We think of hydrogen peroxide as an active disinfectant against a wide range of microorganisms (bacteria, yeasts, fungi, viruses and spores), yet it is also integral to a variety of other endogenous and exogenous activities.¹ Hydrogen peroxide constantly surrounds us in this world, and as such, seemingly takes on a world of its own.

What Is It?
Hydrogen peroxide (H₂O₂) is in the air we breathe, the water we drink and products we use regularly. Even our own bodies constantly produce it. Hydrogen peroxide is biologically created from the breakdown of superoxide anions (O^2-) following exposure of cells to physical, chemical or biological agents.² H₂O₂ is important in normal wound healing, initiating necessary inflammation and pathogen destruction.

We harness the good properties (teeth whitening, contact lens disinfection, hair bleaching), and by eating blueberries, monitoring our vitamin C and using sunscreen, we ward off the ill effects (aging, inflammation, DNA damage, tumor induction).^3,4 The toxicity and disinfectant properties of peroxides, however, are directly related to the concentration and the body’s ability to detoxify itself. A 3% to 6% concentration of hydrogen peroxide is commonly sold over the counter and is used in contact lens disinfecting solutions.¹ Concentrations from 6% to 25% show promise as chemical sterilants.¹

Microorganisms have built-in defense mechanisms against hydrogen peroxide, including the encystment of ameoba and increasing levels of enzymatic degradation. Catalase, among others, is an antioxidant enzyme that protects cells from metabolically-produced hydrogen peroxide by degrading H₂O₂ to water and oxygen. This defense is overwhelmed by increasing the concentrations used for disinfection. Therefore, the disinfection efficacy of a contact lens solution against highly resistant organisms depends on the contact time and concentration of the peroxide over that entire period of time.⁵

How Does It Work?
Hydrogen peroxide disinfects by producing hydroxyl free radicals (OH-) that can attack membrane lipids, DNA, mitochondria and other essential cell components. Hydroxyl radicals are highly reactive, short-lived and remarkably destructive. H₂O₂ is lipid soluble; therefore, when cells come into contact with hyrogen peroxide, the cell is easily penetrated and releases a variety of signaling molecules that play a key role in immune cell activation (inflammation), healing and tumor genesis.

Wound healing has several stages mediated by hydrogen peroxide signaling, including inflammation and new tissue formation. Hydrogen peroxide attracts leukocytes to the wound site, which produce more pro-inflammatory mediators. H₂O₂ also stimulates angiogensis, fibroblasts to create new connective tissue and keratinocytes to close the epidermal layer.⁶ In addition, H₂O₂ increases heparin binding epidermal growth factor, released by the keratinocytes, and encourages closure of the epidermal layer.⁷

The levels of H₂O₂, however, must be kept in check. When available, antioxidants—produced by our body—are insufficent to maintain reasonable levels, DNA and mitochondrial damage results, increasing the likelihood of cancer development.⁸ High levels of oxidative species also have been linked to neurodegenerative diseases (e.g., Parkinson’s, Alzheimer’s, Huntington’s), cardiovascular disease and chronic fatigue syndrome.⁹

Hydrogen peroxide is a potent metabolic regulator in our bodies; it can be either friend or foe. Although our initial reaction is to think of H₂O₂ as a benign disinfectant because of its ability to be broken down to water and oxygen, its mechanisms are powerful and complex and should command respect. 

1. Guideline for disinfection and sterlization in healthcare facilities, 2008. CDC. 2009 Dec. Available at: www.cdc.gov/hicpac/disinfection_sterilization/7_0formaldehyde.html (accessed August 2012).
2. Veal E, Day A, Morgan B. Hydrogen peroxide sensing and signaling. Molecular Cell. 2007 Apr;26(1):1-14.
3. Lisanti M, Martinez-Outschoorn U, Lin Z, et al. Hydrogen peroxide fuels aging, inflammation, cancer metabolism and mestasis: the seed and soil also need “fertilizer.” Cell Cycle. 2011 Aug 1;10(15):2440-9.
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5. Hiti K, Walchnik J, Faschinger C, et al. One- and two-step hydrogen peroxide contact lens disinfection solutions against Acanthameoeba: how effective are they? Eye (Lond). 2005 Dec;19(12):1301-5.
6. Scherml S, Landthaler M, Schäferling M, Babilas P. A new star on the H2O2rizon of wound healing? Experimental Dermatology 2011 Mar;20(3):229-31.
7. Kümin A, Huber C, Rülicke T, et al. Peroxiredoxin 6 is a potent cytoprotective enzyme in the epidermis. Am J Path. 2006 Oct;169(4):1194-205.
8. Kanno T, Nakamura K, Ikai H, et al. Literature review of the role of hydroxyl radicals in chemically-induced mutagenicity and carcinogenicity for the risk assessment of a disinfection system utilizing photolysis of hydrogen peroxide. J Clin Biochem Nutr. 2012 Jul;51(1):9-14.
9. Fang C, Bourdette D, Banker G. Oxidative stress inhibits axonal transport: implications for neurodegenerative diseases. Mol Neurodegener. 2012 Jun 18;7:29.