Air filtration devices


The use of air filtration devices (AFDs) is becoming increasingly common in restoration and cleaning. Their value is particularly critical on jobs that involve high levels of particulates, including most restoration sites plus construction or remodeling environments.

AFDs should always be used when cleaning or drying a structure with a known contaminant. Using AFDs is also recommended when wall cavities, flooring surfaces or sub-surfaces are known to carry a high degree of organic debris, soils, allergens or other undesirable particulates.

Proper air filtration is even more important when building occupants are at significant risk to exposure to particulates — the very young, the elderly, and those who have respiratory problems or a compromised immune system. In these cases, the use of AFDs is absolutely crucial to protect the health and safety of occupants.

AFDs — often called negative air machines, air scrubbers, HEPA units or air filters — work by filtering in several stages. Typically, these stages are arranged in order from coarsest to finest filtration. This helps to increase the useful life of the finer (and more expensive) filter elements and maintain overall filtering efficiency over a longer period of time. Here are the typical filter types and those recommended for use with an AFD.

            1. Pre-filters. Designed to catch large particles, replaced daily or at least with each job. Often available in two stages for varying particle size.

            2. Activated Carbon Filter. Optional. Designed to absorb organic vapors to help remove unwanted odors.

            3. Primary HEPA Filter. High Efficiency Particulate Air filtration media. Designed to capture 99.97 percent of particles down to 0.3 microns in size.

HEPA: The gold standard of air filtration

HEPA, or “high-efficiency particulate air” filter, is the most common rating used to describe air filtration effectiveness. A HEPA rating indicates that the filter removes 99.97 percent of particulates at 0.3 microns in size.HEPA filters and devices

The air movement produced by the unit is described in terms of cubic feet per minute (CFM). This value, however, does not reflect the loss of airflow caused by the filters when they are in place inside the unit, and the actual CFM may be significantly lower. Efficiency also varies based on the amount of air passing through the unit. Filters tested to deliver HEPA at 500 CFM may not achieve HEPA filtration at 1,000 CFM.

When evaluating an air filtration system’s effectiveness, it is critical to know how the unit performs when all filters are in place. Ask your manufacturer to provide this information, or have your unit tested by an independent laboratory.

Why use AFDs?

Any job that involves cleanup of sewage, mold and/or fire damage will generate significant particulates and odiferous (bad-smelling) gases. These contaminants can be a direct result of the damage itself as well as the result of treating and cleaning damaged materials. This includes not only damage in the occupied space, but also contaminants in wall or ceiling cavities that may be disturbed and unknowingly introduced into the occupied space. These contaminants can settle on carpet, upholstery and furnishings. Even worse, they can be drawn into the HVAC system and distributed throughout the structure.

A proper air scrubbing setup will help to ensure a safer, more comfortable environment for the occupants both during the project and after the project is complete. For technicians, the use of an air scrubber may be much more effective, comfortable and less expensive than requiring the use of respirators. Using air scrubbers on every job is both a short-term and long-term investment in the health and productivity of the people who do the work of restoring a clean environment.

Use of a negative air setup may also help limit your legal risks after the job is done. This is especially true with water damage and mold contamination. Sometimes a restored structure develops a problem with indoor air quality (IAQ) due to a pre-existing condition or subsequent damage that is beyond the scope of your responsibility. By using an AFD on every job, you can show that you are taking an extra step to minimize the occupants’ exposure to airborne hazards.

Improving IAQ also benefits equipment performance and quality of your work. For instance, an air scrubber captures dirt, dust and debris that might otherwise clog up and potentially damage other equipment used on the worksite, such as dehumidifiers, airmovers and other equipment. Use of air scrubbing can help to keep your equipment clean and performing at its best.

Determining how many AFDs to use

The number of AFDs to be placed in a given area depends on the cubic volume of the area being treated and the air movement produced by your equipment. The goal is to exchange or filter all the air in the space four to six times per hour or “Air Exchanges per Hour” (AEH). AEH is calculated as follows. Both standard and metric units are provided:

Calculating Air Exchanges per Hour (standard)

ft3 × AEH / 60 = cubic feet per minute of air movement required


ft3 = the volume of air in the area being treated

AEH = Air exchanges required (use a value from 4 to 6, depending on the degree of contamination)

60 = Converts Air Exchanges per Hour to Cubic Feet per Minute

Calculating Air Exchanges per Hour (metric)

m3 × AEH / 60 = cubic meters per minute of air movement required


m3 = the volume of air in the area being treated

AEH = Air exchanges required (use a value from 4 to 6, depending on the degree of contamination)

60 = Converts Air Exchanges per Hour to Cubic Meters per Minute

Divide the result by the amount of air movement (in cubic feet or meters per minute) your AFD machines produce; the result is the number of AFD units to place on the job.

Proper placement

It is important to place air scrubbers evenly throughout the structure in order to capture the most particulates. To maximize the amount of particulates the filtration system captures, it is better to install more units with lower CFMs instead of fewer units with higher CFM. If only one large AFD is available, be sure to place it in a central location. AFDs can also be placed in the air stream of the structure's HVAC system or in the air stream of air movers to increase their capture zone.

Controlling contamination

AFDs can become dangerously contaminated during a restoration project. Failure to properly clean the AFD before transporting it to another location can spread this contamination. To avoid cross-contamination, it is critical to remove all filter elements at the job site, clean the unit inside and out, and put in new filter elements.

Follow these steps to help prevent the spread of contaminants:

1. Wearing PPE as necessary, move the AFD to an outdoor location, plug it in and turn it on. Keep the unit running during the entire process.

2. Remove the first filter, place it in a plastic bag. Place any additional filters into the bag. Seal the bag immediately, and place a second bag over the first and seal it.

3. Turn the unit off and install the new filters in the unit.

Providing the cleanest environment

HEPA filtration is a critical part of maintaining a clean indoor environment duringrestoration. A proper HEPA filtration system is especially important in an environment where dangerous contamination may be present. To protect your technicians and your customers, and to ensure you are providing the cleanest environment possible, be sure to use a HEPA-equipped AFD to establish adequate filtration on every project.  

Brandon Burton is the technical education manager for the Restoration Sciences Academy (RSA), a part of Legend Brands. He has served the restoration community for more than 15 years as an IICRC instructor, ANSI/IICRC S500 chair, RIA restoration council member, and many other industry roles. You can contact Burton at [email protected]

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