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.

HVAC SYSTEM

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

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.

Ventilation

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

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.

CONCLUSIONS

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.

Satish Kumar

See also:Air Conditioning; Building Design, Commercial; Building Design, Residential; Furnances and Boilers.

BIBLIOGRAPHY

American Society of Heating, Refrigerating, and Air Conditioning Engineers (ASHRAE). (1989). Ventilation for Acceptable Indoor Air Quality. Standard 62. Atlanta: Author.

American Society of Heating, Refrigerating, and Air Conditioning Engineers (ASHRAE). (1996). 1996 ASHRAE Handbook: HVAC Systems and Equipment. Atlanta: Author.

American Society of Heating, Refrigerating, and Air Conditioning Engineers (ASHRAE). (1999). Method of Testing General Ventilation Air Cleaning Devices for Removal Efficiency by Particle Size. Standard 52.2. Atlanta: Author.

American Thoracic Society. (1997). "Achieving Healthy Indoor Air." American Journal of Respiratory and Critical Care Medicine156(Suppl. 3):534–564.

Blank, D. M. (1998). "Earning It; What's in the Office Air? Workers Smell Trouble." New York Times, February 22, Sunday Section: Money and Business/Financial Desk.

Barbosa, D. (2000). "3M Says It Will Stop Making Scotchgard." New York Times, May 17.

Dorgan, C. E., and Dorgan, C. B. (1999). "Developing Standards of Performance (SOP) for IAQ in Building." In Proceedings of the 8th International Conference on Indoor Air Quality and Climate. Edinburgh, Scotland.

Fisk, W. J., and Rosenfeld, A. H. (1998). "Potential Nationwide Improvements in Productivity and Health from Better Indoor Environments." In Proceedings of the ACEEE 1998 Summer Study of Energy Efficiency in Buildings. Washington, DC: American Council for an Energy-Efficient Economy.

Institute of Medicine. (2000). "Clearing the Air: Asthma and Indoor Air Exposures." Washington, DC: National Academy Press.

"Is Your Office Killing You?" (2000). Business Week, no. 3684, p. 114.

Liddament, M. W. (1999). "A review of Ventilation and the Quality of Ventilation Air." In Proceedings of the 8th International Conference on Indoor Air Quality and Climate. Edinburgh, Scotland.

Mendell, M. J. (1993). "Non-specific Health Symptoms in Office Workers: a Review and Summary of the Epidemiologic Literature." Indoor Air3(4):227–236.

National Academy of Science. (1999). Health Effects of Exposure to Radon: BEIR VI, Committee on Health Risks of Exposure to Radon. Washington, DC: National Academy Press.

Odom, D., and DuBose, G. H. (1991). "Preventing Indoor Air Quality Problems in Hot, Humid Climates: Problem Avoidance Guidelines." Denver, CO: CH2M Hill, in cooperation with the Disney Development Company.

Persily, A., and Norford, L. (1987). "Simultaneous Measurements of Infiltration and Intake in an Office Building." ASHRAE Transactions 93(2):42–56.

Roulet, C.; Rossy, J.; and Roulet, Y. (1999). "Using Large Radiant Panels for Indoor Climate Conditioning." Energy and Buildings 30:121–126.

Seppanen, O. (1999). "Estimated Cost of Indoor Climate in Finnish Buildings." In Proceedings of the 8th International Conference on Indoor Air Quality and Climate. Edinburgh, Scotland.

Smith, K. R. (1999). "The National Burden if Disease from Indoor Air Pollution in India." In Proceedings of the 8th International Conference on Indoor Air Quality and Climate. Edinburgh, Scotland.

U.S. Department of Energy. (1998). Indoor Air Quality Appendix to International Performance Measurement & Verification Protocol. Washington, DC: Author.

U.S. Environmental Protection Agency. (1991). Building Air Quality—A Guide for Building Owners and Facility Managers. Washington, DC: Author.

U.S. Environmental Protection Agency. (1992). A Citizen's

Guide to Radon, The Guide to Protecting Yourself and Your Family From Radon, 2nd ed. Washington, DC: Author.

U.S. Environmental Protection Agency. (1995). The Inside Story: A Guide to Indoor Air Quality. Washington, DC: Author.

U.S. Environmental Protection Agency. (2000). "EPA and EM." Headquarters Press Release, Washington, DC, May 16.