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For health reasons it is crucial to breathe clean air, free from chemicals and toxicants as much as possible. It is estimated that humans spend approximately 90% of their lifetime indoors [8] and that indoor air pollution in some places can be much worse than that of the ambient air.
Air pollution is the presence of substances in the atmosphere that are harmful to humans and other living beings, or cause damage to the environment. Air pollution can be chemical, physical or biological. [1]
Various definitions of pollution exist, which may or may not recognize certain types, such as noise pollution or greenhouse gases.The United States Environmental Protection Administration defines pollution as "Any substances in water, soil, or air that degrade the natural quality of the environment, offend the senses of sight, taste, or smell, or cause a health hazard.