Indoor Air Pollution, Related Human Diseases, and Recent Trends in the Control and Improvement of Indoor Air Quality

2. Indoor Air Quality (IAQ) and Indoor Air Pollution (IAP)

According to the EPA’s definition, IAQ is the air quality within and around buildings and structures, especially as it relates to the health and comfort of building occupants [8]. IAP, meanwhile, refers to the existence of pollutants, such as volatile organic compounds (VOCs), particulate matter (PM), inorganic compounds, physical chemicals, and biological factors, all of which are at high concentrations in the indoor air of non-industrial buildings, and all of which can have negative impacts on the human body. In order to protect people from such pollutants, IAQ has emerged and been developed as a research field [9]. The main parameters for evaluation of IAQ include pollutant concentrations, thermal conditions (temperature, airflow, relative humidity), light, and noise. Thermal conditions are crucial aspects of IAQ, for two basic reasons [10,11]: (i) Several problems related to poor IAQ can be solved simply by adjusting the relative humidity or temperature, and (ii) building materials in high-temperature buildings can be highly released.

It has been indicated that IAQ in residential areas or buildings is significantly affected by three primary factors [12,13]: (i) Outdoor air quality, (ii) human activity in buildings, and (iii) building and construction materials, equipment, and furniture. It is known that outdoor contaminant concentrations and building airtightness have a great influence on IAQ, due to the possibility of transportation of contaminants from outdoors to indoors [14]. As outdoor pollutants’ concentrations increase, they are transported from outdoors to the indoor environment via ventilation. Hence, the correlation of outdoor air pollution with IAQ highly depends on the ventilation rate additionally to the lifetimes and mixing ratios of such pollutants [15]. Human daily activities generally cause IAP by the discharge of waste gases, tobacco smoke, pesticides, solvents, cleaning agents, particulates, dust, mold, fibers, and allergens [16]. Humans also create favorable conditions for the development of millions of mold, fungus, pollen, spores, bacteria, viruses, and insects, such as dust mites and roaches. Combustion sources and cooking activates contribute to carbon dioxide (CO2), sulfur dioxide (SO2), CO, nitrogen dioxide (NO2), and particulate matter (PM) emissions into indoor air environments [17,18]. In addition, equipment, such as computers, photocopy machines, printers, and other office machines, emit ozone (O3) and volatile compounds. Common building materials, such as poly(vinyl chloride) PVC floor covering, parquet, linoleum, rubber carpet, adhesive, lacquer, paint, sealant, and particle board, can shed toxic compounds (i.e., alkanes, aromatic compounds, 2-ethylhexanol, acetophenone, alkylated aromatic compounds, styrene, toluene, glycols, glycolesters, texanol, ketones, esters, siloxane, and formaldehyde) [10].

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Importantly, the design and operation of ventilation systems also have a significant influence on IAQ. Due to superseding the stale indoor air by the fresh outdoor air, ventilation creates suitable IAQ and a healthy indoor environment. There are several benefits for the operation of ventilation in a building [19], including: (i) Providing oxygen and fresh air for human respiration; (ii) diluting indoor air pollutants to reach the short-term exposure limits of harmful contaminants as well as odors and vapors; (iii) using outdoor air with a low aerosol concentration to control aerosols inside buildings; (iv) controlling internal humidity; and (v) creating proper air distribution and promoting healthy and comfortable environment. Ventilation systems can be classified into two types [20], including: (i) Mechanical ventilation systems that use mechanical equipment, such as fans or blowers; and (ii) natural ventilation systems, which are the exchange processes between indoor air and out indoor without using mechanical equipment. Although natural ventilation systems may be well adopted by the occupants, they are insufficient in some buildings or climates. These days, mechanical ventilation systems have been commonly used in buildings, which significantly increases energy consumption. Thus, hybrid ventilation systems are designed to take advantage of both mechanical and natural ventilation systems, in order to decrease energy consumption and increase the use of sustainable technologies [21]. In hybrid ventilation systems, the shortcomings of natural ventilation will be compensated by mechanical components [22]. In summary, in heating, ventilation, and air conditioning (HVAC) systems of buildings, ventilation plays a key role in creating suitable IAQ, but it is also responsible for energy consumption. Therefore, improving ventilation systems in buildings is the key issue not only for enhancing energy efficiency but also for providing better IAQ to the occupants and minimizing the possibility of health problems as a consequence [21].

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