**2.1 Carbon dioxide (CO2)**

Carbon dioxide (CO2) is a colorless and odorless gas, and it constitutes about 0.03% (300 parts per million) of the total atmospheric gases. It is heavier than other

**Figure 1.**

*Different sources of outdoor and indoor air pollutants.*

non-combustible gas and can accumulate in the lower phase of the environment, resulting in an oxygen deficiency [24]. Carbon dioxide is majorly a by-product of biological respiration or fossil fuel combustion [25, 26]. Fossil fuel-fired power plants contribute about 33–40% of the total CO2 emission globally, with coal-fired power plants being the key contributor [27]. Whereas, anthropogenic CO2 emission is due to activities like forestry, deforestation, land clearing for agriculture [28]. Rising atmospheric CO2 levels result in the greenhouse effect and speed up the global warming process [29]. The average ambient CO2 concentration has been steadily rising and has reaches up to 410 ppm. A CO2 level of 600 ppm is considered acceptable, but a CO2 level above 1000 ppm is detrimental and leads to CO2 toxicity-related side effects [30]. Excessive CO2 concentration in the blood (hypercapnia) give rise to acidosis, which is characterized by a low blood pH (increased acidity). The respiratory, cardiovascular, and central nervous systems are all affected by the lower blood and tissue pH. Other commonly reported symptoms of CO2 toxicity are headaches, lethargy, moodiness, mental slowness, emotional irritation, and sleep disruption [31].

#### **2.2 Carbon monoxide (CO)**

Carbon monoxide is colorless, combustible, and extremely deadly gas [32]. It is emitted from both natural and man-made sources. It is produced when carbonaceous materials are burned in an incomplete manner [33]. The two most common sources of emission of carbon monoxide in ambient air are smoke from fires and exhaust fumes from automobile engines (in the absence of a catalytic converter) [34]. Additionally, the combustion of charcoal and wood can also release carbon monoxide. The minimum exposure limit of carbon monoxide is around 100 mg/m3 for 15 minutes, 60 mg/m3 for 30 minutes, 30 mg/m3 for 1 hour, and around 10 mg/m3 for 8 hours [35]. In most cases, carbon monoxide poisoning causes from inhalation of gases coming out from common household areas such as garages, kitchens, basements, or workrooms and fuel burning. Carbon monoxide toxicity leads to dizziness, headache, weakness, nausea, vomiting, and loss of consciousness [36]. Further, the inability of a cell to use oxygen (e.g., effective oxygen deprivation) leads to chemical asphyxiation and hypoxia, which is the most significant harmful effect of carbon monoxide.

#### **2.3 Volatile organic compounds (VOCs)**

VOCs are categorized as harmful air pollutants because of their relation with carcinogenic, impairing blood production and weakening nervous system in humans [34]. Commonly found VOCs in atmosphere are aromatic hydrocarbons BTEX (benzene, toluene, ethyl benzene, and xylene) and halogenated hydrocarbon like chloroethylene and trichloroethylene [37]. The major sources of BTEX are vehicle/aircraft, processing of petroleum products, paints, thinner, ink, cosmetics, and pharmacy. Benzene/toluene ratio is used more often to know the source of emission. If the ratio is >0.5 shows that the source is other than transport and if the ratio is <0.5, vehicular emission is the major source. Among the six compounds of BTEX, toluene is the one that is most easily degraded due to the presence of a side chain that provides different attack routes for the microbial enzymes to act upon. Isoprene, a naturally occurring biological component, is one of the most significant contributors to emissions of VOC [38]. Anthropologically, VOCs are generated from both domestic and industrial processes including textile cleaning, fertilizers and pesticide application, septic system, traffic, fumigation, building materials,

and pharmaceutical industries [39]. In indoor environments, VOCs are released by combustion, newly constructed or refurbished structures and building materials such as paints, carpets, solvents, various plastics, and wooden furniture [40]. The acceptable level of VOCs concentration in an indoor environment is ranged from 0 to 400 ppb [41, 42]. Short-term exposure of VOCs may induce like nausea, vomiting, and fatigue. However, long-term exposure may cause lung cancer, leukemia, and other forms of malignancy [43, 44].

### **2.4 Particulate matters (PMs)**

Carbon-containing particles along with reactive metals and adsorbed organic compounds belong to the particulate matter components of air pollution [7]. It is a mixture of solid and liquid particles in the air that can be breathed in and may cause serious health problems. According to particle size, PM is further divided into three groups: coarse particles (PM10, diameter less than 10 μm), fine particles (PM2.5, diameter less than 2.5 μm), and ultra-fine particles (PM0.1, diameter less than 0.1 μm) [7]. It can be originating either from natural or anthropogenic activities. Eruptions of volcanoes, dust and wind storms, forest fires, salt spray, rock debris, chemical reactions between gaseous emissions, and soil erosion are some examples of natural sources. PM is also produced by human activities such as burning fuel, making steel, processing petroleum, making cement, making glass, mining, smelting, power plant emissions, burning coal, and disposing of agricultural waste [45]. There is a clear relationship between PM concentrations and seasonal variations [46, 47]. According to the recent air quality guidelines 2021 by WHO, exposure to PM2.5 and PM10 concentration up to 65 μg/m3 for the 24-hour is safe. Black carbon is a carbonaceous component released because of incomplete combustion of fossil fuels (particularly diesel, wood, and coal) (PM 2.5) [10, 48]. Higher exposure to black carbon is a major health issue that can induce heart attacks and strokes. The regulations governing air pollution focus on PM 2.5 as the primary concern. PM10 mostly affects the upper respiratory system, whereas ultra-fine particles detrimental to the lower respiratory tract, lungs, and alveoli.

### **2.5 Polycyclic aromatic hydrocarbons (PAHs)**

These are the vast categories of chemical compounds that include two or more bound benzene rings as diverse arrangements [49]. PAHs are a type of pollutant that can be found almost everywhere in the environment including soil, water, and air. PAHs are the by-product of the incomplete burning of organic substances such as wood, coal, petrol, and oil [44]. Forest fires, garbage incineration, volcanic eruptions, and hydrothermal processes are natural sources of PAHs. Whereas, combustion of timber, waste, and fossil fuels are some examples of anthropogenic activities which are responsible for the emission of PAHs [45, 48]. PAHs have cancer causing and mutation inducing properties in living organisms [47, 50]. The most common ways for people to be exposed to PAHs are through smoking cigarettes or cigars and breathing smoke from open fires or other sources of combustion [46]. Health issues such as skin-related diseases, lungs and gastrointestinal malignancies, and damages in liver and loss of immunity have been resulting in long-term exposure. The National Institute for Occupational Safety and Health (NIOSH) recommends that exposure to PAHs in the workplace should be limited at or below the minimum reliable detectable concentration of 1 μg/m3 .

#### **2.6 Ozone (O3)**

Ground-level ozone is produced by a chemical interaction between oxides of nitrogen and originating from natural sources and/or as a result of human activities. Ozone damages the uppermost skin layers and tear ducts. Short-term exposure is resulted in malondialdehyde production in the epidermis as well as vitamin C and E depletion in mice model [1]. Due of ozone's limited solubility in water, it can enter deep into the lungs when inhaled. During the warm season, an increase in ozone concentration was related to an increase in the daily number of fatalities (0.33%), respiratory deaths (1.13%), and cardiovascular deaths (0.45%). During the winter, no such effect was noticed [51].

#### **2.7 Nitrogen and sulfur oxides**

Nitrogen oxides (NOx) are the gases that are released by natural sources, automobiles, and other fuel-burning actions [1]. NO2 is an odorous, acidic, and extremely corrosive gas that have negative impact on human health and the environment. They are responsible for the yellowish-brown hue of the smog. It causes pulmonary disorders such as obstructive lung disease, asthma, chronic obstructive pulmonary disease, and in rare circumstances acute aggravation of COPD as well as fatalities [52]. High concentrations of NO2 are also detrimental to vegetation causing damage to leaves, stunting growth and diminishing crop yields. The suggested NO2 air quality criteria are 0.12 ppm for a 1-hour exposure duration and 0.03 ppm for a yearly exposure period. These regulations are intended to safeguard vulnerable individuals such as children and asthmatics [53, 54]. Sulfur oxides (SOx) composed of molecules of sulfur and oxygen is an odorless gas detectable by taste and smell at concentrations between 1000 and 3000 micrograms per cubic meter [55, 56]. The majority of SO2 is produced by the combustion of sulfur-containing fuels and metal sulfide ores. Volcanoes are natural sources of SO2 (35–65%) among others. After industrial boilers and nonferrous metal smelters thermal power plants that burn high-sulfur coal or heating oil are generally the largest producers of anthropogenic SO2 emissions on a global scale. The accumulation of SO2 and smoke which reached 1500 mg/m3 resulted in increased number of fatalities [55, 57].
