A recap of “Filter Forensics,” a 2016 NAFA Technical Seminar presentation
By Robert Martin, CAFS, Kimberly Clark Corporation

At the 2016 NAFA Technical Seminar, Dr. Kerry Kinney presented on a collaborative study conducted with other researchers at The University of Texas at Austin and University of Toronto. The study examined the use of HVAC filters as forensic tools to better understand what is happening in indoor environments. This article will provide a non-exhaustive recap of some of the elements of Dr. Kinney’s presentation, including the implications of what is happening in indoor environments, what was found, and what can be learned in the future based on this work.


Better understanding our indoor environments is critical. Americans spend about 90% of their time indoors, and nearly two-thirds of their time in their homes. This fact is especially striking given that the concentration of indoor pollutants is higher than that of outdoor pollutants, including microorganisms & allergens, Semi-volatile Organic Compounds (SVOCs), and heavy metals. These pollutants can manifest themselves in a variety of ways, and one of the most prevalent is childhood asthma, which is the leading cause of disability among U.S. children and rates of incidence are increasing, particularly among children in low-income households.

Asthma should be of particular interest for NAFA members, as there are a variety of environmental triggers for asthma that interact with air and air filtration systems. These factors that have been linked to asthma development or severity include allergens, bacteria, mold & dampness, fungal species, indoor chemical use, and endotoxins, among others.

What were they looking for?

Previous research had focused on settled dust samples and allergens (dust mites, cockroaches, endotoxins, mouse and others). Research involving DNA sequencing has also revealed considerable diversity of bacteria and fungi present in the built environment. Better understanding this microbiome could give researchers the ability to better understand bacterial threats within indoor environments and give new insights on allergens within the space.

Additionally, the research investigated semi-volatile organic compounds, which are contained in building materials and consumer products. SVOCs can be present in high concentrations (10%-40% by weight) and because they’re not chemically bound in polymers, they can move into their surrounding environment. These findings can be alarming given that SVOCs can influence changes in development of the reproductive tract, as well as brain and nerve tissue. They are also linked to prenatal mortality, reduced growth and birth weight, hormone disruption, thyroid function alteration, decreased semen quality, and permanent learning and memory impairment. Biomonitoring data suggest that over 95% of American adults are exposed to phthalates and PBDEs (SVOCs). Even more concerning, the U.S. EPA estimates that the effective intake is significantly higher in infants and children than in adults.

Lastly, the research looked at heavy metals, including lead, which can impact IQ and have neurodevelopment effects in children. Sources of lead exposure can include piping as with the water crisis in Flint Michigan, soil lead, and lead paint.

Filter Forensics

Typical methods for sampling indoor environments have some limitations. When sampling air, the results tend to reflect a short-term view of pollutants in the environment, and be limited by variations in the time and space of where the sampling is conducted. When sampling dust that has settled out of the air, there is an unrepresentative bias towards large particles that are more likely to settle out of the air, and there is the potential for variability between sampling locations.

‘Filter forensics’ seeks to alleviate some of the issues with typical sampling methods. By using HVAC filters as airborne particle samplers, researchers can gain a long-term perspective on what particles are in the air and being capture within the space. Additionally, HVAC filters are widely available and easily accessible, making them a relatively easy tool to implement in studying indoor environments.


In initial studies, the filter forensics generally had lower concentrations of heavy metals than other methods of testing, potentially due to larger particles that are too heavy to make it to the filter having a higher degree of metal concentrations. Even so, heavy metal concentrations in the filter ranged from approximately 1 microgram per gram to approximately 20 micrograms per gram.

Another study examined filters used in residential homes across households that have had occupants with one of three pre-existing experiences with asthma; No asthma, asthma with prior intervention, and asthma without a prior intervention. Interestingly, the study did not find evidence that endotoxins or moldiness varied by season. Nor did endotoxin measurements differ between using filter forensics and settled particle sampling. However, the research did find a higher moldiness index using the HVAC filter sample. The research also found that the filter dust had a level of bacterial diversity that is similar to the type described in academic literature and suggests that filter forensics can provide a good sample of bacterial diversity found within a given home. Perhaps of most interest is the finding that homes of asthmatic children may have higher levels of bacterial diversity, as measured by their HVAC dust.

Some key takeaways

Clearly, indoor environments and exposures are important areas for future study. Dr. Kinney and her collaborators have demonstrated that filter forensics have the potential to be a valuable new tool in those studies by providing a readily available, time-integrated sample of indoor environments. Going forward it can be applied to a variety of exposure and health studies, as well as investigate the impact of buildings on their occupants, and examining how environmental factors impact indoor air quality.

Some of the ongoing and future work in this field will include examining how indoor particulate concentrations change over time on a filter, and how filters can help improve IAQ, including examining the relationship between filter levels and their impact on estimates of airborne particle concentrations. As a subject near and dear to the NAFA membership, I’ll be looking forward to Dr. Kinney and her associates presenting their findings at a future conference!


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