Indoor Humidity and Human Health: Part II--Buildings and Their Systems

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 buildings 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 roof assemblies. The layers lems commonlystem from the proliferation of microbes on include the air films (boundarylayers) at the surfaces, whose moist hygroscopic surfaces within the HVACsystem. In resi- resistance (for a given air speed) is largely fixed. Thus, if dences, moldcontamination is usually found on roomsurfaces wall’s solid materials have low thermal resistance, the resis- (Aberg 1989) and dust mites on carpeting and furnishings tance of the interior boundarylayer (understill air) will be rela-

ASHRAETransactions: Research 213 tively high, and a substantial temperature drop maydevelop Northwest, Tsongas (1991) found substantial numbers across this film whenthe wall is exposedto ar~ overall temper- houses experiencing humidity problems. Condensation prob- ature difference. The wall’s surface temperaturemay be higher lems occurtedprimarily on the inner surfaces of outside walls. or lower than that of the interior air; whenit is lower, conden- Fewof these buildings had functioning moisture exhausts, and sation mayoccur on the surface as described above. This may the spot-check RHmeasurements (which found average values be the situation in cold climates, wherethe thermal gradient from 47%to 56%during the winter) mayhave missed cooking across a wall is outwar’d.It could also occur during transient periods when condensation was produced. TenWoldeet al. conditions, such as whenmechanical systems are shut downor (1984) recommendedthat "few moisture problems will occur switched to the economizer mode. In these cases, hot and whena home’sRH is below 40%." The discussion in the paper humidair maycome into contact with a cooled interior surface, covered mildewin wall corners and (exterior wall) closets, resulting in surface condensation. condensationon windows,and decay in walls or on the under- There is also a water vapor gradient across walls sepa- side of roof sheathing. These are all effects due to thermal rating different vapor pressures. For this gradient, the resis- transmission outward through walls and would not be appli- tance is the permeanceof the building materials. The vapor cable, for’ example,in summercooling situations. gradient describes the vapor’ pressure available at various depths within the wall assembly. The thermal gradient occur- Hot, HumidClimate with Mechanical Cooling: ring at the sametime determinesthe saturation vapor pressure inward Thermal and Vapor Gradient at each depth in the wall. Wherevapor pressures exceed the Gatley (1992), Shakun (1992), and Banks (1992) saturation vapor pressure at a given depth, condensationwill discussedmold problems in buildings (primarily hotels) in hot, occur within the wall assembly.This is the general case of the humidclimates close to the Gulf of Mexico.They all gave inte- surface condensation examplegiven above. rior RHrecommendations, although most of the effects cited Condensationoccurs in building assemblies whenthermal were related to inwardpenetration (by diffusion and infiltra- and vapor pressure gradients are both outward and when tion) of warm,humid outdoor air into mechanically cooled thermal and vapor pressure gradients are both inward. The rooms. A major’ problemis that the most impermeablelayer in formeroccurs under heating conditions in cold climates, while the wall construction is a commonlyused vinyl wallpaper that the latter occurs in hot and humidclimates whenthe interior is acts as a vaporbar~’ier at tire wrongside of the wall. Muchmold mechanicallycooled and dehumidified. Condensationdoes not growth takes place behind such coverings. The other primary occur whenthe thermal and vapor gradients are opposite, problemhad to do with operating the corridors at negative pres- whichis the case for buildings that are located in hot, arid sures relative to the ambient, so that humidair’ tended to climates and use direct evaporativecooling. migrate inwardthrough the envelopeor through-wallair’ condi- tioners. A recent study (Spaul 1993) found that indoor spore Cold Climates: Outward Therma~ counts in buildings with improper’ vapor barrier placement and Vapor Gradient could be controlled by pressurizing the building. Themajority of field studies of"sick buildings" have been Chilled mass encountering warm, moist air may cause performedin cold climates, and results linking (in par’ticular) surface condensation.In humidclimates, this effect can occur mold growth to space humidity are often inapplicable to whenmechanically cooled space is opened to natural ventila- building environments in hot climates. The mold found by tion or economizercycle operation. It also can occur when Pasanen et al. (1991a, 1991b) during extremely dry (10% cold-air duct systemsare turned off at night and on weekends, heating conditions was due to leaks and condensation on cold and warmmoist air can migrate in and condenseon the chilled surfaces. ManyEuropean studies have been done in older, surfaces. Substantial fungal growth can occur in the ducts, poorly insulated housing where cold surfaces are common. particularly to unfacedfiberglass insulation inner linings. (This Interior’ moldgrowth in cold climates is nearly alwaysa winter was described at the ASHRAEforum on humidity, Chicago, effect. Becker(1984), studying mildewin masonrybuildings January 1993.) Israel, found no difference betweenrooms having high arid low Bayer et al. (1992) describes humidity/health problems internal moisture generation: all the mildewproblems occurred southeastern schools where RHlevels were reported above on thermal breaks in outside walls. The lowered interior 75%for the majority of the year, evidently reaching saturation surface temperatures(in winter) dominatedthe surface conden- on occasion. It appears as if the problemsare primarily due to sation process. Similarly, part of the moldeffects discoveredby intermittent operation of the HVACsystem, resulting in large Aberg(1989) in Angolawere due to ceiling surfaces cooled inflows into the cool interior of air that had not been dehumid- nighttime radiation to the sky. Cold-climateeffects are thus ified. Theproblerns werealleviated by providing a continuous possible even in the tropics, wherenocturnal radiative cooling system and keeping interior humiditylevels below70%. is high. Kohloss (1987) reviewedair-conditioning practice in the Humidityproblems are exacerbated whenlow ventilation hot, humid tropics and suggested that the ASHRAEhumidity rates are used for energyconservation and whenthere is inade- limits of that time weretoo low for tropical use~He states that quate exhaust of internal humidity sources. In the Pacific moldand mildeware "usually well under control as long as RH

214 ASHRAETransactions: Research is below 70%"and recommendsraising the humidity limit in (clean) smoothplastics, glass, glazed surfaces, and sheet ASHRAEStandard 55 so that it is 65%at the warmerboundary metal. of the summercomfort zone, rising to 69%at the cooler o Hygroscopic porous media have very small pores (micro- boundary. capillaries). Thesepores are capable of exerting a power- Muchof the literature understandablyfocuses on problem ful mechanical attraction on atmospheric moisture buildings. Onthe other side, it mightbe instructive to observe because once the pores of small diameter begin to fill, existing practice in acceptable spaces. A recent informal their liquid surface area is a strongly concavemeniscus. sampling of manyair-conditioned spaces in Singapore found This has the effect of depressingthe vapor pressure at the no RHvalues below 65%, with many above 80%. The spaces surface of the water, allowing the liquid pocketsto remain were well maintained, showed no superficial evidence of in equilibrium at lower ambient vapor pressures than biological contamination, and were considered acceptable by could a plane surface of water. Hygroscopicporous media their occupants (Arens 1994). tend to swell and shrink as water is gained or lost. Exam- ples givenare wood,natural textile fibers, andclay. Hot, HumidClimate Without Mechanical o In capillary-porous media, pores are visible and the Cooling or Dehumidification: Temperature amountof physically boundwater is negligible~ These do Gradients to Thermal Mass not have lower surface vapor pressures to attract moisture Surface condensationcan occur in passively or naturally and do not shrink or swell. Examplesare bricks, concrete, ventilated buildings whenmoist air encounters a thermally gypsumboard, and packings of sand. massivebuilding element cooled by previous climatic conditions. It can also occur(rarely) duringthunderstorm passage In principle, hygroscopic porous media might appear to cool weather. Becauseneither daily air temperature nor mass permit fungal hyphae access to condensed water at lower temperaturesfluctuate significantly in humidclimates, natu- ambient RHvalues than wouldbe possible on plane surfaces. rally occurring condensationdue to thermal capacitance is not However,the reverse seems to be true at least for dry wood, large. In dry climates, massmay be cooled substantially below where water condensing in pores is adsorbed by cellulose daytimetemperatures by nocturnal ventilation, but this modeof moleculesin the cell walls and becomesunavailable for fungal cooling is unlikely to reduce the mass temperature below the use CWilcox1994). Becauseof this, the hygroscopicnature of subsequent day’s dew-point temperature unless a humid air dry woodactually reduces the availability of water to molds mass movesin outside or muchmoisture is generated indoors. that wouldnormally form on surfaces whenthe ambient RHis In a lightly insulated structure, radiant coolingto the night sky 100%.This is a temporaryeffect, in that permanentexposure to can cause interior surfaces to drop below the dew point and 100%RH would eventually bring the woodto its fiber satura- condensewater (Aberg[1989], as discussed above). tion point, where it loses hygroscopicity. However,wood paneling has been found to adsorb/desorb large amounts of EQUIVALENTRELATIVE HUMIDITIES humidity on a diurnal cycle without surface condensation RELATED TO SORPTiON ONTO SURFACES occurring (Kubler 1982; Okano1977). Ikeda et al. (1993) found that the addition of 24 mmof hygroscopicmaterial to a Moisture Sorption Processes wall exposed daily to several hours of 100%RH reduced the The amountof moisture adsorbedto or absorbedinto (the room RHby 4%to 10%and prevented surface condensation nonspecificterm is "sorbed")a material dependson the physical for periods as long as 60 days. andchemical characteristics of the material. In general, watercan In general, the effects of hygroscopicityon moldgrowth are be held to surfaces by chemicalor physical bonds or by mechan- not well describedin the literature. Data on ERHrequirements of ical attachment.Water that is chemicallybonded to the surface biological organismsare beingdeveloped, as discussedin part I of (usually by covalent bonds, water of hydration) is too strongly this paper. Wilcox(1994), on the other hand, maintainsthat liquid attached to the surface to be useful to biological growth.Water wateron the surface, at least intermittently, is a requirementfor held by physical (van der Waals)bonds coats smoothsurfaces moldgrowth on that surface. There does not appear to be much single or multiple molecularlayers. Thesebonds are roughlyone- informationon the relationship betweensurface condensate,ERH, tenth the strength of chemicalbonds and somelayers maybe avail- and the RHof the ad.jacent air for commonbuilding materials. In able for biological growth. Mechanicallyattached water has no particular, it wouldbe usefulto havethe characteristicsof various bondingbut is attracted by surface tension effects in pores and types of paints, since they coversuch a large fi’action of building capillaries. Mostof this is availableto biologicalorganisms. interior surfaces. It has also beennoted that okter indoorsurfaces Wonget al. (1990) ~ategorized building materials that maybe coveredwith depositedaerosols, affecting their moisture- physically bind water into three types of media: nonporous, absorptioncharacteristics (Fisk 1994). hygroscopicporous, and capillary-porous. i~oisture Sorption on Walls and Duct Surfaces o In nonporous media, condensation of liquid water can A study of mold growth on surfaces of bakeries done by only occur at the surface. Examplesgiven by Wongare Coppocket al. (1951) found that nonporousbrick had more

ASHRAETransactions: Research 215 surface condensation than porous brick, with mold growth treatmentsare available for dust mites, as discussedin part I of beginning at 80%RH, whereas no mold was found on natural this paper. brick until 88%RH. However, porous materials might accumu- late more nutrients over time from atmospheric dusL White- AIR MOVEMENT~N SPACE: wash over natural brick caused mold growth above 80%RH, VELOCITIES AT SURFACES perhaps because of nutrients in this paint. A glossy painted Air movementnear the surface increases the mass transfer woodgrew nothing at all in the range of 70%to 95%RH. of moisture to and from the surface. It appears that mold Coppocket al. recommendedthat, fox’ moldcontrol on all types growthis suppressedin manytypical building situations by the of surfaces, RHshould be kept below 70%~ architectural provision of air movementover surfaces. For Other researchers have noticed differences in the mold example,it is commonpractice in naturally ventilated build- susceptibility of latex as opposedto other paints~ Fox"example, ings in Hawaii and elsewhere to use louvered closet doors to Becker (1984) found less mold growth on inorganic paints eliminate mildewon the clothes inside. If this is not done, (whitewash)than on latex emulsions. Hens (1985 [as cited mildew is knownto occur. The occupied spaces in Hawaiian Aberg])exposed painted plasterboard panels with latex and oil buildings tend to be open and mildewis uncommon°There is paints to 75%and 95%RH at 20°Cfor 50 days. Nogrowth was not muchspecific informationavailable on this subject in the detected on either type of paint at 75%RH. At 95%RH the literature. It is possiblethat air movementhas its primaryeffect latex showedmold while the oil paints did not. It maybe by periodically desiccating organisms on the surface and unwise to generalize from these studies in that manypaints thereby disrupting their’ growth. contain fungicidal additives that could be determining the The smoothness of sutfaces affects the air movement results morethan the paint’s intrinsic characteristics. within the bounda7layer of the surface. This maybe a factor Foaxdeet al. (1992) inoculated acoustic ceiling tiles with in the fine texture of paints andother’ surfacesbut is particularly Penicillium aragonense and exposed them to RHlevels from important at the larger scale offered by carpets, furniture, 33%to 97%for a two-weekperiod. As long as the moisture bedding, and the unfaced fibrous insulation mentionedabove. content of the tiles remainedbelow 3% the inoculated colonies Theprotection offered by the roughnessof the fibers buffers the did not grow~This occurred for all humidities, including 85% surface microclimateand substantially reduces the transfer of RH. At 97%RH the colonies grew, Foarde et al. also did a both moisture and heat to and from the surface. The resulting variant of the experiment in which they soaked the blocks stability is an advantageto biological organismssuch as mites, initially (as with a roof leak) and then exposedthem to the same whichtend to be most populousin carpets and capet backings. humidities with and without fan-supplied air’ movementto dry This mayalso be true for moldsbut evidence was not found in the blocks. Theyfound that if the moisture content could be the literature. Anothereffect of roughnessis the increasedtrap- restored to less than 3%within three days, microbial growth ping of particulate air pollutants. Aswith the topic of air move- was contained. This drying rate was achieved with the fan for mentwithin buildings, there is not muchempirical literature on all but 85%and 97%RH levels. Withoutthe fan, none of the the microenvironmentsat and within building surfaces and on humidities dried the sampleadequately. the ways in which microclimatic fluctuations influence the Thetibers and/or binder in unfacedfiberglass duct insula- growthand spread of’ biological pollutants. tion is hydrophobicper se and does not adsorb or’ absorb atmo- The dynamicbehavior’ of air humidityin rooms,walls, and spheric moisture. Howevex;once dirt has accumulatedor mold wall cavities can be modelednumerically. E1 Diasty et al. has becomeestablished (as after a single flooding), then (1992) demonstrate a simulation of indoor humidity levels, becomeshydrophilic~ QuotingBurge (1987), "Fiberglass-lined moisture transport, moisture absorption/desorption, and ductworkcannot be effectively cleaned if moldgrowth on the surface condensation/evaporationfor’ different wall types, They fiberglass itself has occurred (as opposedto dust and spore also provide references to other such works. Such modelscould accumulation). Microbiologically, tiberglass exposedto humid be used to predict the moistureconditions available fox" micro- air in the supplyairstream is not a goodidea. Fiberglasslining organisms over typical daily and seasonal cycles of indoor should not be used in areas of high humidityor wherewater air temperature and humidity. In the future, they could fbrm a washersare part of the system." Moreyet al. (1991) and West basis for designingand evaluating biological tests and also for et al. (1989a, 1989b) also commenton the hygroscopicity developingmore sophisticated criteria and standards for indoor organic dirt on fiberglass. humidity.

Surface Treatments WATER IN ENVIRONMENTAL Nikutin et al. (1993) found that the boron tire retardant CONDITIONING SYSTEMS addedto cellulose insulation preventedfungal growth(S. atra) Humidifiers: Steam vs. Spray at high humidities (100%) where cellulose would normally have been a natural substrate at humidities above84% RH. The Aerosol-generating ultrasonic and spray-based humidi- preventive mechanismwas not discussed, but boron is known fiers have been implicatedin spreadingdiseases such as humid- to be an exellent fungicide and insecticide. Anumber of surface ifier fever (caused by allergens from humidifier water’

216 ASHRAETransactions: Research protozoansand bacteria). British studies have also linked them HUMIDITY/HEALTH IMPLICATIONS FOR to increased sick building syndromesymptoms (Fisk 1994). EVAPORATIVE COOLING OF BUILDINGS Aerosol-generating humidifiers are generally discouraged in IN A HOT, ARID CLIMATE the literature and steam humidifiers are recommendedinstead Direct-evaporative coolers operate with high rates of because they do not form aerosols. outside air supply--at least three times that of a typical air- conditionedbuilding. In general, high rates of outside air venti- Evaporative Coolers: Solid Mediumvs. Spray lation should reduce the buildup of indoor-generated pollut- Thereare few studieson the air quality effects of direct ants. Various studies of office buildings have shownfewer evaporative coolers. Since most systems use a recirculating complaints whenthey are naturally ventilated (Mendel1993). waterreservoir, there is a potential for biological growthwithin However,if outdoor air pollution is worse than inside, once- the reservoir and on the evaporative pads. However,since the throughventilation wouldincrease the pollutant levels indoors. systemsare designedto have relatively low air velocity across This was the case for ozone in the comparisonof evaporatively the pads, the water evaporatesinto the air without creating an cooled houses in E1 Paso to air-conditioned houses in Houston aerosol, and biological contaminants should, in theory, not (Stocket al. 1993). becomeairborne. This seemsto be supportedby field observa-- Direct-evaporative coolers operate with the thermal tions (O’Rourkeet al. 1993; Macheret al. 1990)as discussed gradient inward while the humidity gradient is outward. Only part I of this paper. one paper was found addressing the problemsof biotic factors Industry guidelines for evaporative coolers using the new in such buildings. This paper, a field study of mites in 190 synthetic solid mediasuggest a bleed rate from the reservoir houses in Tucson, Arizona, 96%of which were evaporatively equal to 30%of the recirculation rate, regardless of the loss to cooled, showed populations of Dermatophagoides farinae evaporation (ASHRAEforum on evaporative cooling, Denver, varying with season but present in morethan half the houses June 1993). Althoughthe bleed is primarily to prevent salt (O’Rourkeet al. 1993). Moldswere not discussed, but personal buildup, making its rate constant presumably also acts to communicationwith the author added that, whenpresent, mold control the amountof growth within the reservoir. A similar appeared to be primarily a result of ubiquitous leaks coming industry suggestion is that evaporative cooling systems should from the roof-mountedevaporative cooling units. completely drain their sumpsdaily, which would also act to O’Rourkeet al. also said that the great majority of the control growth. Tucsonhouses were cooled by old direct-evaporative "swamp coolers," even during the high-humidity "monsoon"(July It is possible to evaporatively cool incomingsupply air through September), during which evaporative cooling is with aerosol-generating sprays. The authors have heard of pushedto its capacity. examplesof this in commercialbuildings but have no experi- Evaporative cooling produces maximumindoor RHs of encewith any of them, either directly or in the literature. Such around 80% during operation. Wu (1990) measured (by systems could presumablypresent the same health hazards as weighing)substantial adsorption/desorptionin the furnishings spray humidifiersand appear to be discouragedin the industry. and structure of the space duringthe cyclical operation typical (This sentiment appearedto be the consensusat the June 1993 of summercooling. The adsorption/desorptiondid not result in ASHRAEforum on evaporative cooling.) ma.jor changesin the space temperature under the ventilation rates used. The moisture gained during the cooling period was Cooling Coils in Air-Conditioning Systems evaporated during off-cycle periods. Kubler (1982) calculated Under dehumidification, the cooling coil becomescoated morethan 20 gallons of daily adsorption/desorptionfor a wood with a film of condensate from the incomingairstream. This house whose interior cycled between 60%and 80%RH. This condensateis led to the drain via the drip pan. Thedrip pan can could represent a quarter to a third of the total daily moisture be a major source of health problemswhen improperly drained. added to the supply air by an evaporative cooler. The higher The standing water is often contaminated with bacteria and adsorptivities by hygroscopicmaterials provide a substantial protozoaand, since its liquid surface is in direct contact with effect in preventingintermittent surface condensation(Ikeda et the supply airstream, it has the potential to contaminatethe al. 1993). building. The literature cites this as a cause of a numberof observed cases of sick building syndrome. The exact mecha- DISCUSSION: HUMIDITWHEALTHIN nism by which the pollutants are injected into the airstream STANDARDSAND DESIGN PRACTICE does not appear to have been described. Comfort standards (ASHRAE1992; ISO 1984; DIN Aerosolscontaining pollutants can enter buildings through 1946) are based on measurementsof temperature and humidity outside-air inlets positioned near aerosol-forming cooling in the occupied space only. The humidities specified on the towers. Since cooling towers contain warmwater, Legionella is warmside of the comfort zone range between 60%and 70% often present. Coolingtower mist was the cause of the large RH. The difference between 60%and 70%RH at this temper- original outbreak in Philadelphia and appears to have been the ature is important in cooling system design and affects the cause of other outbreaksas well. viability of direct-evaporativecooling.

ASHRAETransactions: Research 217 The current ventilation standard (ASHRAEStandard 62 even 80%RH unless there are other factors influencing [ASHRAE1989]) is based on air change rates, with guidance their growth on building surfaces. In setting a maximum language concerning humidity limits in the occupied space limit to air humidityin the space, there is little if anyevi- (60%RH) and in ducts (70%RH). (At the time of this writing, dencefrom field studies that providesa reasonfor distin- it appears as if the 70%requirement is going to be removedin guishing 60%relative humidity fi’om 70%. Reported the Standard62 revision for being impracticallyrestrictive.) problemsat lower RHvalues appearedto be due to causes To directly address biological health influences, an air other than space RH,such as rain penetration or thermal quality standard should be expressed in terms of surface bridges in the envelope. The results tend to support Ten- temperatures and humidities throughout the building and its Woldeand Rose’s recommendedhumidity criteria, which mechanicalsystem (i.e., walls, ducts, and drip pans), as well extend to 80%in winter and 70%in summer’,with possi- the air’ temperaturesand humidities in the occupiedspace. In ble special provisionfor dust mite control. this way the ERHcan be determined for surface materials o In general, the principles and practices of moisture control wherebiological growthis a possibility. For this type of speci- in buildings are knownand available in the professional fication, procedureswill be neededto assess surface moisture literature (Lstiburek and Carmody[1993] is a good exam- properties by measurementand/or calculation. pie). This knowledgehas often been neglected in practice, TenWoldeand Rose (1994) recommendeda set of interim and the results m’e well documentedin the literature of performancecriteria for humidityinside the building as well as both health and biodeterioration. Water’ deposited within within the building envelope. Included in their recommenda- buildings by leaks and inadequate vapor control will tions is the IEA(1991) guidelines that the monthlymean ERH result in mold problems almost completely independent at all interior building surfaces, including building envelope of the level of indoorair humidity. cavities, be less than 80%(for concurrent surface temperatures o Evaporative cooling in hot, arid climates is biologically between 0°C and 40°C). Washablenonporous surfaces such as relatively benign, since building surfaces are warmerand glass and tiles mighthave the looser criterion of stating that drier than conditioned air. Exceptionsmay be (1) light- prolongedsurface condensationshould be avoided, and special weight furnishings that are permeatedby the temperature provisions might be instituted for dust mite control. These andrelative humidityof the interior’ air, providinga habi- recommendationsare not contradicted by evidence reviewed in tat for mites, and (2) floor slabs that are cooler than the this paper’. interior becauseof direct couplingto cooler’ earth temperatures. This latter effect has beensuggested for’ both mites CONCLUSIONS and moldsbut as yet has not been experimentally proven. ° Direct-evaporative cooling through porous media also General Observations appears to be benign in that biological organismsin the o To date, the influence of high humidityon health has not cooling water do not seemto be aerosolized or transmit- been addressed in a way that considers all the relevant ted downstreamin significant concentrations. The wet characteristics of building environments.None of the sev- pads may have benefits over’ dry filters in removing eral types of buildings and environmentalcontrol systems incomingpollutants. However,this needs to be experi- has been comprehensivelyassessed, least of all the subset mentallyinvestigated. In addition, the higher outside ven- of evaporatively cooled buildings. Wherehealth effects tilation rate required by such systemsshould act to dilute are noted, the specific causes are usually not determined. the concentration of indoor-generatedpollutants, includ- This situation impactsour ability to set rational standards ing airborne infectious organisms. and building specifications pertaining to high levels of * For evaporatively cooled building designs, smoothfloors humidity. should be substituted for wall-to-wall carpets in the o Mostof the identified biological health agents growon or homesof mite-susceptible individuals. These floors are within surfaces of the building, its systems, and its fur- easier" to clean. In addition, for coot floor slabs, the nishings or in standing water within or outside the build- smooth surface reduces the temperature difference ing. Noneof the agents growsin the air of the occupied between the room temperature and the surface and space or the mechanicalsystem. Their growthis therefore reduces the ERHat the surface due to increased convec- only indirectly related to the atmospherichumidity mea- tive evaporation. sured in the occupiedspace or the ducts of its mechanical o Carpet treatment with biocides appears to be well-estab- systemTo control these, one needs to ensure that the sur- lished in Europe, although the long-termhealth effects of faces remain dry. There are a number’of waysto achieve such treatment ar’e unknown.One treatment, based on the this in the design, furnishing, and operation of buildings. acaricide benzyl benzoate, is nowapproved for 49 U.S. It is also necessat3, to avoid producingaerosols of water states. from the mechanicalsystem or’ humidifiers. Howthis is o Fiberglass-lined ducts lose their hygrophobicproperties doneis independentof the level of indoor air humidity. after a single immersionand are thereafter’ hygrophilic. o In general, molds do not becomean issue below 70%or The accumulationof organic dust adds to this undesirable

218 ASHRAETransactions: Research effect. Thewidely cited opinionof building health profes- Arens, E.A. 1994. Personal trip log. sionals is that these should be avoided in the future or Arlian, L.G., I.L. Bernstein, J.S. Gallagher et al. 1982. sealed in somemanner from the airstream. The prevalence of house dust mites, dermatophagoids, and associated environmental Specific NeedsIdentified conditions in homesin Ohio. Journal Of Allergy and ¯ ERH(or water activity, w) needs to be determined in typ- Clinical hnmunology 69(6): 527-532. ical building situations and its relationship to atmospheric ASHRAE. 1989. ANSI/ASHRAE Standard 62-1989, humidity tabulated for a range of temperatures. Field Ventilation for acceptable indoor air. Atlanta: studies should attempt to locate the specific sources of American Society of Heating, Ret¥igerating and Air- biological agents and quantify the characteristics (temper- Conditioning Engineers, Inc. ature, ERH,material properties) of the surfaces on which ASHRAE. 1992. ANSI/ASHRAE Standard .5.5-1992, they are growing. Thermal environmental conditions .[or human o Information on the local RHwithin the carpet boundary occupancy. Atlanta: American Society of Heating, layer is needed for studying mites. Such measurements Refrigerating and Air-Conditioning Engineers, Inc. would be analogous to ERHfor molds but on a larger Banks, N.J. 1992. Field test of a desiccant-based HVAC physical scale. system for hotels. ASHRAETechnical Data Bulletin, ¯ In the literature, intermittent moistureexposure is almost Control of Humidity in Buildings 8(3): 39-46. never addressed yet is probably the most commoncondi- Bayer, C.W., C.C. Downing et al. 1992. Indoor tion in building systems. Theeffects on organismsof peri- conditions in schools with insufficient humidity odic moistening and drying out (mites and molds) control. Environments .[or People: Proceedings of influence their growthand survival. Informationis needed IAQ ’92, October, San Francisco, Califo, pp. 115-128. on the effect of daily and longer-termmoisture cycles on Atlanta: American Socieety of Heating, surface moisture, ERH,and on the organismsthemselves. Refrigerating and Air-Conditioning Engineers, Inc. Information is also needed on how such cycles are Becker, R. 1984. Condensation and mould growth in affected by the operation of the building and its mechani- dwellings--Parametric field study. Building and cal system. Dynamicmoisture models might be used in Environment 19(4): 243-250. conjunction with experimentation to provide such infor- Burge, H.A. 1987. Approaches to the control of indoor mation. microbial contamination. Proceedings of IAQ ’87, ¯ Data are needed on the hygroscopic properties of indoor pp. 33-37. Atlanta: American Society of Heating, paints. The studies showinglatex emulsions being rela- Refrigerating and Air-Conditioning Engineers, Inc. tively prone to mold growth were done someyears ago, Coppock, J.B.M.; Cookson, E.D. et al. 1951. "The effect probably before the developmentof latex acrylics and of humidity on mould growth on constructional other current paints. Therecent replacementof oil-based materials." Journal of the Science of Food and paints and even varnishes with water-basedversions sug- Agriculture, vol. 2, pp. 534-537. gests that typical indoorfinishes are very different than in DIN. 1946. (Gesundheitstechnische Anforderungen), the past. Theeffects of fungicidal additives shouldalso be Ventilation and Air Conditioning; Technical Health determined. Requirements (VDI ventilation rules). Raumluftechnik, 1963 (in German). ACKNOWLEDGMENTS E1 Diasty, R., R Fazio, and I. Budaiwi. 1992. Modelling The authors would like to thank Alison Kwok, Ph.D. of indoor air humidity: The dynamic behaviour candidateat U.C. Berkeley,for help with the literature search. within an enclosure. Energy and Buildings 19: 61- Wealso wish to thank WilliamFisk of the Indoor Environment 73. Program at Lawrence Berkeley Laboratory and Professor Fisk, W. 1994. Personal communication, June. Berkeley, WayneWilcox of the University of California Forest Products Calif.: Lawrence Berkeley Laboratory. Laboratoryfor their detailed reviews of’ the manuscript. The Flannigan, B. 1992. Approaches to assessment of the research reported here was funded by the California Institute microbial flora of buildings. Environments for for EnergyEfficiency (CIEE),a research unit of the University People: Proceedings 01~ IAQ ’92, September, San of California. Publication of research does not imply CIEE Francisco, pp. 139-145. 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