Implications for Infection Control
The COVID-19 pandemic, caused by severe acute respiratory syndrome coronavirus 2 [SARS-CoV-2]) has been associated with infections and deaths. Current policies regarding infection control are based on the assumption that the majority of respiratory infections are transmitted by large respiratory droplets—ie, larger than 5 μm—created by coughing and sneezing, then transmitted to exposed fomite or mucosal surfaces.
Proximity is considered a proxy for respiratory droplets, which is supported by the statement “Proximity to the index case was associated with the transmission which is consistent with droplet spread.” Airborne transmission has often been attributed to infectious droplet nuclei produced by the removal of moisture of suspended droplets that are 5 μm or smaller in size. Data are accumulating that severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the virus that causes COVID-19, is transmitted by both small and large particle aerosols
Infectious aerosols are suspensions of pathogens in particles in the air, dependent on both physical and biological laws as other airborne particulate matter. Infectious aerosols are particles with potentially pathogenic viruses, bacteria, and fungi suspended in the air. The predictions of the pathogens’ airborne survival, infectivity, virulence, and other characteristics are based on its biology. The most important determinant of aerosol behavior is particle behavior.
Particles that are 5 μm or smaller in size in indoor conditions can remain airborne for an unlimited period of time unless they are removed because of air currents or dilution ventilation. Particles that are 5 μm or smaller rest in the lower respiratory tract in humans. Particles sized 6–12 μm deposit in the upper airways of the head and neck. Imaging studies concluded plumes of aerosols are caused by sneezing or coughing and contains the highest concentration of particles dissolves in the air over time and distance. Over time the distance the plumes traveled is farther than previously predicted, traveling up to 7–8 m. A re-analysis of the size of particles a person would release falls to the ground within 2 m is 60–100 μm and travels more than 6 m away by sneezing.
Infection control guidelines have said that the majority of respiratory infections are transmitted by respiratory droplets—ie, particles larger than 5–10 μm in size. Airborne transmission has been attributed to a few pathogens through infectious droplet nuclei that are particles sized 5 μm or smaller. The use of Airborne infection isolation rooms and respirator masks are used only to protect against airborne transmission. These recommendations have been based on old data and inferences.
Over the past two decades, investigators have researched particle sizes of infectious aerosols emitted from individuals with respiratory infections from aerosols generated by cough and from exhaled breath. Pathogens from patients with various respiratory infections were isolated in the aerosols generated by coughing. These studies included methods to measure particle sizes have consistently found pathogens in small particles. The study concludes that people produce particles at a wide range of infectious aerosols sizes, but mainly produce pathogens in small particles (<5 μm). Aerosolisation of respiratory pathogens is highly variable. This is partly due to the log-normal distribution of infectious aerosols which is found in super-spreading.
In studies of exhaled breath aerosols with particle size measurements, pathogens were consistently found in small particles (<5 μm). The majority of particles in exhaled breath are smaller than 4 μm, with a median between 0·7 and 1·0 μm. Once the direct measurement of particles containing viruses in exhaled breath was technically feasible, most particles (87%) with influenza viral RNA were found to be smaller than 1 μm.44 Exhaled influenza viral generation rates were estimated to be from fewer than 3·2 to 20 virus particles per min. Further developments enabled detection of so-called fine versus coarse particles (ie, ≤5 μm vs >5 μm).45 Influenza viral RNA was detected in the exhaled breath of 34 (92%) of 37 adults.45 The fine particles contained 8·8-times (95% CI 4·1–19·0) more viral copies than did the coarse ones. Masks can help to reduce exposure to infectious aerosol but are not a substitute for physical distancing and other infection control measures. Face shields can help decrease exposures to and help protect against contamination from large particle aerosols, but do not protect against small particle aerosols.
Infectious aerosols from humans exist in a wide range of particle sizes that are strikingly consistent across studies, methods, and pathogens. Currently, there is no evidence to support the theory that the majority of respiratory infections are associated with mainly large droplet transmission. Small particle aerosols are considered the rule, rather than the exception, opposing current guidelines. These small particles do not need a prolonged period of time to allow for desiccation, and their size is immediately respirable.
The logic that transmission within close proximity defines respiratory droplet spread is false. Small particle aerosols are in the highest concentration close to patients and dissipate with distance. The variability of transmission among respiratory pathogens is less dependent on the physical particle size emitted by the diseased person, as current guidelines suggest. The variability of transmission among respiratory pathogens is more dependent on biological factors such as the size of the emitted inoculum, the ability of the pathogen to survive desiccation, and other stresses of aerosolization and airborne transport, as well as environmental factors such as air movement, temperature and humidity, and host defenses.
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