How Beneficial is Air Filtration?

Common sense: Blowing large volumes of air in a structure will stir up large volumes of particles and cause a mess, if not a health hazard for occupants. In restorative drying, airflow is a vital part of the drying system, but what about the unintended side effect?

Common sense: Blowing large volumes of air in a structure will stir up large volumes of particles and cause a mess, if not a health hazard for occupants. In restorative drying, airflow is a vital part of the drying system, but what about the unintended side effect? Every small particle in the structure is potentially affected by this airflow. Particles that were safely lodged in crevices, on top of carpet and behind the furniture are all likely to be re-introduced to the air – and introduced into the respiratory tracts of our employees and customers. These particles may be large and relatively benign (such as pollen, dust, dander, etc.) and easy for the human respiratory system to filter out. Other particles are much smaller, respirable and potentially harmful. Filtering the air seems to be the obvious solution, according to conventional wisdom.

The IICRC S500 Standard and Reference Guide for Professional Water Damage Restoration recommends considering the use of air filtration for this reason. But how beneficial are air filtration devices in water damage restoration? There is solid evidence that air cleaners reduce particle exposure in residences by as much as 90%*. However, the evidence is incomplete and a controlled study of particles in a high airflow restoration environment was needed.

In mid-2010, the Restoration Sciences Academy (RSA) conducted a study to answer just these questions and the results still ring true today. A structure was set with an average amount of airflow – similar to what would be found in a Class 1 water loss. Particle counts were measured with a laser particle counter at specific elevations and at specific locations in the rooms tested. Tests were repeated multiple times to verify results. Here’s what was found:

Surprising news: Airflow reduces particles

In one set of testing, air movers alone were set in the areas. In these tests, the particles were measured over an hour of run time as well as after the air movers were shut down. One of the surprising results of the research was that particle counts were reduced significantly when airflow was introduced. This finding, at first glance, flies in the face of all common sense – how can there be fewer particles in the air after introducing airflow? The theory (with lots of physics involved) is that the particles are being collected or filtered by the surfaces in the area.

This surface filtration effect is much more pronounced in areas that were carpeted (68% reduction), than in areas with hard surfaces (35% reduction). This result is easy to understand if you consider the surface area of carpeting vs. hard surface flooring. Based on this result, we might be tempted to jump to the conclusion that air filtration is not needed. If a structure acts as its own filter, why should we add filtration? Well, don’t jump to that conclusion too fast!

Bad news: Removing airflow allows the particles to come back

In the testing done by the RSA, particle counts were again tested after airflow was removed to see if there was any lasting reduction in particle count. Not surprisingly, in all testing done with this study, particles significantly increased after equipment was turned off. Again, the theory of particles adhering to the surfaces during airflow, and then “letting go” when airflow is removed, helps to explain this result.

The fact that the particles “let go” of the surfaces argues for leaving air filtration on-site for the entire project. This continues to provide protection for the occupants and employees.

Verified: Air movement aerosolizes particles

In one part of the study, particle counts were lowered in a space with air filtration over a 20-hour period – with no other equipment running. The addition of airflow at that point elevated particle counts. The increase of particles was about 20%. So, airflow simultaneously deposits particles on some surfaces while removing them from others. As this particular test continued, it was found that the increase in particles caused by adding airflow was eventually countered by air filtration.


Verified: Air filtration reduces particles more than airflow alone

When air filtration was added, particles were reduced much more than airflow alone. As a net result of the use of air filtration, overall particle counts were reduced by 96% in a carpeted space. In an area where there was no carpet, the reduction was 86%. In both cases, many more particles were captured when air filtration was used. See Graph Round 3 to get a visual on the significance of air filtration in both carpeted and non-carpeted areas. The dashed line represents the particle counts with air filtration on.


Conclusion: Air filtration is beneficial

This study confirmed that using air filtration during the drying process is clearly beneficial. While drying, undesirable particulate loads in the environment will likely be elevated. Particulate present in the indoor environment may come from spaces that are not normal in the breathable atmosphere within the structure, as air is moved over affected structural materials, beneath flooring, in wall cavities and across other damaged materials.

As these foreign particles are released into the air, this study indicates that those particles are (1) distributed throughout the space and (2) deposited on surfaces. Particulate loads in both the air and on surfaces, especially when considering the potential questionable sources, create a concern for both occupant and worker health. The use of air filtration devices will eliminate the majority of this problem – by as much as 85% in just one hour of operation.

Furthermore, after drying is complete and air movement is discontinued, much of the particulate deposited onto surfaces are likely to be reintroduced into the air, making them an even more significant risk to worker and occupant health.

As indoor air quality becomes a key focus in the restoration industry, technologies and procedures that positively impact the health of the indoor environment both during and after restoration must be used. Because air filtration devices have been shown to improve indoor air quality during and after drying, their use should be a standard practice.

To see the complete data and white paper for the Restoration Sciences Academy test on “Air Filtration Devices and Airflow” visit and click on “RSA Test Labs.”    



*Kang, S. Y., Siegel, J., Novoselac, A. “Effective positioning of portable air cleaning devices in multizone residential buildings.” Indoor Air (2008): 17–22

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