Epidemics of infectious disease such as influenza and tuberculosis cause hundreds of thousands of deaths worldwide annually. This annual death rate can increase to millions with sporadic, unpredictable pandemics, such as the 1918 flu pandemic or potentially the ongoing COVID-19 pandemic. The main routes of infectious disease transmission among humans have long been a matter of debate. Transmission may occur via (i) “direct” physical contact with infected individual, (ii) “indirect” contact with contaminated surfaces (i.e. fomites), such as a contaminated door knob, (iii) sprays of virus-laden respiratory droplets, such as from a cough or sneeze, impacting immediately onto the respiratory mucosa of a susceptible individual, or (iv) by the eventual inhalation of the smaller residual solid cores of evaporated respiratory droplets (i.e. aerosol particles) emitted during breathing, talking, coughing, and sneezing. Controversy remains regarding the relative contribution of these different modes (contact, droplet, or aerosol) in transmitting specific respiratory diseases. For example, documented cases of COVID-19 transmission by asymptomatic and presymptomatic individuals, who do not cough or sneeze to any appreciable extent, suggests that aerosol transmission plays an important role, but little is known about aerosol particle production by asymptomatic individuals.
This dissertation has two broad themes. First, we focus on aerosol particle emission during human expiratory activities. We show that the rate of particle emission during normal human speech is positively correlated with the loudness (amplitude) of vocalization regardless of the language spoken (English, Spanish, Mandarin, or Arabic), and that a small fraction of individuals behaves as “speech superemitters,” consistently releasing an order of magnitude more particles than their peers. We further investigate the effect of voicing and articulation manner on aerosol emission during human speech, and demonstrate that particle emission rates are positively correlated with the vowel content of a phrase; conversely, particle emission decreases during phrases with a high fraction of voiceless fricatives such as /f/. Next, we tested the efficacy of medical-grade and homemade masks in controlling aerosol particle emission from expiratory activities. We find that surgical and KN95 masks reduce the particle emission rates by 90% and 74% on average during speaking and coughing, respectively, compared to wearing no mask. However, the efficacy of homemade cloth and paper masks is confounded by shedding of non-expiratory micron-scale particulates from friable cellulosic fibers present in homemade masks. These results highlight the importance of regular changing of disposable masks and washing of homemade re-usable masks.
The preceding results focused on generation of aerosol particles during human expiratory activities. In the second part of this dissertation, we next consider how aerosol particles can spread through the air in indoor environments. We begin with a theoretical model, using a point source and turbulent diffusivity model valid in the limit of low Stokes numbers. We calculate transmission probabilities versus position and exposure time for different values of the turbulent Péclet number, and we demonstrate that under some certain circumstances speaking can lead to higher probabilities of transmission than coughing. We then turn our attention to experimental animal models of human influenza, performed in collaboration with colleagues at the Icahn School of Medicine at Mt. Sinai. We provide evidence of a mode of transmission seldom considered for infectious disease: airborne virus transport on microscopic particles called “aerosolized fomites.” We show that the airborne particulates produced by infected guinea pigs are mainly non-respiratory in origin by quantitatively characterizing the particles emitted from cages containing guinea pigs using both an Interferometric Mie Imaging system and an Aerodynamic Particle Sizer. Our results surprisingly show that an uninfected, virus-immune animal whose body is contaminated with influenza virus can transmit the virus through the air to a susceptible partner in a separate cage. We further demonstrate that aerosolized fomites can be generated from inanimate objects, such as by manually rubbing a paper tissue contaminated with influenza virus. Our results suggest that besides respiratory droplets and aerosols, aerosolized fomites may contribute to influenza virus transmission in animal models of human influenza, if not among humans themselves, with important but understudied implications for public health. This dissertation concludes with a discussion of potential avenues for future work in the field of airborne infectious disease transmission.
|Advisor:||Ristenpart, William D.|
|Commitee:||Wexler, Anthony S., Shah, Pria S.|
|School:||University of California, Davis|
|School Location:||United States -- California|
|Source:||DAI-A 82/5(E), Dissertation Abstracts International|
|Subjects:||Chemical engineering, Public health, Linguistics, Virology, Pathology, Medicine, Immunology|
|Keywords:||Aerosolized fomites, Airborne transmission, Animal models, Human speech, Infectious diseases, Epidemics, Influenza, Tuberculosis, COVID-19, Asymptomatic , Presymptomatic , Wearing masks|
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