What does negative airflow mean?

Negative airflow means that air is flowing into a room or space from surrounding areas because the air pressure inside that space is lower than the pressure outside, creating a vacuum effect. Think of it like a room that is sucking air in rather than pushing air out. This is created when more air is being exhausted out of the room than is being supplied into it, whether by exhaust fans, HEPA filtered negative air machines, or HVAC imbalances. In practical terms, if you hold a tissue near a slightly open door of a negative pressure room, the tissue will be pulled inward into the room. Negative airflow is intentionally used in hospitals for airborne infection isolation rooms for patients with tuberculosis, measles, or COVID, so that contaminated air does not escape into hallways and infect others, with the exhausted air filtered through HEPA filters before being released outside. In construction and mold remediation, negative airflow containment is used to keep dust and mold spores from spreading to clean areas of a building. In a home, unintentional negative airflow can happen when clothes dryers, bathroom exhaust fans, range hoods, and fireplaces all run at once, pulling air out and causing the house to suck in outside air through cracks, which can cause backdrafting of furnaces and carbon monoxide hazards, cold drafts, and moisture issues. So negative airflow is a directional concept about pressure difference.