Negative air pressure is the engineering control that makes contaminated work safe to perform inside an occupied building. By exhausting more air from the containment than enters it, the enclosure is held at lower pressure than the surrounding space, so air always flows into the work area and never out of it. Combined with HEPA filtration that captures 99.97 percent of particles at 0.3 microns, this is what keeps mold spores, asbestos and lead particulate, sewage aerosols, silica and construction dust confined.
The system is calculated, not estimated. We determine the containment volume, apply the air changes per hour required by the protocol and the contaminant, and size the number and capacity of negative air machines accordingly. Machines are ducted to exhaust outside the building where possible, or filtered and recirculated where exterior exhaust is impractical. Intake and exhaust are positioned to sweep air across the entire work area rather than short-circuiting between openings, and makeup air paths are planned so the containment is not starved.
Pressure differential is verified with a manometer after setup and monitored for the duration of the work, with readings documented. Filters are staged with pre-filters and intermediate filters ahead of the HEPA element so the HEPA is protected and airflow is maintained, and filters are changed on condition rather than on assumption. At completion, machines run through a final clearance and air scrubbing period before containment is removed. Equipment placement, air change calculations and pressure readings are documented in the project file.
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