Building Character: Design For Good IAQ
As indoor air quality (IAQ) professionals, we observe many conditions that can contribute to deteriorated IAQ. The manifestation of these problems is occupant complaints regarding odors, Sick Building Syndrome (headaches; burning eyes, nose, and throat; coughing; and sneezing), building-related illnesses (Legionnaire's disease, asbestosis), and chemical sensitivities of an extreme nature (multiple chemical sensitivity, Twentieth Century Disease, chemical allergies, and other similar diagnoses).
Building problems can cause degraded air quality in all phases of a building's lifecycle: design, operation, and maintenance. The purpose of this article is to examine common design characteristics that may be problematic to the achievement of acceptable IAQ and that are generally easily identified and are usually avoidable.
Sizing SymptomsDepending on the size and scope of the project, building design can be a very complicated process. Table 1 lists a variety of problems which can occur and their possible negative influence on IAQ.
A not-so-uncommon problem is an oversized air conditioning system. The oversized system could be due to a design error or seasonally reduced occupancy/thermal loads such as often occurs in schools in summertime. The oversized system causes rapid response to high temperatures. However, because of oversizing, hot, humid air brought into the system is not adequately dehumidified, producing increased relative humidity (rh) in the space. Sustained rh above 60% increases the likelihood of microbiological growth on environmental surfaces. The air-handling system can serve as a distribution conduit for microbes throughout the zone.
InsulationWetting of hvac interior insulation is problematic with respect to IAQ in several regards. Interior insulation in ductwork comes in a variety of forms.
One of the most common forms is unfaced fiberglass insulation, which is glued and/or pinned to ductwork. When such insulation becomes saturated, this duct lining becomes an amplification site for microbial contamination.
In addition, this fiberglass can be easily eroded because of high-velocity air contact with the surface of the insulation. This erosion can result in the dissemination of fiberglass throughout the zone where occupants can either inhale the fiberglass or, quite commonly, appear to experience dermal irritancy from contact with settled fiberglass. There is concern, and considerable debate, about the carcinogenic potential for inhaled fiberglass. Until the debate is settled, it is a prudent measure to minimize avoidable exposures.
Lastly, fiberglass-lined ductwork is extremely difficult to fully clean. HEPA vacuums will not adequately remove all spores throughout the full depth of the insulation and diligent vacuuming of the surfaces may actually tear or abrade the surface, thus creating an airborne fiberglass exposure problem. Thus, from an IAQ perspective, exterior lining is preferable to interior lining.
No Access, No ReplacementDesign layouts which make maintenance and upkeep of the air-handling systems difficult occur occasionally. There have been instances where handrails have been installed in front of access panels, catwalks have not been installed to access plenum-mounted units, and/or the units are located in plenums that are difficult to reach (e.g., school auditorium ceilings).
The result can be that filters may go years without being changed and are extremely loaded, and other components in the system may fail and become unhygienic. Many of these systems receive no periodic maintenance until the system fails completely. The lack of hygiene in these systems may contribute to IAQ problems.
Poor Air DistributionProblems related to poor distribution of supply air are ubiquitous. For example, this problem is common in schools where some wall-mounted unit ventilators discharge air in a very focused, directed pathway resulting in uneven mixing and distribution. Inspection of commercial office buildings frequently uncovers supply grilles that are poorly situated in offices, thus greatly reducing uniform mixing distribution of air throughout the office.
A common problem in many commercial office buildings is the inability to precisely control heating and cooling in individual offices or occupied spaces.
This problem can be caused by a design that only permits regulation of relatively large zones containing many offices with differing thermal loads and exposures. Under these conditions, it is quite common to have several, if not the majority, of office occupants complaining of thermal discomfort. More zoning would allow for better control of thermal conditions. Another common variant of this problem is controls that are broken, miscalibrated, or misprogrammed. All of these conditions can exacerbate existing temperature control problems.
Building ventilation standards and codes for buildings constructed during the 1970s and much of the 1980s included provisions for what is now generally viewed as underventilation. During much of this era, building codes specified an outside air minimum of 5 cu ft/min/person, which often results in complaints of stuffy, dry air and a perception of objectionable odors. Subsequent revisions to ASHRAE standards and building codes have increased outside air quantities to 15 to 20 cu ft/min/person. In many instances, the only solution to this problem is to upgrade the existing ventilation system.
Smells And GrillesA number of IAQ complaints involve foul or objectionable odors. Investigation frequently pinpoints outside air supplies as a source of these odors, and the location of the outside air intake is often the culprit. We have observed intake grilles located in close proximity to dumpsters, pools of standing stagnant water, idling cars and buses, rooftop toilet stacks, and many other undesirable sources of outside odors. Many times, the only solution to this problem is an expensive relocation of the intake or unsightly ductwork on the exterior of the building, which permits supply air to be obtained away from the contaminant sources.
Rooftop air-handling units that have poor resistance to weather and animal fouling can result in problems. We have observed rain and snow infiltrating both the air handlers and ductwork. Such moisture intrusion is highly undesirable because microbial contamination is likely to result. In addition, we have often observed bird feces, feathers, rodent feces, and nesting materials in and around these air handlers. Contamination from bird feces carries a potential for histoplasmosis and coccidiosis, two potentially deadly diseases.
From an IAQ perspective, engineers should specify equipment and exterior ductwork that has high resistance to moisture intrusion and animal fouling. Alternately, some type of weatherproof enclosure could be constructed around the units to prevent fouling by animals and the intrusion of moisture.
Building inspections frequently uncover placement of supply and return or exhaust grilles in close proximity to each other. Such a condition may result in "short circuiting" of building supply air. ASHRAE 62 ventilation rates assume a well-mixed condition. Without adequate separation between supply and return air, stratification could occur and, a significant fraction of outdoor air may be drawn into the return airstream and not adequately mix with room air. This condition, if not corrected, may necessitate an increase in the outdoor air fraction to compensate for the reduction in mixing.
Humidification DilemmasMany buildings, particularly in the mid-Atlantic region, do not provide heating-season humidification. During cold weather, outside air may not contain enough moisture when heated to room temperature to maintain rh above 30%. Relative humidities in the low 20% ranges and below are very likely to cause bloody noses; eye, nose, and throat irritation; and an increased likelihood of respiratory infections. This is a case where the design professional is caught between a rock and a hard place because use of humidifiers can create their own set of IAQ problems due to poor design and/or maintenance.
Malfunctioning humidifiers can wet internal insulation and ductwork, be contaminated with microorganisms, or cause particulate generation due to evaporation of water sprays. If a design professional decides to specify a humidification system, it should preferably utilize steam according to ASHRAE 62, but care should be exercised to avoid contamination from water treatment chemicals.
Filters: Worth The EffortPoor system air filter quality is one of the most common problems that we encounter. Seldom, if ever, do we find filters in air handlers in excess of ASHRAE 52.2-99 MERV 8 efficiency. Such low-efficiency filters are only capable of capturing the largest particles and thus reducing gross fouling and contamination of the mechanical systems, but do virtually nothing to capture smaller, respirable particles, which have the greatest impact on human health. Particles in the 3-micron range and below are capable of most deeply penetrating the respiratory system and causing the most serious symptoms.
Higher efficiency filters in excess of MERV 11 and up to HEPA filters at MERV 17 efficiency are desirable in instances where human health is a major concern. While such filters are expensive and initial design must accommodate pressure drops caused by high-efficiency filters, such filters need only be replaced every one to two years. A prefiltering system is usually installed to capture the greatest particulate load prior to impingement on the HEPA filters. The prefilters may be changed more frequently, thus allowing the high-efficiency filters a longer lifecycle.
Legionella PrimerLegionella pneumophila is the bacterium capable of causing Legionnaire's disease. This disease causes a high fatality rate, particularly among elderly males with a history of smoking. Legionella pneumophila can only be disseminated in the tiniest of water droplets and thus are generated by such sources as cooling towers, showerheads, faucet aerators, whirlpool tubs, fountains, and the like.
However, this disease is highly preventable with an effective biocide/treatment program. Cooling tower treatment protocols are crucial in preventing growth of Legionella. Most protocols are designed to provide continuous maintenance levels of biocide and periodic "shock doses" to ensure that strains resistant to chronic treatment are overcome by huge doses of a different type of biocide.
Such treatment strategies are quite effective, if appropriately monitored. Currently there is no recommendation to periodically test treatment towers for Legionella pneumophila or the biocide concentrations. Building operators should be aware that tests for total cooling tower bacteria are not generally capable of detecting the presence of Legionella pneumophila. Specialized media must be used to detect the presence of this organism and testing for the biocide concentration is generally performed through pesticide testing laboratories.
While periodic testing programs can be expensive, an outbreak of Legionnaire's disease can be devastating to large-building owners and operators. The author was involved in a major Legionnaire's outbreak in a midsized hospital, and subsequently the hospital was found to be substantially free of any negligence.
Nevertheless, the negative public relations and the cost of defending itself in a lawsuit had a severe crippling effect on the hospital. Legionella is an entirely preventable disease, which, with appropriate care and attention, should never occur.
The Second Time AroundWe are frequently called to analyze and diagnose IAQ problems that arise following hvac design retrofits. A common example is of a research building in which additional exhaust, chemical fume hoods, or biological safety cabinets are added as operations grow in the building. It appears that, in many cases, no one has considered the net impact that such exhaust sources have on total building airflow.
Buildings should ideally be slightly positively pressurized by design, but installation of these devices can quickly change building pressurization from positive to strongly negative. When buildings become negatively pressurized, outside air is then pulled into the building from every building penetration, including doors, windows, cracks, etc. Because such air is not forced through the hvac system, it is neither properly conditioned, nor have any of the outside contaminant burdens been filtered. Such problems are easily preventable if design professionals consider the impact of such process changes.
ConclusionThis article summarizes a number of the most common hvac-related design and installation characteristics that can be problematic to IAQ, but by no means is this list comprehensive. In addition, there are many "variations on the theme" which have the potential to seriously degrade IAQ. Careful design, thoughtful operation, and scrupulous maintenance of building hvac systems will eliminate the vast majority of IAQ complaints.
It further seems that, while not required by local building codes, design professionals without extensive experience related to IAQ issues would be well served by collaborating with firms having greater experience with these issues. After construction of a building, semiannual air testing and inspections are highly advised in most buildings. ES