Air Quality, Indoor
AIR QUALITY, INDOOR
While most of us are aware of the dangers posed by outdoor air pollution, awareness of airborne chemical and biological pollutants present indoors and its implications for human health is more limited. Some indoor air gases and pollutants such as radon, asbestos, carbon monoxide, biological contaminants, and volatile organic compounds (VOCs) pose a serious threat to our health and well-being. Over the past several decades, our exposure to indoor air pollutants is believed to have increased due to a variety of factors, including the construction of more tightly sealed buildings; reduced ventilation rates to save energy; use of synthetic building materials and furnishings; and use of chemically formulated personal care products, pesticides, and household cleaners. Since an average person spends increasing amount of time indoors, it is important to understand the health risks posed by prolonged exposure to indoor pollutants and the energy and comfort implications of different methods to control and mitigate these pollutants in order to ensure acceptable indoor air quality.
Acceptable indoor air quality (IAQ) is defined as "air in which there are no known contaminants at harmful concentrations as determined by cognizant authorities and with which a substantial majority (80%) of the people exposed do not express dissatisfaction" (ASHRAE, 1989). Some of these indoor air contaminants are particulates, vapors, and gases that may be generated by occupants and their activities, building materials, furniture, equipment and appliances present in indoor space, operations and maintenance activities, or brought in from outside. Examples of indoor pollutants are certain gases (radon, carbon monoxide, and carbon dioxide), volatile organic compounds or VOCs (environmental tobacco smoke, formaldehyde, solvents, and fragrances, etc.), bioaerosols (mold spores, pollen, bacteria, animal dander, etc.), and particles from buildings, furnishings and occupants (fiberglass, paper dust, lint from clothing, carpet fibers, etc.).
As the science of indoor air quality has matured, indoor air professionals have realized that many indoor air contaminants and the associated health effects are linked to specific types of buildings and their characteristics. For example, radon is primarily an indoor air concern in homes because of the ease with which it can be transported inside residential construction from the soil beneath. On the other hand, Sick Building Syndrome (SBS) primarily afflicts office building occupants who experience acute health and comfort effects that appear to be linked to time spent in a specific building.
It has been estimated that hundreds of billions of dollars per year is lost due to decreased workplace productivity and increased health costs that can be saved by maintaining good indoor air quality in commercial buildings. The financial benefits of improving IAQ can accrue from reducing costs for health care and sick leave, as well as the costs of performance decrements at work caused by illness or adverse health symptoms and of responding to occupant complaints and costs of IAQ investigations.
Indoor air quality problems have grown with the increased use of heating, ventilating, and air-conditioning (HVAC) systems in commercial and residential buildings. Greater use of HVAC systems have also resulted in the closer examination of the energy impacts of maintaining good indoor air quality. The trade-off between reducing heating and cooling loads of the HVAC system by recirculating as much indoor air as possible and providing an optimum amount of fresh outdoor air forms the underpinnings of many IAQ standards and guidelines in climate-controlled commercial buildings.
A wide variety of HVAC systems are used in residential and commercial buildings to thermally condition and ventilate the occupied spaces. While HVAC systems
can provide enhanced levels of thermal comfort in very hot or very cold weather conditions, thermodynamic processes required to condition outdoor air and deliver conditioned air to occupied spaces to maintain indoor comfort conditions are fairly energy intensive.
A typical HVAC system uses a combination of heating, cooling, humidification (adding moisture) and dehumidification (removing moisture) processes to thermally condition air. This conditioned air, which is a mixture of outdoor air and recirculated indoor air, is known as supply air. The supply airstream typically passes through filters, heat exchangers that add or remove heat from the supply airstream, a supply fan, air ducts, dampers that are used to regulate the rate of airflow, and finally diffusers located either in the ceiling or floor to the occupied space. The return air is drawn from the occupied spaces and flows back to the mechanical rooms either through return air ducts or through the plenum between suspended ceiling and the floor of the next-higher story. A portion of the return air is exhausted to the outdoors, and the remainder is mixed with the fresh outdoor air and resupplied to the space after filtering and thermal conditioning. In general, the supply air contains more recirculated air than fresh outdoor air to keep the energy cost of air conditioning down.
In an all-air system, the indoor temperature can be controlled either by a constant air volume (CAV) system, which varies the temperature of the air but keeps the volume constant, or by a variable air volume (VAV) system, which maintains a constant temperature and varies the volume of the air supplied to internal spaces.
To save energy, many HVAC systems employ a mechanism for regulating the flow of outdoor air called an economizer cycle. An economizer cycle takes advantage of milder outdoor conditions to increase the outside air intake and in the process reduces the cooling load on the system. Controlling the rate of flow of outdoor air appears simple, in theory, but often works poorly in practice. The small pressure drop required to control the flow rate of outdoor air is rarely controlled and monitored. Quite often, the damper system used to regulate the airflow is nonfunctional, disconnected from the damper actuators, or casually adjusted by building operators (Institute of Medicine, 2000).
Outdoor airflow concerns are some of the many reasons that make maintaining good indoor air quality in a controlled indoor environment a particularly challenging task. Maintaining a safe, comfortable indoor environment for workers and residents is challenging under the best of circumstances because apart from temperature control, HVAC systems are also responsible for moisture control and ventilation of buildings. Hot, humid climates, in particular, present some of the biggest challenges, and solutions that are tested and proven in temperate climates may actually worsen IAQ problems in hot, humid climates (Odom and DuBose, 1991).
INDOOR AIR POLLUTANTS: GENERATION, MITIGATION, AND EXPOSURE
To maintain acceptable indoor air quality, the concentration of pollutants known to degrade indoor air quality and affect human health must be controlled. If the origin of the contaminant is known, it is more effective to exercise source control over any mitigation strategy. If the origin of the contaminants is not known, building ventilation and air cleaning and filtration are the two most commonly used processes to dilute or remove all types of contaminants from the indoor air and maintain acceptable indoor environmental conditions.
Source control is one of the most important methods to achieve healthy indoor air. Methods vary depending on the pollutants and can range from simple solutions (not using pressed wood furniture that use formaldehyde) to complex and costly solutions (identifying moisture infiltration through the building envelope). Source control may also require behavioral changes on the part of the affected population, a remedy that may be achieved either through environmental regulation or by raising public awareness. A case in point is the changing perception toward environmental tobacco smoke (ETS). Banning smoking in public spaces or discouraging smoking in homes and in front of children can reduce the risks of second hand smoke greatly. Other examples are selecting furniture that doesn't contain formaldehyde and using paints and carpets that don't emit chemicals that are known to degrade indoor air. If the air contaminants are being transported inside from outdoor sources, precautions should be taken to either plug the pathway to stop such transport (e.g., sealing the construction joints and cracks in the basement to reduce radon
infiltration) or take steps to minimize the infiltration of contaminants (e.g., positioning the fresh air intake away from loading docks and parking spaces).
For radon, moisture, ETS, asbestos, lead-based paint, building materials and products that emit VOCs, pesticides, and household products, one can develop effective source control strategy as well. For example, assuring building envelope integrity at the time of construction of the building would help control and restrict the airflow into the building. The envelope (walls, roof, floor or slab system) should also control moisture infiltration by installing a continuous vapor barrier on the warm side of the insulation system. Operating the building at a slight positive pressure can also help in keeping the moisture outdoors. All these precautions can help control the growth of mold and mildew that thrive under moist conditions and can adversely affect the health of building occupants.
In addition to minimizing the emissions of pollutants from indoor sources, ventilation with outside air must be provided at an adequate rate to maintain acceptable IAQ. The ventilation rate—the rate of outside air supply—is usually defined per unit of floor area (liters per second per sq. meter), number of occupants (liters per person), or indoor air volume (air changes per hour). For indoor-generated particles, the effects of ventilation rate is highly dependent on particle size because the depositional losses of particles increases dramatically with particle size. The predicted change in pollutant concentrations with ventilation rate is greatest for an "ideal" gaseous pollutant that is not removed by deposition or sorption on surfaces (Institute of Medicine, 2000). Ventilation rate may have a very significant indirect impact on indoor concentrations of some pollutants because they affect indoor humidities, which in turn modify indoor pollutant sources.
Buildings are ventilated mechanically with the HVAC systems where it is a controlled process, as well as via air infiltration and through the openable windows and doors where it is largely an uncontrolled process. However, as discussed earlier, mechanical ventilation is one of the most energy-intensive methods of reducing indoor pollutant concentrations primarily because of the need to thermally condition air before it can be circulated inside the occupied spaces. It is estimated that the ventilation needs are responsible for consuming 10 percent of all the energy consumed in buildings in developed countries.
On average, buildings with air conditioning that have inadequate supply of fresh air are far more likely to suffer from poor indoor air quality than naturally ventilated buildings. On the other hand, one can find serious IAQ problems in homes and apartment buildings that are naturally ventilated as well.
There are two commonly used techniques to control odors and contaminants. Both depend on ventilation to achieve their goals. One of them relies on the concept of "ventilation effectiveness," which is defined as the ability of the ventilation system to distribute supply air and dilute internally generated pollutants by ensuring a consistent and appropriate flow of supply air that mixes effectively with room air. The second technique isolates odors and contaminants by maintaining proper pressure relationship between outdoors and indoors and between different indoor spaces. This is accomplished by adjusting the air quantities that are supplied to and removed from each room. In many large commercial buildings, particularly in warm humid climates, the design intent is to pressurize the building slightly with the mechanical ventilation system in order to prevent undesirable infiltration of unconditioned air, moisture, and outdoor air pollutants. On the other hand, smoking rooms, bathrooms, and laboratories are often depressurized so that pollutants generated within these rooms do not leak into the surrounding rooms.
Often, local dedicated exhaust ventilation is used in rooms with high pollutant or odor sources as it is more efficient in controlling indoor pollutant concentrations than general ventilation of the entire space (U.S. Environmental Protection Agency, 1991; U.S. Department of Energy, 1998). In practice, however, indoor-outdoor pressure differences are often poorly controlled, and many buildings are not pressurized (Persily and Norford, 1987). There is considerable uncertainty in predicting the rate of dilution of indoor contaminants in actual complex indoor environment, with rates of pollutant loss by deposition on indoor surfaces being one of the largest sources of uncertainty.
Air Cleaning and Filtration
Particle air cleaning is any process that is used intentionally to remove particles from the indoor air. Filtration and electronic air cleaning are the two most
|Pollutants||Major Sources||Health Effects|
|1. Infectious communicable bioaerosols contain bacteria or virus within small droplet nuclei produced from the drying of larger liquid droplets and can transmit disease.||Human activity such as coughing and sneezing; wet or moist walls, ceilings, carpets, and furniture; poorly maintained humidifiers, dehumidifiers, and air conditioners; bedding; household pets.||Eye, nose, and throat irritation; dizziness; lethargy; fever. May act as asthma trigger; may transmit humidifier fever; influenza, common cold, tuberculosis and other infectious diseases.|
|2. Infectious non-communicable bioaerosols are airborne bacteria or fungi that can infect humans but that have a non-human source.||Cooling towers and other sources of standing water (e.g., humidifiers) are thought to be typical sources of Legionella in buildings.||The best known example is Legionella, a bacterium that causes Legionnaires Disease and Pontiac Fever.|
|3. Non-infectious bioaerosols include pollens, molds, bacteria, dust mite allergens, insect fragments, and animal dander.||The sources are outdoor air, indoor mold and bacteria growth, insects, and pets.||The health effects of non-infectious bioaerosols include allergy symptoms, asthma symptoms, and hypersensitivity pneumonitis.|
|Carbon Monoxide (CO) is a colorless and odorless gas that can prove fatal at high concentrations. High carbon monoxide concentration is more likely to occur in homes.||Unvented kerosene and gas space heaters; leaking chimneys and furnaces; back-drafting from furnaces, gas water heaters, woodstoves, and fireplaces; automobile exhaust from attached garages; environmental tobacco smoke.||At low concentrations, fatigue in healthy people and chest pain in people with heart disease. At higher concentrations, impaired vision and coordination; headaches; dizziness; nausea. Fatal at very high concentrations.|
|Carbon dioxide (CO2) is one of the gaseous human bioeffluents in exhaled air. Indoor concentrations are usually in the range of 500 ppm to a few thousand ppm.||Humans are normally the main indoor source of carbon dioxide. Unvented or imperfectly vented combustion appliances can also increase indoor CO2 concentrations.||At typical indoor concentrations, CO2 is not thought to be a direct cause of adverse health effects; however, CO2 is an easily-measured surrogate for other occupant-generated pollutants.|
|Environmental tobacco smoke (ETS) is the diluted mixture of pollutants caused by smoking of tobacco and emitted into the indoor air by a smoker. Constituents of ETS include submicron-size particles composed of a large number of chemicals, plus a large number of gaseous pollutants.||Cigarette, pipe, and cigar smoking.||Eye, nose, and throat irritation; headaches; lung cancer; may contribute to heart disease; buildup of fluid in the middle ear; increased severity and frequency of asthma episodes; decreased lung function. ETS is also a source of odor and irritation complaints.|
|Fibers in indoor air include those of asbestos, and man-made mineral fibers such as fiberglass, and glass wool.||Deteriorating, damaged, or disturbed insulation, fireproofing, acoustical materials, and floor tiles||No immediate symptoms, but longterm risk of chest and abdominal cancers and lung diseases.|
|Pollutants||Major Sources||Health Effects|
|Moisture is not a pollutant but it has a strong influence on indoor air quality. In some situations, high relative humidity may contribute to growth of fungi and bacteria that can adversely affect health.||Water vapor is generated indoors due to human metabolism, cooking and taking showers, unvented combustion activities and by humidifiers; water and moisture leaks through roof or building envelope; improperly maintained HVAC equipment||Condensation of water on cool indoor surfaces (e.g., windows) may damage materials and promote the growth of microorganisms. The presence of humidifiers in commercial building HVAC systems has been associated with an increase in various respiratory health symptoms.|
|Particles are present in outdoor air and are also generated indoors from a large number of sources including tobacco smoking and other combustion processes. Particle size, generally expressed in microns (10-6 m) is important because it influences the location where particles deposit in the respiratory system (U.S. Environmental Protection Agency 1995), the efficiency of particle removal by air filters, and the rate of particle removal from indoor air by deposition on surfaces.||Some particles and fibers may be generated by indoor equipment (e.g. copy machines and printers). Mechanical abrasion and air motion may cause particle release from indoor materials. Particles are also produced by people, e.g., skin flakes are shed and droplet nuclei are generated from sneezing and coughing. Some particles may contain toxic chemicals.||Increased morbidity and mortality is associated with increases in outdoor particle concentrations (U.S. Environmental Protection Agency 1995). Of particular concern are the particles smaller than 2.5 micrometers in diameter, which are more likely to deposit deep inside the lungs (U.S. Environmental Protection Agency 1995). Some particles, biological in origin, may cause allergic or inflammatory reactions or be a source of infectious disease.|
|Volatile Organic Compounds (VOCs): VOCs are a class of gaseous pollutants containing carbon. The indoor air typically contains dozens of VOCs at concentrations that are measurable.||VOCs are emitted indoors by building materials (e.g., paints, pressed wood products, adhesives, etc.), equipment (photocopying machines, printers, etc.), cleaning products, stored fuels and automotive products, hobby supplies, and combustion activities (cooking, unvented space heating, tobacco smoking, indoor vehicle use).||Eye, nose, and throat irritation; headaches, nausea. Some VOCs are suspected or known carcinogens or causes of adverse reproductive effects. Some VOCs also have unpleasant odors or are irritants. VOCs are thought to be a cause of non-specific health symptoms.|
|Radon (Rn) is a naturally occurring radioactive gas. Radon enters buildings from underlying soil and rocks as soil gas is drawn into buildings.||The primary source of radon in most buildings is the surrounding soil and rock, well water, earth-based building materials.||No immediate symptoms but estimated to contribute to between 7,000 and 30,000 lung cancer deaths each year. Smokers are at higher risk of developing radon-induced lung cancer.|
common examples. Typically, portable air cleaning devices are used in rooms, while in typical commercial HVAC systems the filter is placed upstream of many of the HVAC components in the path of supply airstream to filter particles. Two facts that are applicable to both air cleaners and filters are
- Efficiency of any air cleaner or filter is a function of the particle size present in the indoor air and the velocity and volume of air flowing through the device.
- Pressure drop is a concern wherever filters and, to a lesser extent, air cleaners are employed in the path of normal forced ventilation system.
The technologies for removing particles include mechanical filters; electrostatic precipitators, which charge particles and then collect them onto a surface with the opposite charge; and ion generators, which charge particles and thereby facilitate their deposition. Among mechanical filters, high efficiency particulate air (HEPA) filters are highly efficient in removing particles of a wide range of sizes. However, there is little evidence of either direct health benefits or reduced concentration of pollutants resulting from air cleaning or filtration applications (American Thoracic Society, 1997). New stricter standards that also allow filter selection based on offending contaminants and their particle sizes found in buildings are more likely to show direct health benefits.
Table 1 describes some of the common indoor air pollutants found in buildings, their sources, and their adverse health effects on human beings.
INDOOR AIR QUALITY AND ENERGY
Energy conservation measures instituted following the energy crisis of 1974 resulted in the elimination of openable windows and in the recycling of as much air as possible to avoid heating or cooling outside air. The amount of outdoor air considered adequate for proper ventilation has varied substantially over time. The current guideline, widely followed in the United States, was issued by ASHRAE in 1989. To achieve good IAQ in all-air systems, large but finite amount of fresh air needs to be brought in, heated or cooled depending on the climate and season, and distributed to various parts of the building. The energy implications are obviously huge because the temperature and humidity of the supply air stream must be maintained within a very narrow range to satisfy the thermal comfort requirements of the building's occupants. Furthermore, temperature and humidity are among the many factors that affect indoor contaminant levels. Quality design, installation, and testing and balancing with pressure relationship checks are critically important for the proper operation of all types of HVAC systems and for maintaining good IAQ (U.S. Environmental Protection Agency, 1991; ASHRAE, 1989).
Energy professionals and equipment manufacturers are more cognizant of the indoor air problems and are coming out with new products and strategies that reduce energy use without degrading indoor air quality. According to the U.S. Department of Energy (1998), some of these energy efficient technologies can be used to improve the existing IAQ inside buildings:
- Using outdoor air economizer for free cooling
- —An air "economizer" brings in outside air for air conditioning a commercial building. This strategy is particularly effective in mild weather where the temperature and humidity content of outside air is suitable for increasing the rate of outside air supply above the minimum setpoint. Generally, IAQ will improve due to the increase in average ventilation rate. Care must be exercised in using the economizer cycle in regions where outdoor air is of suspect quality. Also, in very humid regions, one must employ enthalpy-based control systems to take advantages of free cooling with the economizer cycle without encountering mold and mildew problems.
- Heat recovery from exhaust air
- —If a heat recovery system allows an increase in the rate of outside air supply, IAQ will usually be improved. Proper precautions must be taken to ensure that moisture and contaminants from the exhaust air stream are not transferred to the incoming air stream. An innovative way of recovering heat and reducing the dehumidification cost is to use the waste heat to recharge the desiccant wheels that are then used to remove moisture from the supply air. In this method, the energy savings have to be substantial to offset the high cost of the desiccant wheels.
- Nighttime pre-cooling using outdoor air
- —Nighttime ventilation may result in decreased indoor concentrations of indoor-generated pollutants when occupants arrive at work. Once again, proper precautions must be taken to ensure that outdoor air with the right level of moisture content is used for this purpose, otherwise condensation on heating, ventilation, and air conditioning equipment or building components may result, increasing the risk of growth of microorganisms.
- Using radiant heating/cooling systems
- —Because of the higher thermal capacity of water compared to air, water is a better heat transfer medium. In hydronic climate conditioning systems, heat exchangers transmit heat from water to indoor environment, or vice-versa. These heat exchangers can either be convectors or radiators depending on the primary heat transfer process. The decoupling of ventilation from heating and cooling can save energy and improve IAQ. However, one must take appropriate measures to avoid condensation problems.
Research done by experts in the field as well as in laboratories has helped them understand the relationship between IAQ, ventilation, and energy. More research is needed to link specific health symptoms with exposure to specific or a group of pollutants. The policy challenge will be to raise awareness of indoor air quality so that healthy, comfortable environments can be provided by energy efficient technology.
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