**2.1. Types of free radicals or reactive oxygen species**

ROS are classified into two major categories of compounds which includes the free radicals and the non-reactive radicals. The free radical includes nitric oxide radical (NO•), hydroxyl radical (OH•), superoxide ion radical (O• 2 ), peroxyl (ROO•), alkoxyl radicals (RO•), and one form of singlet oxygen (1 O2 ) as shown in **Table 1** [5]. These species are considered as free radicals since they contain at least one unpaired electron in the shells around the atomic nucleus which makes them unstable and therefore can easily donate or obtain another electron to attain stability. As such, they are highly reactive and capable of independent existence [6, 7]. On the other hand, the non-reactive radicals are a group of compounds which are not radicals but are extremely reactive or can easily be converted to reactive species. Examples of these substances include hypochlorous acid (HClO), hydrogen peroxide (H2 O2 ), organic peroxides, aldehydes, ozone (O3 ), and O2 as shown in **Table 1**.
