INDOOR AIR QUALITY: Engineering Considerations

Abstract

From an engineering perspective, successful environmental control must be based on a knowledge of what to control and how to control it. This article provides an environmental characterization of commercial and residential buildings, describes major pathways by which occupants are exposed to allergens in them, and presents technical and managerial concepts that can be employed to reduce indoor allergen exposures through appropriate design, construction, and operations of the buildings. Some data are available that allow environmental characterization of commercial buildings and to hypothesize a continuous degradation of the existing building stock in which 50% to 70% may be characterized as “healthy buildings,” 10% to 20% as undetected problem buildings, 10% to 25% as manifesting “sick building syndrome,” and 5% to 10% manifesting “building-related illness.” Although similar data for residential buildings are not yet available, technical and managerial control strategies used for residential and commercial facilities are not fundamentally different. Therefore, it is hypothesized that 20% to 25% of the US population are probably exposed to both “healthy” residences and commercial buildings, 30% to 70% are likely to be exposed to either “sick” residences or commercial buildings, and that up to 25% may be exposed to both “sick” residences and commercial buildings. Furthermore, as 40% to 50% of the problem buildings (i.e., SBS or BRI) involve exposures to bioaerosols, 70% to 80% involve exposures to chemicals (some of which are allergenic), and approximately 30% involve moisture problems, it is reasonable to assume that the presence of indoor allergens contribute significantly to the hypothesized total exposures. Three mechanisms (i.e., conduction, radiation, and convection) and three strategies (i.e., source control, removal control and dilution control) are identified that provide the technical means by which indoor environmental stressors, including allergens, can be controlled. Then, how “building diagnostics” can be used together with conventional design, construction, and operational procedures to increase assurance of acceptable control at each stage of a building’s life through the managerial concept of “continuous accountability” are described. Also reviewed is a rational procedure for ensuring acceptability, either as a prevention or intervention strategy. It consists of four steps: (1) specification of objective evaluation criteria (i.e., acceptable human response, exposure, system performance, and economic performance); (2) evaluation of thermal and contaminant loads to achieve the specified exposure; (3) evaluation of system performance for its capacity and controllability to meet peak and partial loads; and (4) evaluation of the energy and economic performance of the building and its systems. Three basic conclusions can be derived from this article: 1. Currently, building codes and standards do not adequately address exposure criteria for sensitive populations such as the 20% of the US population that are affected by allergenic rhinitis or the 8% to 12% affected by asthma. Moreover, they do not recognize that sensitization of occupants may develop after indefinite exposure periods. Because of the lack of data upon which to promulgate these standards, control of indoor environmental factors that correlate to the growth or amplification of indoor allergens and that will be perceived as acceptable to a substantial percentage of sensitive occupants must be relied upon. Also, because these factors must be controlled simultaneously, limits of exposure to individual stressors are not sufficient criteria; rather, mean values and allowable deviations (i.e., control precision) for the set of exposure criteria should be specified to achieve the desired level of acceptability for the expected occupants of that space (i.e., including provisions for sensitive populations). 2. Achievement of healthy building conditions is not only a design issue, but requires constant and aggressive management throughout the occupancy of the building. If rigorous housekeeping and preventive maintenance procedures are not performed, the likelihood of continuous degradation is high. During normal occupancy of residential and commercial buildings, dirt and other forms of contaminants will accumulate. It is therefore necessary to employ frequent and thorough housekeeping and preventive maintenance measures, including record-keeping, to ensure acceptable exposure control, especially to those sensitive to indoor allergens. 3. Even with the best of care, it is inevitable that system failures will occur and intervention will be required sometime during the occupancy period. If these problems are detected soon after their occurrence, mitigation costs will usually be less than if action is delayed. A fundamental outcome of building diagnostics is the formulation of a hypothesis which, if confirmed, forms the basis for recommending mitigation. Then, candidate solutions should be carefully analyzed in perspective of the total system performance. Otherwise, the proposed solutions are likely to mitigate one problem but become the source of others. Finally, and of particular importance, another diagnostic investigation should be conducted subsequent to the mitigation to provide assurance that it has been successful.