O CCUPANT C OMFORT AND H EALTH
3.3 INDOOR AIR QUALITY
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Whenever possible, rooms that require the same environmental conditions should be grouped in a single zone. This simplifies the system design and makes it less costly to install and operate. On the other hand, interior and perimeter spaces should almost always be on separate systems or control zones because of the disparity of their thermal loads. Rapid cycling of space temperature—as can occur with ON/OFF control—is yet another system and control con-sideration that can affect occupant comfort.
Decisions regarding zoning requirements and the related cost implications must be resolved through the collaboration of the design team, the owners, and, if feasible, the prospective tenants.
Appropriate zoning is critical to occupant comfort and HVAC sys-tem success. In addition to defining the size of sys-temperature control zones, the degree of control desired has considerable impact on the selection of the air-conditioning system.
man-ner sufficient to dilute or remove contaminants geman-nerated within the space to a satisfactory degree relative to occupant comfort and health. Buildings are usually ventilated by supplying filtered out-door air through the HVAC system. Different HVAC systems pos-sess different capabilities to meet ventilation requirements.
Natural airflow through open windows or through infiltration is the ventilation method of choice for many residences and other small buildings. Such airflow is variable, however, and (to a large measure) hard to quantify and control. Cooling (and indoor air quality) systems designed to utilize natural ventilation are common in Europe, but they are rare in the United States. Mechanical venti-lation dominates design in the United States.
Local and general exhaust systems usually complement a ven-tilation system by containing and removing selected contaminants at the source, as is the case with a bathroom exhaust. It is common practice to supply sufficient outdoor air through an air-conditioning system to make up for air that is exhausted plus an additional amount of air to provide building pressurization to offset infiltra-tion. Energy considerations generally suggest that the ventilation air supply not be increased above the amount needed for dilution of contaminants, except to balance exhaust airflow and provide nomi-nal pressurization. Large-scale infiltration is more effectively lim-ited by ensuring reasonable tightness of the building envelope.
Ventilation is not the only means of limiting contaminant lev-els, and it should not be considered a cure-all. Filtration, for exam-ple, is discussed in Section 3.3.4. Source control, where practicable, is most effective. Building materials, such as carpet and wall cover-ings, should be selected for low emission of volatile organic com-pounds. Physical containment or segregation of emission sources may be appropriate. Indeed, control of some sources may be beyond the capabilities of even a well-designed ventilation system.
The objectives and capabilities of a proposed ventilation system should be understood by all who are concerned with the construc-tion and operaconstruc-tion of the building.
3.3.2 Indoor Air Contaminants
To better understand the methods for controlling indoor air contamination, the designer should understand the general nature of such contaminants. Indoor air contaminants can be solids, liquids, or gases (vapors). Some can be irritants or odiferous, thus affecting occupant comfort. The same contaminants at higher concentrations, as well as others of which occupants may be unaware, can pose
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health risks. People vary in their sensitivity to contaminants.
Minute concentrations of certain fungi and other impurities can cause serious discomfort and impairment in sensitive individuals while not affecting most occupants. Standards for vapors specify a quantity of pollutant per unit volume of air, in parts per million (ppm). Standards for particles often specify the mass concentration of particles expressed in micrograms per cubic meter (µg/m3). The standards typically include all particle sizes—the total suspended particulate concentration (TSP). Large particles are filtered by the nasal passages and generally cause no adverse physiological response unless they are allergenic or pathogenic. Smaller, respira-ble suspended particles (RSP) are important because they can lodge in the lungs. Respirable particles range in size up to 5 µm.
Particles of specific interest include:
• Respirable particulates as a group
• Tobacco smoke (solid and liquid droplets), which also contains many gases (note that Standard 62.1 explicitly assumes that there is no environmental tobacco smoke in establishing typical ventilation rates)
• Asbestos fibers
• Allergens (pollen, fungi, mold spores, and insect feces and parts)
• Pathogens (bacteria and viruses), which are almost always con-tained in or on other particulate matter
Gasses/vapors of interest include:
• Carbon dioxide (CO2)
• Carbon monoxide (CO)
• Radon (decay products become attached to solids)
• Formaldehyde (HCHO)
• Other volatile organic compounds (VOCs)—comprising a wide range of specific compounds
Some contaminants, such as sulfur dioxide (SO2), are brought in along with outdoor air via mechanical ventilation or infiltration.
Other contaminants found in outdoor air, such as nitrogen oxides and carbon monoxide, may have indoor sources as well. Most indoor pollutants, however, emanate from inside sources. People are sources of CO2, biomatter, and other contaminants character-ized as “body odors.” People's activities (cleaning, cooking, gluing, refinishing furniture, photocopying, etc.) also cause pollution.
Building materials and finishes can “outgas” or “offgas” pollutants.
Furnishings, business machines, and appliances (particularly unvented or poorly vented wood- and fossil-fuel-burning heaters and stoves) can be contaminant sources. The soil surrounding a building can be a source of radon and/or pesticides that enter the building through cracks, drains, or diffusion. Standard 62.1 speci-fies maximum concentration levels of common indoor contami-nants. It also sets forth acceptable quality parameters for outdoor air used for building ventilation. If the outdoor air source exceeds the allowable contaminant parameters, it must be cleaned or purified prior to introduction into occupied spaces.
Heating, ventilating, and air-conditioning systems, plumbing systems, and poor construction or maintenance practices can pro-duce “environmental niches” where pathogenic or allergenic organ-isms can collect and multiply to then be introduced into the air. An additional complicating factor in the buildup of contaminants is the variation in dilution rates and effectiveness of the ventilation deliv-ery systems often found in buildings. Contaminant concentrations vary spatially as well as over time. These variations add further nonuniformity to pollutant concentrations.
3.3.3 Determination of Ventilation Rate
Standard 62.1 provides the designer with a means of determin-ing ventilation rates needed to achieve acceptable indoor air quality.
The standard offers the designer two procedures for determining the required ventilation rate—the ventilation rate procedure and the indoor air quality procedure.
The ventilation rate procedure provides prescriptive rates, usu-ally on the basis of cfm [L/s] of outdoor air per occupant and unit area of floor, for an array of applications. Unless unusual pollutants are present, these rates are intended to produce acceptable indoor air quality. The basis for the occupancy ventilation rates is an underlying minimum outdoor airflow per occupant as a means of controlling CO2 to a concentration of 1000 ppm. Although CO2 per se is not a contaminant of concern at this low concentration, it is an easily measurable surrogate for other contaminants, such as body odors. The indoor air quality procedure offers an analytical alterna-tive, allowing the designer to determine the ventilation rate based upon knowledge of the contaminants being generated within the space and the capability of the ventilation air supply to limit them to acceptable levels.
While Standard 62 contains guidelines concerning maximum concentrations of some pollutants, “threshold limit values” for
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industrial applications and other pollutants are published by the American Conference of Governmental Industrial Hygienists (ACGIH 1992). Frequently, local building and occupational codes prescribe threshold limit values and ventilation rates. When they are more stringent than current ASHRAE and ACGIH recommenda-tions, they must be followed unless waived by the governing authority; when they are less stringent, the ASHRAE (or ACGIH) guidelines are recommended since they generally represent a more recent consensus of expert opinion on the subject and may carry legal weight.
3.3.4 Air Cleaning
The selection of air filters, gas or vapor removers, or other methods of air contaminant reduction influences the quality of the ventilation air supply, the dilution capability of the total air supplied to the occupied space, the concentration of room air contaminants, and the resultant occupant health and comfort. Filtration above 50%
efficiency according to the ASHRAE dust-spot determination (ASHRAE 1992) (equivalent to 10 MERV) must frequently be installed if the particulate limits established by ACGIH and Stan-dard 62.1 for ventilation air are to be met. Filters with efficiencies above 80% dust spot (13 MERV) can be effective in removing a sig-nificant portion of respirable particulate contaminants. Dust spot ratings have historically been used to classify filters; minimum effi-ciency reporting value (MERV) ratings are replacing dust spot rat-ings (ASHRAE 1999).
When high-efficiency air filters are installed in an air-handling unit, the total supply airflow helps control particulate concentra-tions within a space. Consequently, all-air constant-volume systems (and, to a lesser extent, variable-volume systems), if equipped with such filters, can produce respirable particulate concentrations in the building environment that are lower than those achievable by sys-tems in which the supply airflow rate is solely limited to the ventila-tion rate (typical of air-and-water systems).
Certain applications demand removal of gaseous contaminants present in the outdoor air or produced within the conditioned space.
Harmful gases and vapors can be removed by adsorption or oxi-dization. Activated carbon is the adsorptive material most com-monly used in HVAC systems. Potassium permanganate impregnated into the carbon or an alumina base is used to oxidize certain chemicals for which carbon has limited effectiveness.
Before being incorporated into a design solution, efficacy
limita-tions, bed depth requirements, and maintenance procedures should be thoughtfully evaluated in light of the need for such a system.
Air washers (see Section 5.2.3) are also effective in reducing SO2 and other acid-forming gases. But such scrubbers need contin-uous maintenance to keep the recirculated water from becoming highly corrosive and the reservoir a breeding ground for biological contaminants. Rare-book rooms and valuable artifact display or storage areas in museums and archival depositories are candidates for gas removal provisions, but the operations and maintenance staff should understand that careful maintenance is required for effective performance. Notwithstanding such gas removal provi-sions, the ventilation rate in these types of spaces should be main-tained at no less than 15 cfm [7.1 L/s] per person (except as otherwise permitted by Standard 62.1) to limit the carbon dioxide concentration.
3.4 ROOM AIR DISTRIBUTION