Indoor Air Quality, Health, and Performance
Indoor air quality, which is a function of outdoor and indoor air pollutants, thermal comfort, and sensory loads (odors, “freshness”), can affect the health of children and adults and may affect student learning and teacher productivity.
Pollutants are generated from many sources. Outdoor pollutants include ozone, which has been associated with absenteeism among students. Pollutants and allergens in indoor air—mold, dust, pet dander, bacterial and fungal products, volatile organic compounds, and particulate matter—are associated with asthma and other respiratory symptoms and with a set of building-related symptoms (eye, nose, and throat irritations; headaches; fatigue; difficulty breathing; itching; and dry, irritated skin). In some cases, outdoor pollutants react with indoor chemicals to create new irritants.
Thermal comfort is influenced by temperature, relative humidity, and perceived air quality (sensory loads) and has been linked to student achievement as measured by task performance. Relative humidity is also a factor in the survival rates of viruses, bacteria, and fungi and their effects on human health (see Chapter 7, “Building Characteristics and the Spread of Infectious Diseases”).
Heating, ventilation, and air-conditioning (HVAC) systems are intended to provide (1) effective outside air delivery to rapidly dilute or filter out air contaminants and (2) thermal comfort for building occupants by heating or cooling outside air coming into occupied spaces. Ventilation can be supplied through mechanical systems, which draw air into and push air out of a building, or “naturally,” through the opening and clos-
ing of doors and windows and by uncontrolled leakage points through a building’s envelope. A variety of mechanical systems is available, including hybrid systems that use both natural and mechanical ventilation.
HVAC systems must be properly designed and sized to handle the sensible and latent heat loads of outside and recirculated air. If not properly designed, operated, and maintained, HVAC systems can themselves generate pollutants and excess moisture, thereby affecting the health of occupants. The principal standards and guidelines for HVAC system design and operation in the United States are (1) American Society of Heating, Refrigeration, and Air-Conditioning Engineers (ASHRAE) Standard 62.1-2004, “Ventilation for Acceptable Indoor Air Quality”; (2) American National Standards Institute (ANSI)/ASHRAE Standard 55-2004, “Thermal Environmental Conditions for Human Occupancy”; (3) the Department of Energy’s EnergySmart Schools guidelines; and (4) individual state codes, some of which are based on or refer to the International Building Code or other codes. Because industry standards for ventilation and energy efficiency have been developed separately, they have, in some cases, had the net effect of increasing relative indoor humidity.
As shown in Figure 4.1, the complex interactions between indoor and outdoor pollutants, moisture/humidity, HVAC systems, operations and maintenance practices can affect occupants’ health, comfort, and productivity. These topics are discussed in greater detail in the rest of the chapter.
Pollutants are generated by many sources both internal and external to a school. External sources include combustion products; biological material; and particulate matter and ozone entering through air intakes and the building envelope. People themselves can carry pollen and allergen sources, such as dust mites and pet dander, into a school on their shoes, skin, and clothes. Internal sources include but are not limited to combustion products; building materials and equipment; educational materials; cleaning products; biological agents; and human activity. In some cases, outdoor pollutants react with indoor chemicals to produce new irritants.
Outdoor Sources of Pollutants
Outdoor air pollutants can affect the health of children and adults in two ways. First, students, teachers, administrators, and support staff are exposed to outdoor pollutants before they enter a building, which can lead to increased respiratory symptoms (Schwartz, 2004). Second, outdoor
sources of pollution can contribute to indoor air pollutant concentrations when outdoor air is drawn into a school building through air intakes located at the rooftop, at ground level, or from below-grade “wells.” Outside air also enters the building through doors, windows, ventilation shafts, and leaks in the building envelope.
Mendell and Heath (2004, p. 9) found that “a substantial literature of strongly designed cohort studies is available on associations between outdoor pollutants and attendance of children at school.” They concluded that there was strongly suggestive evidence that absence from school increased with exposure to ozone at higher concentrations. However, the findings were mixed on the associations of school absence with exposure to outdoor nitrogen oxides, carbon dioxide, and particles <10 µm.
Site location can be an important determinant of outdoor pollutants. Schools next to high-traffic areas or with school buses idling their engines next to school doorways, windows, and air intakes may have higher levels of outdoor air pollutants being drawn indoors (Park and Jo, 2004; Blondeau et al., 2005; Singer et al., 2004; Behrentz et al., 2005).
Other significant sources of outdoor pollutants are plant-derived materials, or biomass, which can generate bioaerosols, including molds, fungi, and pollen. An IOM study (2002, p. 8) found as follows:
Although there is sufficient evidence to conclude that pollen exposure is associated with exacerbation of existing asthma in sensitized individuals, and pollen allergens have been documented in both dust and indoor air, there is inadequate or insufficient information to determine whether indoor air exposure to pollen is associated with exacerbation of asthma.
The IOM study also noted that “there is relatively little information on the impact of ventilation and air cleaning measures on indoor pollen levels, although it is clear that shutting windows and other measures that limit the entry rate of unfiltered air can be effective” (p. 14).
Indoor Sources of Pollutants
Indoor pollutants include chemicals, allergens, volatile organic compounds (VOCs), particulate matter, and biological particles or organisms. Chemicals in indoor environments include combustion products such as nitrogen oxides (NOx), sulfur oxides (SOx), and carbon monoxide (CO). Combustion products can be generated by gas-fired pilot lights in kitchens and laboratories. Other sources of indoor chemical pollutants include building materials (e.g., structural materials such as particleboard, adhesives, insulation); furnishings (carpets, paints, furniture); products used in a building (cleaning materials, pesticides, markers, art supplies); and equipment (copiers and printers).
Indoor allergen sources—house dust mites, pet dander, cockroaches, rodents, and seasonal pollens—can be brought into a building by occupants, can be generated by furry animals kept in classrooms, or can be attracted to food sources in, for example, school kitchens and cafeterias. Daisey et al. (2003) found that a variety of bioaerosols (primarily molds and fungi, dust mites, and animal antigens) could be found in school environments.
Volatile organic compounds (VOCs) and semivolatile organic compounds (SVOCs) are chemical compounds used extensively in building materials such as adhesives for wood products and structural materials, paints, and carpet adhesives. They also are found in art supplies, paints and
lacquers, paint strippers, cleaning supplies, pesticides, office equipment such as copiers and printers, correction fluids and carbonless copy paper, graphics and craft materials, markers, and photographic solutions. In fact, there are no places in schools where VOCs and SVOCs are not found.
Outdoor sources of VOCs and SVOCs include fuels and combustion, biological organisms, and pesticides. Research has shown that concentrations of VOCs are consistently higher indoors than outdoors (Adgate et al., 2004; Wallace, 1991), and studies in homes suggest that indoor concentrations vary depending on the specific VOC (Weisel et al., 2005; Meng et al., 2005). One study also showed that building renovation contributes significantly to total VOC concentrations (Crump et al., 2005).
Particulate matter (PM) includes solid particles ranging in size from ultrafine (<0.1 µm) to relatively large (>10 µm). These particles come from outdoors (including dusts and particles from traffic, stationary sources, and microorganisms), and indoors (humans, building materials, fibers, bioaerosols, mold, pet dander) (Afshari et al., 2005). Larger PM remains suspended in air for relatively short periods of time, instead settling on floors, surfaces, and furnishings. Smaller PM has longer suspension times—i.e., it remains airborne longer. Particulate matter has been implicated in a number of health effects, primarily respiratory and cardiac (Nel, 2005). Particulate matter can absorb VOCs, which may affect occupants’ health and comfort (Nilsson et al., 2004).
Larger PM tends to be related to housekeeping practices, ineffective filtration by HVAC systems, and local activity. Finer PM tends to be more independent of these factors, and a fraction of finer PM will even diffuse through structures and so be not removable by HVAC filtration.
One important group of PM is the airborne allergens, including molds and fungi, dander and other body fragments, dust mites, and cockroach antigens. Because these bioaerosols can induce an immune response, they are capable of causing illness at very low exposure levels and also of causing more severe respiratory disease than PM from nonbiological sources. The strength of the association of each of these bioaerosols with illness was summarized in Clearing the Air: Asthma and Indoor Air Exposures (IOM, 2000), and many of them were found to be more strongly related to asthmatic symptoms than were moisture and mold.
Improperly maintained HVAC systems can themselves be a source of pollutants. Several findings in Damp Indoor Spaces and Health (IOM, 2004) pertain specifically to the design and operation of HVAC systems as a critical factor in the control of moisture and mold growth in buildings:
Although relatively little attention has been directed to dampness and mold growth in HVAC systems, there is evidence of associated health effects (p. 42).
Liquid water is often present at several locations in or near commercial-building HVAC systems, facilitating the growth of microorganisms that may contribute to symptoms or illnesses (p. 42).
Microbial contamination of HVAC systems has been reported in many case studies and investigated in a few multibuilding efforts (p. 43).
Sites of reported contamination include outside air louvers, mixing boxes (where outside air mixes with recirculated air), filters, cooling coils, cooling coil drain pans, humidifiers, and duct surfaces (p. 43).
Bioaerosols from contaminated sites in an HVAC system may be transported to occupants and deposited on previously clean surfaces, making microbial contamination of HVAC systems a potential risk factor for adverse health effects (p. 43).
The Menzies et al. study (2003) of ultraviolet germicidal irradiation (UVGI) of drip pans and cooling coils indicates that limiting the microbial contamination of HVAC systems may yield health benefits. The use of UGVI is discussed in greater detail in Chapter 7.
Indoor Air Chemistry
Ozone (O3) is a primary pulmonary irritant that also plays an important role in indoor chemistry. Although ozone concentrations are generally higher outdoors than indoors, indoor ozone concentrations can be appreciable, infiltrating a building through windows, doors, and the envelope (Weschler et al., 1992). Ozone concentrations might be expected to be higher in naturally ventilated buildings. Indoor ozone sources include printers, copiers, and electrostatic air cleaners if they are not adequately maintained or are improperly exhausted. Sources of indoor terpenes and other unsaturated hydrocarbons are numerous and include cleaning products and air fresheners (Nazaroff and Weschler, 2004).
Reactions among reactive gases (such as ozone) and commonly occurring, nonirritating organic compounds (certain terpenes such as limonene and pinene) can generate products that are highly irritating and can impact human health and comfort (Karlberg et al., 1992; Weschler and Shields, 1997). The process of these ongoing reactions has been termed “indoor air chemistry” (Weschler et al., 1992). Ozone/terpene reaction products have been shown to cause greater airway irritation than either original product (Wolkoff et al., 2006; Weschler, 2004).
Using chamber studies, Weschler et al. (1992) demonstrated the formation of these reaction products from carpets. Weschler later discovered
that concentrations of products generated by reactions among indoor pollutants increased as ventilation decreased (Weschler and Shields, 2000). This increase in reaction products is independent of the diurnal variation in ozone levels or of outside ozone levels. These results suggest that maintaining adequate ventilation rates may reduce the potential for reactions among airborne pollutants that generate even more reactive and irritating products.
Ventilation rate is based on the outdoor air requirements of a ventilation system. Ventilation effectiveness is based on the ability of the system to distribute conditioned air within occupied spaces to dilute and remove air contaminants. The principal standard for ventilation rates is American Society of Heating, Refrigeration, and Air Conditioning Engineers (ASHRAE) Standard 62.1-2004, “Ventilation for Acceptable Indoor Air Quality.” However, Daisey et al. (2003), in a comprehensive review of the literature related to indoor air quality, ventilation, and health symptoms in schools, found that reported ventilation and carbon dioxide (CO2) levels indicated that a significant proportion of classrooms did not meet (then) ASHRAE Standard 62-1999 for minimum ventilation rate.1
A number of studies in schools have reviewed the effect of ventilation rates on health, productivity, and airborne pollutant control. Typically, these studies also look at a second variable, such as temperature or humidity, both of which are components of thermal comfort, to identify any confounding or synergistic effects (Figure 4.2).
Wargocki et al. (2005) conducted a field intervention experiment in two classes of 10-year-old children. Average air temperatures were reduced from 23.6°C to 20°C, and outdoor air supply rates were increased from 5.2 to 9.6 liters per second (L/s) per person in a 2 × 2 crossover design, each condition lasting a week. Tasks representing eight different aspects of schoolwork, from reading to mathematics, were performed during appropriate lessons, and the children marked visual-analogue scales each week to indicate their perception of building-related symptom intensity. In this study, increased ventilation rates corresponded to increased task completion in multiplication, addition, number checking (p < .05), and subtraction (p < .06). Reduced temperature corresponded to increased task completion in subtraction and reading (p < .001) and fewer errors in check-
ing a transcript against a recorded voice reading aloud (p < .07). When reduced temperature was combined with increased ventilation rates, task completion increased in a test of logical thinking (p < .03). Experimental data indicated that increasing ventilation rates from 5.4 to 9.6 L/s per person and decreasing temperatures from 24ºC to 20ºC could improve the performance of schoolwork by children as measured by task completion.
Smedje and Norbäck (2000) investigated the impact of improving school ventilation systems on allergies, asthma, and asthma symptoms in schoolchildren. They issued questionnaires to 1,476 children in 39 schools (mixed primary and secondary schools) from 1993 to 1995. Various exposure factors were measured in 100 classrooms during this time. In 12 percent of the classrooms, new ventilation systems were installed; their effect was to increase the air-exchange rate and reduce humidity. Air pollutant levels were lower in classrooms with the new ventilation systems. This investigation indicated a health improvement for children in the classrooms with increased ventilation, lower humidity, and reduced airborne pollutants. The incidence of asthma symptoms, but not allergies, was reduced in the classrooms with the new ventilation systems.
Shendell et al. (2004b) explored the association between student absences and indoor CO2 levels. These researchers noted that since measuring the actual ventilation rate is expensive and potentially problematic, the indoor concentration of CO2 has often been used as a surrogate for the ventilation rate per occupant, including in schools. They measured the short-term (5 min) CO2 levels in 409 traditional and 25 portable classrooms from 22 schools in Washington State. Attendance data were collected from school records. Their results indicated that a 1,000 parts per million (ppm) increase above the outdoor concentration of CO2 was associated with statistically significant 10 to 20 percent increases in student absences.
Mendell and Heath (2004) looked at the literature and found
Suggestive, although not fully consistent, evidence linking low outdoor ventilation rates in buildings to decreased performance in children and adults and
Suggestive but inconsistent evidence linking lower ventilation rates with decreased attendance among adults.
No studies on the impact of increased ventilation rates and/or effectiveness on teacher health and productivity were identified. However, multiple studies have looked at the effects of increased ventilation rates in office buildings and call centers on adult health and productivity (Milton et al., 2000; Seppänen et al., 1999; Seppänen and Fisk, 2004b, 2005; Fisk et al., 2003; Wargocki et al., 2002; Wyon, 2004). These studies are discussed below.
Studies of Offices and Other Building Types
In a study of 3,720 hourly employees of a large Massachusetts manufacturer in 40 buildings with 115 independently ventilated working areas, Milton et al. (2000, p. 212) analyzed the relationship between the rate at which outdoor air was supplied for ventilation and the amount of sick leave taken. The researchers found “consistent associations of increased sick leave with lower levels of outdoor air supply and IEQ [indoor environmental quality] complaints.” Seppänen and Fisk (2004a) developed a further quantitative relationship by fitting the data from these epidemiological studies using the Wells-Riley model of airborne disease transmission to predict the relationship. The model predicted that there would be a decrease in illness over time with increased ventilation rates.
Seppänen et al. (1999) reviewed the literature on the association of ventilation rates in nonresidential and nonindustrial buildings (primarily offices) with health and performance outcomes. The review included 20 studies investigating the association of ventilation rates with human
responses and 21 studies investigating the association of CO2 levels with human responses. A majority of studies found that ventilation rates of less than 10 L/s per person were associated in all building types with a statistically significant worsening in one or more health or perceived air quality outcomes. Some studies found that increasing ventilation rates up to 20 L/s per person was associated with significant decreases in the prevalence of building-related symptoms or with further significant improvements in perceived air quality. The ventilation rate studies reported relative risks of 1.1 for building-related symptoms at low ventilation rates and 6 at high ventilation rates.
The report Clearing the Air (IOM, 2000, pp. 15-16) stated:
There are both theoretical evidence and limited empirical data indicating that feasible modifications in ventilation rates can decrease or increase concentrations of some of the indoor pollutants associated with asthma by up to approximately 75%. Limited or suggestive evidence exists to indicate that particle air cleaning is associated with a reduction in the exacerbation of asthma symptoms…. It should also be noted that microorganisms can grow on some air-cleaning equipment such as filter media; thus improperly maintained air cleaners are a source of indoor pollutants.
The same report also stated:
Control options for chemical and particulate pollutants in indoor environments include source modification (removal, substitution, or emission reduction), ventilation (exhaust or dilution), or pollutant removal (filtration). The various forms of pollutant source modification are usually the most effective. For most gaseous pollutants—NO2 for example—removal via air cleaning is not presently practical (p. 15).
Ventilation and health in nonindustrial indoor environments were the subjects of a European Multidisciplinary Scientific Consensus Meeting (EUROVEN) review of the scientific literature on the effects of ventilation on health, comfort, and productivity in offices, schools, homes, and other nonindustrial environments (Wargocki et al., 2002). The group reviewed 105 papers and judged 30 as being conclusive, providing sufficient evidence on ventilation, health effects, data processing, and reporting. The EUROVEN group agreed that ventilation is strongly associated with comfort (perceived air quality) and health (building-related symptoms, inflammation, infections, asthma, allergy) and found an association between ventilation and productivity (performance of office work). It concluded that increasing outdoor air supply rates in nonindustrial environments improved the perceived air quality. The EUROVEN group also concluded
that outdoor air supply rates of less than 25 L/s per person increased the risk of building-related symptoms, increased the use of short-term sick leave, and led to decreased productivity among office building occupants.
Wyon (2004) investigated the productivity of office workers when indoor air pollutant loads were reduced by removing pollutant sources or increasing ventilation rate. He showed that during realistic experimental exposures lasting up to 5 hours, the performance of simulated office work was increased 6 to 9 percent by the removal of common indoor sources of air pollution, such as floor-coverings and old air filters, or by keeping the sources in place while increasing the clean air ventilation rate from 3 to 30 L/s per person. Wyon then went on to confirm these laboratory findings in a field investigation during an 8-week period. He concluded that reduction in pollutant loads in buildings can be expected to reduce building-related symptoms.
A recent study of relocatable classrooms found that with either conventional or alternative building materials, ventilation could reduce VOC concentrations to less than 1 ppb (Hodgson et al., 2004). In a study by Reitzig et al. (1998) VOCs were measured in the air of 51 renovated rooms in different types of buildings—private apartments, schools, kindergartens, and office buildings. The only common characteristic was that all of the rooms had been renovated within the last 2 years and complaints had been received about the quality of the indoor air. The investigation found that modern ecological building materials contained less volatile and less common substances but with increased indoor persistence that could partially account for the increasing number of complaints in relation to building-related symptoms.
Seppänen and Fisk (2005) reviewed the scientific literature regarding the effects of ventilation on indoor air quality and health, focusing on office-type buildings. Overall their literature review indicated that ventilation has a significant impact on several important user outcomes, including:
Communicable respiratory illnesses,2
Task performance and productivity,
Perceived air quality among occupants and sensory panels, and
Respiratory allergies and asthma.
Overall, these studies strongly indicate that although compliance with ASHRAE standards for ventilation rates may be the minimum acceptable
See Chapter 7.
standard, increasing the ventilation rate beyond the ASHRAE standard will further improve indoor air quality, comfort, and productivity and may have health benefits (Wargocki et al., 2002, 2005; Smedje and Nörback, 2000; Shendell et al., 2004a,b).3 The incremental effect of an increased ventilation rate may be attributable to a reduction in the pollutant load to which building occupants are exposed. However, the research conducted to date has not established an upper limit on the ventilation rates, above which the benefits of outside air begin to decline.
Human perception of the thermal environment depends on four parameters: air temperature, radiant temperature, relative humidity, and air speed (Kwok, 2000). Perception is modified by personal metabolic rates and the insulation value of clothing. Thermal comfort standards are essentially based on a set of air and radiant temperatures and relative humidity levels that will satisfy at least 80 percent of the occupants at specified metabolic rates and clothing values.
There is a robust literature on the effects of temperature and humidity on occupant comfort and productivity, primarily from studies in office buildings (Fanger, 2000; Sepännen and Fisk, 2005; Wyon, 2004; Wang et al., 2005). These studies show that productivity declines if temperatures go too high (Federspiel et al., 2004). However, there is a paucity of studies investigating the relationship between room temperatures in schools and occupant comfort or productivity (Mendell and Heath, 2004).
ASHRAE has codified the air temperature, relative humidity, radiant temperature, and air movement conditions under which occupants should feel “thermally neutral.” Guidance is found in ASHRAE Standard 55-2004, “Thermal Environmental Conditions for Human Occupancy,” which provides a range of temperatures and relative humidity for winter and summer conditions. When applying current standards, several points are relevant to the school environment:
ASHRAE Standard 55-2004 and the ISO 7730 Standard for “Moderate Thermal Environments” are based on experimental studies of adults, not children.
New “adaptive” models of thermal comfort have not been incorporated into current standards used for the design of mechanical ventilation systems for schools. The metabolic rates of students
vary across a school day as they engage in recess or lunch and move between rooms.
HVAC system design focuses almost exclusively on the thermal and humidity specifications as directed by building codes. Distribution of air diffusers is assumed to satisfy other requirements for air movement and prevention of thermal stratifications. Radiant heat gains and losses at a scale relevant to actual classroom utilization are not considered. Internal mixing, air velocities, and vertical temperature gradients are rarely explicit design considerations and are rarely assessed.
In addition, schools often have a higher occupancy density (more people per square foot of space) than office buildings. For these reasons and others, there is no assurance that school thermal conditions that meet current industry standards are optimal for student comfort or performance.
PERCEPTION OF AIR QUALITY (SENSORY LOADS)
An expanded definition of comfort includes the perception of air “quality.” Occupants may perceive indoor air as heavy, stale, smelly, unpleasant, refreshing, or crisp. As the air is sensed, many attributes are integrated—its temperature, moisture content, odor, and chemical properties. Materials and educational supplies emit odorous compounds as do dirty filters and ducts, cleaning agents, kitchens, bathrooms, gymnasiums, art rooms, moldy surfaces, computers, and copying machines. Chemical reactions that occur indoors also give rise to particles and a host of odorous and irritating compounds (Weschler and Wells, 2005).
Fanger (2000) discusses perceived air quality and ventilation requirements in the context of indoor sensory pollution loads from occupants and materials. Exhaled breath, skin, sweat, dirty clothing, perfume, deodorants, and other body odors make the occupants themselves a source of the sensory pollution load degrading perceived indoor air quality. Using nonsmoking adults at 1 Met (metabolic rate) as a reference, kindergarten children at 2.7 Mets contribute 20 percent more to the sensory pollution load. Teenagers 14-16 years old at 1-2 Met activity levels contribute 30 percent more to the sensory pollution load that ventilation air has to handle to achieve the equivalent acceptance.
Wargocki et al. (1999) demonstrated that the use of low-emissions materials resulted in improved perceived air quality and productivity for typical office tasks and fewer reports of building-related symptoms. These findings were independently verified in the studies of Lagercrantz et al. (2000). Bakó-Biró et al. (2004) demonstrated that sensory pollution
loads from common indoor objects like carpets, building materials, and personal computers decreased text typing performance as the percent of subjects dissatisfied with air quality increased. They reported a 0.8 percent decrease in text typing for a 10 percent decrease in perceived air quality. Wargocki et al. (2000) showed that increasing ventilation from 3 to 10 to 30 L/s per person improved simulated office work (typing rate and computation rate). These and other studies show that sensory pollution loads indoors are perceived by occupants and that dissatisfaction with perceived indoor air quality may have subtle effects on performance.
Moisture and relative humidity also play a role in the perception of air quality. Moisture in the air can lead to oxidation and chemical reactions by hydrolysis and decomposition, including enzymatic digestion by molds. These processes yield compounds that contribute to sensory pollution loads indoors. Fang et al. (1999a,b) found that perception depended on the enthalpy (heat content) of the air. Air that was cool and dry was perceived as “fresh” and “more pleasant” than air that was warm and moist. Figure 4.3, from Fang et al. (1999a,b) shows that in the absence of odorous sources people prefer air that is cooler and drier than the air commonly found indoors.
The introduction of odor sources was perceived to degrade air quality whether introduced individually or in various combinations. Interestingly, when the enthalpy was high, objectionable odors could not be recognized as easily as when the enthalpy was low. In other words, people prefer cooler, drier air but are then more likely to detect odors, which diminish the perceived air quality.
Natural Versus Mechanical Ventilation
The available literature comparing natural to mechanical ventilation is inconclusive as it relates to health, with different authors reporting opposite results (Blondeau et al., 2005; Ribéron et al., 2002). Some evidence indicates that buildings with air conditioning systems may present an increased risk of building-related symptoms among occupants compared to buildings without air conditioning, and that improper maintenance, design, and functioning of air-conditioning systems contributes to increased prevalence of building-related symptoms among occupants (Figure 4.4) (Sepännen and Fisk, 2002; Wargocki et al., 2002; Graudenz et al., 2005).
A large European study that included a wide variety of building types, found that among the potential causes of adverse health effects due to HVAC systems were poor maintenance and hygiene in the ventilation systems, intermittent operation of the HVAC systems, lack of moisture control, lack of control of HVAC system materials, and dirty, loaded filters (Wargocki et al., 2002). This evidence would indicate that it is not so much the presence of HVAC systems that is a factor as it is the quality of HVAC system maintenance and operations.
Sepännen and Fisk (2002) reviewed the available office buildings literature about the relationship of ventilation system types and building-related symptoms and outlined the suspected risk factors of HVAC types and building features (Table 4.1).
Fan-assisted natural ventilation has been investigated in schools (Nördquist and Jensen, 2005) in climates without high humidities, although not in statistically controlled studies. In schools with openable windows, even the best-designed mechanical system may be operated as a hybrid system since teachers and staff often open windows and doors during pleasant weather.
Although there is good evidence that HVAC system characteristics can and do affect occupant health and comfort, including in schools, until recently there have been few studies that attempted to measure the magnitude of health or productivity effects. Sepännen and Fisk (2005) used previous studies (primarily in office buildings) to develop a model relating building ventilation rates, perceived air quality, and temperature
to occupant symptoms and productivity. They estimated that increasing the average ventilation rate from 0.45 to 1.0 exchange per hour4 would reduce the sick leave used by office workers from 5 days per year to 3.9 days per year.
TABLE 4.1 HVAC Systems and Risk Factors for Building-Related Symptoms
HVAC System Type
Natural ventilation with operable windows
No particle removal via filtration; poor indoor temperature and control; noise from outdoors; inability to control the pressure difference across the building envelope and exclude pollutant infiltration or penetration of moisture into structure; low ventilation rates in some weather; possible low ventilation rates in some portions of the occupied space.
Systems with ducts and fans but no cooling or humidification (simple mechanical ventilation)
HVAC components may be dirty when installed or become dirty and release pollutants and odors; poor control of indoor temperature due to absence of cooling; low humidity in winter in cold climates; high humidity during humid weather; noise generated by forced air flow and fans; draft caused by forced air flows.
Systems with ducts, fans, and cooling coils (air conditioning systems)
Additional risk factors from cooling coils: very high relative humidity or condensed moisture (e.g., in cooling coils and drain pans) and potential microbial growth; biocides used to treat wet surfaces such as drain pans and sometimes applied to nearby insulation.
Systems with ducts, fans, cooling coils, and humidifiers of various types
Additional risk from humidifiers: microbial growth in humidifiers; transport of water droplets downstream of humidifiers, causing wetting of surfaces; leakage and overflow of humidifier water; condensation from humid air; biocides in humidifiers; chemical water treatments in steam generators.
Systems with recirculation of return air (recirculation may occur in all mechanical HVAC systems)
Additional risksa from recirculation: indoor-generated pollutants are spread throughout the section of building served by the air-handling system; typically higher indoor air velocities increase risk of draft and HVAC noise; supply ducts and filters of HVAC system may become contaminated by recirculated indoor-generated pollutants.
Sealed or openable windows (windows may be sealed or openable with all types of mechanical HVAC systems)
Additional risk with sealed windows: no control of the environment if HVAC systems fails; psychological effect of isolation from outdoors. Additional risk with operable windows: more exposure to outdoor noise and pollutants.
Decentralized systems (cooling and heating coils located throughout building, rather than just in mechanical rooms)
Additional risk of decentralization: potentially poorer maintenance because components are more numerous or less accessible; potentially more equipment failures due to larger number of components.
aHowever, recirculation facilitates removal of indoor-generated pollutants using air cleaners, e.g., particle filters and may also decrease concentrations of pollutants near pollutant sources.
SOURCE: Sepännen and Fisk (2002).
VENTILATION SYSTEM STANDARDS
ASHRAE Standard committees periodically update the various standards documents. The ASHRAE Standard 62 series addresses ventilation in buildings and the 90.1 series addresses energy efficiency. After initially lowering ventilation requirements in response to the early 1970s energy crisis, ASHRAE Standard 62-2001, “Ventilation for Acceptable Indoor Air Quality,” now requires substantially higher ventilation rates for schools and other buildings. ASHRAE Standard 90.1-2001, “Energy Standard for Buildings Except Low-Rise Residential Buildings,” and other energy-saving measures such as more efficient motors, office equipment, and lighting, along with better thermal insulation for building envelopes, have systematically reduced the sensible heat loads of buildings. This has implications for HVAC design and operations.
In a report commissioned for AirXchange Corporation, TIAX (2003) demonstrated the consequence of systematic changes in buildings as a result of shedding heat loads and increasing ventilation. The net effect of lowering sensible heat loads while increasing ventilation rates without specifically dealing with latent heat loads has been to increase indoor relative humidity. Heat gains from within buildings have decreased. Henderson (2003) and Shirey (2003) report that in certain common conditions the cycling time of HVAC systems is shortened, leaving condensed moisture on coils that can reevaporate, adding moisture to the building supply air. Maintaining optimally comfortable humidity (between 40 and 50 percent) is more difficult. Higher humidity increases condensation on cooled indoor surfaces and thermal bridges. Humidity that remains above 65 percent for appreciable time increases the opportunity for mold growth.
Although there are studies looking at the energy efficiency and health effects of HVAC system operation, few if any studies directly compare the energy efficiency and health trade-offs, if any, of HVAC system operation (Engvall et al., 2005).
Calculations have shown that the estimated costs of poor indoor environmental quality is higher than the energy costs to heat and ventilate the same building (Seppänen, 1999). Additionally, studies show that many measures to improve indoor air quality are cost-effective when the health and productivity benefits of an improved indoor environment are included in the calculations (Fisk, 2000; Fisk et al., 2003; Seppänen and Vuolle, 2000; Tuomainen et al., 2003; Wargocki, 2003), although none of the studies was set in a school environment.
SOLUTIONS/DESIGN REQUIREMENTS FOR INDOOR AIR QUALITY
Ventilation systems are designed to manage the sensible and latent heat loads of buildings. Outside air is needed for ventilation to provide thermal comfort as well as for diluting and removing indoor pollutants, odors, and moisture. Depending on the design and operational parameters of an HVAC system, the air supplied to the spaces can be entirely outdoor air (no recirculated air) or outdoor air mixed with indoor air drawn from the indoor spaces (return air). Using no recirculated air in a space requires more energy because large amounts of air must be conditioned for temperature and humidity levels when a mechanical system is used. In most cases using a percentage of return air mixed with the outside air is desirable for energy conservation. Increasing the amount of outside air in this air mixture to as high a level as is practical could potentially result in higher levels of human health, comfort, and productivity.
Ensuring that the air supplied is as clean as possible requires controlling the sources of pollutants and moisture within the ventilation system itself, cleaning the incoming outside air as much as possible prior to mixing it with the return air (most commonly this is going to be particulate filtration only, but where the outside air is very contaminated, gas-phase air filtration may also be used), effectively and continuously maintaining the hygiene of the HVAC system, and controlling indoor pollutant sources to minimize the spread of airborne pollutants. Additionally, a ventilation system should be capable of effectively distributing the ventilation air into occupied spaces and exhausting the return air from those spaces. Balancing the ventilation system for effective supply and exhaust rates is critical.
Many schools use unit-ventilator systems, a type of decentralized system, because their first costs (design and installation) are generally less than those of central systems: Unit ventilators eliminate the requirement for ducted supplies and returns (plenum or ducted). They distribute all the air from a single location, usually on the external wall of a room, thereby reducing ventilation effectiveness. They also typically do not meet the requirements for low ambient noise, necessary for acoustical quality associated with student learning (discussed in Chapter 6). Teachers often use the top of a unit ventilator as a storage shelf, so if this type of ventilation system is used in a green school, helping teachers understand the importance of not blocking air vents on the system is critical.
Central HVAC systems, which may supply small blocks of classrooms or entire sections of a school, require supply ducts and an air return system (plenum or ducted) to move the air to occupied spaces. Central systems
can have multiple supply and return vents in a single classroom, potentially increasing ventilation effectiveness. Additionally, student comfort might improve since there is a lower probability of air blowing on students sitting on one side of a room.
The design of and materials used in the supply air ducts may have an influence on the long-term health and well-being of the students and on system maintainability. The noise of air moving in the ducts and its potential impact on student learning and teacher health are discussed in Chapter 6. Where the air has a high moisture content, the use of fiberglass-lined ductwork to attenuate noise transmission can support the growth of microbial contamination, if the system is not properly maintained.
The type of ventilation system used may depend on the climate. Throughout much of the United States, ventilation systems need to control humidity as well as temperature and ventilation rates throughout the year. This is particularly true where schools are used year-round. The ventilation system is the primary mechanism for indoor humidity control, particularly in hot and humid climates. Excess humidity in the ventilation system, ductwork, and the building spaces increases the probability of indoor microbial contamination. Active humidity control systems, such as desiccant systems, may be effective for controlling humidity through ventilation systems in hot and humid climates (Fischer and Bayer, 2003). Displacement ventilation is another form of active humidity control in cold climates (Melikov et al., 2005).
School buildings are intended to be used for many years, so it is critical that the ventilation system be designed to allow effective operations and maintenance practices. Sepännen et al. (2004b), in a literature review on the association of ventilation rate and human responses, reported that better hygiene, commissioning, operation and maintenance of air handling systems may be particularly important for reducing the negative effects of HVAC systems. Ventilation may also have harmful effects on indoor air quality and climate if not properly designed, installed, maintained, and operated. To be well-designed, HVAC systems should be easily accessible to facility maintenance staff for maintenance and repair activities.
Indoor and outdoor particulates and certain VOCs can be effectively removed by filtration. Most filters are designed to collect particles larger than 10 m but are relatively inefficient at removing submicron-sized particles. The location of the filters is critical and should ensure that both outside air and recirculated air are effectively filtered for particulate and VOCs removal before the airstream reaches occupants. In addition, filters should be located such that they can be consistently maintained. Efficient
and effective filtration that removes particulate contamination to a level that protects building occupants and not just the equipment is essential (IOM, 2000, pp. 360-382). Particulate filtration having a Minimum Efficiency Reporting Value (MERV) of 11 or higher should be on all HVAC equipment supplying air to the occupied spaces of a building. Filters should fit snugly to prevent the bypass of air around the filter(s). The filters should be changed frequently and regularly to prevent them from becoming a source of indoor air pollution (Clausen, 2004; Hanssen, 2004). Additionally, filters should be kept dry, since wet filters may become microbially contaminated and thereby spread contamination throughout the area served by the ventilation system.
The deployment of electrostatic precipitators as a high-efficiency particulate filtration device may be desirable, particularly in schools near heavy traffic areas (IOM, 2000; Wargocki et al., 2005). If electrostatic air cleaners are used, they should be well maintained to minimize ozone formation. Additionally, indoor ozone sources should be controlled by proper maintenance of copiers and printers and exhausting these to the outside whenever possible. High ventilation rates will help to reduce the formation of by-products of indoor air chemical reactions. Ion-generating air cleaners can be sources of indoor air chemistry by-products (Wu and Lee, 2004; Bekö et al., 2006) and ozone and should be avoided.
Use of gaseous-phase filtration to remove gaseous pollutants from the supply air stream may be desirable in areas with significant amounts of outdoor air pollution. Gaseous-phase filters or filter media should be changed frequently and regularly.
Although to date no systematic research has examined the relationship of cleaning effectiveness to student and teacher health, student learning, or teacher productivity (Berry, 2005), a few studies have related methods for source reduction or control in schools to exposures to pollutants. Smedje and Nörback (2001) observed that classrooms with more frequent cleaning had lower concentrations of cat and dog antigen in settled dust. However, the study could not be repeated. Few studies have looked systematically at changes in exposure, health, or productivity in relation to changes in school building materials, cleaning products, or cleaning practices.
The literature on source reduction and control in homes, particularly those with asthmatic children, is more extensive (Takaro et al., 2004). Integrated pest management techniques have been shown to be effective in reducing antigen levels in homes (Phipatanakul et al., 2004) and have been shown to reduce pesticide levels in schools (Williams et al., 2005). However, whether they result in better health outcomes or improved
productivity has not been determined (Phipatanakul et al., 2004; Williams et al., 2005).
The effects of air pollutants in schools can be reduced through proper design and maintenance practices for HVAC filters, drip pans, cooling coils and other elements. Simple measures such as closing windows during pollen season or prohibiting furry pets in a school may also be effective. In other cases, more subtle design considerations may be needed, for example, limiting food preparation, vending, and eating to certain areas with structural and surface finishes that allow for cleaning and easy pest control.
The choice of cleaning products and methods is also important. A study by Singer et al. (2006) shows that some cleaning products can yield compounds, including glycol ethers and terpenes, that can react with ozone to form a variety of secondary pollutants. Persons whose occupation is cleaning might encounter excessive exposures to those pollutants. Mitigation options include screening of product ingredients and increased ventilation before and after cleaning (Singer et al., 2006). Eliminating the use of air fresheners may also help to reduce the level of pollutants generated by indoor chemical reactions.
CURRENT GREEN SCHOOL GUIDELINES
Current green school guidelines contain many measures intended to improve indoor air quality. They typically endeavor to manage outdoor pollution in a number of ways. One is through the careful location of fresh air intakes to ensure that exhaust and other pollutants generated by trucks, buses, or cars are not fed back into the building; similarly, they emphasize locating air intakes away from loading docks and areas where standing water might pollute the fresh air being taken in. Anti-idling measures for cars, trucks, and buses focus on reducing CO and particulate pollution entering the air stream. The use of walk-off mats and grills, previously discussed under moisture management, is also intended to remove outside dust, dirt, and other pollutant sources that might be on the shoes of anyone entering a school.
Current green school guidelines also contain measures to protect and maintain clean ventilation pathways. They typically encourage the use of high-efficiency filters with a MERV of 10 or higher in all ventilation systems. Plenum air returns that can be contaminated by dust and microbial growth are to be replaced by ducted returns, but exposed fibrous insulation in supply or return ducts is discouraged unless there is double-wall construction to protect the insulation from airborne pollutants. The use of fibrous insulation in air ducts was vigorously debated by the committee because of the importance of mechanical noise abatement. Some green
school guidelines discourage its use based on evidence that it absorbs VOCs and dust and encourages microbial growth inside duct linings that are difficult to clean. Effective noise reduction techniques for ducted HVAC systems and duct cleaning measures are both needed.
Finally, current green school guidelines contain numerous measures to reduce indoor pollutant sources: Two up-to-date industry standards are cited to ensure indoor air quality: ASHRAE 62.1-2004 IEQ (design guidelines) and the SMACNA IAQ Construction Guidelines. Green school measures include eliminating gas-fired pilot lights and discouraging fossil fuel burning equipment indoors to reduce the potential accumulation of exhaust fumes and the development of combustion products and particulate matter. Dedicated exhausts for all spaces that might contain chemicals—for example, storage rooms for cleaning equipment and supplies, photography laboratories, copy/print rooms, and vocational spaces—are encouraged.
Because construction materials create significant dust and outgassing contaminants that can remain in a building after completion, measures have been proposed to reduce indoor pollution from them. These include 72 hours of continuous ventilation of construction areas during the installation of materials that emit VOCs (ideally utilizing open windows and temporary fans rather than the HVAC system, which might absorb some of these contaminants); protecting supply and return ducts during construction; daily HEPA vacuuming for all soft surfaces such as carpets in or near construction areas; replacing all filters at the completion of construction; and providing for 28 days of continuous flushing of the building with outside air (except where high outdoor humidity could lead to mold growth) and then replacing the filters again. All of these measures are intended to reduce human exposure to higher levels of chemical emissions and dust/particulates in newly constructed areas.
Operable windows are typically suggested to allow for natural ventilation and decrease the demand for air conditioning. Such windows may provide a number of benefits: the ability to rapidly control temperature in an overheated classroom or in the event of an HVAC system failure; the ability to significantly increase the quantity of outside air ventilation to dissipate classroom pollution sources without resorting to the whole building HVAC; and the ability to locally ventilate classrooms undergoing renovation without using the whole building HVAC. The committee notes, however, there are concerns with operable windows that should be addressed, including the exposure of children to outside noise, pollution, and pollen, as well as the possible intrusion of rain and unwanted humidity.
To help ensure that longer term indoor air quality problems do not emerge, green school guidelines typically recommend the establishment of an indoor health and safety program for new or renovated schools,
based on resources such as Environmental Protection Agency’s Tools for Schools. This can help to establish operations and maintenance guidelines and clear communication channels so that indoor air quality problems can be prevented or identified and solved.
Despite emerging studies showing that thermal comfort may play a role in student performance, thermal comfort standards in green school guidelines are few. Typically they rely on compliance with the industry standard ASHRAE 55-2004 and do not address the control of humidity. In addition, they do not address radiant overheating by direct sunshine in the classroom in warm months or the potential contribution of solar heat through windows to offset heat losses in cool months. Managing sunshine on a seasonal basis is a critical aspect of ensuring thermal comfort.
FINDINGS AND RECOMMENDATIONS
Finding 4a: A robust body of scientific evidence indicates that the health of children and adults can be affected by indoor air quality. A growing body of evidence suggests that teacher productivity and student learning may also be affected by indoor air quality.
Finding 4b: Key factors in providing good indoor air quality are the ventilation rate; ventilation effectiveness; filter efficiency; the control of temperature, humidity, and excess moisture; and operations, maintenance, and cleaning practices.
Finding 4c: Indoor air pollutants and allergens from mold, pet dander, cockroaches, and rodents also contribute to increased respiratory and asthma symptoms among children and adults. Although limited data are available regarding exposure to these allergens in U.S. schools, studies in both school and nonschool environments support the notion that allergen levels can be decreased through good cleaning practices.
Finding 4d: The reduction of pollutant loads through increased ventilation and effective filtration has been shown to reduce the occurrence of building-associated symptoms (eye, nose, and throat irritations; headaches; fatigue; difficulty breathing; itching; and dry, irritated skin) and to improve the health and comfort of building occupants.
Finding 4e: There is evidence that ventilation rates in many schools do not meet current standards of the American Society for Heating, Refrigeration, and Air-Conditioning Engineers (ASHRAE). Available research indicates that increasing the ventilation rate to exceed the current ASHRAE standard will further improve comfort and productivity.
Finding 4f: Scientific evidence indicates that increased ventilation rates can reduce the incidence of building-related symptoms, reduce pollutant loads associated with asthma and other respiratory diseases, and improve the productivity of adult workers. Increased ventilation rates may also reduce the potential for reactions among airborne pollutants that generate irritating products and may improve perceived air quality. However, the research conducted to date has not established an upper limit on the ventilation rates, above which the benefits of outside air begin to decline.
Finding 4g: Research comparing the effects of natural versus mechanical ventilation on human health is inconclusive. However, there is evidence that improper design, maintenance, and operation of mechanical ventilation systems contribute to adverse health effects, including building-related symptoms among occupants.
Finding 4h: Studies in office buildings indicate that productivity declines if room temperatures are too high. However, there are few studies investigating the relationship of room temperatures to student learning, teacher productivity, and occupant comfort.
Finding 4i: To date, no systematic research has examined the relationship of cleaning effectiveness to student and teacher health, student learning, or teacher productivity. Few studies have looked systematically at changes in exposures, health, or productivity based on changes in building materials, cleaning products, or cleaning practices.
Recommendation 4a: Future green school guidelines should ensure that, as a minimum, ventilation rates in schools meet current ASHRAE standards overall and as they relate to specific spaces. Future guidelines should also give consideration to planning for ventilation systems that can be easily adapted to meet evolving standards for ventilation rates, temperature, and humidity control.
Recommendation 4b: Future green school guidelines should emphasize the importance of appropriate operation and preventive maintenance practices for ventilation systems, including replacing filters, cleaning coils and drip pans to prevent them from becoming a source of air pollution, microbial contamination, and mold growth. These systems should be designed to allow easy access for maintenance and repair. The Environmental Protection Agency’s Tools for Schools program is a well-recognized source of information on methods for achieving good indoor air quality.
Recommendation 4c: Additional research should be conducted to document the full range of costs and benefits of ventilation rates that exceed the current ASHRAE standard and to determine optimum temperature ranges for supporting student learning, teacher productivity, and occupant comfort in school buildings.
Recommendation 4d: Studies should be conducted to examine the relationships of exposures from building materials, cleaning products, and cleaning effectiveness to student and teacher health, student learning, and teacher productivity.