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).