In an enclosed room, if the room air is pumped out of the room, clean air can be "sucked" into the room through the seams of the enclosed room. This scenario allows any air aerosol that has been contaminated to be evacuated from the room, treated and then exhausted to the atmosphere. Think of a hood on a cooking range, negative pressure is created by the hood fan and the air above the stove is vented to the outside. Closing and semi-sealing a kitchen door would create an even greater "suction" in the kitchen with exhausted air being replaced by clean air sucked in from other rooms of the house.
Dental- Doctors of Lynchburg has installed Omni Air 1000 negative pressure units. These are units that would be used in infectious disease wards in hospitals.