UC Berkeley Indoor Environmental Quality (IEQ) Title Indoor humidity and human health: part II--buildings and their systems Permalink https://escholarship.org/uc/item/7tw5877b Authors Arens, Edward A Baughman, Anne Publication Date 1996 License https://creativecommons.org/licenses/by-nc-sa/4.0/ 4.0 Peer reviewed eScholarship.org Powered by the California Digital Library University of California 3952 I door Humidityarid Numar Health: Part F=Sui dings Their Systems Edward A. Arens, Ph.D. Anne V. Baughman Member ASHRAE Student MemberASHRA E ABSTRACT fromtotally unrelated sources. For example,field studies have shownthat mildewcan form at as low as 10%relative humidity This paper continues a review of the humidity effects on (RH)in sorne cases, whereas in others RHvalues as high health as addressed in indoor ventilation and environmental 95%have not produced biological activity (Pasanen et al. standards. Part I identified a numberof health-related agents 1991a, 1991b). Clearly there are other’ factors at work that are affected by indoor humidit3; commonsites of contam- producingthese observations. Evenif the surface moisture is ination with#~ buildings, and commonremediation measures. caused by air humidityalone, the relationship betweenthese Part II discusses the physical causes of moisture-related two maybe a complexfunction of surface temperatures, mate- health problemsin buildings, subdividing them by climate and rials, textures, and exposureto air’ movement.In addition, the mechanicalsystem type. It examinesstudies done on moisture surface maybe in a part of the building or’ its mechanical problems in these differ#zg environments, showing that hz system where temperatures differ’ from those in the main most, if not all, cases the causesof the problemare only indi- airspace, resulting in different rates of condensation/evapora- rectly related to indoor humidity #~ the space. To do a better tion fi’omthose of the moldysurface~ Intermittent operation of’ job of controlling such problems, the building- and system- the mechanicalsystem mayincrease this effect~ specific causes of the problemsshould be studied. A numberof specific researchneeds are identified. Biological agents require appropriate conditions in the building for their’ germination,growth, release to the air, and INTRODUCTION transport to the humanhost. To understand humidity/health effects, one mustideally consider’ the organismsand their lif~ Health-relatedagents that are affected by indoor’ humidity cycles in the context of the building, its surface materials, its include dust mites, fungi, bacteria and viruses, and nonbiolog- mechanicalsystem, its operation schedule, and its surrounding ical pollutants. All of these affect humanhealth, primarily climate (e.g., the entire ecosystemof the organism). This through their inhalation from the air’, although someof them rarely donein the literature. Laboratorystudies have character- have lesser effects through the skin. The primary biological ized someof these pieces of the puzzle in considerable detail, health problemsrelated to higher’ levels of humidityare due to but the applicability of suchstudies to buildingsis often diffi- growth on surfaces or contaminated aerosols produced by cult to determine. At the other’ side of the experimentalspec- spray humidification systems. The interpersonal transfer of trum, field studies have often identified health-affectingagents biotic agents through the occupiedairspace is not determined in buildings but have not discovered or reported either the as extensively by ambienthumidity. Airbornelevels of nonbio- specific location of their origin or the mechanismsby which logical pollutants maybe affected by humidity through its humidityinfluences their growthand release into the human- influences on offgassingand surface reaction rates. occupiedzone. Correlations betweenhealth effects and indoor Humidityin buildings is usually measuredin the airspace air humidityare therefore often building-specific and are risky of the occupiedinterior. Not all of these listed humidity/health to generalize. effects are directly affected by the humidityof the air~ For’ In writing indoor environmentalstandards that use gener- example,fungi dependon the moisture of the surface on which alized criteria, there is a tendencyto set restrictive limits in they are growing.The moisture content of this substrate maybe order to includeall classes of problemcases~ Restrictive limits a function of the humidityof the surroundingair’ or maycome can have the undesirable effects of increasing energy required EdwardA. Arensis a professorand AnneV. Baughmanis a graduatestudent researcherin the Departmentof Architectureat the University of California,Berkeley~ ASHRAETransactions: Research 212 for conditioning spaces and reducing options for of space within the occupiedspace (Arlian et al. 1982). Whichpartic- conditioning. For example,in arid climates such limits affect ular surfaces are affected dependson specific characteristics of the use of evaporative cooling, whichcan offset energy-inten- the building’s construction and operation, the climatic charac- sive compressorcooling. Theyalso directly affect a substantial teristics of the region, the type of HVACsystem used, and the fraction of the cooling load in hot, humidclimates. Todevelop maintenanceof the building, its system,and its furnishings. more precise standards, it is useful to examinethe types of For molds, the key issue for growthoccurring on surfaces processes that lead to health problemsand to categorize types is the equivalentrelative humidity(ERH) or water activity. This of buildings, climates, etc., by howmany of these processes parameteris influenced by the following processes: they contain. It appearslikely that the biotic/health problemsat any given level of interior air humidityare different for build- ings in different types of climates and for different types of ¯ surface temperatures and the adjacent air humidity, environmentalcontrol systems. o hygroscopic properties of the materials, and In this paperthe different climate types are categorizedas: ¯ air movementat surfaces. o hot, dry climates (thermal flows through the building Althoughdust mites are not directly dependenton the ERHof envelopeare inward and water vapor flows outward); the substrate, the fungi that somemites require for digestion of ¯ hot, humidclimates with interior cooling and/or dehumid- skin scales are (Flannigan, 1992; Hart et al. 1990)~The mites ification (both thermal and vapor flows through the enve- themselveslive within textiles of furnishings and carpets in lope are inward); microenvironmentsbuffered from and quite different from the ¯ hot, humidclimates without cooling or dehumidification space environment.It is the relative humidity within these (thermal flows to and from the building’s thermal mass); microenvironmentsthat is most important, since mites extract and water vapor directly fromthe surroundingair. In their studies o cold climates (both thermal and vapor flows through the of houses in Tucson, O’Rourkeet al. (1993) observed mites envelopeare outward). most commonlyin ground-floor carpets on cold slab floors. This is probably explained by the increased local relative In addition, the following building-/system-relatedcategories humidity within the cooled carpet pile. If the floor is cool are useful in examiningpotential humidity/healtheffects: enoughor the space environmentmoist enough, the dew point will be reached and condensation will occur. Even if this occurs only occasionally, the condensedwater will be retained ¯ surface properties in rooms and in heating, ventilating, in the carpet and its backingfor an extendedperiod, providing and air-conditioning (HVAC)ducts, including tempera- a higher local vapor pressure and higher relative humiditiesin ture, hygroscopicity, and air movement; the carpet. ¯ water in cooling and humidification systems; ¯ intermittency of operation in cooling systems; and EQUIVALENTRELATIVE HUMIDITIES RELATED ¯ moisture sources not resulting from the humidity in the TO SURFACE TEMPERATURESAND THE interior air, includingrain penetration into the building’s HUMIDITY OF ADJACENTAIR structure, rising dampthrough foundations, and plumbing leaks. Thermal Gradients and Vapor Pressure Gradients Vaporpressure is proportionalto temperature.If a surface Each of these categories represents a set of different becomescold relative to the adjacent air, its relatively opportunities for humidity/healtheffects. The various health depressed vapor pressure can allow water to condenseonto the agents will be discussed in this contexLGiven the complexity surface fromthe air. For a given type of material, the amountof of the biotic/health problemin buildings and their mechanical water eventually condensed(and its ERH)are functions of both systems,the links of humidityto biotic/health factors shouldbe the surface temperature and the vapor pressure of the determinedas explicitly as possible. surroundingair. Different types of surface materials will reach different ERHvalues; this is described in the section on mois- EFFECTS OF MICROENVIRONMENTS ture absorption. ON BIOTIC GROWTH For a given temperaturedifference across a material, the The surfaces that suffer biotic growth differ among temperaturegradient is proportional to its thermal resistance building types. West et al. (1989a, 1989b) suggest that and its thickness. Resistances are summedfor multiple layers, commercialbuildings, moisture-associated air quality prob- as found in building wall and