Reactive Oxygen Species: Friend or Foe?

Written by Ahmed Zayed | Last updated on August 4, 2023

Reactive oxygen species also known as free radicals are the oxygen paradox.

The Oxygen paradox means that although there are organisms that cannot survive without oxygen nevertheless oxygen is potentially dangerous to their survival. 

Reactive oxygen species had been previously reported to emerge from mitochondrial metabolism that was initially recorded as poisonous.

In recent years it has grown to be apparent that reactive oxygen species play an important role in controlling approaches like growth development and their particular response to biotic and abiotic environmental stimuli.          

Reactive oxygen species (ROS) act as secondary messengers in the body. It plays an important role in influencing the physiological capabilities of the body but its overproduction under extreme environmental conditions can result in deleterious effects on human health.         

In the human body, the mitochondria and nicotinamide adenine dinucleotide phosphate oxidase NADPH OXIDASE are the two well-known sites for the production of reactive oxygen species.

The major type of reactive oxygen species that are discussed here are superoxide and hydrogen peroxide, both of which play an important role in your health.           

At the end of this article, you'll learn the positive as well as the negative sides of ROS and different strategies you can use to reduce oxidative stress.

It also provides a brief review of antioxidant properties in the management of human disease.

What are reactive oxygen species? 

Reactive oxygen species are oxygen-containing elements that are highly reactive chemicals formed from diatomic oxygen.            

The consumption of oxygen in various physiological and pathological processes results in the production of unstable oxygen radicals that have unpaired electrons in their outer shelf.

These unstable oxygen radicals react with other molecules.  The oxygen-containing radicals are capable of their independent existence with one or more unpaired electrons.           

The superoxide free radicals that are formed as a byproduct of mitochondrial oxygen metabolism can result in excessive damage to the cells DNA, lipid, and protein and can even cause cell death.

The harmful effects of these oxygen-containing radicals are due to the overproduction of the particles resulting from environmental stresses like UV radiation and heat exposure.          

Reactive oxygen species can be oxygen-derived radicals as well as nonradical derivatives of oxygen.  The range of reactivity of oxygen species is important for their effect at the molecular level.

Any change in the level of reactive oxygen species can result in their overproduction causing oxidative damage to the cells.                           

Therefore, a balance between overproduction and utilization of reactive oxygen species is essential for maintaining redox processes in cells.

Type of reactive oxygen species 

Reactive oxygen species are categorized as free-radical and nonradical derivatives of oxygen. These are the unwanted byproducts derived from 1-2% of the total oxygen consumption. Reactive oxygen species include superoxide radicals, hydrogen peroxide, singlet oxygen, and hydroxyl radicals.

types of ros

Superoxide radical

Superoxide radical is a highly reactive form of free radical that results from partial reduction of O2 which in turn is a derivative of other reactive oxygen species. Superoxide with a relatively short half-life does not cause much damage by itself.

Superoxide radicals are considered to be the underlying cause of many diseases such as cancer, cardiovascular disease, dementia and chronic inflammation.

Hydrogen peroxide

Hydrogen peroxide is formed by the reduction of superoxide as well as by the dismutation of superoxide under low pH circumstances. It has a higher half-life and can result in much more oxidative damage.  

High concentrations of hydrogen peroxide can cause serious eye and throat irritation, breathing difficulties, skin burns and coughing.

Singlet Oxygen

Singlet oxygen has a short half-life and results from the reaction of chlorophyll triplet with superoxide.  It’s the highly reactive form of oxygen that results in extensive damage to photosystems.

Hydroxyl radical

Hydroxyl radical being reactive of all the members is the most toxic ROS known. Hydroxyl radical is formed from the partial reduction of hydrogen peroxide. It is formed at a relatively neutral pH and results in cellular compartment destruction by lipid peroxidation, protein damage, and membrane destruction.           

Hydroxyl radicals result in platelet activation and thrombosis in the blood vessels leading to cardiovascular diseases.

How are reactive oxygen species formed in the body

Reactive oxygen species are formed by the mechanism of mitochondrial oxidative phosphorylation and by xenobiotic cellular responses.          

Reactive oxygen species are produced through the biomechanical process of respiration and photosynthesis.

During respiration, the mitochondrial energy is converted into the cell for the formation of Adenosine triphosphate (ATP) through the process called oxidative phosphorylation.

Oxidative phosphorylation involves the transfer of protons through the electron transport chain which is the major source of generation of ATP.

The electron transport chain consists of 3 complexes and ATP synthase enzymes that help the electrons travel their way to oxygen which is then reduced to produce water and about 2% of oxygen gets isolated in mitochondria resulting in the formation of free radicals.

The adenosine triphosphate in return causes the formation of reactive oxygen species.           

Therefore any alteration in oxygen concentration can affect Reactive oxygen species formation and can cause oxidative stress

Oxidative stress can result in molecular damage and can be a reason for various diseases like cancer, neurodegeneration, and aging. 

Sources of reactive oxygen species 

sources of ros

Reactive oxygen species can be of endogenous as well as exogenous sources. The endogenous sources include mitochondria, peroxisomes, and endoplasmic reticulum.          

Mitochondria is the main center for intracellular ROS production. Complex I NADH dehydrogenase and Complex III ubiquinone cytochrome c reductase are the two major sites for the production of superoxide free radicals in mitochondria.          

Peroxisome is the other source in which the electrons are transferred to oxygen and leads to the formation of free radicals.            

In endoplasmic reticulum, the main domains for reactive oxygen species production are cytochrome p-450 and b5 enzymes. These p-450 enzymes are dependent on the electron of NADPH. 

Some electrons during their transfer get leaked out and react with O2 producing superoxide.

Exogenous sources include Ultraviolet radiation, extreme temperatures, heavy metal ions, and drugs. Ionizing radiation undergoes a process known as radiolysis producing damaging particles through their interaction with water.

As a result, water loses an electron and becomes sufficiently reactive which later gets converted into hydroxyl radical, hydrogen peroxide, and superoxide. 

Some lifestyle factors such as a diet high in fat, alcohol consumption, lack of exercise, smoking, and tobacco use can result in the production of reactive oxygen species.

Role of reactive oxygen species in human health 

As we know reactive oxygen species are the natural byproduct of mitochondrial oxidative metabolism; they have both a positive as well as negative impact on the human body.  

Schematic illustrating the roles of OS and nutrient antioxidants in COPD

Most of the population is aware of the “oxidative stress” nature of reactive oxygen species but very few are familiar with its protective mechanism.

The Positive role of reactive oxygen species 

Reactive oxygen species play an essential role in hemostasis. Reactive oxygen species under normal levels are important for the proper function of the cardiovascular as well as immune systems of the body.

It’s important to know that balance in levels of reactive oxygen species is essential for their better functioning. Any overproduction or decrease in regulation can result in oxidative stress.          

Reactive oxygen species are essential for the proper signaling process of the body like proper regulation of cell cycle and programmed cell death through the process of cell proliferation and apoptotic pathway.

Reactive oxygen species help the immune system in fighting against foreign bacterial invasion. So any impairment to ROS levels can result in a lack of immune responses in the body.           

Nitric oxide, which is a reactive oxygen species that can diffuse through the tissue and has a fast diffusion rate, can react with the superoxide resulting in the formation of the potent oxidant that is essential for the immune system to tackle the bacteria and many other foreign particle invasions.          

Whenever the toll-like receptor-4 detects any foreign particle invasion they send a signal to the immune system to ensure the production of reactive oxygen species which help the immune system to tackle the foreign bacterias.           

The vascular smooth muscles like PDGF and thrombin also require reactive oxygen species for better and appropriate cell growth. Hydrogen peroxide causes vasorelaxation in the pulmonary and coronary systems of the body.           

Reactive oxygen species also have an essential role in the body's inflammatory process which requires reactive oxygen species for their active performance in the systems.           

Balanced levels of reactive oxygen species in the body help to inhibit the collateral blood vessels that are the main source of many vascular diseases.           

The hemostatic levels of ROS also ensure the accurate production of antioxidants which helps in maintaining the accurate balance of ROS in the body and maintaining the defense mechanism of the body.        

Reactive oxygen species are also important for embryo development and formation.  

Negative role of reactive oxygen species 

Reactive oxygen species seem to be found in both physiological as well as pathological conditions. Since it's relatively important for the body to maintain a balance in ROS levels any interference in its activity can lead to harmful deleterious effects on human health.         

Its high concentration can lead to autoimmune diseases, cancer, diabetes, and neurodegenerative disease.

Cells need ROS for the regulation of cell signaling. Its excessive levels can lead to the activation of an apoptosome protein complex that results in mitochondrial damage and cell apoptosis leading to inflammatory conditions like atherosclerosis, multiple sclerosis, and rheumatoid arthritis.           

Hydrogen peroxide, a reactive oxygen species, causes the production of hypochlorous acid through the enzyme myeloperoxidase which at relatively high levels causes oxidative damage and inflammatory disease.

MPO plays a major role in the excessive production of reactive oxygen species that inactivates the neutrophils in inflammatory processes and leads to a respiratory shutdown.           

Reactive oxygen species in their relatively high concentration cause oxidative damage to lipids, proteins, and DNA. It causes breakage of the lipid chain and results in increased membrane permeability to foreign bacteria.          

The presence of excessive ROS also has a powerful impact on Protein which is the specific site for amino acid modification.

Reactive oxygen species cause protein enzymes to deactivate resulting in proteolysis. The interaction between reactive oxygen species, reactive nitrogen species and amino acid residue can lead to aging and protein dysfunction.

The abiotic stress on reactive oxygen species can also cause DNA strands to break resulting in excessive DNA damage.

Therefore, the amount of mitochondrial reactive oxygen species ranging from normal physiological level to pathological level can result in a series of complications within the cell leading to oxidative damage and cell necrosis. 

Role of reactive oxygen species in aging and age-related diseases 

To understand the role of reactive oxygen species in aging, it is better to get hold of the concept of the process of aging and oxidative stress in one’s mind.

comparison of a woman's face during her youth and old look

Aging is characterized by the progressive loss of tissue and organ function and gradual impairment in all physiological functions. During aging mitochondrial function is impaired which leads to less ATP production and more reactive oxygen species accumulation. A key player in this process seems to be oxidative stress.

Reactive oxygen species induce cell differentiation and apoptosis, thus contributing to the natural aging process. Reactive oxygen species also contribute to muscle contractions, and regulation of vascular tone, and determine bactericidal and bacteriostatic activity

Antioxidants being the natural defense mechanism of the body plays an important role in ROS homeostasis. Any interference in its mechanism can lead to oxidative stress damage. Oxidative stress is a condition in which there is variation between reactive oxygen species and antioxidants in the body. 

Oxidative stress plays a major role in inducing oxidative damage and cellular impairment by damaging the cell’s protein, DNA, and lipid resulting in a general decline of the physiological functions of the body.

The oxidative stress theory of aging is based on the hypothesis that age-associated functional losses are due to the accumulation of RONS-induced damages which increases with age. 

RONS, whether they are endogenous or exogenous, cause oxidative modification of each of the major cellular macromolecules (carbohydrates, lipids, proteins, and DNA), which can also be used as markers of oxidative stress.

Antioxidants and reactive oxygen species 

What are antioxidants? 

Antioxidants are substances that protect cells from oxidative stress caused by free radicals. As we know antioxidants play a great role in terminating these free radicals from the body and effects of the oxidative stress caused by the reactive oxygen species in our body. 

An antioxidant is a stable molecule that can donate an electron to free radicals and neutralize it.

Antioxidants are of two types :

  1. enzymatic also known as natural or primary
  2. non enzymatic also known as synthetic or secondary

Enzymatic antioxidants are superoxide dismutase, catalase, glutathione peroxidase and thioredoxin which are produced during normal mechanisms in the body. 

Nonenzymatic antioxidants include ascorbic acid, melatonin, tocopherols, and tocotrienols(Vitamin E),

and uric acid. Most of the natural antioxidants arise from fruits, herbs, grains and vegetables. Antioxidants also help to lower the risks of chronic diseases.

They are the secondary metabolites produced by plants and not by humans. Hence incorporation of such foods and beverages that are rich in phenols, increases the antioxidant capacity of phenol. 

Mechanism of action of antioxidants 

There are two mechanisms of action of antioxidants.

The first mechanism states the chain-breaking mechanism. In this mechanism, the antioxidants donate their electrons to the free radicals present in the systems, or decay in a harmless product and neutralize them. 

The second one is the preventive mechanism. In this mechanism, the antioxidants remove the RONS initiator through a quenching chain-initiating catalyst. Enzymes like catalase and superoxide dismutase prevent oxidation by reducing the reaction rate. 

The antioxidants and free radicals are second-order reactions. 

Therefore, they not only depend on the concentration of antioxidants and free radicals but are also dependent on factors related to the chemical structure of both reagents, the medium and the reaction conditions.

pH is the most important determinant of the reducing capacity of an antioxidant. There is a hypothesis that states that the higher the pH, the faster the reaction between antioxidants and free radicals. 

Type of antioxidants 

Antioxidants are classified as enzymatic and nonenzymatic


An enzyme is a substance that speeds up chemical reactions in the body.

The enzymatic antioxidants include 

Superoxide dismutase: superoxide dismutase is formed through a process called oxidative phosphorylation. This superoxide dismutase transforms into hydrogen peroxide which is then reduced to water. Three families of superoxide dismutase include Cu/Zn, Fe, and Mn types and at last the Ni type.

Superoxide dismutase speeds up Certain chemical reactions in the body. It helps break down potentially harmful oxygen molecules in the cells. This prevents damage to the tissue. Deficiency of this enzyme can result in a wide range of pathologies like hepatocellular carcinoma, the earlier incident of cataracts, and an acceleration of age-related muscle loss.      

Catalase: Catalase is one of the most known enzymes found nearly in all living organisms. Catalase causes the decomposition of hydrogen peroxide into reactively less reactive oxygen and water molecules.

Catalase has a prime role in regulating the cellular level of hydrogen peroxide. Lack of catalase can result in many diseases like diabetes mellitus, vitiligo, hypertension, anemia, and some dermatological disease.

Glutathione: Glutathione is a powerful antioxidant made of molecules known as amino acids Glutathione includes glutathione peroxidase, glutathione reductase, and glutathione S-transferase. These enzymes are effective scavengers of hydrogen peroxide and are found at particularly high levels in the liver. These enzymes also serve in detoxification metabolism.

Glutathione is involved in making chemicals and proteins needed in the body and immune system is a powerful antioxidant that may help protect the body from disease, improve insulin sensitivity, form sperm cells, transport mercury out of the brain and assist in regular cell death. 


The nonenzymatic antioxidants include 

Ascorbic acid: Ascorbic acid or vitamin C is the exogenous form of antioxidant. Ascorbic acid is the reducing agent that can neutralize reactive oxygen species such as hydrogen peroxide. 

Melatonin: Melatonin is also known as N-acetyl-5-methoxy tryptamine. Melatonin is a powerful antioxidant that can cross the blood-brain-barrier. 

Tocopherols and tocotrienols(Vitamin E): these are the fat-soluble vitamins that protect the membrane from oxidation by their reaction with lipid radicals. 

Uric acid: Uric acid is the mediator of active oxygen species. It is the major antioxidant that protects against aging and oxidative stress.

Importance of maintaining a balance between ROS and antioxidants 

Reactive oxygen species can have positive as well as negative effects on the body depending on its concentration. Excessive amounts of ROS can induce inflammation, chronic diseases and cancer.

Whereas relatively low levels of ROS contribute to aging by limiting the replication of cells. 

An antioxidant is a substance that is present at lower concentrations in the body. It readily absorbs and prevents excessive free radical formation at physiologically normal levels.  

With increasing age, there is a decrease in the defense mechanism of the body and an increase in reactive oxygen species. This imbalance between the reactive oxygen species and antioxidants results in progressive cellular damage. 

Therefore antioxidant defense systems should minimize the harmful levels of reactive oxygen species on one side while still forming a considerable amount of ROS required for cell signaling and redox regulation.  

Factors affecting ROS levels in the body

Reactive oxygen species is a normal byproduct of physiological metabolism. Its production in the body is greatly influenced by stress-related factors.

These stress factors include extreme environmental conditions, ultraviolet radiation, pollutants/irritants, drugs/medicines, smoking/tobacco, bacteria, nutrient deficiency, and metal toxicity.

All these factors result in an imbalance between ROS and antioxidants. This imbalance results in the overproduction of reactive oxygen species that cause oxidative damage to the building blocks of the cell.

This may result in significant damage to the cell structure. pH also affects the levels of reactive oxygen species. 

Methods to measure ROS levels

Detecting reactive oxygen species that play an important role as redox modulators in biological systems requires invasive methods like ROS- specific indicators for imaging. 

Several tests have been utilized for the measurement of reactive oxygen species. These tests are classified into direct and indirect methods. 

Direct methods include chemiluminescence assay, nitroblue tetrazolium assay, and cytochrome c reduction while myeloperoxidase test and lipid peroxidation measures are examples of indirect assay.

One way to detect the cellular level of reactive oxygen species is through the use of membrane-permeable fluorogenic probes.

This membrane permeable probe can measure the free radicals like hydrogen peroxide, superoxide, and hydroxyl radical through staining with 5-(and -6)-carboxy-2, 7 -dichlorodihydrofluorescein diacetate (DCFDA).  

Strategies to reduce ROS levels

High levels of reactive oxygen species as compared to antioxidants can result in diverse chronic age-related diseases. The reduction of oxidative stress is relatively important for proper cell functioning. The reduction of oxidative stress is achieved through three levels

  1. First by  lowering exposure to environmental pollutants with oxidizing properties
  2. Second by increasing the production of endogenous and exogenous antioxidants
  3. Third by stabilizing mitochondrial energy production and efficiency that lowers the generation of oxidative stress.

Vitamin C plays an important role in lowering ros levels in the body. Vitamin C protects the cell against DNA damage and damage to lipids that occurs through peri-oxidative stress.

Some foods high in vitamin C include Oranges, Cabbage, Papaya, Kiwi, Strawberries, Mango, Snow peas, Broccoli, and Kale.

Vitamin E is also an essential nutrient with antioxidant properties that prevent the production of reactive oxygen species.

Food sources high in vitamin E include Eggs, Sunflower seeds, Spinach, Red peppers, Grain products, Tomatoes, Almonds, Peanuts, and Vegetable oil.

Frequently Asked Questions 

What are four types of reactive oxygen species?

Reactive oxygen species are highly reactive chemicals formed from diatomic oxygen. Ros consists of radical and nonradical derivatives of oxygen formed by partial reduction of oxygen.

ROS consists of four main types namely superoxide, hydrogen peroxide, singlet oxygen and hydroxyl radical.  

These reactive oxygen species are generated through mitochondrial oxidative phosphorylation processes. 

Are reactive oxygen species good or bad?

Reactive oxygen species have both positive as well as negative roles in the body. At relatively low or normal physiological levels, ros play an important role in cell signaling and redox governing processes.

ROS has an important role in the immune system.

Lack of ROS in immune systems can affect the individual ability to fight against foreign particles On the other hand high concentrations of reactive oxygen species can cause damage to the building blocks of the body such as DNA, protein, and lipid.

How do reactive oxygen species cause damage? 

Reactive oxygen species cause damage in the form of protein per oxidation, lipid peroxidation, and DNA damage. Ros causes damage to the DNA through strand breakage and base oxidation.

Protein oxidation damages antioxidants that can further propagate maladaptive inflammation. Lipid peroxidation generates pro-inflammatory products through a lipid chain reaction, possibly releasing damaging enzymes.

What are the benefits of ROS in the body?

Reactive oxygen species are natural by-products of oxidative metabolism. Ros plays an important role in regulating cell growth, differentiation, inflammation and immune responses of the body.

It helps the immune system in fighting against foreign pathogens. Ros also promotes autophagy, a lysosomal pathway, for the degradation of unnecessary cellular components.


The phagocytic cells, like macrophages, form reactive oxygen species. Mitochondria is the major endogenous source of reactive oxygen species while UV radiation, pollutants, and drugs are the major exogenous sources. 

The reactive oxygen species consist of four types namely superoxide, hydrogen peroxide, singlet oxygen, and hydroxyl radical. All of these species react with the bio-molecules and oxidize or reduce them. 

In this context, it is worthwhile not only to investigate the ROS's role in cell signaling and redox governing processes but also to provide how ROS contributes to the progression of the diseases such as cardiovascular and inflammatory disease, cataracts, and cancer. Antioxidants play an important role in the prevention of free radical-induced tissue damage. 

Therefore, the dual nature of ROS with their beneficial and detrimental characteristics indicates the sophistication of their specific roles in a biological compartment.


  1.  Role of Reactive Oxygen Species in Aging and Age-Related Diseases: A Review . Retrieved from
  2. Reactive oxygen species in cancer. Retrieved from
  1. Oxygen, Reactive Oxygen Species and Tissue Damage: Ingenta Connect Retrieved from
  2. Reactive oxygen species in cell signaling | American Journal of Physiology-Lung Cellular and Molecular Physiology Retrieved from

About the author 

Dr Ahmed Zayed is a medical resident specializing in plastic surgery with years of experience in the field. He is also a writer for top-rated websites including Washington Post, Chicago Tribune, ConsumerHealthDigest, and Huffington Post

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