High Negative Pressure Limits Dispersion of Airborne Contaminants

Maintaining a high negative pressure in hospital isolation rooms effectively limits the dispersion of airborne contaminants, according to a new study.

The study, part of a doctoral dissertation by a researcher at the University of Eastern Finland (UEF; Viestintä), was designed to determine the containment capability of hospital airborne infection isolation rooms (AIIRs) by measuring air change rates of the patient room and anteroom, pressure differences, contaminant removal, and contaminant transmission during door openings and human movement. The researchers used a tracer gas method to simulate the release of infectious agents from a patient.


The results showed that high air change rates do not ensure efficient removal of infectious agents in the breathing zone, but that local in AIIR airflow patterns are more important. Applying high mean negative pressure between the AIIR and the surroundings significantly limited particle transmission outside of the AIIR. But while an anteroom helped control particle transmission, the dilution was not effective enough, based on observed air change rates of the anterooms studied. The dissertation was presented at UEF on December 14, 2018, and published in the December 2018 issue of Annals of Work Exposures and Health.

“Generally, a healthcare worker does not stay in the anteroom more than 2–3 minutes, thus to achieve at least 90% contaminant removal, the minimum air change rate requirement of 40 liters per hour would be needed after entering the anteroom,” said dissertant Anna Kokkonen, MSc. “The findings of the present study can be utilized as a starting point when setting consistent negative pressure and ventilation design guidelines for contaminant containment.”

AIIRs are single-occupancy patient spaces designed to isolate airborne pathogens to a safe containment area. AIIRs are a specialized application of a hospital’s heating, ventilating, and air conditioning (HVAC) system, where the airflow supplied into the room is balanced with exhaust airflow to create negative differential pressure with respect to an adjacent space, usually the hallway or an anteroom, so that no airborne particulates escape into nursing staff or public areas. The exhaust air is expelled through dedicated ductwork to rooftop ventilation stacks, where atmospheric air provides sufficient dilution to make the resulting air safe.

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University of Eastern Finland