Page 432 - Fundamentals of Air Pollution 3E

384 23. Indoor Air Quality

II. FACTORS INFLUENCING INDOOR AIR QUALITY

Several factors influence the quality of air indoors: the rate of exchange
of air with air from outdoors, the concentration of pollutants in outdoor
air, the rate of emissions from sources indoors, the rate of infiltration from
soil gases, and the rate of removal in the indoor environment (Fig, 23-1).
The source of indoor air pollutants may be inside the building, or they
may be transported into the interior space from the outside. Sources located
indoors include building materials, combustion sources, furnishings, and
pets. Emissions of organic gases are higher with increased temperature
and humidity but usually decrease with age of the structure or furnishings.
Construction materials and the composition of furnishings inside the build-
ing may give off or outgas pollutants into the interior airspace, e.g., glues
or adhesives. Natural gas for cooking and kerosene space heaters release
NO and CO 2 even when operating properly. Molds may grow in the ventila-
tion ducts and be distributed throughout a building.
Radon from the soil can enter buildings through cracks in the foundation
when the pressure inside is lower than in the soil. The rate of infiltration
depends on the soil type, the building structure, and the pressure differen-
tial between the soil and the building.
Air is exchanged between indoors and outdoors by several ways: natural
ventilation, mechanical ventilation, and infiltration or exfiltration. Natural
ventilation involves movement of air through building openings like doors,
windows, and vents. Mechanical ventilation involves fans and heating and
air-conditioning systems. Infiltration and exfiltration represents undesir-
able movement of air in and out of the structure. Buildings are characterized
as "tight" when infiltration rates are low.

Fig. 23-1. Representation of home with various sources and sinks for indoor air pollutants:
(A), exchange; (B), indoor concentration; (C), outgassing of building and furniture materials;
(D), infiltration from soils; (E), removal on interior surfaces.