When recommended in the report, a negative air pressure environment is maintained within the containment continuously during the gross material removal and during non-working hours on the days that gross material removal is being conducted. The negative pressure is evident by the inward bowing of any plastic sheeting used as a barrier for the containment. 

The negative air environment is created by the use of an air filtering machine exhausted to the outdoors.  The negative air machine is equipped with a HEPA filter. The HEPA filters are regularly changed in accordance with the manufacturer's recommendations.  A negative pressure of at least minus 0.02 inches of water column relative to the area outside of the containment is maintained for the duration of the work.  A sufficient number of negative air filtration units are used to ensure a minimum of four air changes per hour inside of the containment.