Air pollution, indoor

The presence of gaseous and particulate contaminants in the indoor environment. Most pollution is due to human (anthropomorphic) sources. Natural sources do exist, including plants, animals, and other living organisms, and water sources that release various chemical aerosols. Contamination can occur from infiltration indoors of atmospheric pollutants generated outdoors, and thus the indoor environment is affected by meteorological conditions.

Natural sources are soils and water that release radon progeny, volatile organic compounds, fungi, and such. Chemical releases (emissions) originate from various types of appliances, combustion sources (including those in garages), building materials and water, and living organisms that release allergens (for example, dander from pets). Carbon dioxide (CO₂) is generated by combustion and living organisms. Other sources involve molds, microorganisms (such as bacteria and viruses), insects and arthropods, and pollen from outdoor and indoor plants. Human activity has been noted to increase air contamination from all of these sources. Some pollutants are individually generated such as tobacco smoke or are generated through use of consumer products. See also Particulates; Radon.

The generation and behavior of pollutants from sources in enclosed environments are also affected by most meteorological factors. Indoor environments are often sealed to the extent that the weather is cold or hot. Infiltration of pollutants is affected also by climate, temperature, humidity, barometric pressure, and wind speed and direction. The small particles and the gases that infiltrate most, that is, nitrogen oxide (NO_x) and volatile organic compounds, are affected by outdoor concentrations, tightness (being sealed, with little air exchange), operation of heating and cooling systems, convection currents, full growth (or lack thereof) of local trees and shrubs, and so forth. Indoor environments also possess individual characteristics of ventilation, dispersion, and deposition. Some gases, for example, sulfur dioxide (SO₂) and ozone (O₃), infiltrating the indoor environment are absorbed readily by materials and exist in high concentrations only when the outdoor concentrations are very high. Chemical-physical mass-balance models are used to estimate pollutant concentrations, as are statistical models. See also Aerosol; Ozone.