Understanding airborne transmission : experimental and observational studies during covid-19 pandemic

Contrary to previous understanding, airborne transmission is a significant route for the spread of pathogens in the respiratory tract. Assumptions about the infection route determine how we protect ourselves and control the spread of infections. Previously, infectious aerosol particles were thought to occur mainly in connection with individual pathogens, such as tuberculosis, or in medical procedures that produce aerosols. In the medical literature, the division into droplets (which fall within a distance of one meter) and aerosol particles (which can travel further in the air) has been rigidly based on the size limit of 5 micrometers (μm). However, the COVID-19 pandemic caused by the SARS-CoV-2 virus has revealed several gaps in knowledge and differing views on the generation of particles and the effect of particle size on their movement. The previous division does not reflect reality. In normal breathing and speaking, a distribution of particles is generated where the majority are small aerosol particles. This thesis aims to expand our understanding of airborne transmission and the control of airborne infectious diseases from four different perspectives. Firstly, the research aimed to measure particle exposure generated during medical procedures classified as aerosol-generating procedures during general anaesthesia, and compare it to particle exposure occurring during normal patient contact (studies I-II). Secondly, the goal was to investigate the spread of aerosol particles indoors and analyse the effectiveness of protective measures (study III). Thirdly, the study aimed to investigate the airborne and surface contamination of SARS-CoV-2 in patients' environments, both at home and in the hospital (study IV). Fourthly, the study analysed occupational exposure to SARS-CoV-2 and the relation of personal protective equipment used (studies V-VI). The research combined expertise in aerosol physics, particle dispersion modelling, virology, occupational health, and exposure research in medicine. Methods, studies I-II: Aerosol particle measurements were conducted in surgical operations at Helsinki University Hospital (HUS). We measured particle exposure during preoxygenation, mask ventilation, intubation, and extubation, comparing it to exposure during coughing and background concentrations. Study III: We simulated the spread of aerosols in restaurant dining situations using a model virus Phi6 and Large-Eddy Simulation (LES) supercomputer modelling. We examined the effect of air purifiers and space dividers on the spread of the virus. We developed an infection risk model that considers turbulence, showing the probability of infection as a function of time and space. Study IV: We collected air, surface, and deposition samples, as well as saliva and blood samples from patients with acute COVID-19 infection in both hospital and home environments. The samples were analysed for the presence of SARS-CoV-2 using quantitative real-time reverse transcription polymerase chain reaction (qRT-PCR). Studies V-VI: We collected information on occupational exposure to SARS-CoV-2 from nurses and doctors working at HUS using questionnaires. Infections were confirmed by laboratory tests and classified as work-related, likely work-related, leisure-related, or unclear. Results, studies I-II: Anaesthesia procedures generated particle exposure comparable to coughing. The procedures mainly generated particles smaller than 1 μm. Study III: We observed aerosol particles and infectious model viruses spreading throughout the studied indoor space. The particles spread with indoor air turbulence, creating local concentration differences. Air purifiers effectively reduced particle counts, while space dividers did not. Study IV: We detected SARS-CoV-2 virus RNA in air, surface, and deposition samples from hospitals and homes. Studies V-VI: Of the respondents to the survey, 41 (4.7%) were confirmed to have contracted COVID-19, and of these, 22 (54%) were work-related or likely work-related. About one-third of infections originated from colleagues. No work-related infections were observed if personal protective equipment based on airborne precautions, including FFP2/3 respirators, was used. Conclusions: The particle exposure from general anaesthesia procedures is comparable to exposure during normal patient contact. The generation of aerosol particles as part of natural respiratory activities should be better considered. Aerosol particles spread throughout the entire space, but there are significant differences in their concentrations depending on the ventilation of the space. This partly explains why a significant proportion of infections are detected near the source, where concentrations are high. Air purifiers effectively reduce the number of particles. Both air and surface contaminations were detected in the environments of COVID-19 patients, emphasizing the importance of protecting against both routes of transmission regardless of the environment. To prevent airborne infections, it is important to use respiratory protection that prevents microorganisms from entering the respiratory tract, and to consider the risk of infection among personnel.