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Minnesota Document Shows Hospitals How to Set-Up Stop-Gap Negative Pressure Rooms

Back in 2007, the Minnesota Department of Health, Office of Emergency Preparedness, Healthcare Systems Preparedness Program created a document that couldn’t be more appropriate for the COVID-19 pandemic. The guide, “Airborne Infectious Disease Management: Methods for Temporary Negative Pressure Isolation,” shows hospitals, with some help from HVAC contractors and their in-house facilities management personnel, how to create negative pressure rooms. It’s amazing what one can accomplish with plywood and duct tape.

To create a negative pressure room requires lowering the room pressure below the corridor pressure so air flows into the room under the door with a pressure differential of about 0.01-in. w.g. or 2.5 Pascals (Pa). The recipe to do this has simple ingredients: A HEPA filter, a fan, flex duct, a box to house the device, and duct tape.

First, construct a way to blow air outside, preferably through a window opening. You’ll need a pre-filter to catch dust and lint and then a HEPA filter. Filters for clinical spaces should be able to remove at least 90% of particles that are 0.5 microns and larger from inside and outside air.

You’ll need a fan. CDC has published guidelines for air changes per hour. At 2 ACH, it takes 69 minutes to filter the air to 90% efficiency. Probably not enough. At the high end of the range, 50 ACH, the air is filtered to 99.9% efficiency in eight minutes. That’s overkill. At 10 ACH, the room air is filtered to 90% efficiency in 14 minutes and 99.9% efficiency in 41 minutes. Once you determine the volume of the room, work with the hospital staff on what fan capacity they desire.

Once the components are selected, you and the hospital facilities staff can build plywood boxes to house the filters, the fan and flex duct that exhausts out through the window opening. Tape over the return air grille in the room. The Minnesota Department of Health document estimates that this process should take about 12 hours per room. The catch is that this is a stopgap measure. There’s no additional supplied heating or cooling and it only takes care of a single room. It is, however, fast and cheap.
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