Droplet nuclei are sometimes defined only by size. The Centers for Disease Control and Prevention (CDC) defines droplet nuclei as “dried residues of less than 5 microns in size” [1]. Additionally, it is sometimes referred to as “small drops of moisture carrying infectious pathogens” [2]. The World Health Organization (WHO) defines droplet nuclei as follows: “when the droplet particles are >5–10 μm in diameter, then they are referred to as respiratory droplets, and when then are <5 μm in diameter, they are referred to as droplet nuclei” [3], and they do not require dessication.
The term “droplet nuclei” must be clearly defined in terms of desiccation. For the purpose of this study, droplet nuclei will be discussed using CDC's definition, which includes a requirement for desiccation. The WHO and CDC's current position is based on the conventional medical definition that in human exhalation origin, only droplet nuclei can cause airborne transmission [1]. Presently, the WHO and CDC suspect but do not officially recognize airborne severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) transmission [4]. According to the CDC, airborne transmission is currently limited to measles, chickenpox, disseminated shingles, and smallpox [5]. Airborne transmission is suspected in certain other viruses but has not been confirmed yet. Suspected cases of SARS-CoV transmission through apartment building air [6], outbreak of influenza in an airplane delayed on the ground with an inoperative ventilation system [7], and reports of simultaneous coronavirus disease (COVID-19) outbreaks in large enclosed spaces [8] suggest that these enveloped viruses are transmitted through dry droplet nuclei because it is unlikely that the fine droplets will stay moist for long distances and time. Wells found that droplets smaller than 100 μm would completely dry out before falling approximately 2 m to the ground [9].
Morawska reported that droplets with sizes of the order of 1 μm evaporate within a few milliseconds even under conditions of high relative humidity, droplets with size of the order of 10 μm exist for up to a few tens of a second, while very large droplets that are 100 μm in diameter survive for up to a minute [10]. SARS-CoV-2 and SARS-CoV exhibited similar half-lives in aerosols, with median estimates of approximately 2.7 hours [11]. In this study, all aerosols are considered to be dried out during the course of the process and turned into droplet nuclei. A couple of hours of half-life is sufficient for the dry droplet nuclei to be dispersed and become clinically infectious, especially in an enclosed environment. The presence of infectious, replicating virions in <1 µm of aerosol samples has been evident, and there were increases in viral RNA during cell culture of the virus from aerosol samples [12]. It has been shown that droplets up to 1μm in size evaporate within a few milliseconds [10]; this indicates that SARS-CoV-2 can be transmitted through droplet nuclei. SARS-CoV-2 is not inactivated on plastic and stainless steel surfaces for a couple of days [11]. Generally, enveloped viruses with a lipid envelope tend to be more persistent at lower relative humidity, while non-enveloped viruses are more stable at higher relative humidity [13].
Airborne transmission is meant by most authors to be synonymous with aerosol transmission [14]. The WHO states the following: “Airborne transmission is defined as the spread of an infectious agent caused by the dissemination of droplet nuclei (aerosols) that remain infectious when suspended in air over long distances and time” [7]. This statement includes two definitions. First, the WHO defines that the term “airborne transmission” is limited to transmission by droplet nuclei. Second, the WHO also mentions that airborne, droplet nuclei, and aerosol transmissions are all synonymous. The CDC states, “The definition of an aerosol, as used here, is a suspension of tiny particles or droplets in the air, such as dusts, mists, or fumes.” [15]. CDC’s definition requires droplet nuclei to be dry, and airborne transmission is further limited to transmission by droplet nuclei.
Therefore, aerosol transmission is not synonymous with airborne or droplet nuclei transmission in CDC’s definition. Additionally, most reports stating that SARS-CoV-2 spreads through air and/or aerosol do not clearly specify whether the definitions of the terms “airborne” and “aerosol” refer only to dry droplet nuclei or fine, moist droplets floating in the air [14].
The fact that the definitions of aerosols and droplet nuclei are not uniform worldwide, even between the WHO and the CDC, is a huge detriment.
Equilibrium moisture content (EMC) is the moisture content at which the material is neither gaining nor losing moisture in the air. The droplet nuclei is considered dry when the EMC state has been reached.. Because the value of the EMC depends on the relative humidity and temperature of the air, the moisture content of droplet nuclei varies with relative humidity and temperature and their infectivity is also likely to vary. This is an issue to consider when discussing whether or not droplet nuclei are infectious.
Indeed, it is difficult to determine how small and how dry a particle should be to be referred as a droplet or droplet nuclei. To prevent this ambiguity, a suitable alternative is to abolish the current classification of droplets and droplet nuclei and reclassify them as droplets and aerosols based, solely, on how long they are suspended in the air, regardless of their size and dryness.
Along with the global unification of the definitions of aerosols and droplet nuclei, it is essential to recognize the above theoretical evidence showing that SARS-CoV-2 has a drying-resistant nature and is likely to have airborne transmission through dry droplet nuclei.
References
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