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The Benefits of Clean Air

November 17, 2008
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Microscopic particles much too small to see with the naked eye are released into the air from cars, trucks, power plants, factories, and other sources. These microscopic fine particles travel deep into the lungs where they irritate the tissue, cause inflammation, and make existing illness of the airways and circulatory system worse.

The U.S. Environmental Protection Agency (EPA), the National Academy of Sciences (NAS), the American Lung Association (ALA), and the American Heart Association (AHA) all agree that exposure to fine particles impairs breathing and increases the risk of asthma attack, stroke, heart attack, and other serious illnesses. The EPA estimates that controllable levels of fine particles contribute to 17,000 premature deaths and over 12,000 hospitalizations in the United States each year.1

Highly regarded scientific studies in the United States and elsewhere have shown that lower exposure to fine particles reduces the risk of serious respiratory and cardiovascular events and lengthens the average lifespan.2, 3, 4, 5 The benefits of lower exposure will be greatest for people who are most vulnerable to the effects of fine particle pollution, including children, the elderly, and others with asthma, diabetes, and heart conditions.

So, what is the best way for a person to reduce their exposure to fine particles of outdoor origin? Perhaps surprisingly, the answer is to lower the concentrations of these fine particles that seep from the outside into the indoor air of their home. The objective of this white paper is to explain why this is the case and how it can be accomplished. We begin by explaining where people are most exposed to fine particle pollution, next describe the practical means of reducing exposure in those locations, and wrap up by addressing the health benefits associated with lower exposure.

Figure 1. Scale of fine particles in relation to similarly-sized materials. (Click on the image for an enlarged view.)


Airborne particles (also referred to as particulate matter or PM) are tiny solid substances and liquid droplets that are light enough to remain suspended in air for days and weeks at a time. PM is generated from a wide variety of sources each of which has a unique signature of chemical composition and size.

Size is the most important characteristic of PM in terms of air pollution standards established by the U.S. Environmental Protection Agency and the 50 states. Particle size is commonly measured in units of micrometers. A micrometer is one-millionth of a meter and is about 70 times smaller than the width of a human hair. Another way to think about a micrometer is that one million of them in a line would extend only about three feet. Clearly, a micrometer is microscopic and much too small to see with the naked eye. Particles less than 2.5 micrometers in size or PM2.5 have received the greatest attention from scientists and government agencies due to concerns about its effects on human health. Figure 1 illustrates the size of a human hair, beach sand, and different sizes of particulate matter, including PM2.5.

PM2.5 or fine particles originate primarily from the burning of fossil fuel in cars, trucks, off-road vehicles, power plants, manufacturing operations, and burning of biomass from events such as wildfires. Several components of fine particles exit a vehicle tailpipe or smokestack as a hot gas and cool to form extremely small aerosols less than 0.1 micrometer that are called UFP. Exposure to UFP and potential health effects are the focus of a great deal of current research. UFP commonly increase in size as a result of chemical and physical transformations until they reach about a micrometer in diameter. Particles in this size range are so small that they move through the atmosphere very much like oxygen and other gases except that they are washed from the sky by rain and can stick onto trees, buildings, and other objects when blown by the wind.


Environmental health scientists have demonstrated very clearly that the amount of fine particles in the air that a person breathes tracks with fine particle levels in outdoor air over time.6 This may seem counter intuitive given that the average American spends 19 hours a day indoors and 15 to 16 hours a day indoors at home. However, fine particles of outdoor origin are the primary contributor to PM2.5 concentrations inside most homes and other buildings. Fine particles are so small that they easily move around door and window frames as well as through small openings in the walls and roofs of homes. Because people typically spend most of their time indoors at home, residential indoor concentrations are the primary contributor to personal exposure to PM2.5.


Concentrations of fine particles of outdoor origin indoors can be reduced by lowering the amount of outdoor air that comes into a home and using an indoor air cleaning system to remove particles from the air of a home.

The amount of outdoor air that enters a home every hour is commonly referred to as the ventilation rate. The amount of ventilation varies within a home over time and among homes based on weather, age and type of home, and the extent that windows and doors are opened. During heating season, about 10 percent to 50 percent of the air inside of a home will be typically be replaced by outdoor air each hour. The same is true during the cooling season for homes with air conditioning when windows are shut most of the time. Ventilation rates are much greater when windows are open. In that case, the air inside of a home may be replaced by outdoor every 15 to 60 minutes.7, 8, 9

All in all, we see that a relatively large amount of outdoor air comes into homes on a routine basis. In fact, a minimum amount of ventilation is beneficial, because air leaving a home carries away air pollutants that are generated by indoor sources such as cleaning agents, pet allergens, and other materials. In the final analysis, even the most tightly constructed homes will have about half of the indoor air replaced by outdoor air every hour on average throughout a year. As a result, lowering the amount of outdoor air that enters a home has only limited effectiveness as a means to control concentrations of fine particles of outdoor origin indoors.

A more effective way to control exposure to fine particles of outdoor origin is to remove them from the air inside of a home with an indoor air cleaning system. For this approach to be effective, the technology must have a high clean air delivery rate. Clean air delivery rate or CADR (sometimes pronounced “kā-dăr”) is a standard measure of performance for indoor air cleaning devices. CADR can be calculated from the airflow rate through the device and the removal efficiency for a given size or type of particle. For example, an air cleaner with a flow rate of 1,000 cubic feet per minute and removal efficiency of 90 percent would have a CADR of 900.

Research conducted by Environmental Health and Engineering Inc. (EH&E) and investigators from the Harvard School of Public Health demonstrates that high efficiency air cleaners that operate within a forced air ventilation system provide the largest CADR available on the residential market today.10 Lower efficiency in-duct systems and portable air cleaners did not perform as well in our study. For example, the high efficiency in-duct system that we tested had a CADR of 1170, the highest among the six types of air cleaners evaluated by EH&E. In comparison, standard in-duct filters and electronic air cleaners that we tested had a CADR of 3 to 750 and removal efficiencies of less than 1 percent to 55 percent.

Some details from our evaluation of indoor air cleaners in a test home are provided in Table 1. The results shown in the table are for particles between 0.3 to 0.5 micrometers, the most abundant size of particles in outdoor and indoor air. The lowest CADRs that we measured were near zero and occurred when three Ionic Breeze® units (Sharper Image Inc., San Francisco) were operated simultaneously in the test home and when a conventional 1-inch fabric filter was used in the central ventilation system. Operation of a single portable air cleaner with a HEPA filter (Hunter Fan Co., Memphis, Tenn.) produced a CADR of 230, while five portable HEPA air cleaners operated simultaneously and distributed throughout the test home had a CADR of only 630. The high efficiency in-duct system known as CleanEffects™ (Trane Residential Systems, Tyler, Texas) had a CADR of 1170, nearly double that achieved by five portable HEPA devices operating at once.

Figure 2. CONTAM modeling results comparing daily PM2.5 concentrations in three ventilation configurations (High Efficiency, Conventional, and Natural). Whiskers represent the 10th and 90th percentiles, circles represent the 5th and 95th percentiles. (Click on the image for an enlarged view.)

CADR provides a measure of how fast particles can be removed from air. This information is useful, but is not a direct measure of how much fine particle levels can be reduced within a home. Levels of fine particles in indoor air reflect a balance between the rate at which particles enter the house and the rate at which they are removed. The balance between the competing flows can be determined from measurements taken in actual homes or from calculations within a validated model.

We recently completed a detailed modeling study to estimate concentrations of fine particles of outdoor origin present in indoor air of homes with various types of filtration devices. In this study, we used a validated indoor air quality model developed by the National Institute of Standards and Technology (NIST) to simulate 1-hour average concentrations of fine particles of outdoor origin in homes of Cincinnati, Cleveland, and Columbus, Ohio, for an entire year.11 We chose those metropolitan areas because of the detailed information available on demographics, housing stock, residential ventilation systems, fine particles in outdoor air, and weather.

The findings from this study show that a high CADR whole-house air cleaning system will reduce indoor exposure to particles of outdoor origin substantially compared to conventional filtration and natural ventilation. As shown in Figure 2, operation of a high CADR whole-house system is estimated to lower PM2.5 levels by 70 to 80 percent over homes with a conventional in-duct filter or naturally ventilated homes.


The evidence of negative health effects associated with fine particle pollution is well accepted by governmental and scientific agencies, including the U.S. Environmental Protection Agency (EPA) and the National Academy of Sciences (NAS) and organizations such as the American Lung Association (ALA) and American Heart Association (AHA). The flip side of this knowledge of course is that lowering exposure will lower risks and improve health.

There are a number of clear-cut examples of where lower exposure to fine particles results in improved public health. For example, a number of years ago in Dublin, Ireland, the majority of homes were heated by burning coal. This practice led to high levels of fine particle pollution in the city. When a ban on coal burning was instituted, air pollution decreased and so did respiratory and cardiovascular-related deaths in the city.12 A similar situation occurred in Utah when a local steel mill closed and fine pollution levels in the area decreased. The reduction in pollution was associated with reductions in local hospital admissions.13

Two studies that are directly relevant to the health benefits of reduced fine particle exposure indoors showed that the risk of particle-related hospital admissions was lower in cities with a high proportion of air conditioned homes than in cities with less air conditioning.14, 15 Researchers believe that the lower health effects levels seen in these cities were because homes with air conditioning typically had lower indoor fine particle levels. In a study that directly addresses this issue, researchers in Europe showed that installing air cleaning devices in the homes of elderly participants improved the ability of their blood vessel cells to control blood pressure, a factor related to heart disease.16

In addition to the studies mentioned above, there are hundreds, if not thousands, of other research studies that have shown that short and long-term exposures to fine particle air pollution damages health. Outdoor fine particle exposures, for example, have been linked to a variety of adverse health effects, including early death, hospital admissions, asthma attacks, and more recently low birth weight, indicators of heart disease, and increased airway inflammation. The findings of these research studies linking exposure to fine particle pollution to adverse health impacts are supported by results from toxicological and clinical studies, which consistently link exposures to fine particle air pollution with early indicators of cardiovascular damage, including autonomic dysfunction and pulmonary and systemic inflammation. Similarly, studies led by researchers from the Harvard School of Public Health have been conducted where small groups of people have been exposed to high levels of fine particles. In these studies, indicators of heart disease, as you might expect, increased due to the increased exposure to fine particles. Together, these findings provide consistent and compelling evidence of fine particle pollution health impacts. These studies also demonstrate that reducing the levels of fine particles indoors has the potential to reduce adverse health effects.17

Some of the largest studies of health effects related to fine particles were conducted on healthy adults living in areas across the United States. For example, in one of the first and most important studies in the area, a research group from the Harvard School of Public Health followed over 8,000 healthy adults living in six U.S. cities for over 14 years and concluded that living in a city with higher pollution levels resulted in higher death rates.18 The association between fine particles and early death was confirmed with a much larger study of over 500,000 adults located in 151 cities across the country in a study funded by the American Cancer Society.19

We used results from those studies of human populations to quantify the likely health benefits of reduced exposure to PM2.5 of outdoor origin achieved by a high efficiency whole-house air cleaner. For this purpose, we followed a standard methodology for health impact assessments established by the U.S. Environmental Protection Agency and recommended by the National Academy of Sciences and U.S. Office of Management and Budget. Our health impact assessment built upon the exposure modeling study for the three largest cities in Ohio this is described above.

The results of the health impact assessment indicate that a high CADR in-duct air cleaner that operated with the characteristics of the Trane CleanEffects system measured in our test home would lower the risk of clinically significant health outcomes and particle-related mortality by 34 percent compared to the use of a standard furnace filter. The public health benefits for individual residences are quite substantial when generalized to the populations of the Ohio cities that live in single-family homes with central heating (see Table 2).


Exposure to fine particle air pollution is related to a number of serious health effects, such as early death, increased hospital admissions, asthma attacks, low birth weight, and indicators of both respiratory and heart disease. While it may not be possible to control your exposure to fine particles outdoors, ventilation systems with high clean air delivery rates can reduce indoor levels of fine particles in the place where you spend the majority of your time, your home. In studies conducted by EH&E, the only research to compare different types of filtration systems in a full sized home, a high efficiency, whole-house system (Trane CleanEffects) performed the best at removing fine particles from indoor air. Use of an appropriately sized whole-house system with a CADR of approximately 0.7 per square foot of home is expected to reduce the levels of indoor fine particles of outdoor origin by 70 to 80 percent over conventional filtration and natural ventilation. Lower exposure to particles of outdoor origin is associated with a substantial reduction in the risk of particle-related morbidity and mortality for individuals and meaningful decreases in the public health burden of air pollution.


1 U.S. Environmental Protection Agency, 2005. Regulatory Impact Analysis for the Final Clean Air Interstate Rule, USEPA Office of Air and Radiation, 2060-AL76.

2 Dockery, D. W., et al. 1993. An association between air pollution and mortality in six U.S. cities. New England Journal of Medicine, 329, 1753-9.

3 Jerrett, M., et al. 2005. Spatial analysis of air pollution and mortality in Los Angeles. Epidemiology, 16, 727-36.

4 Laden, F., et al. 2006. Reduction in fine particulate air pollution and mortality: Extended follow-up of the Harvard Six Cities study. American Journal of Respiratory Critical Care Medicine, 173, 667-72.

5 Pope, C. A., 3rd, et al. 1995. Particulate air pollution as a predictor of mortality in a prospective study of U.S. adults. American Journal of Respiratory Critical Care Medicine, 151, 669-74.

6 Wilson WE, Suh HH. 1997. Fine particles and coarse particles: concentration relationships relevant to epidemiologic studies. Journal of the Air and Waste Management Association, 47(12):1238-1249.

7 Sarnat, S. E., et al. 2006. Factors affecting the association between ambient concentrations and personal exposures to particles and gases. Environmental Health Perspectives, 114, 649-54.

8 Suh, H., et al. 1992. Personal exposures to acid aerosols and ammonia. Environmental Science and Technology, 26, 2507-2517.

9 Murray, D. M., et al. 1995. Residential air exchange rates in the United States: Empirical and estimated parametric distributions by season and climate region. Risk Analysis, 15, 459-465.

10 MacIntosh D, Myatt T, Ludwig J, Baker B, Suh H, Spengler J. 2008. Whole house particle removal and clean air delivery rates for in-duct and portable ventilation systems. Journal of the Air and Waste Management Association, 58: 1474-1482.

11 MacIntosh D, Minigishi T, Levy J, Myatt T. In-Duct Air Cleaning: A Modeling and Health Impact Assessment. Presented at the 136th American Public Health Association Annual Meeting. San Diego, CA, October 27, 2008.

12 Clancy, L., et al. 2002. Effect of air-pollution control on death rates in Dublin, Ireland: an intervention study. Lancet, 360, 1210-4.

13 Pope, C. A., 3rd. 1991. Respiratory hospital admissions associated with PM10 pollution in Utah, Salt Lake, and Cache Valleys. Archives of Environmental Health, 46, 90-7.

14 Franklin, M., et al. 2007. Association between PM2.5 and all-cause and specific-cause mortality in 27 US communities. Journal of Exposure Science and Environmental Epidemiology, 17, 279-87.

15 Janssen, N. A., et al. 2002. Air conditioning and source-specific particles as modifiers of the effect of PM(10) on hospital admissions for heart and lung disease. Environmetal Health Perspectives, 110, 43-9.

16 Brauner, E. V., et al. 2008. Indoor particles affect vascular function in the aged: an air filtration-based intervention study. American Journal of Respiratory Critical Care Medicine, 177, 419-25.

17 Adar, S. D., et al. 2007. Focused exposures to airborne traffic particles and heart rate variability in the elderly. Epidemiology, 18, 95-103.

18 Dockery, D. W., et al. 1993. An association between air pollution and mortality in six U.S. cities. New England Journal of Medicine, 329, 1753-9.

19 Pope, C. A., 3rd, et al. 1995. Particulate air pollution as a predictor of mortality in a prospective study of U.S. adults. American Journal of Respiratory Critical Care Medicine, 151, 669-74.

Reprinted with permission from the white paper, “The Benefits of Clean Air,” by Environmental Health and Engineering Inc., Needham, Mass. For a copy of the complete white paper, go to www.trane.com/prevention.

Publication date: 11/17/2008

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