Air filtration devices are machines that are used to filter the air. An air scrubber is an AFD that is being used to filter re-circulated air within a workspace. A negative air machine is an AFD that is being used to filter air and create negative air pressures within a workspace. In many cases the AFD can be used interchangeably to "scrub" the air, create "negative air" or create a "positive air" pressure. Therefore, an AFD can be a versatile piece of equipment that is easily adapted to perform some or all of these functions.
Some restorers are setting up AFDs as a negative air machine on any category of water loss as a precaution. This practice is beneficial for several reasons including minimizing contractor's liability.
Others are setting up AFDs as air scrubbers. This practice may provide some benefit, but the end result may not be a clean indoor environment for several reasons. There is an area around the intake of an AFD that is called the "capture zone". Within this area, the AFD can capture 100 percent of any particulates or bio-aerosols that enter into it.
Typically the capture zone of a large HEPA filtered AFD does not extend much more than 3 feet out from the intake. Because the capture zone is so small relative to the room, it may not capture as much of the contaminants as you might think. Keeping in mind that an "air scrubber" re-circulates the filtered air within the room, the exhaust side of the AFD may do more harm than good by blowing contaminants, if present, to other parts of the building. This problem could be minimized by the use of a diffuser on the exhaust side of the AFD. On the other hand, AFDs used as negative air machines can help control an environment by keeping the moisture and potential contaminants from going elsewhere.
The U.S. Environmental Protection Agency has published a document called "Residential Air Cleaning Devices: A Summary of Available Information." This document was prepared for residential air purifiers, but many of the same comments apply regarding commercial AFDs being used as scrubbers. This document states:
"The three strategies (in order of effectiveness) for reducing pollutants in indoor air are source control, ventilation, and air cleaning. Source control eliminates individual sources of pollutants or reduces their emissions, and is generally the most effective strategy. Some sources, like those that contain asbestos, can be sealed or enclosed; others, like combustion appliances, can be adjusted to decrease the amount of emissions. Unfortunately, not all pollutant sources can be identified and practically eliminated or reduced. Ventilation brings outside air indoors. It can be achieved by opening windows and doors, by turning on local bathroom or kitchen exhaust fans, or, in some situations, by the use of mechanical ventilation systems, with or without heat recovery ventilators (air-to-air heat exchangers). However, there are practical limits to the extent ventilation can be used to reduce airborne pollutants. Costs for heating or cooling incoming air can be significant, and outdoor air itself may contain undesirable levels of contaminants. Air cleaning may serve as an adjunct to source control and ventilation. However, the use of air cleaning devices alone cannot assure adequate air quality, particularly where significant sources are present and ventilation is inadequate."
As you can see, air cleaning is considered to be the least effective method for reducing pollutants in indoor air. In a situation where a Category 1 water loss has occurred, your goal is to dry the wet areas, prevent secondary damage from moisture, and control pre-existing contaminants. Most buildings have substantial dust and debris accumulated in and under carpeting. It is also possible to have concealed mold growth that is not part of the water loss. If you don't know the source, then "source control" is impossible; therefore, the use of an AFD is a precaution.
Ventilation, on the other hand, may be possible by using the AFD as a negative air machine and allowing filtered outdoor air to enter into the workspace. Of course, this can only be achieved in an open drying system.
There are several advantages to drying a building under negative pressure. First, it will prevent dust and debris from being forced, by air pressure, into other parts of the building. Also, moisture flows from areas of higher air pressure to areas of lower air pressure. By creating a negative air pressure, the moisture in wall cavities or other spaces will be more likely to be drawn into the negatively pressurized room where dehumidification can remove it. A final advantage is that negative air pressure is less likely to cause particulates and bio-contaminates to become aerosolized.
In order for an AFD to be effective in removing particulates from the air, it must be of adequate size and the filters must be able to remove a sufficient amount of the particulates that pass into the unit. An efficient filter that removes 100 percent of particles is not helpful if it is only able to move a low amount of air. Some of the factors that influence the ability of an AFD to remove particles from the air include the size and mass of the particle, filter loading, air that bypasses the filter and is drawn around it, and how well the air mixes in the room.
There are several methods that have been adopted for measuring the effectiveness of filters. The weight arrestance test is generally used to evaluate filters that have low efficiencies. These typically are the common mechanical filters used in HVAC systems and do not provide an adequate evaluation of smaller respirable-sized particles such as molds or bacteria.
The atmospheric dust spot test is generally used to evaluate medium efficiency filters. This also does not provide an adequate evaluation of smaller respirable-sized particles such as molds or bacteria.
Dioctylphthalate testing (DOP testing) has been used to rate high efficiency air filters. This uses a particle of a known size that is released into one side of a filter and then measured on the clean side of the filter. By measuring the amount of particle that penetrates the filter you can determine the filters efficiency. Typically a HEPA filter is 99.97 percent efficient at 0.3 microns. DOP testing provides a better assessment of the ability to capture the smaller respirable-size particles. It is important to understand whether the test was performed on the filter alone or the AFD with the filter installed.
There is currently a movement away from DOP testing to other methods. Since AFDs can have bypass, merely testing the filter outside of the machine isn't a sufficient indication of how well the total system works. There are other factors that impact the filters efficiency including particle shape and flow rate.
Another filter rating system that is beginning to gain some popularity is the MERV system established by the American Society of Heating Refrigeration and Air Conditioning Engineers (ASHRAE). This rating system has achieved some level of acceptance since the EPA, in their document "Mold Remediation in Schools and Commercial Buildings," recommends using a MERV 8 or higher. However, very few manufacturers are offering MERV ratings for their equipment at this time. Until a standardized system of rating air cleaners is utilized, it is frequently up to the buyer to decide what their evaluation criterion should be.
Most of you are familiar with the Association of Home Appliance Manufacturers (AHAM) rating system for dehumidifiers. AHAM has also developed an American National Standards Institute (ANSI)-approved standard for portable air cleaners. This rating system is referred to as the "clean air delivery rate" or CADR. It is used to rate the effectiveness of room air cleaners for tobacco smoke, dust and pollen. Unfortunately at this time, they do not rate systems for other small particles such as mold or bacteria, which are one reason why an air cleaner might be used. The CADR takes into consideration the filter efficiency, air flow and pressure drop. Currently, the CADR is not commonly being used to rate AFDs for biological contaminants.
Under the right conditions, some air-cleaning systems can effectively remove certain particles, although the particles must be suspended in the air. The suspended particles must then be able to move to or be drawn to the capture zone of the AFD in order to be removed.
At this time there is controversy about how effective AFDs are, when used as scrubbers. An ad hoc committee convened by the FDA in 1987 concluded, "The data presently available are inadequate to establish the utility of these devices in the prevention and treatment of allergic respiratory disease."1
It is also recognized that not all air-filtration systems work equally well. Unfortunately, there is no one single method for evaluating these systems so they can be compared. At this time, no government standards exist for rating air cleaners or scrubbers - although private standards have been established. Furthermore, only particle removal is evaluated, with no guidelines for the comparative ability of air cleaners to remove gases. Air cleaning may achieve an additional reduction in the levels of certain pollutants when source control and ventilation do not result in acceptable pollutant concentrations. However, air cleaning alone cannot be expected to adequately remove all of the pollutants present in the typical indoor air environment. Consider these factors:
If you are going to use an AFD as an air scrubber or as a negative air machine, consider using a particle counter prior to, during and after turning off the AFD to measure the efficiency of the system that you have set up. By measuring the particle levels, you can determine whether or not and to what extent the system is beneficial.