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After 10 days, about 99% of the VOCs were releasedto indoor air. The substrates with high adsorption capacity act as a sec-ondary source for VOCs emission, and such effectsmay pro-long the decay of VOCs.Although the emission and sorption of many VOCspollution sources have been investigated experimentally andnumerically, the following need to be researched further: Some hypothesises have been adopted in the numericalmodels, such as one-dimensional mass transfer, constantphysical properties, and uniform materials. Many materialsactually do not meet these hypothesises. Excessive VOCs do damage human health, but the damage ofinpidual VOCs in pathology is lack of research.Moreover, it isnot clear that the effect mechanism of VOCs to human body, and the influences of exposure concentrations and exposuretime on human health are uncertain.2.2.2. Secondary gaseous pollutantsThe mix of pollutants in indoor environments can be trans-formed as a consequence of chemical reaction. Reactionbetween ozone and some unsaturated hydrocarbons is animportant source of indoor secondary pollutants whichmainly include free radicals, aldehydes, ketones, alcohols,carboxylic acids, and fine particulate matter (Sarwar et al.,2003). Secondary pollutants may be more irritating than theoriginal reactants (Wainman et al., 2002; Rohr et al., 2003).During the past few years, many investigations wereconducted on indoor secondary pollution due to ozone react-ing with limonene (Clausen et al., 2001; Schell et al., 2001;Knudsen et al., 2002; Nøjgaard et al., 2005; Sirakarn et al.,2005; Tama ´s et al., 2006; Sarwar and Corsi, 2007), terpene(Knudsen et al., 2002; Weschler and Shields, 2003; Kleno andWolkoff, 2004; Sarwar et al., 2004; Fiedler et al., 2005; Nøjgaardet al., 2005), a-pinene (Fick et al., 2003; Pommer, 2003; Pommeret al., 2004), VOCs (Fan et al., 2003; Liu et al., 2004), organismdeposited on a dirty filter (Beko ¨ et al., 2003; Hyttinen et al.,2003), and airborne particulate (Mølhave et al., 2004, 2005).Much significant research has occurred in three subtopics: Confirming the importance of hydroxyl radical in indoortransformations. Hydroxyl radical is a product of ozone/terpene reactions and goes on to react with otherproducts. Hydroxyl radical is responsible for a largefraction of oxidized products, including certain productsthat cannot be made by ozone pathways alone. Chemical reactions that occur on indoor surfaces. Suchreactions may have larger impact on IAQ than those thatoccur in the gas phase because of the large indoor surface-to-volume ratio. The impact of secondary pollutants on occupants. A majorlimitation in evaluating the impact of secondary pollutantsis the inability to measure many of the reaction products.Sensory measurements are useful in detecting changesderived from indoor chemistry and changes missed by theanalytical methods routinely used to evaluate indoor air.Indoor secondary pollutants have significant impact oncomfort and human health, but the degree of impact andthe frequency of occurrence are uncertain at present. Inaddition, many secondary pollutants cannot be measuredbecause of the complexity of composition, and it is necessaryto improve the measure level.3. Air-conditioning systems
3.1. Air-conditioning systemsMany kinds of AC systems are used to improve indoor thermalcomfort and IAQ. Recent research is focused on dedicatedoutdoor air system (DOAS), independent control of tempera-ture and humidity system (ICTHS), and cooling ceiling anddisplacement ventilation systems (CC/DV).3.1.1. Dedicated outdoor air system (DOAS)With the occurrence of SARS, avian flu and anthracnose insome countries, the safety of AC systems becomes more im-portant. DOAS, an effective measure to realize ‘‘immunebuilding’’, has gradually been appreciated by internationalAC industry (Mumma, 2001). The reason why DOAS is called‘‘new concept’’ AC system is that the techniques used inDOAS have widely been applied but are subtly combined toshow its broad prospect. DOAS results in a significantrevolution in air-conditioning industry.Typical DOAS, shown in Fig. 1, consists of the followingseveral parts (Yin and Mumma, 2003): Cold source: As the outlet air temperature of the outdoor airproducer is required to be no more than 7 C, its inlet watertemperatureshouldbenomorethan4 C. Though the conven-tional chiller could be used as the cold source ofDOAS, the out-let water temperature of the chiller should be no more than5 C. So the chiller should be redesigned. The optimal coldsource will be the ice-storage system. Outdoor air processor: To ensure that indoor terminal de-vices run in dry condition, outdoor air heat load, total latentheat load and partial sensible heat load are removed by outdoor air processor whose outlet air temperature shouldbe no more than 7 C. Sensible heat removing terminal device (SHRTD): Theresidual sensible heat load is removed by SHRTD which in-cludes cooling ceiling (CC), fan coil unit (FCU), and unitaryair conditioner. Among themCC is the best one. DOAS usingCC as SHRTD can save the primary energy by 18%, comparedwith DOAS using FCU as SHRTD (Li and Zhang, 2007). Total heat exchanger: Outdoor air is dehumidified by soliddehumidizer in desiccant wheel, and then exchanges totalheat or sensible heat with exhaust air in heat recoverywheel to recover the energy of exhaust air. Automatic control system: Automatic control system isnecessary for DOAS. The controlled parameters mainly in-clude the outlet water temperature and the cold water fluxof outdoor air processor, cold water flux and inlet watertemperature of SHRTD, indoor dry-bulb temperature anddew-point temperature.The inlet water temperature of SHRTD is controlled basedon indoor dew-point temperature, and the total latent heatload is removed by outdoor air processor. Therefore, SHRTDruns in dry condition completely and there is no need toworry about the condensing water. Moreover,