Most respiratory infections are caused by respiratory viruses, a significant cause of morbidity and mortality for humans. Respiratory viruses are transmitted through by four major transmission modes:
Mass, composition, and particle diameter are major drivers or aerosol behavior. Meanwhile, temperature, humidity, and air pressure are also significant factors in determining how far a virus can travel in the air, and will also influence how long that virus can remain viable (capable of delivering a payload) after being introduced to the indoor environment.
Particles with lower mass tend to stay in the air much longer than droplets. Smaller particles can more easily bypass natural respiratory defense mechanisms and penetrate deeper into the lungs without being intercepted. Although aerosolized contamination may carry a smaller payload per unit, the quantity, abundance, and hang time of these aerosols make them particularly troublesome in environments where staff, patients, and guests frequently interact in close proximity.
In healthcare environments, aerosolized droplets produced by coughs and sneezes are extremely effective in delivering infection-causing payloads. Because the virus can traverse the environment on a jet stream, surface-level cleaning and disinfection methods are inadequate to curb transmission. Aerosolized droplet transmission is typically associated with breathing in contaminated through the nose and mouth, as well as indirect or self-induced infections caused when a virus is transferred from secondary contact with contaminated surfaces, clothing, or objects.
If viral genomes are detected in an air sample, there is a high probability that bioaerosols are present at the sampling locations. Theoretical experiments suggest that the minimum size of particles containing SARS-CoV-2 is calculated to be 0.09 μm, which corresponds to the size of a single virion. However, it's not well understood if a single virion would deliver enough viral payload to cause infection in the host.
Transmission risks are evaluated by determining how many viral genomes are present in each liter of air. The number of viral genomes per liter is largely influenced by the average ratio of viruses in the oral fluid. Virus to fluid ratio implies that each airborne droplet may only include a fractional percentage of viable virus particles (virions). Adding to the complexity, the minimum size of the particles can decrease due to the evaporation of water on the particle surfaces.
A complete virus particle is called a virion. The main function of the virion is to deliver its DNA or RNA genome into the host cell so that the genome can be expressed (transcribed and translated) by the host cell.
Peer-reviewed research suggests, "There has been no discernible evidence on the minimum infectious viral load for COVID-19 pandemic, but many researchers speculate that a few hundred of SARS-CoV-2 virus would be enough to cause the disease among susceptible hosts".
Airborne particles and droplets function differently based on their mass and velocity. Large particles and droplets remain airborne for only short periods of time before settling on a nearby surface. Notably, during a cough or sneeze that generates large droplets, the aerosol spray also includes a finer mist of ultra-small droplets.
The distinction between a droplet and an aerosol is not without disagreement. The World Health Organization defines respiratory droplets as having a size set in the range of 5-10μm, whereas droplets <5μm in diameter are referred to as droplet nuclei or droplet aerosols.
Particles and droplets between 0.3 µm to 2.5 µm) are of particular interest when evaluating the epidemiological impacts of contaminated or polluted air. When studying particles generated by influenza-infected patients, particles with a size range of 0.35–2.5 μm were of higher number concentration,
Once inhaled, PM 2.5 particles are small enough to evade physiological defense mechanisms and penetrate deep into the lower respiratory tract. Questions remain about what role particle size plays when determining virus transmissibility. Airborne particles smaller than 1.0 um, also referred to as PM 1.0 sub-micron particles, are more effective at penetrating deeper into the lungs than their larger counterparts.
"In a simplistic and practical fashion, PM2.5 can be considered the sum of two distinct components, namely ultrafine particles (UFPs, ≤100 nm in aerodynamic diameter) and accumulation-mode particles (AMPs, ∼100 nm to 1.0 μm). UFPs make up a large number concentration but contribute little mass to PM2.5. Furthermore, results from animal studies have suggested that inhaled UFPs deposit more deeply into the lung and may even directly translocate into the circulatory system, thereby exerting adverse health effects via different pathophysiological pathways than larger particles."
Large droplets comprise most of the total volume of expelled respiratory droplets. Sneezing may produce as many as 40 000 droplets between 0.5–12 μm. Coughing may produce up to 3000 droplet nuclei, about the same number as talking for five minutes
When speaking, coughing, or sneezing, humans generate various particles and respiratory droplets that fall along three prototypical size clusters (smallest | median | largest diameter):
In any case, understanding the size and characterization of a particle is important when determining which methods of filtration will offer the highest reduction in overall air contamination and pollution.
For expanded information on HEPA filtration solutions and considerations, continue on to other articles in this series.
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Gong J, Zhu T, Kipen H, et al. Comparisons of ultrafine and fine particles in their associations with biomarkers reflecting physiological pathways. Environ Sci Technol. 2014;48(9):5264-5273. doi:10.1021/es5006016
Gong, Jicheng et al. “Comparisons of ultrafine and fine particles in their associations with biomarkers reflecting physiological pathways.” Environmental science & technology vol. 48,9 (2014): 5264-73. doi:10.1021/es5006016
Gelderblom HR. Structure and Classification of Viruses. In: Baron S, editor. Medical Microbiology. 4th edition. Galveston (TX): University of Texas Medical Branch at Galveston; 1996. Chapter 41. Available from: https://www.ncbi.nlm.nih.gov/books/NBK8174/
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