Quantifying infectious disease mitigation and HVAC system performance using tracer particles on USNS Mercy class hospital ships

To reduce airborne infectious disease risks, older clinical spaces need to reevaluate their HVAC design and infection mitigation strategies, considering modern recommendations. Infectious aerosols can spread from patient to patient, staff to patient, and vice versa, with nosocomial infections jeopardizing patient recovery. This study compared aerosol removal rates across care wards on a Navy hospital ship, the USNS Mercy, which went into service in 1986. Experimentally, baseline HVAC performance was characterized in three open care wards at five patient beds. The baseline conditions contrasted with those resulting from multiple portable air purifier (HEPA filter unit) arrangements. Real-time data was provided by fluorescent aerosol spectrometers placed to detect contaminant spread of fluorescent microspheres. Modeling aerosol concentration decay enabled calculation of an effective air-change-rate, or aerosol clearance rate, while integrating particle counts over a test allowed calculations of exposure risk relative to a simulated patient. Generally, additional portable HEPA units demonstrated improved aerosol clearance, allowing the older design to reach modern 6 air change per hour standards. Locating patients closer to the HVAC exhaust, and further from recirculation vents reduced spread of contamination to nearby spaces, but retained elevated concentrations within the ward. Conversely, patients closer to recirculation returns reduced exposure within the ward but increased spread to nearby wards. Similarly, a tradeoff exists with placement of HEPA units closer to recirculation returns more protective of nearby spaces, and less protective within the ward. Many of the datapoints here can be extended to open-ward style facilities with patients with communicable airborne diseases. Copyright © 2025 American Association for Aerosol Research