National Academies of Sciences, discusses transmission by droplets and wearing protective equipment during droplet-generating procedures, but not for exposure to all patients with respiratory illnesses. The report did recognize that there were cases of SARS-CoV-1 whose transmission could not be explained using the droplet transmission paradigm. The CDC stated, “In some instances, however, true airborne transmission (via droplet nuclei) cannot be excluded as a possible mode of SARS-CoV transmission.”

A recent commentary in the Lancet by Trisha Greenhalgh and colleagues stated that handwashing has been the mainstay measure to prevent disease transmission. Strategies to prevent the spread of pathogens through the air, such as wearing a fitted N95 mask and improving indoor ventilation, are often “ignored or downplayed.”

Many feel we need to wipe the slate clean and effectively start over. Hence the new terminology.

The WHO consultation report stresses that:

  • “These potentially infectious particles are carried by expired airflow, exit the infectious person’s mouth/nose through breathing, talking, singing, spitting, coughing or sneezing and enter the surrounding air.”
  • “IRPs (infectious respiratory particles) exist on a continuous spectrum of sizes, and no single cut-off points should be applied to distinguish smaller from larger particles. “ And that, “The updated terminology no longer includes a cut off of particle size, but rather a continuum of particle sizes of IRPs.”

Another new term, “direct deposition,” is similar to “droplet transmission” but without size considerations. Any particle of any size transmitted by air can eventually be deposited on surfaces.

The WHO’s new description of pathogens that spread through the air also aligns with the October 2020 National Academy of Sciences report regarding the “Airborne transmission of SARS-CoV-2.”

In other words, when deciding upon the appropriate action, you no longer must consider particle size and spread by large droplets. If you will be exposed to a pathogen that spreads through the air, wear an N95 mask. If it spreads by the air, at some point, it will deposit directly on surfaces.

One may ask how we got so off-target with pathogen control in the first place. The answer lies in the narrow and limited nature of expertise on advisory committees. Most recently, we have seen the CDC plan to expand the expertise of their HICPAC committee and send recommendations back to them for reconsideration.
The same may be occurring within the WHO regarding pathogens that spread by the air. Trisha Greenhalgh and colleagues pointed out that one possible explanation is that “dominant voices in the infection prevention and control community did not grasp the basics of airborne transmission and failed to listen to people who did.” Others have also discussed the historical resistance to effectively stopping airborne transmission.

Infection disease professionals must not only advocate but also act. A good first step is to carry a portable CO2 monitor to help evaluate indoor air quality at your health care workplace. One should advocate for continuous CO2 monitoring in your facility and make sure HEPA filtration is used with adequate ventilation. It is ironic that locked in our high-tech, energy-efficient buildings, we may be less safe than in third-world countries in tropical climates where windows are left open. Finally, remember that N95 masks are designed to stop pathogens that spread through the air; surgical masks are not.

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