What is negative air?

Negative air is a term used in construction, mold remediation, asbestos abatement, and healthcare to describe a system that creates lower air pressure inside a contained work area compared to surrounding clean areas, so that air flows into containment and not out, preventing contamination from escaping. A negative air machine, also called a negative air scrubber or HEPA filtered air filtration device, is a powerful fan that pulls air from the contaminated zone through HEPA filters that capture 99.97 percent of particles down to 0.3 microns including mold spores, asbestos fibers, dust, and infectious droplets, and then exhausts clean air outside the building. By continuously exhausting filtered air outside, it maintains negative pressure, typically -0.02 to -0.04 inches of water column relative to outside, which can be verified with a manometer. The contained area is sealed with plastic sheeting and tape, and the negative air machine ensures that any leaks flow inward. In hospitals, negative air is used for airborne infection isolation rooms for TB and measles patients. In remediation, OSHA and IICRC require negative air for mold areas over 10 square feet and for asbestos removal to protect workers and occupants. The term negative air can also refer to the concept of negative pressure or negative airflow, meaning air moving inward due to lower internal pressure. It is not about negatively charged ions, but about pressure difference and filtered exhaust to contain hazards.