Molecular oxygen is activated by both enzymatic and nonenzymatic mechanisms to produce ROS (reactive oxygen species) as by products in mitochondria and non-mitochondria processes. The most important of ROS is hydrogen peroxide. ROS have detrimental effects when in excess. Oxidative stress is an imbalance between oxidants (ROS) and antioxidants in favor of the oxidants, leading to a disruption of redox signaling. The reactivity of ROS with biomolecules leads to the damage of lipids, carbohydrates, proteins, and DNA and the induction of different diseases. ROS lead to the production of cytokines that mediate and control immune and inflammatory responses. Complex interactions exist between cytokines, inflammation and the adaptive responses in maintaining homeostasis, health, and well-being. Glutathione as an anti-oxidant mechanism has central role in the pathophysiology of human disease. People are continually exposed to chemicals in their everyday life. The toxicity of metals is related mainly to oxidative stress and epigenetic changes.