IISection
4.2 IndooR aIR pollutIon: theIR souRCes and Causes IAQ is defined as “air in an occupied space towards which a substantial majority of
occupants express no dissatisfaction and in which there are not likely to be known contaminants at concentrations leading to exposures that pose a significant health risk” (ASHRAE, 1989). The IAQ is closely associated with the outdoor air quality.
However, occupants and their activities also tend to generate contaminants in the indoor spaces. A range of conditions and the interactions of “sources,” “sinks,” and air movement among rooms and between the building and outside determine IAQ.
Indoor air pollutants represent a complex array of constituents made up of gases, vapors, and particles. The determination of health effects related to these pollutants collectively, individually, or in certain combinations requires extensive information about the exposure of an individual to this mixture. The major indoor air pollutants that affect human health are classified broadly into three categories: particles, vapors, and gases, and their sources are broadly classified as the activities of build- ing occupants and other biological sources; the combustion of substances for heat- ing or fuel; and emissions from building materials. For some contaminants, infiltration from outside, either through water, air, or soil, can also be a significant source. The indoor air pollutant sources can be external, internal, biological, or chemical in nature. The external sources include industries and construction sources:
exhausts from heating, ventilation, and air-conditioning (HVAC) equipments, vehi- cles, and soil gases such as radon. External biological contaminants can come from standing water that promotes mold growth. Internal sources of pollutants include building materials such as pressed wood board, glues, insulation, paints, stains, sol- vents, and other furnishings like carpet, furniture, and cabinets, HVAC systems, office equipment such as laser printers and copiers, activities like smoking and cook- ing, and other combustion sources such as fireplaces and furnaces, cleaning materi- als, both their use and storage, and pesticides. Biological sources in indoors are pets, plants, and humans. They produce dust mites, molds, pollen, animal dander, and bacteria. They often come from damp or water-damaged walls, floors, ceilings, and bedding and from poorly maintained air-conditioners and humidifiers. Environmental tobacco smoke (ETS) contains over 3800 compounds, including VOCs, inorganic gases, and metals, many of which are carcinogenic or can promote the carcinogenic properties of other pollutants. “Sinks” are high-surface-area or porous sites on or within which odor or other gaseous contaminants deposit. They may be located in the rooms or systems and may ultimately become secondary sources themselves. Air movement in a building consists of (a) natural air movement among rooms, some- times fostered by the occupant’s movement, (b) air movement driven by a forced air system, namely an HVAC system; air movement between the building and outside through ventilation, infiltration and exfiltration; and air movement driven by elevator piston action, the thermal stack effect, and air pressurization differentials.
The poorly designed ventilation systems and airtightness may lead to “inade- quate” supply of fresh air in buildings. As a result, negative pressure develops, which may cause the outside pollutants to be drawn inside the buildings from vents, cracks, and openings. Uncontrolled temperature and humidity conditions indoors may also generate odor and bioaerosols—the fungi, molds, and other sickness-causing microbes. The indoor pollutant flow in Figure 4.1 provides an overview of the contaminant’s “life” in the building. The indoor air pollutants include bioaerosols, particulates, VOCs, and inorganic and organic gases.
The bioaerosols are airborne microbiological particulate matters, derived from viruses, bacteria, mites, pollen, and their cellular or cell mass components.
Bioaerosols are present in both indoor and outdoor environments. Floors in a hospi- tal can be a reservoir for organisms that may subsequently be re-entrained into the air. While carpeting appears to trap microorganisms firmly, conditions within the carpet may promote their survival and dissemination. Water is a well-known source of infective agents even by aerosolization.
The particulates represent a broad class of chemical and physical contaminants found in the air as discrete particles. These are defined as mixtures or dispersions of solid or liquid particles. Typical examples of particulates include dust, smoke, fumes, and mists. They are broadly classified as suspended particulate matter (SPM) and respirable particulate matter (RSPM). RSPM are generally defined as 10 μm or less in size (PM10), although the Environmental Protection Agency (EPA) has expressed
Re-enters building Contaminant enters building
(or is generated within the building) Building material Furnishings/equipments
HVAC equipment Consumer products
Occupants Outside air/soil Purposeful activities
While in the building
Exhaled by
occupant Leaves with
exhausted air Inhaled by
occupants
fIguRe 4.1 Indoor air pollutant flow.
concern that they are available even in less than 2.5 μm (PM2.5) and less than 1.0 μm (PM1.0) sizes and are the primary cause of lung cancer. Particulates found in the work environments are generated as a result of work-related activities, that is, adding batch ingredients for a manufacturing process, applying asphalt in a roofing operation, or drilling an ore deposit in preparation for blasting. The external environment is a major source of particulates because of ambient pollution. The introduction of this source is through ventilation, infiltration, and occupant traffic. In the indoor environ- ment, particulate sources may include cleaning dirt accumulation in carpets and on other fleecing sources, construction and renovation debris, paper dust, and deterio- rated insulation. ETS, kerosene heaters, humidifiers, wood stoves, and fireplaces are the common sources for RSPM indoors.
The VOCs exist as a gas, or can easily off-gas under normal room temperature and relative humidity (RH) conditions. A range of VOCs is always found in all nonindustrial indoor environments. After ventilation, VOCs are probably the first concern when diagnosing an IAQ problem. The list of potential sources of VOCs is lengthy and growing. Some of the major and common sources include wet emissions that have very high emission rates initially; after application, wet emissions can be present in newly constructed buildings, photocopying material, carpets, wall cover- ings and furnishings, refrigerants, gasoline, cosmetic products, biological matter, molded plastic containers, disinfectants, cleaning products, and ETS. While direct VOC emissions from primary sources are predominant, some materials act as sinks for emissions and then become secondary sources as they re-emit adsorbed chemi- cals. Floor dust, which is different from the dust in the air, has been found to be a sink and a secondary emission source for VOCs.
Inorganic gases include oxides of nitrogen, sulfur, carbon monoxide (CO), carbon dioxide (CO2), ozone (O3), and chlorofluorocarbons (CFCs). Oxides of nitrogen result mainly from cooking appliances, pilot lights, and unvented heaters. Underground or attached parking garages can also contribute to concentrations of indoor NOx. An unvented gas stove contributes approximately 0.025 ppm of NO2 to a home. Nitric oxide is an odorless, tasteless, colorless gas. Inhalation of NO causes the formation of methemoglobin, which adversely affects the body by interfering with oxygen transport at the cellular level. Nitrogen dioxide is a corrosive gas with a pungent odor, the odor threshold of which is reported to be between 0.11 and 0.22 ppm. NO2 has low water solubility and therefore can be inhaled into the deep lung where it causes a delayed inflammatory response.
Sulfur dioxide (SO2) can result from the emissions of kerosene space heaters, the combustion of fossil fuels, or burning any material containing sulfur. SO2 is a color- less gas with a pungent odor detected at about 0.5 ppm. Because SO2 is quite soluble in water, it can react with moisture in the upper respiratory tract to produce irritant effects on the upper respiratory mucous membrane. Contaminants’ exposure to fine particulates, the depth and rate of breathing of an individual, and the presence of preexisting disease can influence the degree of SO2 toxicity.
CO is an odorless, colorless, and tasteless gas produced by the incomplete com- bustion of hydrocarbons. Common indoor sources of CO include gas stove, kerosene lanterns and heaters, tobacco smoke, wood stoves and unvented or improperly vented combustion sources. CO is a chemical asphyxiate. Inhalation of CO causes a
throbbing headache brought about by CO having a competitive preference for hemo- globin. Carbon monoxide inhibits oxygen transport in the blood through the formation of carboxy hemoglobin.
CO2 is produced by human respiration. It is not normally considered to be a toxic air contaminant, but it can be a simple asphyxiate. A level of 1000 ppm has been sug- gested as being representative of delivery rates of 10 L/s per person of outside air when CO2 is measured at equilibrium concentration and at occupant densities of 10 people per 100 m2 floor space. CO2 can become dangerous not as a toxic agent but as a secondary asphyxiate. At concentrations between 2500 to 5000 ppm, CO2 can cause headache. At extremely high levels of 100,000 ppm, people lose consciousness in 10 min, and at 200,000 ppm, CO2 causes partial or complete closure of the glottis.
O3 arises from the electrical or coronal discharges from office equipment includ- ing laser printers and photocopiers. Ozone is a pulmonary irritant and causes changes in human pulmonary function at concentrations of approximately 0.12 ppm. Exposure to ozone at 60–80 ppb causes inflammation, bronchoconstriction, and increased air- way responsiveness.
CFCs are halogenated alkaline gases that have been used as heat transfer gases in refrigeration applications, blowing agents, and propellants in aerosol products, and as expanders in plastic foams. Inhalation exposures to CFCs can cause cardiotoxicity at chronic, low-level exposures. Chronic exposures to 1000 ppm for 8 h per day for up to 17 days caused no subjective symptoms or changes in pulmonary function.
Formaldehyde is a VOC. It is a ubiquitous chemical used in a wide variety of products and is most frequently introduced in newly constructed buildings. It is a colorless gas at room temperature and has a pungent odor at higher concentrations.
The infiltration of outdoor air is one source of formaldehyde in the indoor environ- ment, but the primary sources are in the indoor environment itself: building materi- als (thermal insulation in the side walls of buildings, plywood and particle board, floor coverings, and carpet backing), combustion appliances (gas stoves and heating systems), tobacco smoke, and a large number of consumer products (paper for wax paper, facial tissues, napkins, and paper towels).
Asbestos is a widespread component of the structural environment in schools, homes, and private and public buildings. Its release in the indoor environment depends on the cohesiveness of the asbestos-containing material (ACM) and the intensity of the disturbing force. Asbestos and similar fibrous minerals have been used in construction materials, consumer products, and appliances. ACM is most frequently found in boiler insulation, pipe insulation, sprayed-on fireproofing, breaching insulation, and floor and ceiling tiles.
ETS comes from the sidestream smoke emitted from the burning end of ciga- rettes, cigars, and pipes and second-hand smoke exhaled by smokers. Breathing in ambient ETS is generally referred to as passive or involuntary smoking. The con- taminants arising from tobacco combustion can be distinguished into mainstream smoke and sidestream smoke. Both smokers and nonsmokers are exposed to side- stream smoke. Mainstream smoke is undiluted and is pulled through with the tobacco into the smoker’s lungs. Sidestream smoke is directly from burning tobacco.
Depending on the smoking behavior, burning temperature, and type of filter, the composition of mainstream smoke exhaled by the smoker varies substantially.
Smoking is the major source of indoor particulates (having variable composition) and numerous irritating gases. Because tobacco does not burn completely, other con- taminants are given off, including sulfur dioxide, ammonia, nitrogen oxides, vinyl chloride, hydrogen cyanide, formaldehyde, radionuclides, benzene, and arsenic.
Odors are a class of contaminants in gaseous form that can bring about discom- fort, irritation, stress, complaints, and even fear, panic, and mass hysteria. They arise from occupants, and their effects figure in IAQ issues predominantly on the basis of comfort rather than health. Therefore, it is very difficult to track down odor com- plaints, and the facility managers often relegate odor complaints to the bottom of the priority pile. Routine activities such as cooking, smoking, bathroom use, and main- tenance give rise to odors that are often disagreeable and in some cases offensive. To varying degrees, almost all building materials and furnishings are sources of odor.
Radon and its progeny are the only natural airborne radionuclides. The series begins with radon-222 and ends with the alpha decay product of radon-226 and is formed from the decay of radium, which in turn results from the decay of uranium.
Radon is a noble gas that can move from its site of formation, giving it a substantial opportunity to reach air that is inhaled by humans. It is an odorless and colorless gas that is always present at various concentrations in the air. The short-lived decay products of radon—polonium, lead, and bismuth—are chemically active and thus can be collected in the lungs either directly or through particles with which they attach. Approximately 90% of radon daughters attach to larger airborne particles before they can be inhaled. The most important dose arises from the alpha decay of polonium isotope. The main sources of radionuclides and radiation are building materials, soil, and ground water. It typically enters through cracks, voids, or other openings in the foundation of buildings. Conditions affecting the flow of radon are the soil factors, the building factors, and the pressure differentials.
Air temperature and humidity are two of the most important comfort variables out of six primary comfort variables, which affect the quality of the indoor environment and are important indicators of IAQ. They are also extremely important to the occupant’s perception of IAQ. The hot and humid climate of India might have an adverse impact on the comfort of the occupants. Therefore, controlling the air temperature and humidity is the primary factor for achieving comfort in indoor environments. In an enclosed space, air temperature generally increases from the floor to the ceiling (vertical temperature gradient). If this temperature difference is sufficiently large, a person’s head can have local warm discomfort and/or cold discomfort at the feet even though the overall average is thermally neutral. To prevent this local discomfort, the standard calls for a maximum temperature difference between the head and the feet of 3°C (5°F). Similarly, the humid- ity level should be below 55% or, more exactly, below a dew point of 62°F to avoid discomfort in enclosed spaces. The American Society of Heating, Refrigerating and Air- Conditioning Engineers (ASHRAE) Standard 55-2004 (ASHRAE, 2004a) notes that for thermal comfort purposes, temperature could range between approximately 67°F and 82°F. A more specific range can be determined from the standard but depends on RH, season, clothing worn, activity levels, and other factors. The standard notes that HVAC systems must be able to maintain a humidity ratio of exactly or below 0.012. This corresponds to an upper RH level as high as about 80% at low dry bulb temperatures but can be lower depending on factors such as temperature and the other factors listed above.
The standard does not specify a lower humidity limit but notes that nonthermal comfort factors may place limits on the acceptability of very-low-humidity environments. The acceptable temperature and humidity ranges for winter and summer are 68.5–76°F and 74.0–80.0°F, respectively, for a dry bulb at 30% RH, and 68.5–74.5°F and 73.0–79.0°F, respectively, for a dry bulb at 50% RH. ASHRAE Standard 62.1-2004 (ASHRAE, 2004b) recommends that RH in occupied spaces be controlled to less than 65% to reduce the likelihood of conditions that can lead to microbial growth.
The predominant sources of indoor pollutants are listed in Table 4.1.
table 4.1
sources of Indoor air pollutants
principal pollutants sources
predominantly outdoor
SO2, SPM/RSPM Fuel combustion
Pollens Trees, grass, weeds, plants
Lead, manganese Automobiles
Calcium, chlorine, silicon, cadmium Suspension of soils or industrial emission VOCs, poly aromatic hydrocarbons (PAHs), etc. Petrochemical solvents, natural sources,
vaporization of unburned fuels both Indoor and outdoor
Nitrogen oxide (NO), NO2, CO Fuel-burning, vehicular exhaust emission
CO2 Fuel-burning and metabolic activity
Ozone (O3) Photochemical reactions, photocopying machines
SPM/RSPM ETS, resuspension, condensation of vapors and
combustion products
Water vapor Biological activity, combustion, evaporation
Organic substances (VOCs) Volatilization, combustion, paint, metabolic action, pesticides, insecticides, fungicides
Spores Fungi, molds
predominantly Indoor
Radon Building construction, materials (concrete stone),
water
Formaldehyde Particle board, insulation, furnishings, ETS
Asbestos, minerals, and synthetic fibers Fire-retardant, acoustic thermal, or electric insulation
Organic substances Adhesives, solvents, cooking, cosmetics, solvents
Ammonia (NH3) Metabolic activity, cleaning products
PAH, arsenic, nicotine, acrolein, etc. ETS
Mercury Fungicides, in paints, spills in dental care facilities
or laboratories, thermometer breakage Aerosols, allergens Consumer products, house dust, animal dander
Viable organisms Infections
Source: National Research Council. 1981. Indoor Pollutants. National Academy Press, USA.
4.2.1 parameterSof iaQ
The types and quantities of pollutants found indoors vary temporally and spatially.
Depending on the type of pollutant and its sources, sinks, and mixing condition, its concentration can vary a lot even in a small area. Ventilation plays an important role in deciding the IAQ of an indoor space. However, the exposure of human to indoor pollutants depends on various factors that are listed in Table 4.2.
4.2.2 Ventilationand iaQ
Ventilation refers to the “provision of sufficient quantities of outside air in the build- ing for the occupants to breathe and to dilute the concentration of the pollution gener- ated by the people, equipment and material inside the building.” It plays an important role in providing good IAQ. It drives the transport of pollutants between indoor and outdoor environments and determines the residence time of pollutants indoors.
Smaller the ventilation rate, longer will be the residence time of pollutants. The pol- lutant concentration inside the spaces is inversely proportional to the ventilation rates, that is, doubling the ventilation rate will halve the pollutant concentration.
It is necessary for both mechanical HVAC and naturally ventilated buildings (Awbi, 1991). A poorly designed HVAC system may be the culprit of IAQ problems.
Biological contaminants like bacteria, virus, mold, and so on are often found in areas that provide food and moisture or water. Humidifiers, condensate pans, and table 4.2
factors affecting exposure to Indoor air pollutants
Human activities Working time and activities related to work
Day duties, night duties, number of hours, etc.
Domestic work Kitchen activities, cleaning activities Social life
Geographical factors
Regional variation Weather: RH, wind velocity, temperature Urban, suburban, and
neighborhood variation
City size, density of buildings, population correlate with temperature, wind velocity, pressure (e.g., urban heat island)
Variation in IAQ in buildings
Pollutant concentration in kitchen are usually greater than the rest of the rooms
Building factors Site characteristics Airflow around buildings, proximity to major sources of outdoor pollution
Occupancy Type and intensity of human activity (smoking, cleaning, etc.), spatial characteristics of a given activity, operation schedule of a building Design Interior space design, envelope design (primary
elements: foundations, floors, walls, and roofs;
secondary elements: facing, cladding, and sheathing) Source: National Research Council. 1981. Indoor Pollutants. National Academy Press, USA.