# Environmental Pollution and Oxidative Stress: Health Effects During Pregnancy: A Review
## 1 Introduction  
Reactive oxygen species (ROS) include compounds formed by electron receptivity of molecular oxygen. They also include oxygen radicals, organic radicals, and peroxides produced from compounds that have reacted with oxygen radicals [1]. Thus, ROS include the following radicals: superoxide, hydroperoxide radical, hydroxyl radical, peroxide radical, and alkoxide radical, as well as non-radicals: hydrogen peroxide, single oxygen, ozone, hypochlorous acid, and organic acid. The activity of ROS varies, i.e., superoxide is more active than molecular oxygen, while the hydroxyl radical is more active than both superoxide and hydrogen peroxide. Organisms have a number of antioxidant mechanisms to neutralize the excessive increase in ROS [2]. These mechanisms include antioxidant proteins, but also low-molecular-weight antioxidant compounds. Among antioxidant proteins (enzymes) is superoxidedismutase (SOD), which catalyzes the conversion of superoxide to hydrogen peroxide. In human mitochondria, SOD is bound with its active center with Mn (Mn-SOD), while in the cytoplasm, there is mainly Cu-SOD and/or Zn-SOD [3]. Peroxides are found mainly in catalase, another antioxidant enzyme responsible for converting hydrogen peroxide to water and oxygen, thus protecting the body from hydrogen peroxide. However, the most important antioxidant enzyme responsible for the removal of hydrogen peroxide is glutathione peroxidase (GSHPX), which requires the binding of selenium. GSHPX removes hydrogen peroxide by converting glutathione (GSH) to oxidized glutathione (GSSG). GSSG can then be reduced by glutathione reductase, which is the major GSSH-reducing enzyme using nicotinamide adenine dinucleotide phosphate (NADPH) [4]. Another antioxidant defense is the conversion of metal ions into forms that do not catalyze free radical reactions. This occurs mainly in the extracellular space, where the levels of SOD, GSH, GSHPX, and catalase are often very low [5; 6; 7]. The action of antioxidant enzymes is essential for the life of an aerobic organism. However, in addition to antioxidant enzymes, some low-molecular-weight molecules that act as free radical scavengers also contribute to the body's antioxidant defense. These molecules have the ability to neutralize free radicals, that is, to convert them to less active forms. A typical example is GSH, which has an active sulfhydryl group (-SH) of cysteine. GSH reacts with free radicals, causing a series of reactions, resulting in the reduction of free radicals into products that are easily metabolized. In addition, it is a substrate for GSHPX, thus leading to the detoxification of the body from active forms of oxygen such as hydrogen peroxide and organic hydroperoxides. A-Tocopherol is the most widely used free radical scavenger in membranes. More specifically, it contributes to the inhibition of lipid peroxidation [8] caused by peroxide radicals when they invade cell membranes. In this reaction, however, tocopherol itself is converted to a radical, confirming the principle of free radical chemistry, which states that \"when radicals react with non-radicals, new radicals are formed\". Only when two radicals react with each other and join their unpaired electrons do non-radicals occur. It is worth mentioning here that antioxidants interact with each other, and ultimately the body's antioxidant protection is the result of the synergistic action of various antioxidants. For example, glutathione reduces dehydroascorbic acid to ascorbic acid, and the latter then reduces \(\alpha\)-tocopherol [1]. In addition to endogenous antioxidants, exogenous antioxidants are also recognized for their ability to eliminate free radicals. These antioxidants collaborate to renew each other. Many antioxidants are consumed through nutrition, and various dietary factors have been linked to their involvement in inflammatory responses. Certain elements like vitamins, \(\beta\)-carotene, polyphenic compounds, omega-3 PUFA, and others are considered key factors [9].