Indoor air pollutants include a wide variety of materials, including organic and inorganic pollutants [3], and particulate matter (PM) [28,35,36]. Some of the more important pollutants are briefly discussed below.

Nitrogen oxides are combustion by-products, produced by the burning of natural gas or oil in oxygen-rich environments such as kitchen stoves and ovens, furnaces, and unventilated gas and kerosene heaters. When a fireplace or wood stove is used, some of these pollutants will enter the room. Cracks in the stovepipe, downdrafts, or wood spillage from a fireplace can worsen the condition [3,37]. A recent study shows that nitrogen dioxide (NO₂) in kitchens with a gas cooker were three times higher than outdoors [22]. Adverse effects of NO₂ exposure are breathing symptoms, bronchoconstriction, growing of bronchial reactivity, airway painfulness, and reduced immune protection leading to increased susceptibility to respiratory infection [9]. High levels of NO₂ are linked to an increased sequence of respiratory symptoms and poorer respiratory action in asthmatic children [38].

Trace elements: generally, toxic trace elements are related to PM and are Fe, Al, Mg, Zn, Co, As, Cr, Cd, Mn, Cu, Ni, and Pb. Trace elements such as Mg, Fe, and Al are greatly released from crustal sources such as parent rocks, metallic minerals, seas, and oceans. Fossil fuel combustion, forest and biomass burning and metal processing are also sources releasing many trace elements [28]. These toxic pollutants either are ingressed from outside of the buildings or generated inside because of fossil fuel combustion.

Mercury (Hg) is a persistent, poisonous, and bio-accumulative heavy metal. It can discharge into the atmosphere from a diversity of anthropogenic and natural sources. A substantial amount of observed mercury is transmitted from the burning process of fuels (36%) and biomass (33%) [39,40].

Ozone can cause the muscles in the airways to constrict, trapping air in the alveoli. This leads to wheezing and shortness of breath. Ozone has a strong, pungent odor. The source of ozone in a building is electrostatic copying devices, mercury-raised light bulbs, and electrostatic air cleaners [3,41,42].

Inhalable particulate matter is classified into three groups according to their sizes: coarse particles (2.5 < d_p < 10 μm), fine particles (≤2.5 μm) and ultrafine particles (UFP, <0.1 μm) [43,44]. Fine particles are more potent when inhaled in comparison to the coarse fraction since they can penetrate more into the lungs. UFP can penetrate alveoli and enter the blood, which can be very harmful. Numerous epidemiological and clinical research works exist that establish the relationship between particulate matter [26] exposure and different health effects and references therein [45]. The Department for Environment, Food and Rural Affairs (Defra, UK) estimated that the health costs incurred by particulate matter (PM₁₀) pollution in the UK is in the range of £9.1 and £21.4 billion per year [45,46,47]. Sources that can increase the PM₁₀ concentration are Earth’s crust elements that are the result of oil burning and human activities, and motor vehicles [48]. An increase in the exposure to PM leads to increased hospital admissions, certainly in the sensitive group cohorts such as the old and individual with cardiopulmonary and respiratory illness. PM concentration inside a building is basically governed by indoor sources of fine particles, outside PM concentration, the rate of air circulation, and the particles’ depositional speed [45]. Bozlaker et al. [49] and Mohammadyan et al. [50] studied the relation between indoor and outdoor particulate materials. The result shows the indoor concentration PM_2.5 is usually higher than outdoor [45].

Asbestos exposure for an extended period of time could lead to lung cancer known as mesothelioma and asbestosis. Insulation and other building materials such as floor tiles, drywall compounds, and reinforced plasters are sources of asbestos [51].

VOCs are chemicals that mostly vaporized easily at room temperature, and their concentration is higher than other pollutants in the indoor air. Aerosols, cleaning agents, polishes, varnishes, paints, pressed-wood products, and pesticides are some of the VOC sources at homes and offices [3,52,53].

Toluene and ethylbenzene: toluene exists in many materials such as gasoline, paints, and fingernail polish. Ethylbenzene is also present in paints, lacquers, and insecticides. These compounds are a hazard for human health and can have adverse effects on the nerve, liver, kidneys, and respiratory system [9,54].

Formaldehyde is a class of aldehydes that is a colorless gas. The source of formaldehyde is different building materials, household products, or combustion processes. Indoor sources include pressed-wood products, including particleboard, paneling, fiberboard, resins, and wallboard as well as textiles, such as carpet backings, drapes, and upholstery fabrics, linens, and clothing; urea–formaldehyde foam insulation; adhesives; paints; coatings; and carpet shampoos plus tobacco smoke. Decreasing ventilation rate will increase the level of formaldehyde [53,55,56]. Formaldehyde enters the body via the respiratory system, skin, or gastrointestinal tract. Formaldehyde absorbed in the respiratory tract is rapidly metabolized. Formaldehyde exposure could cause respiratory symptoms, reductions in lung function, headaches, and asthma, and it can affect the nervous system [53].

Carbon dioxide and carbon monoxide result from poorly ventilated kitchens, rooms over garages, and unvented combustion appliances (stoves, ovens, heaters, and the presence of tobacco smoke) [10]. Sneezing, coughing, and minor eye irritation are symptoms of exposure [51].

Acetaldehyde is toxic to the cilia of respiratory epithelia and may interfere with respiratory clearance mechanisms. Acetaldehyde is also a central nervous system depressant and a proven carcinogen in animals, and a potential carcinogen in humans. The acetaldehyde source of indoor is construction materials, furnishing materials such as vinyl, polyvinyl chloride (PVC) and rubber floorings, nylon carpets, particleboard furniture, plywood, fiberboard, flooring adhesives, wood paneling, caulking, paint removers, and other consumer products. Also, it emitted by printers and photocopiers [9,57].

Acrolein is a very potent eye irritant, causing lacrimation at concentrations of approximately 2 mg/m³. At high concentrations, acrolein can cause significant lung injury, including dyspnea, asthma, congestion, edema, and persistent respiratory insufficiency with decreased lung function [9].

Naphthalene is a volatile white solid. It is an aromatic hydrocarbon, including a fused pair of benzene rings [9]. Naphthalene is mostly used in toilet deodorant and also as moth repellents. Extended exposure to a large amount of naphthalene may damage or destroy some of the red blood cells; 10 parts per million (ppm) for the level of naphthalene in workplace air over an 8 h workday is the limit set by the Occupational Safety and Health Administration (OSHA) [58].

Trichloroethylene (TCE) is a clear, non-flammable liquid used mainly for vapor degreasing and cold cleaning of manufactured metal parts and to a less degree as a solvent for a variety of organic materials. The primary sources of TCE in the indoor air include varnishes, finishes, lubricants, adhesives, wood stains, paint removers, cleaning liquids containing TCE, and contaminated food and water [9]. The EPA classified the TCE as carcinogenic to humans [59]. TCE can affect the central nervous system (CNS), eyes, kidney, liver, lungs, mucous membranes, and skin [60].

Tetrachloroethylene (PCE) is a colorless liquid mostly used for dry cleaning fabrics, as a solvent for organic materials, and to degrease metal parts in the automotive and other metalworking industries. Another source of PCE is dry cleaned clothes. Exposure to PCE vapor could cause damage to the following organs: kidneys, liver, the peripheral nervous system [9], upper respiratory tract, skin, the central nervous system (CNS) [61].

The WHO has categorized indoor VOCs into various classes, as seen in Table 1. Based on the available literature, some of the major sources of VOCs are listed in Table 2 in which a guideline for the maximum exposure is provided. It should be noted that the concentration of TVOC should not exceed 300 μg/m³ [9,62]. Furthermore, there is no safe level of exposure for some of these pollutants (asbestos and radon).

Polybrominated diphenyl ethers (PBDEs) include materials such as plastics, television sets, textiles, synthetic building materials, computers, and cars. Human exposure to PBDEs includes food consumption and ingestion of polluted air and house dust. Studies show that computer clerks have higher PBDE levels in their blood than others. It should be noted that exposure to PBDEs via inhalation is of minor importance [66]. PBDEs are toxins that disrupt developing fetuses and infants. PBDEs can act as endocrine disruptors by the change of thyroid hormones homeostasis [67,68].

Insecticide affects the environment, depending on their physical and chemical properties [5]. For example, each year, malaria is responsible for 584,000 deaths worldwide. Hence, indoor residual spraying (IRS) is an important source of Insecticide indoor pollution [69].

Radon (²²²Rn) is a radioactive gas that is odorless and colorless. It is the result of the radioactive decay of radium-226. The soil and rock in the building can be the primary source of radon in indoor air. Another source of indoor radon is groundwater—Rn concentration changes with seasonal and daily variation [70]. A high concentration of radon leads to lung cancer [71].

Tobacco smoke is the largest source of air pollutant in indoor environments. It has more than 4000 chemical compositions, which could lead to pneumonia and bronchitis in childhood [9,72,73].

Biomass fuels and coal are a source of energy for cooking and heating. Almost 3 billion people use biomass (wood, charcoal, crop residues, and animal dung) and coal worldwide as their primary and other household needs [74]. The CO₂ and NO₂, arsenic, fluorine and organic matter such as polycyclic aromatic hydrocarbons emit from biomass and coal combustion. Chronic obstructive pulmonary disease, asthma, respiratory infections, lung cancer and eye diseases are the exposure effects of biomass and coal [75,76].