On many water damage projects, we should create a condition of negative pressure also called negative air or a pressure differential. The main purpose of this negative pressure is to eliminate cross-contamination to unaffected spaces.
We create negative pressure by pushing or pulling air out of our contained space, causing air from the surrounding spaces to come into the space, replacing the air being removed. Think of how turning on the exhaust fan in a bathroom would cause air to come under the door from the hallway to replace the air being exhausted out. This keeps contamination from moving from the workspace to those surrounding areas as they cannot move against an incoming airflow.
The ANSI/IICRC S500 2021 tells us “If the …water is Category 1, the restorer can proceed without contamination controls (e.g., erecting containment barriers, establishing pressure differentials). With regard to Category 2 or 3 water intrusions…restorers shall use contamination controls” (10.6.7)
Regarding Category 2 and Category 3 water intrusions, the S500 also states “Restorers should maintain negative pressure in relation to uncontaminated areas.” (12.3.5)
The S500 provides examples of contamination controls such as “erecting containment barriers and establishing negative pressure.” which is typically established with an AFD utilizing HEPA filtration. AFDs remain in place until it is confirmed that the contamination has been removed. Remember, there is a difference between demolition and decontamination, but that conversation is for another Mitigation Minute!
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