indoor air quality-vallero 2008

8/13/2019 Indoor Air Quality-Vallero 2008

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6 8 0 25. Indoor Air QualityFor many industrialized countries, e fforts to improve the outdoor air qualityhave been under way for the majority of this century. Inmany locations aroundthe world, Significant improvements have taken place. Air quality ill manymajor cities such as London, New York, and Chicago has improved from tl\econditions present ill the first half of the twentieth century. Mechanisms and

control programs are in place ill the developed countries to continue theimprovement ofambient airquality. Considerable effort and energy have beenexpended to characterize, evaluate, and control air pollution emissions to theatmosphere.Buildings and their design have undergone major changes. Fifty years agoicentral heating and windows which could be opened and closed dependingon the season were the norm for commercial buildings. Now we have multi-story buildings with central heating and air conditioning and sealed glassexterior walls. Residential housing has undergone similar design and struc-tural changes, ill some cases resulting ill dwellings that may have poorerindoor air quality.New residences and commercial buildings are designed and built withenergy conservation as a major design criterion. New materials have beendeveloped and are being used ill construction. Although these modificationshave helped to save energy, a consequence of some of these modifications isslower exchange of air with the outside. This helps considerably with the heat-ing or cooling system because energy must condition this new air which isintroduced into the structure. However, the decreased air exchange rates usu-ally translate into dirtier air.

A second consideration is the change illlifestyle for individuals in industrial-ized societies. We are no longer a society dependent on occupations whichrequire us to be outdoors for a Significant part of our day. Over the past twodecades, studies of daily activities have consistently shown for urban popula-tions that, on average, we spend about 90% of our time indoors ill our homes,cars, offices, factories, public buildings such as restaurants, malls, and others.Any given individual activity profile may differ significantly from this average.

Exposure assessment techniques now attempt to include as many as possi-ble of the locations in which individuals now spend time. The conceptinvolves identification of micro environments which are important for poten-tial exposure. For example, exposure to CO would include time spent illcom-muting, parking garages, ill residences with gas stoves, as well as time spentoutdoors. This approach classifies time spent ill these rnicroenvironments andthe typical concentrations of CO U 1 these locations.

II_ FACTORS INFLUENCING INDOOR AIR QUALITYSeveral factors influence the quality of air indoors: the rate of exchange ofair with air from outdoors, the concentration of pollutants illoutdoor air, therate of emissions from sources indoors, the rate of infiltration from soil gases,

and the rate of removal in the indoor environment (Fig. 25.1).

II. Factors Influencing Indoor Air Quality 6 8 1

A[B]

c A

D

Fig. 25.1. Representation ofhorne with various sources and sinks for indoor air pollutants:A, exchange; B, indoor concentration; C, outgassing of building and furniture materials; D,infiltration from soils; E, removal 011 interior surfaces.

The source of indoor air pollutants may be inside the building, or theymay be transported into the interior space from the outside. Sources locatedindoors include building materials, combustion sources, furnishings, andpets. Emissions of organic gases are higher with irtcreased temperature andhumidity but usually decrease with age of the structure or furnishings.Construction materials and the composition of furnishings inside the build-ing may give offor outgas pollutants into the interior airspace, e.g. glues oradhesives. Natural gas for cooking and kerosene space heaters release NOand CO2 even when operating properly. Molds may grow in the ventilationducts and be distributed throughout a building.

Radon from the soil can enter buildings through cracks in the foundationwhen the pressure inside is lower than in the soil. The rate of infiltrationdepends on the soil type, the building structure, and the pressure differentialbetween the so il and the building.Air is exchanged between indoors and outdoors by several ways: naturalventilation, mechanical ventilation, and irtfiltration or exfiltration. Natural

ventilation involves movement of a ir through building openings like doors,windows, and vents. Mechanical ventilation involves fans and hea ting andair-conditioning systems. Infiltration and exfiltration represents undesirablemovement of air ill and out of the structure. Buildings are characterized astight when infiltration rates are low.The air exchange rate influences the concentration of indoor pollutantsill two ways. At higher air exchange rates, the pollutants inside a structureare removed from the interior, As long as the ambient outside air has lowerpollutant concentrations, high exchange rates help lower indoor air pollu-tant levels. However, if the pollutant concentration outside is elevated,then an increase ill the air exchange rate will bring these materials intothe building: e.g. an idling vehicle adjacent to an air intake will transferexhaust fumes into the building, At lower exchange rates, pollutants


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69 2 25. Indoor Air Qualitydesigned to bring sufficient fresh air into a building to minimize the buildupofcontaminants and odors.

REFERENCES1. A C itizen s Guide to Ra do n: W hat It Is an d W hnt to D o A bout It. US Envirorunental Protection

Agency and US Department of Health and Human Services, OPA-8-004. GovernmentPri.nting Office, Washington, DC, 1986.

2. Ve l1ti latiol1 for Ac ce pt ab le In do or A ir Qua lity. American Society of Heating, Refrigerating andAir-Conditioning Engineers, Atlanta, GA, 1981.

SUGGESTED READINGGodish, T., illdool A ir P Ol/utiOI1 Co ntrol. Lewis Publishers, Chelsea, MI, 1989.Nazaroff, W. w ., and Teichmann, K., Indoor radon. E'lVirol1 . Sci. T echnol. 24,774--782 (1990).US Environmental Protection Agency, lJ ldoo l Air Q ual it y 2007, http://www.epa.gov/iaq/

index.html 2007. Includes separate links to asbestos, mold and other contaminants.US Environmental Protection Agency and US Consumer Product Safety Commission, Th e In side

Sto ry : A G uid e to Indoor Air Qu nlity. EPA/400/1-88/004, September 1988.

QUESTIONS1. Define sick building syndrome.2. What c on trols are available to control indoor air quality?3. Why will the command and control approach not work for residential indoor air quality?4. Describe an implication of high radon levels during the sale of a h ome.5. The mass balance can be determined for a ny control volume. So, let us consider a house to be

such a control volume. We can construct a simple box m od el for this control volume:

Infiltration rate = Q (m3 s -1)Outdoor airconcentration = CA(g m-3) I Sources I I Sinks I

Control volume = V (m3)Concentration = C (g m-3)

Rate ofdecay =

~1) r r = >

indoor air quality-vallero 2008

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