Numerous indoor air pollutants have been recognized to have harmful impacts on IAQ and human health [23]. The main indoor air pollutants include NOx, volatile and semi-volatile organic compounds (VOCs), SO₂, O₃, CO, PM, radon, toxic metals, and microorganisms. The sources and health effects of some common pollutants are listed in Table 1. Some of them can be present in both indoor and outdoor environments, while others originate from the outdoor environment. Generally, indoor air pollutants are able to be classified into organic, inorganic, biological, or radioactive [24].

Particulate Matters

PM is defined as carbonaceous particles in association with adsorbed organic chemicals and reactive metals. PM’s main components are sulfates, nitrates, endotoxin, polycyclic aromatic hydrocarbons, and heavy metals (iron, nickel, copper, zinc, and vanadium) [7]. Depending on the particle size, PM generally is classified into (i) coarse particles, PM₁₀ of diameter <10 µm; (ii) fine particles, PM_2.5 of diameter <2.5 µm; and (iii) ultrafine particles, PM_0.1 of diameter <0.1 µm. PM is especially concerning, as it is sometimes inhalable, affecting the lungs and heart and causing serious health effects. It has been shown that indoor PM levels often exceed outdoor ones [25]. Indoor PM sources include (i) particles that migrate from the outdoor environment and (ii) particles generated by indoor activities. Cooking, fossil fuel combustion activities, smoking, machine operation, and residential hobbies are the main reasons why PM is distributed inside of buildings. Compared with PM₁₀ and PM_2.5, PM_0.1 created by fossil fuel combustion represents a greater threat to health due to its penetrability into the small airways as well as alveoli [26,27]. According to research about the concentration of major indoor pollutants, it has been indicated that cooking and cigarette smoking are the largest sources of indoor air PM, whereas cleaning activities often have a lesser contribution to indoor PM [28]. Smoking is known as a major source of indoor PM_2.5, with estimated increases in homes with smokers ranging from 25 to 45 µg /m³ and the concentration in winter is greater than in summer [29]. For cooking activity, it was shown that cooking activities enable the emission of millions of particles (~10⁶ particles/cm³) through the burning of oil, wood, and food and most of them are ultra-fine particles [30,31]. In addition, these fine particles can distribute not only to the kitchen but also spread to the living room and other areas in the building, thereby causing adverse effects to the occupants’ health [31,32]. Meanwhile, other normal human activities, such as walking around and sitting on furniture, are likely to resuspend house dust and contribute to 25% of the indoor PM concentrations [29]. In summary, it has been found that the source strengths for human activities ranged from 0.03 to 0.5 mg.min⁻¹ for PM_2.5 and from 0.1 and 1.4 mg.min⁻¹ for PM₁₀ [33].

VOCs

Volatile organic compounds (VOCs) are recognized as gases containing a variety of chemicals emitted from liquids or solids [34]. Formaldehyde, a colorless gas with an acrid smell and which is released from many building materials, such as particleboard, plywood, and glues, is one of the most widespread VOCs. VOC concentrations in indoor environments are at least 10 times higher than outdoors, regardless of the building location [34,35]. Generally, indoor VOCs are generated from four main sources: (i) Human activities, including cooking, smoking, and the use of cleaning and personal care products; (ii) generation from indoor chemical reactions; (iii) penetration of outdoor air through infiltration and ventilation systems; and (iv) originating from building materials [35‒39]. The VOCs concentration is able to be affected by air exchange rates, house age and size, building renovations, outdoor VOC levels, and door and window opening [40]. Moreover, it has been demonstrated that about 50 different VOCs are identified during 90-min cooking periods [35]. Because VOCs are organic chemicals that possess a low boiling point (T_b) and are easily volatilized even at room temperature, the WHO classified them into four groups: (i) Very volatile organic compounds (VVOCs) with T_b:
50–100 °C; (ii) volatile organic compounds (VOCs) with 100 °C < T_b < 240 °C; (iii) semi-volatile inorganic compounds (SVOCs) with 240 °C < T_b < 380 °C; and (iv) particulate organic matter (POM) with T_b > 380 °C [41,42]. Normally, exposure to VOCs released from consumer products is incurred via three main pathways: Inhalation, ingestion, or dermal contact. Most people are not seriously affected by short-term exposure to low concentrations of VOCs, but in cases of long-term exposure, some VOCs are considered to be harmful risks to human health, potentially causing cancer [43]. As for SVOCs, transdermal uptake directly from the air has a higher contribution compared with intake via inhalation [44,45].

NO_X

The two principal nitrogen oxides are nitric oxide (NO) and nitrogen dioxide (NO₂), both of which are associated with combustion sources, such as cooking stoves and heaters [46]. Ambient concentrations of NO and NO₂ vary widely depending on local sources and sinks. Their average concentration in buildings without combustion activities is half that in the outdoors, but when gas stoves and heaters are used, indoor levels often exceed outdoor levels. Under ambient conditions, NO is rapidly oxidized to form NO₂; hence, NO₂ is usually considered as a primary pollutant. The reaction of NO₂ with water produces nitrous acid (HONO), a strong oxidant and common pollutant of indoor environments [47]. It has been indicated that indoor levels of NO₂ are a function of both outdoor and indoor sources; hence, indoor levels can be influenced by high outdoor levels originating from combustion or local traffic sources. It was reported that the distance between buildings and roadways has a significant influence on indoor NO₂ levels [48]. Besides, the air exchange between outdoors and indoors also affects NO₂ levels in buildings [49]. Additionally, the major indoor sources include smoking and wood-, gas-, oil-, coal-, and kerosene-burning appliances, such as stoves, space, ovens, and water heaters and fireplaces [47].

Ozone

Ozone is a powerful oxidizing agent mainly produced by photochemical reactions of O₂, NOx, and VOCs in the atmosphere. However, it cannot be used to eliminate other indoor chemical pollutants, due to its slow reaction with most airborne pollutants [50,51]. Ozone enables rapid reaction with several indoor pollutants, but the reaction products can irritate humans and damage materials. The main sources of indoor ozone mainly come from the outdoor atmosphere and the operation of electrical devices [52]. The machines commonly emitting indoor ozone gas include photocopiers, disinfecting devices, air-purifying devices, and other office devices [53–56]. The ozone emission mechanisms of these devices can be divided into two categories: Corona discharge and photochemical mechanisms. It has been shown that indoor ozone levels depend on various factors: (i) The outdoor ozone level; (ii) indoor emission rates; (iii) air-exchange rates; (iv) surface-removal rates, and (v) reactions between other chemicals and ozone in the air [50]. Indoor ozone levels generally fluctuate between 20% and 80% of the outdoor ozone level according to the air-exchange rate [57]. Humans are exposed to ozone primarily by inhalation, but skin exposure is also a recognized vector [58].

SO₂

Sulfur dioxide (SO₂) is the most common gas among the group of sulfur oxides (SO_x) present in the atmosphere. SO₂ is primarily produced by the combustion process of fossil fuels and combines with aerosols and PMs to form a complex group of distinct air [59]. Indoor sources of SO₂ emissions include vented gas appliances, oil furnaces, tobacco smoke, kerosene heaters, and coal or wood stoves [60]. In addition, outdoor air is also regarded as the main source of indoor SO₂ [61]. Indoor SO₂ levels are often lower than outdoor levels. SO₂ emission indoors is usually small, owing to its reactivation, which can be easily absorbed by indoor surfaces. It is known that the hourly concentration of SO₂ in buildings is often below 20 ppb [62]. Human exposure to SO₂, which can impair respiratory function, is only via inhalation.

COx

Carbon monoxide (CO) in indoor air is produced mainly by combustion processes, such as cooking or heating. Besides, CO can also enter into indoor environments through infiltration from outdoor air [63]. The important sources of indoor CO emissions include unvented kerosene and gas space heaters; leaking chimneys and furnaces; back-drafting from furnaces, gas water heaters, wood stoves, and fireplaces; gas stoves; generators and other gasoline-powered equipment; and tobacco smoke [47]. The average concentration of CO in a building without any gas stoves is about 0.5–5 ppm, while the concentration in areas near gas stoves ranges from 5 to 15 ppm and even 30 ppm or higher. CO exposure can cause adverse health effects, such as (i) at low concentrations, there are impacts on cardiovascular and neurobehavioral processes; and (ii) at high concentrations, unconsciousness or death [64].

Carbon dioxide (CO₂), a colorless and odorless gas, is a well-known constituent of the earth’s atmosphere and also a major human metabolite [65]. The average CO₂ concentration in ambient air is about 400 ppm, which is primarily the result of the combustion of fossil fuels [65,66]. Recently, the indoor CO₂ level has been applied as a reference for the assessment of IAQ as well as for ventilation control [66–68]. According to the ASHRAE standard, it is recommended that indoor CO₂ concentrations are below 700 ppm to ensure human health [69]. It is established that exposure to a CO₂ concentration of 3000 ppm increases headache intensity, sleepiness, fatigue, and concentration difficulty [65,70].

Toxic Metals

Heavy metals are released into the atmosphere through either human activities or natural processes [71]. IAP by heavy metals has various causes, including infiltration of outdoor pollutants (dust and soil), smoking, fuel consumption products, and building materials [55]. Heavy metals in indoor dust, entering the human body through inhalation, ingestion, or dermal contact, can have adverse effects on human health [72,73]. According to the International Agency for Research on Cancer (IARC), heavy metals in indoor air are classified into two main groups based on their effects on humans: (i) Non-carcinogenic elements, including cobalt (Co), aluminum (Al), copper (Cu), nickel (Ni), iron (Fe), and zinc (Zn); and (ii) both carcinogenic and non-carcinogenic elements encompassing arsenic (As), chromium (Cr), cadmium (Cd), and lead (Pb) [74]. These common heavy metals (i.e., As, Cr, Cd, Pb) are likely to cause cancers [75,76], while Cd and Pb, along with some others, can cause carcinogenic effects, such as cardiovascular disease, slow growth development, and damage to the nervous system [73,77,78]. It has been reported that Pb levels in indoor air can fluctuate from 5.80 to 639.10 μg/g, while the highest levels of As, Al, Cr, Cd, Co, Cu, Ni, Fe, and Zn are about 486.80, 7150.00, 254.00, 8.48, 43.40, 513.00, 471.00, 4801.00, and 2293.56 μg/g, respectively [74].

Aerosols

Indoor aerosols are either primary aerosols originating from different indoor sources or secondary aerosols formed by indoor gas-to-particle reactions [79]. Moreover, outdoor particles infiltrating indoors are also likely to be a source of indoor aerosols. Secondary inorganic aerosols are PMs consisting of inorganic elements, including anthropogenic or crustal sources and water-soluble ions [6], while secondary organic aerosols (SOAs) are formed in the gas-to-particle conversion process of VOCs [80]. Additionally, carbonaceous aerosols, which comprise SOAs and elemental carbons released in incomplete combustion, are well-known species in PM_2.5 [81]. Biological aerosols (bioaerosols) are a subset of atmospheric PMs comprising dispersal units (fungal spores and plant pollen), microorganisms (bacteria and archaea), or cellular materials [82]. Due to their diversity in terms of compounds and phases (gas, liquid, or solid), aerosols can be regarded as dynamic systems [83]. As such, their particle size distribution varies from the nucleation mode (<30 nm in vacuum cleaning condition) to the accumulation mode (~100 nm, indoor combustion aerosols from smoking, cooking, or incense burning), and to the fine and coarse modes (>1 µm, resuspension aerosols) [84,85]. Aerosol exposure through inhalation in the indoor environment has been linked to numerous adverse health effects, mainly in the lungs (the entrance to the human body) and other important target organs, such as the heart and brain [79].

Radon

The primary sources of indoor radon include building materials, soil gas, and tap water [86]. As soil contains radium at trace concentrations, radon is likely to be one of the constituents in the gas filling soil pores. As for radon emissions from building materials, all materials holding trace amounts of radium can release radon. Among building materials, masonry materials (i.e., stone, concrete, and brick) are the main sources for indoor radon emission, in that tons of such materials are used in building construction. Indoor radon can be released through the usage of water from underground water sources containing granite or other radium-bearing rock, and such water sources commonly contain radon concentrations above 10,000 pCi/L [87]. Finally, outdoor air is also regarded as a source of indoor radon [88]. Human exposure to radon in buildings is incurred mainly through the permeation pathways of underlying soil gas [89]. Epidemiological studies have demonstrated that indoor radon can cause lung cancer risk increases of 3% to 14%, depending on the average
radon level [90]

Pesticides

These days, inorganic and organic pesticides have commonly been utilized as protectants for wooden building materials by impregnation or surface coating [91]. Pesticides are also used to control and prevent pests, including bacteria, fungi, insects, rodents, and other organisms [92,93]. In the indoor environment, pesticides are usually semi-volatile compounds that may exist in either gas or particulate form according to properties, such as the vapor pressure, product viscosity, and water solubility [94]. In addition, it has been indicated that carpet and textiles are likely to play the role of long-term reservoirs for organochlorine pesticides [95,96]. It is supposed that when used in carpets, textiles, and cushioned furniture, pesticides in fibers will migrate into polyurethane foam pads [97,98], and thus carpet, textiles, and cushioned furniture can reflect an integrated pesticide exposure during their lifetime. Moreover, pesticides are able to enter buildings from outdoors. Once inside, they can persist for months or years due to their protection against sunlight, extreme temperatures, rain, and other factors [97]. Dermal uptake, ingestion, and inhalation of particles or volatile compounds containing pesticides are believed to be potential exposure routes in the indoor environment [92]. Pesticide exposure is associated with adverse health risks, including (i) short-term skin and eye irritation, dizziness, headaches, and nausea; and (ii) long-term chronic impacts, such as cancer, asthma, and diabetes [99].

Biological Pollutants

Biological pollutants in indoor environments include biological allergens (e.g., animal dander and cat saliva, house dust, cockroaches, mites, and pollen) and microorganisms (viruses, fungi, and bacteria) [100]. Biological allergens, known as antigens, originate from a number of insects, animals, mites, plants, or fungi, and will induce an allergic state in reacting with specific immunoglobulin E (IgE) antibodies [101]. Indoor sources of allergens mainly include furred pets (dog and cat dander), house dust mites, molds, plants, cockroaches, and rodents [102], and there are outdoor sources as well [101]. Viruses and bacteria often originate from or are carried by people and animals. It has been demonstrated that exposure to biological allergens can result in sensitization, respiratory infections, respiratory allergic diseases, and wheezing [103], while exposure to bacteria and viruses indoors is likely to cause noninfectious and infectious adverse health outcomes [104].

Table 1. Common indoor pollutants and their effects on human health.

This entry is adapted from the peer-reviewed paper 10.3390/ijerph17082927