**1. Introduction**

Essential oils (EOs) are invaluable secondary metabolites of complex components and liquids mixtures obtained from different parts of aromatic plants. EOs obtained from different plant parts such as the leaves, stem, seed, bark, and root undergo other extraction processes: azeotropic distillation or solvent extraction. Hydrodiffusion, hydrodistillation, and steam distillation extraction, which is a commonly used method of extraction because it is cheap and easily achievable, is a type of azeotropic distillation [1]. These EOs are volatile organic compounds and over 300 different such compounds with a relatively molecular weight below 300 [2]. EOs comprise several chemical constituents, terpenes and phenylpropanoids, the most constituents of the EOs in aromatic plants. The metabolic pathways responsible for forming these major chemical constituents of EOs are methylerythritol, mevalonate and shikimic acid [3].

EOs type, yield, composition or chemistry are determined by different factors. Notably, plant variety, plant nutrition, harvest season, geographical locations, climate and seasonal variations, stress factors, and post-harvest handling and storage are the attributable factors [4]. Naturally, the EOs in plant function to protect the plant from various attacks of insects, fungi, bacteria and viruses. In addition, EOs tends to facilitate pollination in plants because of the odor or smell [5].

The importance of EOs must be considered in different fields of cosmetics, food, and pharmaceutical industries which necessitated and resuscitated the growth of the EOs market. Globally, the approximate contribution of EOs in terms of market value is USD 10.3 billion in 2021. The projection of USD 16.0 billion is the offing for the year 2026 [6] Hence, EOs contribute massively to the GDP because of the awareness of the health benefits of consuming natural food or products. Generally, the bioactive component of EOs from medicinal plants contains an armamentarium of antioxidants that confer an array of potent therapeutic properties. At the same time, the antioxidants contained in EOs have a better safety profile compared to synthetic antioxidants such as butylated hydroxyanisole (BHA), and butyl hydroxytoluene (BHT), among others [7]. The suspected toxic or harmful nature of synthetic antioxidants in human health draws concerns. In addition, natural antioxidants from EOs are very essential in the food industry preventing rancidity and improving the shelf-life of food products. The use of natural antioxidant from EOs become very sacrosanct with increasing or growing interest in medicines because many or nearly all diseases conditions finds their root or are linked to oxidative stress.

Generally, the physiological process allows cell death without resulting in any pathological state or condition; however, most oxidative cell death and tissue damage from the overarching effect of excess oxidants or free radicals or reactive oxygen species (ROS) beyond the body's mechanism antioxidants defense system (endogenous and exogenous) results to oxidative stress. Free radicals or ROS are highly unstable with a propensity to accept or donate electrons which cause structural and functional modifications to important cellular biomolecules such as the DNA, protein, lipids, and carbohydrates that lead to oxidative stress [8, 9].

The resultant effect of oxidative stress is dangerous to human health as envisioned in the occurrence of autoimmune diseases, cardiovascular diseases and heart attack, cancer, kidney disease, infectious disease, diabetes, and rheumatic diseases, particularly with more imminent concern in neurodegenerative diseases [10]. The disturbed homeostasis in oxidative stress conditions forms the basis of the etiology and pathogenesis of many diseases above. Hence, the right to way go in treating and managing such conditions is preventing and halting the cause of oxidative stress that would ultimately prevent or delay pathological changes or alleviate the disease occurrence.

Given that oxidative stress is associated with the etiology and pathogenesis of many diseases, eliminating the causes of oxidative stress may prevent or delay pathological changes and reduce the occurrence of diseases. The antioxidant is pivotal to

*Essential Oils and Their Antioxidant Importance: The* In Vitro *and* In Vivo *Treatment... DOI: http://dx.doi.org/10.5772/intechopen.113031*

ensuring normal homeostasis condition and its actionable characteristics help or come to bear to prevent, halt and ameliorate free radicals or ROS such as superoxide anion (O2−), per hydroxyl radical (HOO.), hydroxyl radical (HO.) singlet oxygen ( 1 O2 ), and hydrogen peroxide (H2O2) that are responsible for oxidative stress.

Antioxidants refer to molecules that fight the imbalances caused by the excess oxidants, free radicals or ROS, thus significantly delaying and preventing the oxidative damage process and conferring defense mechanism against such oxidative damage process in cells. ROS prevention, capture, and blockade, as well as the repair processes that expunge the damaged biomolecules that ROS has initiated, form the basis of the mechanisms of action of antioxidants in combating oxidative stress [11]. Protective or defense mechanism offered by both endogenous and exogenous antioxidants help combat oxidative stress linked to many diseases, particularly neurodegenerative diseases (NDs).

The etiology and progression of NDs are linked to oxidative stress, among other vital factors. The neurons of both the central and peripheral systems are implicated; however, NDs primarily affect neuronal brains. The factors that make neurons in the brain prone to oxidative stress leading to NDs include high oxygen demand, increased peroxidation-susceptible lipid membrane-bound cells, and modest content of antioxidant defense system and related enzymes [12–14]. Till today many researchers have opined that oxidative stress plays a significant role in different NDs, such as Amyotrophic lateral sclerosis (ALS), Alzheimer's disease(AD), Parkinson's disease (PD), Huntington's disease (HD), Multiple sclerosis (MS) [15].
