A negative air machine moves air out of a controlled space so a pressure difference can be maintained relative to adjacent areas, when the system is properly arranged.
What this means at your property
The equipment name does not guarantee containment. Ducting, openings, airflow paths and operation all affect performance. A unit used to recirculate air is not performing the same pressure-control function as one exhausting from an enclosure.
What should the work plan explain?
Ask where air goes, how the work-zone boundary is maintained and how performance will be checked. Confirm occupant access arrangements and the response if power, ducting or the enclosure is disrupted.