The Coronavirus disease (COVID-19) pandemic has highlighted the severity of airborne virus transmission, especially in hospital environments, where it poses additional risks to vulnerable patients. Infectious aerosols of small size can remain suspended in the air, facilitating the distant spread of pathogens and enhancing the risk of nosocomial infections. The standard air purification method, which relies on built-in mechanical ventilation remains expensive and can sometimes prove inadequate for effective air purification.
Portable Air Cleaners (PACs) provide supplementary support by delivering enhanced air quality that minimizes infection risks. PAC-induced atmospheric changes must be cautiously assessed to avoid the spread of unintentional contamination. This research evaluates the movement of airborne particles within a National Health Service (NHS) outpatient reception area under different ventilation setups.
The study uses an approved aerosol generator to replicate airborne respiratory disease transfer methods. An Omron CompAIR C28P nebulizer (Japan) delivered aerosolized saline solution at 1 bar using compressed air to maintain constant streaming rates through its saline (9 g NaCl/L deionized water) holding chamber. The aerosolized particles were discharged through the VirtuAL hUman exhalation replicaTOR (VALUATOR) at 1.2 m elevation replicating seated adult exhalation. Portable laser diode particle counters measured aerosol dispersion by assessing particles ranging from 0.3 to 25 micrometers in size and density (AeroTrak 9306 by TSI Incorporated, USA).
The experiments took place at UCLH NHS Foundation Trust’s outpatient clinic within controlled ventilation facilities at their London location in the UK. The three studied mitigation methods included aerosol movement between adjacent rooms followed by dispersion throughout the entire clinic and long-distance transport. The experiment used different combinations of mechanical ventilation systems and portable air cleaner devices. Throughout experiments, researchers monitored air exchange rates using cumulative and normalized measurements of particle counts to obtain results.
Research conducted within a clinical setting showed important results describing how particles travel through space. During mechanical ventilation operation, the application of portable air cleaners in CR8 promoted higher particle transport to CR7, the waiting area, and the nurses’ station. Closing doors reduced aerosol movement.
Across the clinic area, the radius of aerosol travel from CR5 to CR1, together with the nurses’ station, changed according to ventilation and PAC positioning. The operation of supply ventilation systems resulted in aerosol count reduction levels from 41%-62% in CR1 and 48%-69% in the nurses’ station areas. Aerosol counts increased up to 27% when PACs operated without supply ventilation. Research indicates that the combined use of ventilation with air cleaners produces complicated results for airborne particle regulation in healthcare facilities.
The research examined how ventilation and air purifying devices affected the dispersion of airborne particles in hospital outpatient facilities. The aerosol spread was successfully decreased by 97% through the closure of two doors, but PACs proved ineffective. The airflow adjustment caused by these PACs sometimes led to worsened aerosol movement patterns. Raising the Air Changes Per Hour (ACH) rate did not automatically lead to reduced transmission, even though PACs were able to reduce aerosol spread by up to 19% in consulting rooms, sometimes causing increased aerosol transmission in waiting areas.
The specific patterns of mechanical ventilation played an essential role in particle distribution, which proved unpredictable. Based on computational fluid dynamics simulations, researchers support the implementation of thorough ventilation system placement and air purifier strategizing to minimize transmissions in these spaces.
The experimental process required minimal hospital staff activity combined with patient immobility as well as complete prevention of door usage for stabilizing variables. Future research must evaluate airflow changes caused by human movement and doors in operation. The study maintained a focus on aerosol migration without assessment of droplet size temperature, temperature levels, and humidity conditions.
The use of improved saliva surrogates combined with multiple particle counters would enhance analytical precision. Research findings demonstrate that PACs decrease aerosols, but their placement may lead to unintentionally increased migration levels and, therefore, demand precise deployment strategies and simulation models for optimal airflow management.
References: Salmonsmith J, Ducci A, Guo L, et al. The influence of mechanical ventilation and portable air cleaners upon aerosol spread in a hospital outpatient clinic. Aerosol Sci Technol. 2025;1-12. doi:10.1080/02786826.2024.2446587
