Quantifying Aerosol Effective Air Change Rate in the USNS Mercy Hospital Ship, and Optimizing Placement of Infectious Patients and Portable HEPA Filter Mitigations

SEAN KINAHAN, Donald Huston, David Silcott, Gabriel A. Lucero, Rick Arestad, Ryan Silcott, Braden Silcott, Blake Silcott, Peter Silcott, Daniel N. Ackerman, Joshua L. Santarpia, National Strategic Research Institute

     Abstract Number: 603
     Working Group: Health-Related Aerosols

Abstract
The United States Navy has two hospital ships, USNS Mercy and USNS Comfort, which responded to the COVID-19 pandemic by deploying to Los Angeles and New York City. These 1000+ bed ships have multiple intermediate and limited care wards that used to house potentially infectious patients. These wards have older HVAC systems that were not designed to modern ASHRAE recommendations of 6 Air Changes Per Hour (ACH), and were not designed with treatment of communicable infectious disease patients as a primary mission.

In the wake of these deployments, and the pandemic response, we characterized the existing functionality of three open wards upon the USNS Mercy across two decks, capturing particle escape and penetration to nearby hallways and wards, and the wards effective air change rate for test aerosols at multiple locations. Further, testing included five release locations, and four different portable HEPA filter configurations to determine which placement, and how many, was optimal.

Particle transport and removal times were captured using IBAC real-time fluorescent particle spectrometers, with a fluorescent threshold set to allow for distinguishing 1 µm fluorescent PSL particles from naturally occurring background populations, including most biological aerosols.

Generally, the lower ward 6 had an order of magnitude less penetration of test aerosols into the HVAC system and distributing to nearby hallways and spaces. Increasing the number of 1000 CFM HEPA filters within the ward improved the effective air change rate, with between 3 and 5 HEPA filters necessary to achieve the 6 ACH recommended today, depending on which ward. Effective air change rates and integrated particle counts were utilized to explore exposure rate differences. Using a Wells-Riley model, increasing from 0 and 5 HEPA filters could increase the time required to reach a given probability of infection by 2-3 times, depending on ward and placement.