INTRODUCTION
Hypochlorous acid (HOCl) is one of the latest skincare ingredients to gain widespread popularity on social media. Despite newfound interest by some, the use of HOCl is certainly not new; its medicinal use, as a wound disinfectant, dates back to World War I.1 Since then, HOCl has earned FDA approval for several indications, including wound care and management of eyelid and oral infections.1 In dermatology, specifically, several stabilized HOCl formulations are in the marketplace and have been studied for various indications.2 Now, HOCl spray has made its way to social media with users incorporating it into their daily skincare routines, applying it as a post-workout disinfectant, and using it to manage acne. Herein, we review the evidence for HOCl to determine the validity behind this skincare trend.
Mechanisms of Action
HOCl is naturally produced through the body's innate immune response. It is formed through the respiratory burst pathway after myeloperoxidase-expressing cells phagocytose pathogens. Its microbicidal actions are mediated through oxidative damage to pathogens' proteins, DNA, and lipids, which can gradually lead to cytotoxicity and cell death.3 HOCl is microbicidal to many pathogens including methicillin-resistant S. aureus, S. epidermidis, E. coli, and P. aeruginosa.4 HOCl also has numerous anti-inflammatory mechanisms such as reducing the activity of histamine, leukotriene B4 (LTB4), and interleukin-2 (IL-2) and increasing the activity of transforming growth factor-beta.4
Applications in Dermatology
There are numerous dermatologic applications for HOCl, with wound care being the most common. HOCl reduces the microbial load in infected wounds in a concentration-dependent manner; at a concentration of 500 mg/L, the extent of microbial reduction is around six times greater than at 100 mg/L.2,5 HOCl can also directly contribute to wound healing success by increasing the percentage of epithelialization compared to other media, such as sterile saline solution.6 Additionally, HOCl may have a role in reducing scarring of wounds. A study comparing HOCl gel to 100% silicone gel found that HOCl gel decreased the vascularity and height of scars to a greater degree after 16 weeks.7
HOCl is also effective in the treatment of atopic dermatitis (AD)-associated pruritus. In animal studies, topical HOCl reduced scratching behavior to the same extent as 0.1% betamethasone dipropionate ointment.8 In human studies, topical HOCl reduced itch within three days with no serious adverse events.9 Another study demonstrated significantly reduced S. aureus colony counts within three minutes and after one week of treatment with HOCl.10 Reduction of S. aureus and diminished activity of histamine, IL-2, and LTB4 have been hypothesized to be primary mechanisms by which HOCl reduces AD-associated pruritus.4 HOCl has both anti-inflammatory and pro-inflammatory effects, but anti-inflammatory effects predominate.4 Specific antiinflammatory mechanisms, namely HOCl's effect on proteases, are concentration-dependent. At high concentrations, HOCl decreases protease activity causing anti-inflammatory effects, but at low concentrations, it increases protease activity causing the opposite effect.4
Finally, the role of HOCl in treating acne and seborrheic dermatitis has been previously explored. One study evaluating the efficacy of HOCl solution versus benzoyl peroxide in acne management found equivalent improvement in facial inflammatory lesions with both treatments.11 Another study investigating the efficacy of HOCl gel in managing mild-to-moderate facial and scalp seborrheic dermatitis observed a 33% improvement in Investigator Global Assessment scores within 14 days.12 More studies are needed to quantify the efficacy of HOCl in treating these conditions.
Stability of Hypochlorous Acid
While HOCl has many beneficial applications, its efficacy is highly dependent on its stability. Several variables can impact the stability of HOCl in solution, including pH and environmental factors such as sunlight, temperature, contact with air, and other compounds in solution. HOCl is stable between a pH of 3.5 and 5.5; at pH ranges above or below this, concentrations of HOCl decrease, and concentrations of reaction products increase.4 For example, at higher pHs, the concentration of sodium hypochlorite, which has less microbicidal activity than HOCl, increases.4 In addition to pH, controlling environmental factors is also essential to maintaining the stability of HOCl. One study evaluated the effect of sunlight on the shelf life of HOCl and found that chlorine reduction started on day 4 in the sunexposed solution and by day 14 in the sun-sheltered solution, highlighting that sunlight significantly decreases the stability of HOCl.13 In addition to sunlight, irradiation from an
