Indoor Air Quality -- Fundamentals of Building Science

Indoor Air

It is estimated that most people spend as much as 90% of their time indoors (about 70% in our home). U.S. Environmental Protection Agency (EPA) studies of human exposure to air pollutants suggest that pollutants in indoor air may be 2 to 5 times greater, and occasionally more than 100 times higher, than out door levels. Indoor air quality affects both comfort and health.

U.S. EPA is a federal agency created in 1970 to permit coordinated governmental action for protection of the environment by systematic abatement and control of pollution through integration of research, monitoring, standards setting, and enforcement activities.

According to the U.S. EPA, poor quality of indoor air is the third leading cause of death and claims an estimated 335 000 lives/year. Additionally, deaths attributed to poor indoor air quality have risen faster than most other major diseases in the last decade. Worldwide, according to the World Health Organization, 1 out of 3 workers are in a workplace that is making them sick. Thus, the quality of indoor air has become a primary concern in all types of buildings. As a result, in addition to tending to the thermal comfort of occupants in the building, the designer must address the quality of indoor air.

In normal breathing, a typical human breathes in about 0.25 ft^3 (7 L) per min; the equivalent of about 360 ft^3 (10 000 L) per day. One ft^3 (about 4 L) of indoor air contains about 400 million particles, which equates to a typical human inhaling about 40 billion particles a day. Some of these particles adversely affect comfort and health.

Indoor Air Quality

Indoor air quality (IAQ) refers to the physical, chemical, and biological characteristics of air in the interior spaces of a building or facility. The quality of indoor air can be influenced by many factors, even though a building does not have the industrial processes and operations found in factories and plants. A building heating, ventilating, and air conditioning system that is properly designed, installed, operated, and maintained can promote indoor air quality. When proper procedures are not followed, indoor air problems may result. Indoor air must satisfy three basic requirements, including having normal concentrations of respiratory gases, adequately diluted airborne contaminants, and be thermally satisfactory.

As a result of efforts to save energy and lower utility costs, buildings are being built tighter than ever, with thicker walls, more insulation, and better windows and doors. Making structures more energy efficient does, however, exact an unexpected price: Clean outdoor air stays out and indoor air pollutants such as smoke particles, dust, bacteria, fungus, mold spores, and chemical vapors become trapped inside. This leaves the potential for pollutants to build up to harmful levels.

Poor IAQ can lead to serious health, productivity, and financial consequences. Pollutants found in the indoor air have been shown to heighten many of the health conditions from which people suffer. People exposed to indoor air pollutants over a long period of time may become more vulnerable to outdoor air pollutants, especially groups that include the young, the elderly, and the chronically ill who suffer from respiratory and cardiovascular diseases. Additionally, re search indicates that poor air quality results in several negative effects on the productivity of employees in office buildings.

Tbl. 10 substances that potentially cause indoor air pollution in buildings. Compiled from various sources.

Source:

  • Cleaning and maintenance
  • Equipment and work activities
  • Building materials and furnishings
  • Human activities
  • Outside pollution

- - - -

Substances:

  • Cleaning chemicals /Pesticides
  • Copy machines (ozone)
  • Office supplies such as glues and correction fluids
  • Printing machines
  • Laboratory use of chemicals
  • Damaged asbestos insulation, fireproofing, and flooring lead from paint
  • Formaldehyde from furniture, curtains, and carpeting
  • Smoking
  • Cosmetics, soaps, and lotions
  • Preparing food
  • Exhaust from vehicles industrial pollution
  • Dumpsters and other unsanitary debris near air intakes
  • Leakage from underground fuel tanks and landfills

Ill. 15 Evidence of mold growth in joints of bathtub surround tile.

Ill. 16 Evidence of mold growth under wallpaper; the cause was leakage in the return ducts.

Indoor Air Contaminants

Several indoor air contaminants exist in buildings that fall into roughly three categories:

• biologically active organisms (i.e., bacteria, viruses, mold, and spores)

• gases and other odor-causing compounds (products of combustion, radon)

• fine particulate matter (i.e., asbestos, dust) Substances that potentially cause indoor air pollution in buildings are listed in TBL. 10. Indoor air contaminants include the following:

Biological Contaminants

Biological contaminants, also referred to as microbials, are a living organism; was living; or was a product of something living. A bioaerosol is a biological contaminant that is airborne and causes indoor air problems. Some release spores into the air and present the biggest health concern. Examples of biological contaminants are the following:

Bacteria

Bacteria are simple, one-celled microscopic organisms of which less than 1% are infectious and cause disease in humans. When infectious, bacteria enter the body and cause illness by invading "good" cells, rapidly reproducing, and producing toxins, which are powerful chemicals that damage specific cells in the tissue. Bacteria can flourish in dust, HVAC systems, swimming pools, physical therapy equipment, showers, whirlpool spas, and potting soil.

Viruses

Viruses are tiny microscopic capsules that contain genetic material (DNA or RNA). They can be inhaled, absorbed through the skin, or ingested and need a suitable host to reproduce.

Types of viruses include the common cold (rhinovirus _ 200 more), influenza (flu), viral hepatitis, avian influenza (bird flu), mononucleosis (mono), Ebola, and human immunodeficiency virus (HIV). Viruses may be spread around an office via the mechanical ventilation system.

Molds

Molds are microscopic fungi. Fungi grow everywhere. To thrive, mold requires air, moisture (40 to 60% relative humidity), and food (paper, wood, drywall, insulation, and natural fibers). It manifests itself in a variety of appearances, such as black, grey-brown, grey-green, white with orange spots, or pink or purple splotches. All homes and buildings have molds, with about 160 species commonplace. Evidence of mold growth is shown in Ill. 15 through 17.

Mold spores vary in shape and size (2 to 100 m). They become airborne and may travel in several ways. They may be passively moved (by a breeze or water drop), mechanically disturbed (by a person or animal), or actively discharged by the mold (usually under moist conditions or high humidity). Spores can be inhaled, absorbed through the skin, or ingested with our food. People with allergies, immune suppression, or underlying lung disease are more sensitive and susceptible to infections.

Toxic black mold and other fungi produce volatile organic compounds (VOCs) that irritate mucous membranes and the central nervous system.

Ill. 17 Evidence of mold growth caused by small leaks and inadequate airflow.

Ill. 18 Grains of pollen (magnified).

Allergens

Many indoor sources produce allergenic particles including the feces of dust mites, which are minute creatures that feed on the dead skin that humans shed every day; the excrement of cock roaches; the dander and hair from animals (dogs, cats, rats, and mice); and pollens from flowering plants. Magnified grains of pollen are shown in Ill. 18.

Biological contaminants can cause or exacerbate many undesirable health effects. Allergic reactions are the most common health problem. Symptoms often include watery eyes, runny nose and sneezing, nasal congestion, itching, coughing, wheezing and difficulty breathing, headache, dizziness, and fatigue. Respiratory disorders (e.g., runny nose, cough, nasal congestion, and aggravation of asthma), hypersensitivity diseases, and infectious diseases can also result. There have been many reports linking health effects in office workers to offices contaminated with moldy surfaces and in residents of buildings contaminated with fungal growth.

Many IAQ concerns tied to microbials are related to moisture problems such as water intrusion from plumbing and roof leaks, and floods, or condensation buildup from excessive humidity within a space. The need for energy conservation in buildings over the past several decades has introduced new construction materials and techniques. It has also introduced concerns with molds. The building does not breathe as easily, trapping moisture vapors inside the building envelope cavities. Additionally, leaky roofs, windows, and plumbing from poor construction or lack of timely repairs result in the growth of mold and mildew and release of spores. The microbial spores become airborne, spreading into the air and settling inside wall cavities, behind cabinets and wallpaper, and throughout ductwork.

When moisture is present, explosive growth of microbials can occur in a surprisingly short period of time (e.g., 12 to 48 hr). Mold, mildew, and other fungi and bacteria need only oxygen (air), moisture, food, and an ideal temperature to thrive.

Nourishment comes from organic building materials such as gypsum wallboard, ceiling tile, cardboard, wood, carpets, furniture, and clothing. Liquid water is not necessary because most species propagate well in conditions of 60 to 70% relative humidity. An ideal temperature is in the range of 68° to 86°F (20° to 30°C).

One of the first examples of a biological contaminant is Legionnaires' disease. This term was coined in 1976 after a respiratory disease affected many delegates attending a convention in Philadelphia held by the American Legion of Pennsylvania.

Eventually, 182 people became ill and 32 died. Symptoms begin with a headache, pain in the muscles, and general flu-like symptoms. These symptoms are followed by high fever (some topping 107.4°F) and shaking chills. Nausea, vomiting, and diarrhea may occur. This can lead to dry coughing, breathing difficulties, and chest pain. Mental changes, such as confusion, disorientation, hallucination, and loss of memory, can occur to an extent that seems out of proportion to the seriousness of fever.

Complete recovery can take several weeks. About 10% of known cases of Legionnaires' disease have been fatal. Legionella bacteria have been found in hot water tanks, hot water propelled from showerheads and faucets, and in whirlpool spas. The use of hot water with production of aerosols allows Legionella, if present in the water, to get into the lungs.

Carbon Dioxide

Carbon dioxide (CO2) is a naturally occurring gas that is produced by combustion and is a by-product of the natural metabolism of living organisms. A small amount of CO2 exists in indoor air at all times and is acceptable. At moderately high concentrations, however, CO2 causes discomfort. It raises people's breathing rate and may cause minor eye irritation, particularly in people who wear contact lenses. Problems associated with high CO2 levels are drowsiness, fatigue, and the sick building syndrome.

HVAC engineers often use indoor CO2 concentrations to estimate air exchange with the outdoors. Controls used for this purpose mostly reflect comfort issues, not health concerns. CO2 concentrations should be maintained at levels below 1000 ppm (parts per million).

Tbl. 11 physiological response to various concentrations of carbon monoxide (CO). Extracted from governmental sources.

Physiological Response

Threshold limit value

  • Concentration that can be inhaled for 1 hr 4 without appreciable effect.
  • Concentration causing unpleasant 1 symptoms after 1 hr of exposure
  • Dangerous concentration for exposure of 1 hr
  • Concentrations that are fatal in exposure of 4 less than 1 hr

Parts of CO per Million / Parts of Air

  • 50
  • 400 to 500
  • 1000 to 2000
  • 1500 to 2000
  • 4000 and above

Environmental Tobacco Smoke

Environmental tobacco smoke (ETS), commonly termed as "second hand smoking" or "passive smoking," is one of the major sources of indoor air pollution. Environmental tobacco smoke is a complex mixture of more than 4000 chemicals that exist in vapor and particle states. Many of these chemicals are known toxic or carcinogenic agents. Nonsmoker exposure to ETS-related toxicants would occur in indoor spaces where there is smoking. The U.S. EPA has classified ETS as a known human carcinogen and estimates that it is responsible for approximately 3000 lung cancer deaths every year in the United States.

Children's lungs are even more susceptible to harmful effects caused by ETS. There is also strong evidence of increased middle ear effusion, reduced lung function, and reduced lung growth in children exposed to ETS. Although the trend in the United States is toward elimination of smoking, which reduces the concern, the rate of tobacco smoking remains high in many parts of the world. In Europe almost one-half of the men and one-quarter of the women smoke, and in the western Pacific area of Asia two in five men smoke.

Tbl. 12 reported risks of living with different levels of radon gas over a lifetime. From u.s. epa literature.

Combustion Pollutants

Combustion pollutants from stoves, space heaters, furnaces, and fireplaces that may be present at harmful levels in the home or workplace stem chiefly from malfunctioning heating devices, or inappropriate, inefficient use of such devices. Another source can be motor vehicle emissions from, for example, proximity to a garage or a main road. A variety of particulates acting as irritants or, in some cases, carcinogens may also be released in the course of combustion. Possible sources of contaminants include gas ranges that malfunction or are used as heat sources, improperly ventilated fireplaces, furnaces, wood or coal stoves, gas water heaters and gas clothes dryers, and improperly used kerosene or gas space heaters. The gaseous pollutants from combustion sources within a building follow.

Carbon monoxide (CO) is an odorless, colorless gas that can cause asphyxiation. At high concentrations, it can kill people in minutes; at lower concentrations, it can worsen the symptoms of heart disease, or cause headache, dizziness, nausea, fatigue, and vomiting. Low-level CO poisoning is often mistaken for the flu. Susceptible groups who are especially vulnerable to low-level CO effects include pregnant women and developing fetuses, and people with cardiovascular or cerebral vascular illnesses.

The physiological reaction to various concentrations of carbon monoxide is summarized in TBL. 11.

Each year, over 200 people in the United States die from CO produced by fuel-burning appliances (furnaces, ranges, water heaters, room heaters). Others die from CO produced by cars left running in attached garages. Several thousand people go to hospital emergency rooms for treatment for CO poisoning.

Nitrogen dioxide (NO2) is a corrosive oxidant gas that can injure lung tissues at high concentrations. At lower concentrations, there is evidence that NO2 can hamper the body's immune defenses, creating increased susceptibility to respiratory infections. Researchers also report indications that the gas reduces lung function and increases the allergic response of some asthma sufferers.

Sulfur dioxide (SO2) is a colorless gas that acts mainly as an irritant, affecting the mucosa of the eyes, nose, throat, and respiratory tract. Continued exposure to high SO2 levels can contribute to the development of acute or chronic bronchitis.

Additional ventilation air is not the solution for eliminating high levels of combustion contaminants in a space. Instead, eliminating or repairing the source of combustion pollutants is the best approach. Measures to prevent high levels of combustion contaminants in a space include ensuring that fuel-fired appliances are installed according to manufacturer's instructions, and local building codes and make certain they are inspected and serviced regularly.

Ozone

Ozone (O3) is a type of oxygen molecule that has three atoms per molecule instead of the usual two (O2). It is produced from oxygen by UV radiation and is naturally present in air. It can be produced indoors by electrical discharges from electrical equipment such as photocopiers and electrostatic precipitators. Symptoms include irritation to mucous membranes in eyes, nose, and throat, headaches, dizziness, and severe fatigue. Long-term effects are lung damage at higher exposures, potential genetic damage, and premature death in people with heart/lung disease.

It mimics effects of ionizing radiation (x-rays and gamma rays).

Volatile Organic Compounds (VOCs)

VOCs are emitted as gases from certain solids or liquids, including building materials, fabric furnishings, carpet, adhesives, fresh paint, new paneling, pesticides, solvents, and cleaning agents. VOCs are consistently found at higher levels indoors than outdoors. Products used in home, office, school, and arts/crafts and hobby activities emit a wide array of VOCs including scents, hair sprays, rug, oven cleaners, dry-cleaning fluids, home furnishings, and office material like copiers, certain printers, correction fluids, graphics, and craft materials.

Methylene chloride, which is found in some common household products like paint strippers, can be metabolized to form carbon monoxide in the blood. Formaldehyde, another industrial product, can off-gas from materials made with it, such as foam, insulation, and engineered wood products (e.g., ply wood, OSB, and so on) and has been classified as a probable human carcinogen. Pesticides sold for household use are technically classified as semi-volatile organic compounds. Symptoms like nose and throat irritation, headache, allergic skin reaction, and nausea may indicate the presence of these VOCs.

Airborne Lead

Airborne lead affects children in the form of cognitive and developmental deficits, which are often cumulative and subtle.

Lead poisoning via ingestion has been the most widely publicized, stressing the roles played by nibbling of flaking paint by toddlers and by the use of lead containing foodware like glass and soldered metal-ceramic ware by adults. Among children, long-term lead poisoning is linked to irreversible learning disabilities, mental retardation, and delayed neurological and physical development. Lead toxicity may alternatively manifest itself as acute illness. Signs and symptoms in children may include irritability, abdominal pain, emesis, marked ataxia, and seizures or loss of consciousness.

The U.S. Department of Housing and Urban Development (HUD) reports that 65 million homes built before 1978, especially those built before 1950, contain some lead paint.

HUD also estimates that over 1.7 million children nationwide have elevated blood lead levels.

During the middle of the 1900s, paint containing 30 to 40% lead pigment was used on interior and exterior surfaces of buildings because of its colorfastness and hiding power. Lead was also used in pipes and soldered pipe joints for carrying drinking water in older buildings. In 1955, the paint industry adopted a voluntary standard of no more than 1% lead by weight in interior paints. The U.S. Consumer Product Safety Commission lowered the maximum allowable lead content in paint to 0.5% in 1973, to 0.06% in 1977, and to zero in 1978.

However, the presence of lead-containing soil, dust, and paint chips can continue to create potential health hazards.

Radon

Radon is a naturally occurring gas produced by radioactive decay of radium. According to the U.S. Surgeon General and EPA, when propagated in air and inhaled into the lungs, the low-level radiation emitted by the products of decay of radon (polonium) damages lung tissue and increases the risk of lung cancer over the long-term. The U.S. Surgeon General has warned that radon is the second leading cause of cancer. This adverse health effect is significantly compounded if you smoke. U.S. EPA literature advises that radon causes between 7000 and 30 000 deaths per year in the United States alone. The U.S. EPA reported risks of living with different levels of radon gas over a lifetime are summarized in TBL. 12.

Because radon is a gas, it can move freely through soil pores and rock fractures and eventually into air. Movement into a building is driven by a difference in soil gas pressure and air pressure; the soil gas attempts to equalize the lower pressure of indoor air in the building. Radon easily migrates through foundation cracks, utility (pipe and wiring) entries, seams between foundation walls and basement floor slabs, sumps, drains, and uncovered soil in crawl spaces.

Radon levels in indoor air can vary considerably from floor to floor and from day to day. Variations in levels in a building are from fluctuations in the radon's ability to freely migrate through the soil and into the building. These fluctuations are caused by changes in occupancy patterns and weather conditions that influence a building's indoor air pressure. Variations in test measurements between neighboring buildings are generally because of differences in the physical features of the buildings and geological variations at the building sites. Differences in radon quantities from region to region are generally from differences in the abundance of radium found in the underlying soils and bedrock below the building.

Several methods of mitigation for high levels of radon are used. Most involve sealing and ventilating the building envelope. Shown in Ill. s 3.19 and 3.20 is a subfloor depressurization system that ventilates a crawl space below a structural basement floor. A blower built into the system draws in gases from the soil, including radon, and exhausts them to the out doors before they enter the living space of the home.

Ill. 19 A subfloor depressurization system used to reduce radon levels by ventilating a crawl space below a structural basement floor.

Ill. 20 An inline blower draws soil gasses, including radon, and exhausts them to the outdoors before they enter the living space of the home.

Ill. 21 Asbestos fibers (magnified).

Suspended Particulate Matter

Suspended particulate matter (SPM) includes airborne particles from smoke and dust. Indoor particulate pollution may come from sources within the building or infiltrate from outdoors. In most buildings, particulate pollution is a major problem because most of the air-handling units use synthetic filters that only remove particles of sizes larger than 20 µ. The smaller sized particles enter these buildings freely, and are more dangerous because these are respirable.

Asbestos

Asbestos is the name of a group of naturally occurring materials that separate into strong, very fine fibers. Asbestos fiber masses tend to break easily into a dust composed of tiny particles about 1>1200 the size of a human hair. Magnified asbestos fibers are shown in Ill. 21. Any material that has asbestos as an ingredient is known as an asbestos-containing material (ACM). ACMs were used in older buildings as pipe, duct, furnace and boiler insulation, linings, gaskets or wrap pings; sprayed- or toweled-on surface finishes on ceilings and walls; wall and roof cement boards and shingles; flooring and ceiling tiles and sheets; and components of cooking appliances and heaters.

ACMs are of concern when the fibers are friable, that is, they can be easily crumbled or crushed and become airborne.

Stable, well-bonded ACMs are a concern only when they begin to deteriorate, can be easily disturbed, or are disturbed by drilling, sawing, sanding, or removal that causes release of asbestos fibers into the air. Released fibers remain suspended in air and can easily penetrate body tissues as they are inhaled or ingested. The durability of these fibers causes them to remain in the body for many years.

Long-term exposure to asbestos may increase the risk of several serious diseases: lung cancer; asbestosis, a chronic lung ailment that can produce shortness of breath and permanent lung damage and increase the risk of dangerous lung infections; and mesothelioma, a relatively rare cancer of the thin membranes that line the chest and abdomen. Symptoms of these diseases did not typically manifest themselves for decades. Since 1979, use of asbestos in building materials has been reduced substantially.

Sick Building Syndrome

The term sick building syndrome (SBS) was first coined in the late 1970s. SBS describes a situation in which reported symptoms among a population of building occupants can be temporarily associated with their presence in that building. Today, SBS is also known as the tight building syndrome. Typical com plaints from occupants include lethargy, headache, dizziness, nausea, eye irritation, nasal congestion, and inability to concentrate. The cause of SBS is frequently tied to poor design, maintenance, and operation of the building's ventilation system.

Other contributing elements may include poor lighting and poor ergonomic conditions, temperature extremes, noise, and psychological stresses that may have both individual and interpersonal impact. Building-related illness (BRI) is the general term for a medically diagnosable illness, which is caused by or related to occupancy of a building.

SBS should be suspected when a substantial proportion of occupants who spend extended times in a building report or experience acute on-site discomfort. SBS is the condition of a building in which more than 20% of the occupants are suffering from adverse health effects, but with no clinically diagnosable disease present. It is the condition of the building-not of the occupants. Examples include Legionnaire's disease if caught from a building's cooling tower, CO poisoning from a malfunctioning water heater, and so forth.

IAQ problems that cause building related illnesses and /or SBS include the following:

Lack of Fresh Air

If insufficient fresh air is introduced into occupied spaces, the air becomes stagnant and odors and contaminants accumulate. Lack of fresh air in occupied areas is the number one cause of SBS.

Inadequately Maintained or Operated Ventilation Systems

Mechanical ventilation systems must be properly maintained and operated based on original design or prescribed procedures. If systems are neglected, their capability to provide adequate IAQ decreases. For example, missing or overloaded filters can cause high levels of dust, pollen, and cigarette smoke to enter occupied spaces. Clogged condensate drain pans and drain lines in HVAC systems allow water to accumulate, which can lead to microbial contamination and BRI.

Disruption of Ventilation Air

The quantity of air depends on the effectiveness of air distribution. File cabinets, bookshelves, stored boxes, dropped ceiling tiles, added office walls, cubicles, and partitions can block or divert the supply of air in occupied spaces. If air circulation is disrupted, blocked, or otherwise not introduced to occupied areas, room air can become stagnant, causing increased levels of contaminants.

Poorly Regulated Temperature and Relative Humidity Levels

If the temperature and /or relative humidity levels are too high or too low, employees may experience discomfort, loss of concentration, eye and throat irritation, dry skin, sinus headaches, nosebleeds, and the inability to wear contact lenses. If relative humidity levels are too high, microbial contamination can build up and can cause BRIs.

Sources of Contamination

Chemical emissions can contribute to building related illnesses and SBS. Chemical contaminants in a building environment either originate from indoor sources or are introduced from out door sources. Common sources include emissions from office machinery or photocopiers, cigarette smoke, insulation, pesticides, wood products, synthetic plastics, newly installed car pets, glues and adhesives, new furnishings, cleaning fluids, paints, solvents, boiler emissions, vehicle exhaust, roof renovations, and contaminated air from exhaust stacks. Contaminants found in indoor environments can include radon, ozone, formaldehyde, VOCs, ammonia, CO, particulates, nitrogen and sulfur oxides, and asbestos.

Methods for Improving IAQ

Methods for improving IAQ fall into four categories: eliminating the source, ventilation, design, and operations and maintenance procedures. Practical measures for IAQ control are listed as follows:

Eliminating the Source

• Elimination: complete removal of biological agent, toxic substance, hazardous condition, and /or contamination source.

• Substitution: the intentional use of less hazardous materials wherever possible.

• Isolation: such as by encapsulation, shielding, sealing, and the use of distance.

• Housekeeping and dust suppression: keep surfaces clean of contaminants, prevent their re-dispersion, and /or eliminate personal contact.

Ventilation

• Ventilation: increase outside air or exhaust for dilution.

• Filtering and purification: filtering system and its proper maintenance.

Design

• New construction: design steps to prevent problems from occurring; procedures to commission a new building (such as air purging).

• Renovation design: avoid changes that restrict original airflow and that increase contaminant load beyond HVAC capabilities.

Operation and Maintenance

• Maintenance and work practices: specification for the proper work procedures to reduce or control contaminant releases.

• Replacement: insulation, carpeting, wall coverings, and so on, which when wet can serve as breeding grounds for microorganisms, need to be checked regu larly and replaced when damaged.

• Education, training, labeling, and warning procedures for building occupants and maintenance staff.

• Sanitary procedures and personal protective devices for building maintenance staff.

• Proper storage and disposal of some contaminants: care and good control practices.

Control of pollutants at the source is the most effective strategy for maintaining clean indoor air. Control or mitigation of all sources, however, is not always possible or practical. Ventilation, either natural or mechanical, is the second most effective approach to providing acceptable indoor air.

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