Causes and Sources of Indoor Air Pollution
Improved building construction and insulation, including weather stripping, caulking, and storm and thermopane windows, reduce infiltration and air exchange, which results in less air dilution and an increase in the concentration of indoor air pollution. Inadequate ventilation and the recirculation of contaminated used air to save on energy costs for heating and cooling further aggravate the problem. Good practice would dictate that at least one-third of the recirculated air should be clean fresh air, even though this would increase energy costs, unless an air-to-air heat exchanger is used.
Household appliances, aerosol applications, cleaning products, pesticides, photocopying machines (ozone), interior furnishings and building materials (formaldehyde and volatile organic compounds), tobacco smoke, dry-cleaned clothing, and radon may also contribute to the indoor air pollution problem. Noise might also be included. Table 2.10 lists major pollutant/sources, specific contaminants, and acceptable levels. The contaminants may be found in the new or rehabilitated home, office, or other workplace; in the automobile, airplane, or
INDOOR AIR QUALITY 139 TABLE 2.10 Sources and Exposure Guidelines of Indoor Air Contaminants
Asbestos and other fibrous aerosols: friable asbestos—fireproofing, thermal and acoustic insulation, decoration; hard asbestos—vinyl floor and cement products, automatic brake linings (O) Biological aerosols: human and animal metabolic activity products, infectious agents, allergens, fungi, bacteria in humidifiers, bacteria in cooling devices Carbon monoxide: kerosene heaters, gas stoves, gas space heaters, wood stoves, fireplaces, smoking, and automobiles (O) Formaldehyde: particleboard, paneling, plywood, ceiling tile, urea-formaldehyde foam insulation, other construction materials
Inhalable particulates: smoking, vacuuming, combustion sources (O), industrial sources, fugitive dust (O), and other organic particulate constituents Metals and other inorganic paniculate contaminants: Lead—old paint, automobile exhaust (O)
Mercury—old paint, fossil fuel combustion (O) Cadmium—smoking, use of fungicides (O) Arsenic—smoking, pesticides, rodent poisons Nitrates—outdoor air Sulfates—outdoor air
Nitrogen dioxide: Gas stoves, gas space heaters, kerosene space heaters, combustion sources (O), automobile exhaust (O) Ozone: photocopying machines, electrostatic air cleaners, outdoor air Pesticides and other semivolatile organics: Sprays and strips, drift from area applications (0) Polyaromatic hydrocarbons and other paniculate constituents: woodburning, smoking, cooking, coal combustion, and coke ovens (O)
0.2 fibers/ml for fibers longer than 5 ^m (based on ASHRAEb guidelines of 1/10 of U.S. 8-hr occupational standard) None available
9ppm for 8hr (NAAQSc); 35ppm for 1 hr (NAAQS)
0.1 ppm (based on Dutch and West German guidelines as reported in ASHRAE Guidelines, 1981, and National Research Council report, 1981) 55-110 ^g/m3 annuald
150-350 ^g/m3 for 24hrd
(NAAQS) 2 ^g/m3 for 24 hr (ASHRAE) 2 ^g/m3 for 24 hr (ASHRAE) None available None available 4 ^g/m3 annual, 12 ^g/m3 for
24 hr (ASHRAE) 0.05 ppm annual (NAAQS)
Not exceeding 0.12 ppm once a year (NAAQS) 5 ^g/m3 for chlordane (NRC)e
140 RESIDENTIAL AND INSTITUTIONAL ENVIRONMENT TABLE 2.10 (continued )
Radon and radon progeny: diffusion through floors and basement walls from soil in contact with a residence, construction materials containing radium, untreated groundwater containing dissolved radon, combustion of natural gas used in cooking and unvented heating, radon from local soil emanation (O) Sulfur dioxide: kerosene space heaters, coal and oil fuel combustion sources (0) Volatile organics: Cooking, smoking, room deodorizers, cleaning sprays, paints, varnishes, solvents and other organic products used in homes and offices, furnishings such as carpets and draperies, clothing, furniture, emissions from waste dumps (O)
0.01 working level (ASHRAE guidelines)
for 24 hr (NAAQS) None available a (O) refers to outdoor sources.
bAmerican Society of Heating, Refrigerating and Air Conditioning Engineers. cU.S. National Ambient Air Quality Standards.
d These numbers indicate the probable range for the new NAAQS for particulates of 10 ^m or less in size. Based on "Recommendations for the National Ambient Air Quality Standards for Particulates—Revised Draft Paper," Strategies and Air Standard Division, Office of Air Programs, U.S. Environmental Protection Agency, Washington, DC, October 1981.
eNational Research Council, 1982, "An Assessment of Health Risk of Seven Pesticides Used for Termite Control," National Academy Press, Washington, D.C.
Source: N. L. Nagada, H. E. Rector, and L. A. Wallace, Project Summary Guidelines for Monitoring Indoor Air Quality, EPA-600/S4-83-046, U.S. Environmental Protection Agency, Office of Monitoring Systems and Quality Assurance, Washington, DC, January 1984.
bus; or in the school, auditorium, indoor ice skating rink, restaurant, enclosed shopping center, commercial and public building, hospital, and nursing home.
Most urban dwellers spend as much as 80 to 90 percent of their time indoors including transportation vehicles. The primary types of indoor air quality problems are inadequate ventilation (52 percent), contamination from inside the building (17 percent), contamination from outside the building (11 percent), microbiological contamination (5 percent), contamination from building fabrics (3 percent), and unknown (12 percent).21
Bacteria, viruses, fungi, pollens, house dust, and mite droppings are found in indoor air. Fungi, including spores and molds, multiply in the presence of increased humidity level (greater than 70 percent, some say 45 to 60 percent).22
Pollens, fungi, and other allergens are also brought indoors by ventilation systems, clothing, tracking, and open doors and windows. Substantial reduction in ventilation rates will tend to increase concentrations of contaminants and the probability of infection and allergy to the extent contaminants remain viable and airborne.
Sources of biological contaminants include air-conditioning systems; humidifiers; air ducts; cooling towers; grass, tree, and weed pollens; occupants; and household pets. Keep air-conditioning systems clean and empty, clean humidifiers and sanitize frequently, and minimize household dust.23 Some unit air cleaners are effective in removing particulates but may also incubate fungi and microorganisms. Air cleaners, such as electrostatic precipitators, ionizers, or filters, are not designed to remove radon or other gases. Humidifiers and filters require scheduled cleaning or filter replacement. Prevent the accumulation of water in equipment; ensure proper drainage. Recirculating or independent steam humidification is said to be preferable to the filter-type humidifier for room humidification. Ensure that the water used is not contaminated with toxic volatile compounds.
The spread of respiratory diseases is facilitated by infectious agents and particulates in contaminated air. Overcrowding and the recirculation of contaminated air, if not adequately diluted, cleaned, or disinfected, permit continual seeding and accumulation of pathogenic microorganisms at a rate exceeding the natural die-off rate. A study at U.S. Army training centers showed a 45 percent increase in respiratory infection in energy-efficient buildings providing 1.8 ft3 per minute per person outside air. This was compared to older barracks providing 14.4 ft3 per minute per person outside air where the infection rate was lower.24
Legionnaires' disease, meningococcal meningitis, the common cold, influenza, and other respiratory diseases may be transmitted by airborne aerosols. Comprehensive studies on the health effects of long- and short-term exposure to indoor (and outdoor) contaminants are limited. Young children, the elderly, and people suffering from respiratory diseases will be the first to show signs of discomfort from indoor air contamination. Some common complaints are headache; fatigue; eye, nose, and throat irritation; fever; and dizziness. See also "Respiratory Illness Control."
Some air contaminants are are associated with noninfectious and communicable disease and are environmentally related. They may also aggravate respiratory and heart diseases and cause nausea, headache, eye, nose, and throat irritation, discomfort, and allergies. Death from chronic or acute exposure may also result. More information on the health effects of specific indoor contaminants is needed, but this does not preclude taking preventive action, particularly where information is available, such as for carbon monoxide, formaldehyde, asbestos, radon, and biological aerosols. Review of a few air contaminants and sources are given next. More detailed information can be found from public health literature.
Radon Radon is an odorless, colorless, and tasteless chemically inert radioactive gas released in the decay of radium from uranium in most soils and rocks. It is found naturally in soil gas, underground water, and outdoor air. It is 60 times more soluble [at 50oF (10oC)] than oxygen in water. Radon has a half-life of 3.8 days. Thorium, one of the uranium decay products, also releases radon. Radon and primarily its alpha-emitting decay products (especially polonium) contribute a major portion of the biologically significant dose associated with natural background radiation. The beta and gamma emissions are not significant. The alpha particles, however, adhere to dust particles that, when inhaled, can become attached to the lungs and remain to irradiate the surrounding tissue, contributing to the cause of cancer.
It has been estimated that exposure to "one working-level month" over a lifetime (assumed to be 70 years) would result in about 350 additional lung cancer deaths per million people exposed. Exposure to a radon level of 4 pCi/1* for 12 hours per day would result in an annual exposure of 0.5 working-level months.25^ Working level is explained later. It has also been estimated that radon causes 13,000 cancer deaths per year.* Smokers are at much greater risk. In view of this, a screening program to identify problem areas and recommend mitigation alternatives to homeowners is indicated. Also indicated is radon measurement before the purchase of a new or existing home.
The hazard associated with radon is related to the concentration and time of exposure. Radon should not exceed 2 to 5 pCi/l indoors. The U.S. Environmental Protection Agency (EPA) has set a guideline limit of 4 pCi/l per 24 hours for homes (this is believed to be conservative) and a standard of 20 pCi/l in underground uranium mines. Special problems exist at uranium tailings and phosphate slag sites. The EPA estimates 20 million homes exceed the 4-pCi/l limit.§ The action level for existing dwellings in the United Kingdom is 10 pCi/l and 20 in Canada, with 2.5 in new dwellings in the United Kingdom.27
Major potential entry sources of indoor radon from the soil are cracks in dwelling concrete floor slabs and basement walls; pores and cracks in concrete blocks, mortar joints, and floor-wall joints; spaces behind brick veneer walls that rest on uncapped hollow-block foundations; floor drains; footing drains; and exposed soil in the bottom of drainage sumps. Radon-contaminated water, when agitated, aerated, or splashed as in dishwashing, clothes washing, showering, toilet flushing, and opened faucets or when water is heated, permits the release of radon. In addition to rock and soil underlying dwellings, construction materials (some stone masonry, concrete blocks, bricks, concrete), and some well and seepage waters and gas supplies may be the source of radium and radon. In the average dwelling, 10,000 pCi/l of radon in the water can be assumed to release
*Picocuries per liter: 4 pCi/1 = 150 Bq/m3 (bequerels per cubic meter).
t Source: Health Risks of Radon and Other Internally Deposited Alpha-Emitters: Bier IV, National Academy Press, Washington, DC.
^National Research Council estimate, 1988 (see ref. 26). Lung cancer may be causing 5000 to 10,000 lung cancer deaths per year in the United States based on an average annual dose of 2.4 rem. §See W. W. Nazaroff and K. Teichman, "Indoor Radon," Environ. Sci. Technol. (June 1990): 777.
1 pCi/l to the air, but the actual indoor concentration will be dependent primarily on the amount of radon entering from the soil and on the extent to which the indoor air is diluted by outside air.28
The EPA is proposing a level of 200 to 500 pCi/l for drinking water, which might be increased to perhaps 1,000. A level of 20,000 pCi/l in water is considered a significant concentration. If the water is high in radon, it can be removed by filtering through a granular activated-carbon (GAC) filter, by storage until the radon has decayed, or by aeration before it enters the dwelling water system. But the carbon becomes radioactive and in decay releases gamma radiation, which can be a health hazard. Aeration appears to be the most cost-effective procedure for public water systems and a GAC filter for a private dwelling having its own well-water supply, if needed. Activated carbon concentrates the radon and decay products and, hence, poses a disposal problem. Consult with the equipment dealer, the state or local health department, and radiation protection office for the proper way to dispose of the used carbon.
Radon contamination in an existing dwelling, if it is a problem, can be reduced by preventing its entry or by removing the radon. It can be reduced by closing and caulking all cracks, joints, and openings of the structure in the basement or in contact with the ground, or in the flooring above the crawl space, and by tightly covering open drains and sumps as previously noted. Good insulation of water pipes and underflooring beneath living areas would be required in the crawl space in areas subjected to subfreezing temperatures29 and to reduce heating or cooling costs. If this is not sufficient to reduce the indoor radon level, natural or mechanical forced-air ventilation into basement and crawl spaces can be provided, with openings to allow radon-laden air to exit. Exhaust ventilation would be needed for tightly covered sumps and footing drains. In Florida, a vent area of 1 ft2 for each 150 ft2 of floor area for wooden flooring or at least 1.5 ft2 of opening for each 15 feet of linear perimeter wall for nonwooden flooring is required by the housing code. To reduce radon levels in basements and enclosed crawl spaces, bring in outside air to dilute and displace the inside air. Forced-air ventilation may be necessary; exhaust fans in living areas and combustion air for warm-air furnaces and fireplaces would depressurize the dwelling and draw in radon from the basement and should not be used. Provide outside air vent for furnace and hot-water gas heater and outside air duct for wood stove and fireplace.
In a new building, the gravel under the basement floor or floor slab could have perforated pipe embedded in it to intercept and vent radon gas above the roof using a mechanical exhaust fan. Wind turbines and natural convection are not effective. A polyethylene sheet would be placed under the basement concrete floor slab above the gravel before it is poured. The ventilation method used must not reduce the air pressure within the dwelling. Sealing major potential sources of radon entry, as already stated, and ventilation should greatly reduce radon concentrations to "safe" levels in most cases. The need for radon protection, such as built-in ventilation under the basement floor or floor slab, is best provided in new construction and required in building codes where needed. Local geological information and in-home radon measurements will give an indication of need.
Measurement for radon exposure is in working-level units, which include radon's first four daughter products that will result in MeV of potential alpha energy per liter of air. A working level of 1.0 is assumed to be equivalent to a one-month total of 200 pCi/l of radon in most indoor environments (at 50 percent equilibrium).30 A working-level month (WLM) is equivalent to an average of 173 hour spent in a mine by a uranium worker. Exposure must not exceed 4 WLMs in any calendar year. The occupational limit for workers is 4.8 WLMs according to the 1985 recommendation of the International Commission on Radiological Protection.31 The National Council on Radiation, Protection, and Measurement says radon level should not exceed 2 WLMs per year, or 8 pCi/l.32 The U.S. Mine Safety and Health Administration has set a maximum exposure level of 8 pCi/l for miners.
State and local health departments or state radiation protection offices can usually provide a list of companies supplying radon testing services. Federal and state publications are available to assist the homeowner to understand the problem and take corrective action if indicated.33 Seek professional advice if a significant problem exists.
Formaldehyde Formaldehyde, a colorless gas, may cause extreme discomfort and contact dermatitis indoors. The odor can be detected at less than 1 ppm. Exposure to 1.0 to 5.0 ppm or less can cause burning of the eyes, tearing, and general irritation of the upper respiratory passages. Levels of 0.3 to 2.7 ppm have been found to disturb sleep and to be irritating to some persons. Exposure to 10 to 20 ppm may produce coughing, tightening in the chest, a sense of pressure in the head, and palpitations. Exposures of 50 to 100 ppm and above can cause serious injury, including pulmonary edema and pneumonitis, and possibly death when above 100 ppm. Exposure to formaldehyde solutions, or urea-formaldehyde-containing resins, is a well-recognized problem.34 However, a four-year study at the National Cancer Institute concluded that there is "little evidence that mortality from cancer is associated with formaldehyde exposure at levels experienced by workers in this study."35
Sources of formaldehyde are resins and glues to bond particle board and plywood, urea-formaldehyde foam insulation, permanent press fabric, embalming fluid, drugs, disinfectants, and cosmetics as well as chemicals used in pathology and anatomy laboratories and in the manufacture of automobiles, furniture, paper, and electrical equipment.36 Formaldehyde problems are also related to materials in mobile homes and prefabricated housing. Users of formaldehyde should wear protective clothing, use protective equipment, and apply engineering controls such as hoods and separate exhaust systems. The workplace should provide a minimum ventilation of five air changes per hour. Some ameliorative measures suggested, where urea-formaldehyde is a problem, are to remove the product; seal with a specially formulated coating, vinyl covering, latex paint, or varnish after two years; and increase ventilation. Sealing will prevent the penetration of moisture, contact with urea-formaldehyde, and release of formaldehyde gas. The gas release from materials tends to decline in time. Improper formulation of urea-formaldehyde foam insulation is believed to exacerbate the problem. It is no longer used in the United States. Phenol-formaldehyde resins are generally used in outdoor materials and do not release significant quantities of formaldehyde; however, they cost more than urea-formaldehyde products.
The Occupational Safety and Health Administration (OSHA) 8-hr time-weighted average occupational exposure has been reduced from 3.0 to 1.0ppm with a maximum short-term exposure level of 2.0 ppm for any 15-min period.37* The National Research Council has established a limit of 0.1 ppm for space flights. The Department of Housing and Urban Development (HUD) has set a limit of 0.4 ppm for indoor air.
Polychlorinated Biphenyls (PCBs) PCBs are considered "probable" human carcinogens based on animal studies. Although manufacture was banned in 1979, many products containing PCBs remain in use. Possible major exposure routes to PCBs are inhalation when electrical transformers and other equipment containing PCBs are ruptured or burned, breathing PCB-contaminated air or skin contact in the work environment, the ingestion of food (fish) or drinking water containing PCBs, and spills or illegal dumping of fluids containing PCBs. Fluorescent light ballasts and vinyl-coated paper are also a common source of PCBs. It is best to use caution and seek advice immediately from your health or environmental protection department should there be an actual or potential exposure to PCBs.
The Occupational Safety and Health Administration has established an airborne exposure limit of from 0.5 mg/m3 (54 percent chlorine content) to 1 mg/m3 (42 percent chlorine content) as an 8-hr time-weighted average (skin).* The National Institute of Occupational Safety and Health (NIOSH) recommends that the airborne exposure to PCBs in the workplace be 1 f g/m3 or less.* The EPA has proposed a limit of 100 f g/m2 in areas where frequent and regular skin contact with surfaces is possible.
Tobacco Smoke Environmental tobacco smoke consists of a suspension of 0.01 to 1 f m particles leaving the burning tobacco condensate. Also produced are numerous hazardous gases including carbon monoxide. The involuntary chronic exposure to cigarette smoke, also referred to as passive smoking, is associated with an increased risk of lung cancer, according to a 1986 report of the National Academy of Science (NAS).38 In addition, children of smoking parents have increased respiratory illnesses compared with children whose parents do not smoke; however, data on other diseases such as other cancers and cardiovascular diseases are insufficient. According to the NAS report, passive smoking causes irritation of the eyes, nose, and throat of many nonsmokers. Ventilation rates
* Average level is proposed to be reduced to 0.75 ppm.
^Potential contribution to overall exposure by the cutaneous route, including mucous membranes and eyes.
*See "NIOSH, CDC, Recommendations for Occupational Safety and Health Standards, 1988," DHHS, PHS, NIOSH, CDC, MMWR. Supplement, August 26, 1988, pp. 23 and 24.
of up to five times higher in smoking areas are suggested to achieve acceptable indoor air quality. Office spaces should provide at least 20 ft3/min per occupant of clean outside air where smoking is permitted and at least 5 ft3/min per occupant in nonsmoking areas.39 Effort should be directed to the prohibition of smoking in enclosed spaces and the discouragement of smoking.
Volatile Organic Compounds (VOCs) VOCs are a broad range of chemical compounds, with boiling points in the range of approximately 120 to 480°F (50° to 260°C) and vapor pressures greater than about 4 x 10-5 to 4 x 10-6 in. Hg.40 Several hundred VOCs have been identified in the indoor environment.41 The Large Buildings Study by the EPA developed a VOC sample target list that includes aliphatic hydrocarbons, halogenated hydrocarbons, and oxygenated hydrocarbons such as aldehydes, alcohols, ketones, esters, ethers, and acids.42 Sources of VOCs in nonindustrial environments include building materials, furniture, furnishings, ventilation systems, household and consumer products, office equipment, and outdoor-related activities (e.g., traffic, neighborhood industry).
Little is known about the symptoms of overexposure to VOCs, but some are suspected of causing adverse health effects such as sensory irritation, odor, and the more complex set of symptoms of sick-building syndrome. Also researchers have found that neurotoxic effects may follow from low-level exposures to gaseous air pollutants.43 Reactions include runny eyes and nose, high frequency of airway infections, asthmalike symptoms among nonasthmatics, along with odor or taste complaints. There is also a possible link between the increase in allergies throughout the industrialized areas of the world and exposure to elevated concentrations of VOCs.
Emission source control, gas-phase air filtration using activated-carbon filters or photocatalytic reactors, and ventilation are common ways of controlling indoor VOCs.44 Sophisticated models have been developed to simulate the VOC emission, sorption, and transport in the built environment.45 These models can help explain the physics of VOC transport process, and can be used to guide the building design.46
Other Emissions Unvented kerosene, fuel oil, and wood stove space heaters, gas cooking and heating appliances, power equipment including automobiles, and gas clothes dryers lead to the emission of particulates, in addition to hazardous gases. Portable heaters also present risks of burns, injuries, fires, and explosions. Their use should be prohibited. Unvented kerosene space heaters can emit organic compounds, in addition to nitrogen dioxide, carbon dioxide, carbon monoxide, and sulfur dioxide. The concentrations can exceed the EPA ambient air standards, particularly in small spaces and where ventilation is inadequate. Poor-quality kerosene exacerbates the problem. Gas cooking appliances are also sources of carbon dioxide, nitrogen dioxide, formaldehyde, and other organic compounds, in addition to carbon monoxide. Carbon monoxide, nitrogen oxides, and particulates from automobile exhaust in garages can produce increased and hazardous concentrations in office buildings above the garage and in public areas. Gasoline-powered ice resurfacing machinery can cause the same effect in indoor ice-skating rinks and forklifts in enclosed spaces.
The smoke from cooking and heating with open fires in houses in some underdeveloped countries is the cause of serious respiratory illness in infants. Pregnant women exposed to the smoke produce lower-birthweight children.47
Good ventilation requires that the air contain a suitable amount of moisture and that it be in gentle motion, cool, and free from offensive body and other odors, poisonous and offensive fumes, and large amounts of dust. Comfort zones for certain conditions of temperature, humidity, and air movement are given by the American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE).48 Air movement, radiant heat, the individual, and the tasks being performed must also be taken into consideration.
There is no one temperature and humidity at which everyone is comfortable. People's sensations, health, sex, activity, and age all enter into the comfort standard. McNall49 recommends a temperature range between 73 and 77oF (23 and 25oC) and humidity between 20 and 60 percent for lightly clothed adults engaging in sedentary activities in residences. Lubart50 suggests a comfort level within the range of 68oF (20oC) at inside relative humidity of 50 percent to 76oF (24oC) with 10 percent relative humidity. The EPA recommends a relative humidity of 30 to 50 percent for homes.51 Indoor relative humidity of 60 percent or higher would cause excessive condensation and greater mildew, corrosion, and decay. Excessive moisture can cause condensation and increasing deterioration of building materials. Humidities above 70 percent promote germination and growth of fungal spores. Ordinarily, however, only temperature and ventilation control is used in the home, with no attempt made to measure or control the relative humidity. Humidity control becomes more important with reduced ventilation because the products of respiratory and metabolic processes and certain indoor operations and activities contribute moisture vapor. In a confined space, this can lead to the accumulation of contaminants and moisture and discomfort or illness.
Indoor air should be free of objectionable odors, unhealthy levels of microorganisms, allergens, and chemical contaminants. The design of a ventilation system should avoid uncomfortable drafts and large temperature variations. National, state,52 and local building and energy conservation codes specify ventilation requirements for various space uses and should be consulted. It has been proposed that the generally accepted minimum supply of fresh air per occupant be 15 or 20ft3/min.53 Much higher fresh air supply is needed in rooms where smoking is permitted or where other polluting activities are permitted. The OSHA requirements must be met in the workplace. In true air conditioning, air is treated to simultaneously control its temperature, humidity, cleanliness, and distribution to meet requirements of the conditioned space. Room air inlets and returns should be arranged to ensure proper air mixing and ventilation of the space and to avoid drafts.
The tendency to reduce air infiltration and fresh air makeup in ventilation has increased the buildup of air contamination, with resultant occupant complaints. To alleviate the problem, minimum ventilation standards, including fresh air intakes, have been established or proposed in building codes and other publications.53 In addition, the use of certain indoor products such as urea-formaldehyde insulation and unvented kerosene space heaters has been banned in some jurisdictions. However, owner- or occupant-provided equipment, materials, and furnishings not under regulatory control and ambient air quality may nevertheless contribute to indoor air pollution. Ventilation may be provided by natural or mechanical means. Investigation of an indoor air pollution problem could well start with carbon dioxide tests and interrogation of management, employees, and custodial and union people. If carbon dioxide concentrations are below 600 ppm, with comfortable temperature and humidity levels, complaints about air quality should be minimal.
Natural Ventilation A minimum of one or two air changes per hour can often be secured by normal traffic and leakage through walls, floors, and ceilings and through or around doors and windows, but previously mentioned energy conservation measures may reduce air infiltration and air change by 50 percent or more. Under ordinary circumstances, adequate ventilation can be obtained in residences by natural means with properly designed windows. Openable windows, louvers, or doors are needed to ventilate and keep attics, basement rooms, pipe spaces, and cellars relatively dry. The tops of windows should extend as close to the ceiling as possible, with consideration to roof overhang, to permit a greater portion of the room to be exposed to controlled sunlight. The minimum total window or skylight area, measured between stops, for a habitable room should be at least 8 percent of the floor area and the openable area at least 45 percent of the window or skylight area.54 The ventilation of modern buildings is usually dependent on mechanical air conditioning and air recirculation, including controlled fresh air intake. Some examples and design criteria are discussed next.
Schools Separate venting of each classroom to the outside is preferred. Good standards specify that the mechanical ventilating system provide a minimum air change of 15 to 20 ft3/min per student to remove carbon monoxide and odors, without drafts. The air movement should not exceed 25 ft/min, and the vertical temperature gradient should not vary more than 5°F (3 o C) in the space within 5 feet of the floor and 2 feet or more from exterior walls. Temperature should be automatically controlled.
Public Areas In recreation halls, theaters, churches, meeting rooms, and other places of temporary assembly, a system of mechanical or induced ventilation is usually needed to meet the requirement of at least 15 ft3 of clean air per minute per person. Any system of ventilation used should prevent short circuiting, uncomfortable drafts, and the buildup of unhealthy levels of air contaminants. Approximately one-third of the recirculated air should be clean outside air.
Correctional Institutions Where dependence is on natural ventilation, windows or other openings should provide an area of at least 12.5 percent of the floor space of the sleeping, living, educational, and work areas and be located to provide cross-ventilation. Gyms and swimming pools require special temperature, humidity, and ventilation controls. If dependence is on mechanical ventilation, 15 to 20 ft3/min per person is recommended. Where air is recirculated, approximately one-third should be fresh, clean outside air.55
Toilets and Bathrooms Bathroom and toilet room ventilation is usually accomplished by means of windows or ventilating ducts. The common specification for natural ventilation is that the window or skylight area be at least 8 or 10 percent of the floor area and not less than 3 ft3, of which 45 percent is openable. For gravity exhaust ventilation, vents or ducts at least 72 in.2 in area per water closet or urinal and a minimum of 2 ft3/min of fresh air per square foot of floor area should be provided. A system of mechanical exhaust ventilation providing at least five air changes per hour of the air volume of the bathroom or toilet room during hours of probable use is usually specified for ventilation where windows, ducts, or vents are not relied on or are not available for ventilation. ASHRAE53 recommends 50 ft3/min per water closet and per urinal for a public restroom. Exhaust fans activated by the opening and closing of doors or by a light switch do not provide satisfactory ventilation. The recirculation of air supplied to toilets, lavatories, toilet rooms, bathrooms, and restrooms (also kitchens, laboratories, and garages) is generally not permitted.56
Air Change Measurement Air in an enclosed space normally diffuses out and outdoor air filters in at a rate dependent on the tightness of the space or building and wind direction and velocity. The air change can be determined by dividing the volume of air entering an enclosed space or room by the volume of the space or room. For example, if 100 ft3/min enters a room having a volume of 1,000 ft3 occupied by five people, there would be six air changes per hour (100 x 60 ^ 1,000) and 20ft3/min per occupant.
The air change may be measured by use of tracers. Desirable qualities of a tracer gas are detectability, nonreactivity, nontoxicity, neutral buoyancy, relatively low concentration in ambient air, and low cost.57 The commonly used tracers include nitrous oxide (N2O), carbon dioxide (CO2), helium (He), and sulfur hexafluoride (SF6). Several tracer gas measurement procedures exist, including an American Society for Testing and Materials (ASTM) standard.58 In the measurement, the tracer is released into the building in a specific manner, and the concentration of the tracer within the building is monitored and related to the building's air change rate. Standardization of devices is necessary.
Monitoring The monitoring and measurement of the quality of indoor air can be accomplished by modification, as needed, of equipment used to sample ambient air and occupational exposure and by adapting laboratory equipment and procedures. Passive measuring devices for carbon monoxide, radon, formaldehyde, and asbestos, although not accurate, are acceptable. The Anderson impactor sampler may be used to collect indoor airborne fungi supplemented by plate incubation for colony count and identification. Psychrometers for measuring temperature and humidity and smoke tubes for determining air movement are also generally used. Samplers for volatile organic compounds and continuous samplers are also available. Standardized methods for the determination of air pollutants in indoor air are listed in Table 2.11.
Respiratory Illness Control
The NIOSH suggests seven steps to minimize respiratory illness:
1. Promptly and permanently repair all external and internal leaks in the heating, ventilation, and air-conditioning system (HVAC).
2. Maintain relative humidity below 70 percent in occupied spaces and in low-air-velocity plenums. (At a higher level of humidity, the germination and proliferation of fungal spores are enhanced.)
3. Prevent the accumulation of stagnant water in cooling-deck coils of air-handling units through proper inclination and continuous drainage of drain pans.
4. Use steam rather than recirculated water as a water source for humidifiers in HVAC systems; however, such steam sources should not be contaminated with volatile amines.
5. Replace filters in air-handling units at regular intervals. (These should have at least a moderate efficiency rating—50 percent or more—as measured by the atmospheric-dust spot test and should be of the extended-surface type; prefilters (e.g., roll type) should be used before passage over the higher efficiency filters.)
6. Discard, rather than disinfect carpets, upholstery, ceiling tiles, and other porous furnishings that are grossly contaminated.
7. Provide outdoor air into ventilation systems at minimum rates per occupant of at least 20ft3/min in areas where occupants are smoking and at least 5ft3/min in nonsmoking areas. (ASHRAE Recommended Standard 62 specifies a minimum of 15 or 20ft3/min per person.53)
These activities should be considered in ongoing preventive maintenance
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