Experience Files

Proper setup and use of Air Filtration Devices (AFDs), often called negative air machines, are critical for effective containment and airborne contaminant removal during remediation projects. They allow for ductwork to be connected to either the inlet or outlet.

Understanding the "Entrainment Zone" and Proper Placement

An AFD's "entrainment zone"—the area where particles are effectively captured and drawn into the filters — is roughly the size of its inlet cubed. For instance, a 24" x 24" inlet creates a capture zone approximately the size of a dishwasher (assuming clean filters). This means AFDs should be strategically placed and operated near where dust-generating activities are being performed. Otherwise, much of the airborne debris will remain in the respirable range until another force acts upon it.

Using ductwork on the inlet side dramatically reduces the entrainment zone to the size of the air handler opening. Furthermore, using hoses on the inlet side almost always leads to recontamination of the structure when the hoses are disconnected and moved.

Smart Filter Changes and Containment Integrity

An AFD setup must allow for easy and safe pre-filter replacement. If a unit is directly attached to the clean side of the containment plastic by cutting a hole and inserting the inlet port, changing filters will often result in contaminating the unaffected portion of the property.

A better solution: Remove the outer filter housing, cut the containment plastic, and then seal the containment plastic directly to the AFD's main housing. Install the pre-filters, and then reinstall the outer cover. This allows for filter changes without breaching the clean side.

Achieving Optimal Air Changes and Negative Pressure Control

Most Industrial Hygienists (IHs) and Certified Indoor Environmentalists (CIEs) typically recommend 4 to 6 air changes per hour (ACH) for effective air filtration. It's important to note that an AFD rated at 2000 CFM was likely tested without the HEPA filter in place; their actual CFM with a HEPA filter will usually range between 1500-1600 CFM. A 500 CFM unit, however, is generally quite accurate to its published rating.

As a rule of thumb:

• One 1600 CFM unit will cover approximately 3000 sq. ft. of contained area with 8' ceilings, achieving 4 air changes per hour.

• One 500 CFM unit will cover approximately 937.5 sq. ft. of contained area with 8' ceilings, achieving 4 air changes per hour.

IHs will also require the use of a manometer to maintain negative pressure within the containment, typically around 0.02" of water column. The containment plastic should show a light indentation. Pressures exceeding 0.1" of WC are likely to cause containment failure; more is not always better when it comes to negative pressure. Not all the air exhausted from AFDs needs to be vented outdoors; some can and should be recirculated. Cutting small slits in the top of the exhaust duct (preferably close to the unit inside the contained area) can also help to reduce excessive negative pressure.

Maintaining Filter Performance

AFDs perform best when their filters are fresh. You should be prepared to swap pre-filters at the end of each day, or more frequently depending on generated dust levels. Technicians should use a HEPA vacuum to remove the dust from outside of filters and perform the filter "hot swaps" with the machine running to prevent the dispersal of captured debris. While extreme care when swapping filters is less critical during times of heavy dust-generating activities, this practice is absolutely vital during the wet-wiping phases of mold remediation.