Is That House an Air Filter?

January 02, 2013
January/February 2013
A version of this article appears in the January/February 2013 issue of Home Energy Magazine.
Click here to read more articles about Indoor Air Quality

Over the last few decades, epidemiological research has consistently shown that elevated outdoor concentrations of airborne particulate matter are associated with increases in a variety of adverse health effects, including respiratory symptoms, stroke, heart attack, and mortality. Pope and Dockery (2006) provide an excellent review of many of these previous studies, although many other excellent studies also exist. Most of these important associations have been found using outdoor particle concentration data, but only recently have we begun to realize that much of our exposure to outdoor particulate matter actually occurs inside buildings, particularly in residences where we spend most of our time. Additionally, differences in indoor exposures to outdoor particulate matter may actually drive some of the differences in adverse health effects observed in different regions (Chen et al. 2012; Hodas et al. 2012).

This home, built in 1917, had the third highest outdoor particle infiltration rate in this study. (Brent Stephens)

Not even the plastic flamingos in this home's front lawn could stop outdoor particulate matter from infiltrating indoors. (Brent Stephens)

This 1950 home had the 4th highest penetration factor and outdoor particle infiltration rate. (Brent Stephens)

This 1955 home had the 5th highest penetration factor of homes from this study. (Brent Stephens)

This 2011 net-zero-energy-capable home had the lowest penetration factor and air exchange rate of any home in this study when relying on natural infiltration. (Brent Stephens)

This 1940s home had recently been renovated down to the studs and its penetration factor and particle infiltration rate were both much lower than other original homes of similar vintage. (Brent Stephens)

Field equipment consisted of a blower door, CO2 tank, CO2 analyzer, mixing fans, and particle instrumentation. (Brent Stephens)

To date, only a handful of researchers have experimentally investigated how efficiently outdoor particulate matter transports into indoor residential environments, in part because measuring particle infiltration is a time-consuming, invasive, and costly venture. So I spent the last few years working to refine particle penetration/infiltration (I use these terms interchangeably) test methods and applying them in a variety of single-family homes.

Here I will summarize some of my recent findings on how residential building envelopes can act as particle filters, how that action may vary across the building stock, and how it relates to the home energy profession.

(Note that I performed this research as a Ph.D. student at the University of Texas at Austin and am now a faculty member in the Department of Civil, Architectural and Environmental Engineering at Illinois Institute of Technology, IIT, in Chicago, Illinois.)

Outdoor Particle Infiltration

Airborne particles are emitted from a variety of both natural and anthropogenic sources, and their associated health impacts depend largely on their size and chemical composition. EPA sets ambient air quality standards for outdoor particulate matter using two forms of measurement: (1) the mass concentration of all particles less than 10 micrometers in diameter (PM10) and (2) the mass concentration of all particles less than 2.5 micrometers in diameter (PM2.5). PM2.5 is also referred to as fine particulate matter and can penetrate deep into the human respiratory system where gas exchange occurs with the blood. Ultrafine particles (those less than 100 nanometers, or 0.1 micrometers) are another important class of particulate matter, particularly near busy roadways, where concentrations are typically elevated due to traffic sources.

In buildings that rely on infiltration for ventilation air, which still represent the vast majority of single-family detached residences in the United States, outdoor particles can transport indoors via leaks in the building envelope. I should note that particles can also transport through open doors and windows, as well as through mechanical ventilation systems, as discussed in a previous article in this magazine (see “Preventing Particle Penetration,” HE Mar/Apr ’04, p. 12), but my work focused only on the building envelope with doors and windows closed. In this situation, the penetration of outdoor particles across leaks in building envelopes and into indoor environments depends upon several factors, including the geometry of openings, indoor-outdoor pressure differences, the amount of airflow through openings, air exchange rates, and particle size (Liu and Nazaroff 2001). Most of these parameters are difficult (or impossible) to measure in real buildings, so we must rely on experimental investigations to learn something more meaningful about outdoor particle infiltration.

Building envelopes can be described most fundamentally in terms of their penetration factor. A penetration factor of 1 means that 100% of outdoor particles infiltrate without depositing inside the building envelope; in other words, the envelope has a 0% removal efficiency and all particles of outdoor origin can infiltrate indoors. Conversely, a penetration factor of 0 means that no outdoor particles infiltrate indoors and the envelope provides 100% removal efficiency. Therefore, lower values of penetration factors are preferred in terms of indoor exposures to outdoor particulate matter. Previous researchers have measured the penetration factors of building envelopes using a variety of methods, finding that the building envelope typically prevents anywhere between 0% and 90% of outdoor particles from infiltrating indoors, depending on particle size and some largely unidentified building characteristics (Chen and Zhao 2011, and references therein).

To determine the absolute importance of outdoor particle infiltration into a home, penetration factors can also be multiplied by the air exchange rate (the air exchange rate, or AER, is the rate at which indoor air is replaced with outdoor air, in units of inverse hours). The product can be used to describe the particle infiltration rate, or the rate at which outdoor particles infiltrate into a space (also in units of inverse hours). The term particle infiltration rate thus accounts for both the efficiency of the envelope for removing particles and the amount of particle-laden air flowing through the envelope at any given time.

Specific particle penetration tests have been performed in only a few buildings worldwide, so we do not yet know for certain what building characteristics drive differences in particle infiltration. However, one study of note, Thatcher et al. (2003), measured particle infiltration into two homes in California in the early 2000s and, importantly, found that particles of nearly all sizes infiltrated more easily (that is, had higher penetration factors) into the leakier of the two buildings (air leakage was measured by blower door tests). This study provided motivation for the major question in my work, which was: How closely related are particle penetration and envelope air leakage?

We’ve learned a whole lot over the years about air leakage in residential buildings. For example, researchers at the Lawrence Berkeley National Laboratory (LBNL) maintain a large envelope leakage database. They have blower door test data for nearly 150,000 homes in the United States. Previous modeling studies have shown us that particle penetration through building envelope leaks should vary, quite intuitively, with the size and shape of leakage pathways, but this has not been experimentally verified. Therefore, my contribution to this work was to refine a particle infiltration test method such that it could be performed relatively quickly in a larger number of homes, so I could then explore the relationship between outdoor particle penetration and envelope air leakage. The ultimate goal of this work was to learn a lot about something that is quite difficult to measure (particle penetration) from something that is easy to measure and ubiquitous (envelope leakage measured with a blower door).

Particle Infiltration Test Method

Measuring the penetration factor of the building envelope requires accurate time-resolved measurements of both indoor and outdoor particle concentrations, as well as the air exchange rate of the building during the time of testing. Solving for particle penetration factors mathematically also requires knowledge of another parameter: the indoor particle loss rate, which accounts for deposition to indoor surfaces and removal by an HVAC filter if one is installed and the HVAC system is operating. To solve for two variables with only one mass balance equation on particle concentrations in a home requires either (1) sufficient measurement time to observe large changes in indoor and outdoor particle concentrations (Rim et al. 2010) or (2) some tricky building manipulations to measure indoor particle removal and outdoor particle infiltration independently in separate stages (Chao et al. 2003; Thatcher et al. 2003). Because it is much easier to perform infiltration measurements accurately in unoccupied buildings where people and pets can’t disturb the measurements, I decided to rely on building manipulations to minimize the amount of time spent in the test homes that I visited.

My particle penetration test method consisted of measuring indoor and outdoor particle concentrations simultaneously at one-minute intervals. Upon arriving at each house (after I had politely arranged for homeowners to vacate the house after letting me inside), I installed particle counters indoors in a central location (typically the kitchen or living room) and outdoors in a convenient location. I used condensation particle counters, which measure the number concentration of particles from 20 nanometers to 1,000 nanometers (or 1 micrometer) in real time. These instruments, while not size resolved, are almost entirely representative of submicron particles (those less than 1 micrometer) and are also considered mostly representative of ultrafine particles (those less than 100 nanometers, or 0.1 micrometers).

Once particle instrumentation was installed, I installed a blower door fan and frame in a doorway and performed a multiple-point depressurization test, followed by a multiple-point pressurization test. These tests served two purposes. The first was to characterize envelope leakage. The second was to artificially elevate indoor particle concentrations near outdoor levels. I kept the blower door operating and, while pressurizing the building, opened a door or window on the other side of the house. This was necessary to (1) artificially elevate indoor particle concentrations near outdoor levels and (2) replace whatever existing indoor aerosol I encountered with particles only of outdoor origin. I had to do this because the particle monitors I used did not provide size-resolved measurements, so I needed to be measuring from essentially the same particle distribution indoors and outdoors to ensure accuracy.

During this elevation procedure, I also turned on the central HVAC system, ceiling fans, and installed box fans around the house to encourage mixing, because the equations that I used to solve for penetration factors, loss rates, and air exchange rates require the assumption of complete mixing in the indoor environment. After the initial particle elevation, I removed the blower door fan and frame and closed all doors and windows. I quickly injected CO2 indoors from a small tank typically used for soda machines. This elevated indoor CO2 levels relative to outdoors, which allowed me to use CO2 decay to measure the air exchange rate in the homes. Once both particle and CO2 concentrations were elevated, I left the house unoccupied for a period of two to four hours while the instruments measured the subsequent decay of both particles and CO2. That time period typically left enough time for indoor particle concentrations to decay (due to deposition to surfaces, removal by air exchange, and removal by HVAC filters) to a baseline level where they were then affected only by particles infiltrating from outdoors. An example test result is shown in Figure 1. The equipment I used in the field is shown in the photo below.

For a period of approximately two months, I took my field equipment and performed these tests in 19 single-family homes in Austin, Texas, some of which are shown in the photos throughout the article. The goal was to quickly add to our existing knowledge of outdoor particle infiltration into homes, and then use those data to explore the ability of building characteristics such as envelope leakage and building age to actually predict particle penetration.

Figure 1. An example of particle penetration test data from one home. The inset shows measured values of the deposition rate (k), the air exchange rate (?), and the outdoor particle penetration factor (P).

Penetration Factors of 20-1,000 Nanometer Particles

Figure 2. Penetration factors measured in 19 homes in Austin, Texas.

Penetration Factors and Leakage

Figure 3. Association between penetration factors and blower door leakage coefficients. Note that Site 18 is excluded from this analysis as an outlier.

Particle Infiltration Results

Results of the particle penetration tests are shown in Figure 2. Penetration factors (P) ranged from 0.17 to 0.72 in the 19 homes, with an average of 0.47. These results mean that, on average, the envelope blocked about half of the outdoor particles and about half were allowed to enter, but with considerable variation between homes. For example, the most protective building envelope allowed only 17% of outdoor submicron particles to pass through the building envelope and enter indoors (that is, 83% of particles were removed). The least protective envelope allowed 72% of outdoor submicron particles to enter the indoor environment (that is, the envelope removed only 28% of outdoor particles). Note that there are some estimates of uncertainty associated with each of these measurements, but they generally hovered near ±10%, as discussed in the full paper (Stephens and Siegel 2012).

To put these numbers into perspective, the least protective envelope (with a removal efficiency of 20–1,000 nanometer particles of ~28%) can be thought of as performing like a medium-efficiency HVAC filter (that is, a MERV 6–8 filter). The most protective envelope (with a removal efficiency of 20–1,000 nanometer particles of ~83%) can be thought of as performing similar to a very high efficiency HVAC filter (that is, a MERV 15–16). Note that these ballpark estimates are made using average removal efficiencies of 20–1,000 nanometer particles for the filters modeled in Kowalski and Bahnfleth (2002); ASHRAE Standard 52.2 covers a different range of particle sizes (0.3 to 10 micrometers) than what I actually measured. Even with this uncertainty, we can see that particle removal efficiencies of building envelopes can vary drastically between homes.

Blower Door Air Leakage and Vintage

These measurements in 19 homes substantially increased our knowledge of particle infiltration into homes, but we can also learn more from the data. Data from the blower door depressurization tests at each home were used to calculate several envelope leakage parameters in accordance with ASTM E 779. Parameters included the leakage coefficient (C, m3 s-1 Pa-n), the estimated leakage area (ELA, cm2), normalized leakage (NL, dimensionless), and the air changes per hour at 50 Pa (ACH50, hr-1). Note that the leakage coefficient, C, is used to establish the empirical relationship between airflow through leaks in the envelope and the pressure difference across the envelope (and thus has strange, mostly meaningless, units). A large leakage coefficient means there are a large amount of leaks in an envelope. Similarly, the estimated leakage area, ELA, provides an estimate of the area of leaks in the envelope, and the normalized leakage parameter, NL, provides an estimate of the area of leaks in the envelope relative to the size of the building. Finally, ACH50 provides an estimate of the flow through the leaks when the house is depressurized to 50 Pa divided by the volume of the house.

In addition, I looked up the homes in the county tax appraisal database to record their initial year of construction. The newest home was built in 2011 and the oldest home was built in 1917 (the average home was 45 years old). One home, built in 1948, was excluded as an outlier in the rest of this analysis because it had been drastically renovated (down to the studs) in the previous year, and its leakage characteristics had obviously changed.

The first relationship I explored using data from the remaining 18 homes was between measured particle penetration factors and results from blower door tests. Interestingly, penetration factors were significantly correlated most strongly with the leakage coefficient, C, or the parameter that provides an estimate of the amount of leaks in an envelope. This means that leakier buildings were significantly less effective at blocking outdoor particles from infiltrating indoors. However, the actual predictive ability of the leakage coefficient alone was not very strong, as shown in Figure 3.

Linear Regressions of Outdoor Infiltration Rates

Figure 4. Linear regressions of outdoor particle infiltration rates (penetration factor × air exchange rate) versus three blower door leakage parameters. Note that Site 18 is excluded from this analysis as an outlier.

Penetration Factors and Infiltration Rates Versus Year of Construction

Figure 5. Particle penetration factors and outdoor particle infiltration rates versus year of construction of 18 homes in this sample. Note that Site 18 is excluded from this analysis as an outlier.

learn more

Portions of this article were first published in Indoor Air, Stephens, B. and Siegel, J. A. (2012), “Penetration of ambient submicron particles into single-family residences and associations with building characteristics.” Indoor Air, 2012, 22(6):501-512.

The article is available for download through,

To learn more about Dr. Stephens' Built Environment Research Group at Illinois Institute of Technology, go to

Lawrence Berkeley National Laboratory (LBNL) maintains a database of envelope leakage data as measured by blower doors. You can find that information at

Chan, W., et al. “Analyzing a Database of Residential Air Leakage in the United States.” Atmospheric Environment 39, no. 19 (2005): 3445–55.

Chao, C. Y. H., M. P. Wan, and E. C. K. Cheng. “Penetration Coefficient and Deposition Rate As a Function of Particle Size in Non-smoking Naturally Ventilated Residences.” Atmospheric Environment 37, no. 30 (2003): 4233–41.

Chen, C., and B. Zhao. “Review of Relationship Between Indoor and Outdoor Particles: I/O Ratio, Infiltration Factor and Penetration Factor.” Atmospheric Environment 45, no. 2 (2011): 275–88.

Chen, C., B. Zhao, and C. J. Weschler.. “Indoor Exposure to ‘Outdoor PM10.’” Epidemiology 23, no. 6 (2012): 870–78.

Gunier, R. B., et al. “Traffic Density in California: Socioeconomic and Ethnic Differences Among Potentially Exposed Children.” Journal of Exposure Analysis and Environmental Epidemiology 13, no. 3 (2003): 240–46.

Hodas, N., et al. “Variability in the Fraction of Ambient Fine Particulate Matter Found Indoors and Observed Heterogeneity in Health Effect Estimates.” Journal of Exposure Science and Environmental Epidemiology 22, (2012): 448–454.

Kowalski, W. J., and W. P. Bahnfleth. “MERV Filter Models for Aerobiological Applications.” Air Media Summer 2002: 13–17.

Liu, D., and W.W. Nazaroff. “Modeling Pollutant Penetration Across Building Envelopes.” Atmospheric Environment 35, no.26 (2001): 4451–62.

Pope, C. A., and D. W. Dockery. “Health Effects of Fine Particulate Air Pollution: Lines That Connect.” Journal of the Air & Waste Management Association 56, no.6 (2006): 709–42.

Rim, D., L. Wallace, and A. Persily. “Infiltration of Outdoor Ultrafine Particles into a Test House.” Environmental Science & Technology 44, no. 15 (2020): 5908–13.

Sacks, J. D., et al. “Particulate Matter-Induced Health Effects: Who Is Susceptible?” Environmental Health Perspectives 119, no.4 (2010): 446–54.

Stephens, B., and J. A. Siegel. “Penetration of Ambient Submicron Particles into Single-Family Residences and Associations with Building Characteristics.” Indoor Air 22, no. 5 (2012).

Thatcher, T. L., et al. “A Concentration Rebound Method for Measuring Particle Penetration and Deposition in the Indoor Environment.” Aerosol Science & Technology 37, no.11 (2003): 847–64.

Figure 4 shows another interesting way to look at the data by combining particle penetration factors (P) with measured air exchange rates (AER). Both of these parameters were individually higher in leakier buildings, so the product of the two parameters (the particle infiltration rate, in units of inverse hours) was jointly much greater. Using this term, it appears that blower door test results can actually offer some predictive ability for estimating the rate at which outdoor particles enter the indoor environment.

As shown in Figure 4, each of the three blower door parameters was strongly correlated with outdoor particle infiltration rates. In fact, ACH50, the parameter in this graph with which you are all probably most familiar, actually showed the strongest correlation. So there you have it: Outdoor submicron particles infiltrated in significantly greater numbers into leakier buildings than into tighter buildings due to the combined effects of higher penetration factors and air exchange rates. This is a very intuitive result, but the magnitude of differences between the leakiest and tightest homes is important. Outdoor particle infiltration rates varied from 0.02 per hour in the tightest home to 0.62 per hour in the leakiest home—a factor of 31 difference in a sample of only 18 buildings!

In addition, as any home energy professional can attest, older homes are also typically leakier than newer homes, so we can also explore these data by the initial year of construction. Figure 5 shows penetration factors and outdoor particle infiltration rates plotted versus the year in which each home was built.

Again, the correlation was stronger when comparing outdoor particle infiltration rates, where both air exchange rates and particle penetration factors were higher in older (and leakier) buildings. Although there is a certain amount of scatter in the data, the overall trends are apparent. The largest three outdoor particle infiltration rates were measured in the three oldest homes, while the smallest outdoor particle infiltration rate was measured in the newest home.

What it Means for Home Energy Professionals

These results suggest a few important things. For one, these data might actually help explain some of the socioeconomic disparities that have been observed in associations between adverse health effects and elevated outdoor particle concentrations, particularly for lower-income and minority groups (Sacks et al. 2010). This research suggests that not only do low-income and minority groups disproportionately live near busy roadways with elevated outdoor concentrations of submicron particles (Gunier et al. 2003), but they probably experience higher indoor exposures to outdoor particles because they also live in disproportionately older and leakier homes. For example, according to a study by researchers at LBNL, approximately half of all homes in the United States have normalized leakage (NL) values from blower door tests less than 0.5, while only about 20% of low-income homes in the United States are that airtight (Chan et al. 2005). In the eight homes in this study that had an NL less than 0.5, I measured an average outdoor particle infiltration rate of 0.11 per hour. Conversely, approximately half of all low-income homes in the United States are estimated to have an NL greater than 1; I measured an average particle infiltration rate of 0.40 per hour in the six homes in this sample that were leakier than NL = 1 (a difference of nearly a factor of four). Therefore, these data suggest that all else being equal, a greater percentage of low-income occupants may be exposed to higher indoor levels of outdoor submicron particles than the rest of the population.

As home energy professionals who suggest or perform envelope retrofits and air sealing in existing homes, it turns out that you may be affecting your clients’ indoor exposure to outdoor particulate matter without knowing it, potentially for the better. If measurements like these continue to show strong relationships between particle infiltration and envelope leakage, home energy professionals may be able to use reduced indoor exposure to outdoor particulate matter as another reason to advocate for energy efficiency retrofits (see “Creating Healthy and Energy-Efficient Housing—What Does the Research Tells Us?” HE Sept/Oct ’12, p. 26). This holds true particularly for those of you working in low-income homes as part of weatherization assistance programs. Logical next steps in research that we intend to conduct at IIT include (1) taking more detailed size-resolved measurements of these same parameters in a wider variety of homes in the city of Chicago; (2) taking these same measurements before and after energy efficiency retrofits are performed on the envelope; and (3) exploring the potentially negative effects of air sealing on the ability of buildings to dilute indoor concentrations of indoor-generated airborne pollutants.

Brent Stephens is an assistant professor in the Department of Civil, Architectural and Environmental Engineering at Illinois Institute of Technology in Chicago, Illinois, where he runs the Built Environment Research Group.

I would like to extend thanks to all of the participants in this study. I would also like to acknowledge Jeffrey A. Siegel, my Ph.D. adviser at the University of Texas at Austin, and Lance Wallace, who provided valuable advice on quality assurance in this study. This work was funded in part by the National Science Foundation (IGERT Award DGE 0549428) and a Continuing Fellowship from the Graduate School at the University of Texas at Austin.</p

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