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With the improvement of standard of living, air-conditioning has widely been applied.However, health problems associated with air-conditioning systems and indoor air qualityappear more frequently. In this paper, recent research is reviewed on air-conditioningsystems and indoor air quality control for human health. The problems in the existingresearch are summarized. A further study is suggested on air-conditioning systems andindoor air quality control for healthy indoor air environment. 1. IntroductionAir-conditioning systems have been used in many parts ofthe world. The purpose of most systems is to provide ther-mal comfort and an acceptable indoor air quality (IAQ) foroccupants. With the improvement of standard of living, oc-cupants require more and more comfortable and healthfulindoor environment. People spend 80 90% of their time in-doors, and indoor environment has important effects on hu-man health and work efficiency. The factors affectingindoor environment mainly include temperature, humidity,air exchange rate, air movement, ventilation, particle pollut-ants, biological pollutants, and gaseous pollutants (Grau-denz et al., 2005). By analyzing recent studies, Seppanenand Fisk (2002) found that there was an increase in preva-lence of sick building syndrome (SBS) between 30% and200% in the buildings with air-conditioning systems whencompared with natural ventilation systems.33291
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Death causedby Legionnaires’ disease even occurred in air-conditionedbuildings. In addition, SARS occurred in 2003. All of theseevents are a warning for indoor environment problems re-lated to AC systems. It is fair to say that indoor environ-ment problems still exist in many air-conditioned andmechanically ventilated buildings, even though existingstandards may be met.One of the consequences of the worldwide energy crisis in1970s is the public recognition of the importance of energysaving. The buildings built since then are more airtight anduse a great deal of insulation materials to minimize the lossof energy through the building envelope. Fresh air is reducedin air-conditioning systems in order to reduce the energy con-sumption. Meanwhile, synthetic materials and chemicalproducts (e.g., building materials and decorating materials)have widely been used indoors. The combination of lowventilation rates and the presence of numerous syntheticchemicals results in elevated concentrations of indoor particlepollutants and volatile organic compounds (VOCs) (e.g.,benzene, toluene, and formaldehyde). This is deemed to bea major contributing factor to compound hypersensitiveness(Wang et al., 2004a).However, it is exciting that some comfortable and healthyair-conditioning systemswere proposed in the past few years.In order to control the concentration level of indoor pollutantsand to improve IAQ, many researchers have investigated thecontrol methods of IAQ. In this paper, recent research willbe reviewed on air-conditioning systems and indoor airquality control for human health.2. Indoor air environmentIndoor air environments must meet the requirement of ther-mal comfort and IAQ. Thermal comfort is affected by manyfactors, which mainly include air temperature, air humidity,air velocity, mean radiant temperature, human clothing, andactivity levels. The wide use of air conditioning helps toimprove thermal comfort, but health problems associatedwith poor IAQ appear more frequently (e.g., SBS) (Niu, 2004).
Many experts believe that IAQ may be the most importantand relatively overlooked environmental issue of our time(Gao, 2002). It is indoor pollutants that lead to poor IAQ. Indoorpollutants include particle pollutants and gaseous pollutants.2.1. Particle pollutantsThe sources of indoor particle pollutants can be pided intoindoor pollution sources and outdoor pollution sources, andthe concentrations and composition of indoor particlepollutants are different with different pollution sources. Inresidential buildings, particles released by indoor pollutionsources (e.g., cooking, smoking) were mostly fine particlesand ultra-fine particles which were about 80% of the particlesin terms of particle counts (See and Balasubramanian, 2006).And PM2.5 concentrations could be up to 3 and 30 times higherthan the ordinary levels during smoking and cooking, respec-tively (He et al., 2004). The sources such as sweeping andvacuum cleaner tended to elevate concentrations in thecoarse fraction (Howard-Reed et al., 2003). Outdoor particlespenetrate into indoor environment through the aperture ofdoors and windows, and the fresh air of air-conditioningsystems. In an urban environment the most abundant partic-ulate matter in terms of number was in the ultra-fine size,smaller than 0.1 mm. There was only a very small share ofparticles (less than 10%) with diameters larger than 0.1 mm(Thomas and Morawska, 2002; Gramotnev and Ristovski,2004; Morawska et al., 2004).Chemical characteristic of indoor particles is anotherresearch topic. Sawant et al. (2004) and Cao et al. (2005)investigated the chemical characteristics of PM2.5. The maincomposition of PM2.5 and mass percentage inside the resi-dences are organic carbon (40–60%), nitrate (13–14%), traceelements (11–12%), ammonium (8%), elemental carbon (6%),and sulfate (4%).