title: Secondary exposure to aerosol emitted during respiratory therapy: An experimental and modelling study for exposure evaluation and control
Creators: name: O'Toole, Ciarraí
Publisher: name: University of Galway
Year: 2024
Subjects: subject: NATURAL SCIENCES
subject: Physics
subject: Aerosol
subject: Respiratory therapy

description: Background: Fugitive emissions are defined as medical aerosol released from a nebuliser during respiratory therapy, and include exhaled medicinal aerosol. Fugitive emissions occur during nebulisation of medical drugs used for the treatment of respiratory diseases such as asthma, COPD, and cystic fibrosis. However, while the rise in anti-microbial resistance, solely due to exposure of nebulised antibiotics, is a global concern, there is a dearth of knowledge regarding exposure to other aerosolised medication that is used for the treatment of a range of respiratory illnesses. The research conducted in this body of work aims to address this knowledge gap. Methods: In vitro testing of simulated adult and paediatric patients during respiratory therapy was conducted for a variety of clinical and home healthcare scenarios. Firstly, for treatment of non-spontaneously breathing adult patients using a mechanical ventilator, two tidal volumes of 820 and 270 ml, were assessed regarding their effect on fugitive aerosol mass emitted and dispersed. Aerosol mass concentrations were measured at distances of 0.8 m and 2.0 m from the nebuliser, using an Aerodynamic Particle Sizer (APS). The use of exhalation filters to reduce fugitive emissions was also tested. For spontaneously breathing adult and paediatric patients, four nebuliser types, were tested, with a range of interfaces e.g. mouthpieces and facemasks. Over 180 experimental runs, comprising over 36 combinations of patient, nebuliser and interface types, were carried out. Aerosol mass concentrations were measured at 0.8 and 2.0 m. The inhaled dose of the patient was also quantified via UV spectrophotometry. The extent of fugitive emissions was investigated during high flow therapy for spontaneously breathing adult and paediatric patients during high flow nasal cannula usage, and for non-spontaneously breathing patients via tracheostomy. The effect of patient type, interface, additional supplemental gas flow rate and distance (0.8 m and 2.2 m) from the nebuliser on fugitive aerosol emitted was evaluated by mass concentration measurement. Finally, aerosol mass concentrations were analysed and used to calculate emission and removal rates during standard nebulisation procedures. A mass balance model for a single-zone environment was employed, and from this, the potential inhalation exposure was calculated for health care workers (HCWs) and bystanders, depending on distance from the nebuliser, patient type treated, nebuliser and interface used, occupancy level and natural ventilation status of the treatment room. Results: It was confirmed that fugitive emissions are present when a patient uses a nebuliser for respiratory treatment. The tidal volume setting on a mechanical ventilator was found to directly influence fugitive aerosol, with higher tidal volumes (Vt) leading to higher emissions, reported as median (IQR1, IQR3) (0.077 (0.073, 0.091) mg m-3 for Vt =820 ml vs. 0.062 (0.056, 0.065) mg m-3 for Vt = 270 ml), when an exhalation filter was not used. When a filter was employed, fugitive emissions reflect ambient aerosol concentrations. Nebuliser type affects the inhaled dose to the patient, and also the fugitive aerosols available for secondary inhalation by HCWs and bystanders. Vibrating mesh nebulisers (VMN) usage resulted in higher inhaled doses to patients, and lower fugitive emissions, compared to other jet nebulisers tested (e.g. 45.1 (42.6, 45.6)% for VMN vs. 11.0 (10.1, 11.9)% for a small volume nebuliser for an adult patient). Patient type influenced fugitive aerosol concentrations with greater fugitive aerosol evolving from adults than from paediatric patients. Interface selection impacted upon inhaled dose and fugitive emissions, with a mouthpiece delivering higher inhaled dose to the patient and lower fugitive aerosols to the local environment, compared to a facemask (45.1 (42.6, 45.6)% vs 27.5 (26.3, 29.6)%). The practice of filter usage in interface and nebuliser combinations was found to be beneficial, as aerosol emissions were reduced to local ambient aerosol concentrations. During high-flow therapy, interface type, patient type and gas flow rate were all influencing factors on fugitive emissions and it was generally observed that as flow rate increased, the percentage of nominal dose inhaled and fugitive emissions decreased e.g. 5.35 ± 2.81% and 0.302 ± 0.027 mg m-3 for 10 L/min vs 1.01 ± 0.26% and 0.050 ± 0.003 mg m-3 for 60 L/min for an adult patient. Computational simulations predicted that reducing patient occupancy levels and increasing natural ventilation could have a positive impact in reducing potential secondary aerosol exposure to HCWs and bystanders in naturally-ventilated care settings. A maximum reduction of 91% in inhalation exposure to a bystander was seen when increasing the air change rate to 5 hr-1. Conclusion: In conclusion, this work confirms the presence of fugitive aerosol emissions during respiratory therapy of patients using nebulisers. In the most comprehensive study to date, the influencing factors upon fugitive emissions are investigated and inter-compared, and these include nebuliser, patient and interface type, as well as additional gas flow rate and mechanical ventilator parameters; the impact of these factors on potential secondary inhalation exposure of HCWs and bystanders is also evaluated in this research. Based on the experimental research, it is recommended to employ filtration to interfaces and nebulisers where possible, as this is a simple mitigation measure in reducing emissions in a wide range of environments. Careful consideration of nebuliser type and interface used should be made if filtering is not an option. The computational research indicates that decreasing patient occupancy levels within a single zone environment, and additionally, increasing natural ventilation, are also noteworthy mitigation strategies that should be considered in protecting HCWs and family members from inhalation exposure during respiratory therapy in the unregulated homecare environment.