Defilement of internal air occurs mainly due to the occupation of space by people, plants and pets (exhalation, body odours, excretion products odours, food preparation), gadgets, equipments, and building and furnishing materials. The quality of air is usually determined by people’s sensation to various odours present in the air. But certain harmful pollutants like carbon dioxide and radon cannot be perceived by people at even high concentrations.
Quality of air is determined in two ways. There are Absolute standards that provide for ideal conditions for comfort and bio-survival. Relative standards provide ways for determining the qualitative difference between out door air and indoor air. Quality of outside air is generally superior because an infinite space and high speed winds are available for dilution to occur. Quality of internal air can be improved by diluting the proportion of pollutants in air, by replacing part of the fouled air with comparatively cleaner air; or by various mechanical and chemical scrubbers.
Particulate matter is a major source of air pollution, which mainly but not necessarily, originates outside and penetrates inside through various cracks and openings. Particulate can be dust, fumes, mist or biogenic matter. Particles of diameters greater than about 75 microns settle down rapidly and are termed Grit. Particles of smaller than 50 microns may remain suspended and constitute Aerosols. An aerosol is a liquid or solid particle which is in a quasi stable suspension in air. Very fine aerosols may remain suspended for weeks, whereas larger aerosols may get deposited in minutes. The deposition (and movement) of very small particles (2 microns) is influenced by temperature gradient (through convective currents). The effect on health due to airborne particulate matter of biogenic origin such as fungi, moulds, bacteria, viruses, pollens are well known.
In modern artificially controlled environment buildings are well designed and sealed to eliminate waste leakages. Such spaces function well so far as the environmental systems operate. However, pollutants arising from building materials, aerosols settling down, degradation of biotic materials, evaporation of condensed moisture from air handling plants, etc. continue to be added to the internal environment. The process of addition becomes severe when the environmental systems are switched off, such as at night time, on off days and when there are power breakdowns. This situation also occurs when there is no casual ventilation of the space due to shutting of nominal openings like doors and windows.