**2.2 Ozone (O3)**

Ozone or ground-level ozone is formed due to reaction of flue gases (particularly VOCs and other hydrocarbon) released during the incomplete combustion of fossil fuels. This is a highly reactive gas that can cause coughing, wheezing as well as difficulty on enough air. This is considered as 52% more stronger oxidant as compared to chlorine [31–35].

### **2.3 Oxides of nitrogen (NOx) and sulfur (sox)**

Oxides of nitrogen and sulfur are formed during uncontrolled and incomplete combustion of fossil fuels. Oxides of nitrogen include NO (nitrogen monoxide), NO2 (nitrogen dioxide), and N2O (di-nitrogen oxide). These gases through different chemical reactions are responsible for smog formation as well as acid rain. Similarly, oxides of sulfur are formed due to the impurities found in fuel being burnt in the firing system. It is very important to prevent these gases before they enter in the environment [36–38].
