Ventilation, Pollutant Source Control, Health, and Performance
The primary purpose of a building’s heating, ventilation, and air-conditioning (HVAC) system is to provide comfort for the occupants by meeting thermal requirements and diluting contaminants. HVAC systems accomplish this through the conditioning of outside air coming into occupied spaces and the removal of irritants and pollutants. The principal standards and guidelines for HVAC system design and operation include (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 EnergySmart Schools guidelines; and (4) individual state codes, some of which are based on or reference other codes such as the International Building Code.
Green school guidelines typically address HVAC systems as they relate to energy efficiency, indoor air quality (ventilation), moisture control, filter efficiencies and maintenance, and the elimination of CFC-based refrigerants.
In a comprehensive review of the literature related to indoor air quality, ventilation, and health symptoms in schools, Daisey et al. (2003) found the following:
Reported ventilation and carbon dioxide (CO2) levels indicated that a significant proportion of classrooms did not meet the (then) ASHRAE standard 62-1999 for minimum ventilation rate;4
A variety of volatile organic compounds (VOCs) and bioaerosols (primarily molds and fungi, dust mites, and animal antigens) could be found in school environments; and
Although there were a number of studies in which typical building-related symptoms5 were measured, there was a relative paucity of literature in which specific building conditions or measured pollutants were then linked to specific symptoms.
The following sections address some aspects of indoor air quality and pollutant source control, ventilation rates, and moisture management in HVAC systems. The committee plans to address additional issues related to HVAC systems, including temperature and humidity, in its final report.
INDOOR AIR QUALITY AND POLLUTANT SOURCE CONTROL
There are many sources of exposure to pollutants internal and external to a school. Internal sources include stationary and (potentially) mobile combustion sources; building materials and equipment; educational materials; cleaning products; biological agents; and human activity. External sources include combustion sources (stationary and mobile); biological material; and soil gases (e.g., radon, VOCs from municipal waste sites) particulate matter, and ozone entering through air intakes and the building envelope.
Outdoor sources of air pollution can affect the health of children and adults in two ways. First, students, teachers, and administrative 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 through several routes: outdoor air is drawn into a school building by the ventilation system through air intakes located at the rooftop, at ground level, or from below-grade “wells.” Outside air also enters the building through doors and windows and through leaks in the building envelope. People themselves can carry viruses, bacteria, pollen, and pollutants, such as dust mites and pet dander, into a school on their shoes and clothing.
Mendell and Heath (2004) found that “a substantial literature of strongly designed cohort studies is available on associations between outdoor pollutants and attendance of children at school” (p. 9). 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 associations of school absence with exposure to outdoor nitrogen oxides, carbon dioxide, and particles <10 µ in aerodynamic diameter.
Other significant sources of pollutants outside schools are plant-derived materials, or biomass, which can generate bioaerosols, including molds, fungi, and pollen. The 2000 IOM study Clearing the Air 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. (p. 8)
The 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).
There are numerous pollutants in a school building. These pollutants include particulate matter, chemicals, and biological particles or organisms. Sources include building materials (e.g., structural materials such as particleboard, adhesives, insulation); furnishings (carpets, paints, furniture); products used in the building (cleaning materials, pesticides, markers, art supplies); and in some cases the occupants themselves (CO2, pet hair).
There are a limited number of studies in which source reduction or control methods in schools have been related to pollutant exposures. Smedje and Norbäck (2001) observed that classrooms with more frequent cleaning had lower concentrations of cat and dog antigen in settled dust. However, few studies have looked systematically at changes in exposure, health effects, or productivity based on changes in building materials or products used in schools.
The literature on source reduction and control in homes, particularly 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 productivity in schools has not been determined (Phipatanakul et al., 2004; Williams et al., 2005). A recent review by Shendell and others (2004a) examined the various sources and types of controls available for these sources. The committee will address those issues in greater detail in its final report.
INDOOR AIR QUALITY AND VENTILATION RATES
Ventilation rate is based on the outdoor air requirements of a ventilation system. A number of studies have reviewed the effect of ventilation rates on health, productivity, and airborne pollutant control. Although the majority of these studies have been in environments other than schools, there have been several more recent studies and reviews either specifically on schools or including schools in the overall study. Typically, these studies also look at a second variable, such as temperature or humidity, components of thermal comfort, to identify any confounding or synergistic effects.
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.
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 building-related symptom intensity. Increased ventilation rate increased work rate in addition, multiplication, number checking (p < .05), and subtraction (p < .06). Reduced temperature increased work rate in subtraction and reading (p < .001), and reduced errors in checking a transcript against a recorded voice reading aloud (p < .07). Reduced temperature at increased ventilation rate increased work rate in a test of logical thinking (p < .03). Their experimental data indicated that increasing ventilation rates from 5.4 to 9.6 l/s per person and decreasing temperatures from 24°C to 20°C potentially would improve the performance of schoolwork by children.
Smedje et al. (2000) investigated the impact of improving the ventilation systems in schools on allergies, asthma, and asthma symptoms in schoolchildren. They issued questionnaires to 1,476 children in 39 schools (mixed primary and secondary schools) during the 1993-1995 period. Various exposure factors were measured during this time period in 100 classrooms. In 12 percent of the classrooms, new ventilation systems were installed, increasing the air-exchange rate and reducing the humidity levels in the schools. The air pollutant levels in these schools were lowered with the installation of the new ventilation systems as compared with the levels in the classrooms without the new ventilation systems. Also, the incidence of asthma symptoms, but not allergies, was reduced in the classrooms with the new ventilation systems. Their results indicated an improvement in the children’s health in the classrooms with increased ventilation, lower humidity levels, and reduced airborne pollutant levels.
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 carbon dioxide (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-ppm increase above the outdoor concentration of CO2 was associated with statistically significant 10 percent to 20 percent increases in student absences. Although student absences are not a direct measurement of student performance, an increased number of absences may contribute to poorer student performance.
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) 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” (p. 212). Seppanen and Fisk (2004) further developed a quantitative relationship by fitting the data from these epidemiological studies by 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 increasing ventilation rates.
Seppanen 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 to 6 for building-related symptoms for low compared to high ventilation rates.
VENTILATION AND HEALTH
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, short-term sick leave) and found an association between ventilation and productivity (performance of office work). They concluded that increasing outdoor air supply rates in nonindustrial environments improves the perceived air quality, and that outdoor air supply rates of less than 25 l/s per person increase the risk of building-related symptoms, increase short-term sick leave, and decrease productivity among occupants of office buildings.
Seppanen 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,
Task performance and productivity,
Perceived air quality among occupants and sensory panels, and
Respiratory allergies and asthma.
The literature review also indicated 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. The committee will address the literature on ventilation and health in greater detail in its final report.
VENTILATION AND COMFORT
Fang et al. (1998) and Toftum et al. (1998) showed that perceived air quality is strongly influenced by the humidity and the temperature of inhaled air even when the chemical composition of the air is constant and the thermal sensation for the entire body is kept neutral. These findings address the importance of the second purpose of ventilation systems, which is to condition the air to comfortable levels of temperature and humidity. There is a robust literature on the effects of temperature and humidity on both comfort and productivity, although it is based primarily on studies in office buildings (Fanger, 2000; Seppanen and Fisk, 2005; Wyon, 2004). Studies show that productivity declines if temperatures go too high (Federspiel et al., 2004). The committee will address the literature on temperature, health, and performance in greater detail in its final report.
Quantifying Relationships Among Ventilation, Health, and Productivity
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 either the health or productivity effects. Several recent reviews, however, have attempted to quantify the relationship of ventilation rate and pollutant transport to the health and productivity of people indoors.
Wyon (2004) investigated the productivity of office workers based on reducing indoor air pollutant loads by source removal or increasing ventilation rate. Wyon showed that during realistic experimental exposures lasting up to 5 hours, the performance of simulated office work was significantly increased (by approximately 6 percent to 9 percent) by the removal of common indoor sources of air pollution, such as floor-coverings, old supply air filters, and personal computers, 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. A reduction in pollutant loads in buildings can be expected to reduce building-related symptoms.
Seppanen 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 exchanges per hour would decrease the sick leave prevalence in an office from 2 percent (5 days per year) to 1.6 percent (3.9 days per year).
MOISTURE MANAGEMENT IN HEATING, VENTILATION, AND AIR-CONDITIONING SYSTEMS
Several findings in the 2004 IOM study Damp Indoor Spaces and Health pertain specifically to the design and operation of HVAC systems as a critical factor in the control of moisture 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).
Menzies study of ultraviolet germicidal radiation of drip pans and cooling coils shows that limiting microbial contamination of HVAC systems may yield health benefits, and follow-up research is recommended (p. 44).
In addition to moisture and mold, other allergens and irritants found in school buildings may affect a substantial minority of students, teachers, and staff through the same pathways described above. Indoor environments contain varying quantities of allergens from a variety of sources: house dust mites, cat and dog dander, cockroaches, rodents, and seasonal pollens. Indoor concentrations of particulates and various gaseous pollutants (ozone, NO2, VOCs) may be higher than those found in outside air. The strength of the association of each of these agents 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. Through proper design and maintenance practices, green schools can minimize the effects of these exposures. This may involve simple measures such as closing the windows during pollen season or prohibiting furry pets in a school. In other cases, more subtle design considerations may be needed, for example, limiting food preparation, vending, and eating to certain areas with structural and finish features that allow easy pest control.
Taken together, these findings indicate the significant role that HVAC system design and operation play in controlling moisture and pollutants inside buildings.
Finding 3: In regard to ventilation, pollutant source control, health, and performance, the committee has found the following:
Numerous pollution sources and building system characteristics affect air quality in a school. The most important determinants of indoor air quality are (1) design and operation of the ventilation system to limit the buildup of pollutants and humidity and achieve thermal comfort, (2) control of indoor sources of pollutants, and (3) control of outdoor sources of pollutants.
There is a robust body of evidence indicating that the health of children and adults can be affected by air quality in a school.
A growing body of evidence suggests that teacher productivity and student learning, as measured by absenteeism, may be affected by indoor air quality as well.
Indoor pollutants and allergens from house dust mites, pet dander, cockroaches, and rodents also contribute to increased respiratory and asthma symptoms among children and adults.
The reduction of pollutant loads, both sensory and not, has been shown to reduce the occurrence of building-associated symptoms and to improve the health and comfort of people occupying the buildings.
Although compliance with the American Society for Heating, Refrigeration, and Air-Conditioning Engineers (ASHRAE) standards for ventilation rates may be the minimal acceptable standard for green schools, there is good evidence that increasing the ventilation rate beyond the ASHRAE standard will further improve comfort and productivity. However, an upper limit on the ventilation rates, indicating when the benefits of outside air begin to decline, has not been established.
The committee will address additional issues related to heating, ventilation, and air-conditioning systems and their associations with health and productivity in its final report. Until this review is completed and the results are synthesized, the committee will defer making specific recommendations.