A Potential Benefit of Hypochlorous Acid-Facial Sanitisation: A Review

Sanitisation has become a major component of everyday life, with emphasis on the hands and surfaces. The face remains unsanitised. This is due to the lack of an acceptable sanitiser. The use of masks has been implemented in order to protect the spread of the pathogens by covering the face, however there remain issues associated with the use of PPE.. The face remains a harbour for upper respiratory tract infections, with constant deposition and replication of microbes. With SARS-CoV-2 being so prominent, it was postulated that there is an inoculum dose-dependant relationship with severity. By reducing microbial load of the face, the risk of both infection and severity of infection are reduced. HOCl has proven antimicrobial and antiinflammatory activity and was tested for efficacy against SARS-CoV-2, demonstrating a 99.99% reduction (50ppm; 1 min contact). A facial sanitiser, added to the arsenal of hand sanitisers and masks, further improves protection and prevention against SARS-CoV-2. The advantages of regular sanitising of the face and mask include a reduced level of microbial contamination, reduced risk of biofilm formation, and respiratory tract and skin infections. HOCl was reviewed as a face and face mask sanitiser, concluding that it was an ideally


Introduction
The face, namely through the oral and nasal mucosa and the conjunctiva, is a major point of entry and point of infection for many pathogens involved in human diseases and serves as a harbour for the growth of microorganisms. This is particularly true for SARS-CoV-2, among other significant infections (e.g., S. pyogenes, S. aureus, M. tuberculosis) involving the respiratory tract. The pathogenic contamination of the face is by both airborne pathogens and surface borne pathogens, and via unsanitised hands. On average, individuals who are welleducated on infection control involuntarily touch their faces 23 times per hour where 44% of the face touches involved mucosal areas, with the remainder 56% being non-mucosal surfaces [1] . With the ongoing Covid-19 pandemic, regular hand and surface sanitisation and regular hand washing has been emphasised in order to reduce cross contamination and risk of infection. While this is beneficial, involuntary face touching, especially around the mucosal regions, alongside the presence of airborne pathogens, increases the risk of infection. Consequently, the use of disposable or reusable face masks has been made mandatory. Although it is true that the use of a face mask is beneficial in reduction of the risk of infection, there remains certain issues with use of the face maskthe main issues being improper usage of face masks, prolonged usage of disposable face masks, and infrequent and inadequate washing of reusable masks. It should also be considered that poor handling of masks and frequent removal and reapplication of the same masks further increases the surface area of the face mask which is contaminated. As face masks only cover the nose and mouth, there remains an exposure risk of the eyes and conjunctiva. This poses a risk of contact and entry for pathogens even whilst wearing a mask. In addition to this, upon removal of face masks and other personal protective equipment (PPE), there is still the risk of contamination of skin surfaces [2] . Errors in doffing (removal) procedures of PPE was studied by Okamato et. al., (2019) whereby 125 participant healthcare workers (HCW) were enrolled. The findings demonstrated that 39.2% made multiple doffing errors and 36% were found to be contaminated with multidrug resistant organisms [3] . Lim et. al. (2015) performed a study in Samsung Medical Center in South Korea, whereby they identified a 20.6% error rate of donning of respirators or hoods. Contamination on the doffing process identified a 48.3% rate of contamination of the head, 6.9% contamination rate of the face, and 72.4% rate of contamination of the neck [4] . Even though it is clear that the primary focal point of many infections is the face and facial mucosal surfaces, there is a need for disinfection products that can be used to sanitize the face and to rid the face and mucosal surfaces of pathogens. A facial sanitiser would be a great tool to add to the current arsenal against the Covid-19 pandemic, as well as against other respiratory tract infections. HOCl is a secondary reactive oxygen species produced by neutrophils, macrophages, and masts cells. Physiologically HOCl can be said to be present in all tissue systems, including the skin [5] . HOCl has proven to be a highly effective disinfectant, demonstrating rapid activity against a wide spectrum of microbes, including drug resistant bacteria and bacterial spores, fungi, viruses, prions, and amyloid seeds [6,7] . In this review, the mechanism of action of HOCl will be briefly discussed alongside the safety of use of HOCl on sensitive areas such as the face. The potential benefits of using HOCl to reduce the number of pathogens will be discussed, and other potential benefits which may be associated with the use of HOCl will be identified.

Mechanism of Action
Hypochlorous acid is a secondary reactive oxygen species (ROS). It is a naturally occurring molecule present in the human body, produced during the respiratory burst of phagocytosis. The composition of the molecule is HOClthe molecule itself being the secondary reactive oxygen species. The molecule then dissociates into free radicals -H + and OCl -. Hypochlorite (OCl -) is a potent free radical (oxidant) which rapidly causes damage to bacterial cell walls and cell contents [8] . The following summarises the antimicrobial action of HOCl: [9] 1. Hypochlorite (OCl -) react with membrane lipids and cell walls of pathogens resulting in: a. Peroxidation of polyunsaturated fatty acids and destruction of cross-linkages in cell walls b. A chain reaction of peroxidation of membrane lipids and decreased membrane fluidity 2. Cellular dysfunction occurs due to inhibition of enzymes required for glycolysis, thus, pathogens, specifically bacteria, are unable to metabolise glucose to produce ATP resulting in: a. An increase in the presence of free radicals, resulting in a change in intracellular redox potential further inhibiting glycolysis b. The complete disruption of essential cellular functions and depletion of adenosine triphosphate (ATP) 3. Additionally, there is activation of enzymes resulting in cellular lysis. Human cells make use of a group of defence mechanisms collectively known as the Antioxidant Defence System (ADS) [10] . Microbial cells, however, do not have this defence mechanism and for this reason HOCl is effective against microbes.
HOCl demonstrates efficacy against a wide range of microorganisms. Efficacy has been demonstrated against pseudomonas aeruginosa and escherichia coli [11,12] , poultry derived salmonella spp. [13] , staphylococcus spp. [12] , enterococcus faecalis [14] , among other bacteria as mentioned later in this review. Sporicidal activity has also been demonstrated [15] . Virucidal activity is also well noted, with efficacy against resistant organisms such as norovirus spp. [16] .

Safety
The potential for HOCl to damage mammalian cells appears to be present. However, inherent mechanisms in human cells are in place in order to prevent cellular damage. The physiological protective mechanism is based on the antioxidant defence mechanism. As HOCl is a ROS (oxidant), the antioxidant defence mechanism of human cells protects against the oxidative stress caused by HOCl. This specific quality is what renders HOCl safe for use in humans while making it toxic to microbes. SkinSafe, developed by Mayo Clinic, designates HOCl as hypoallergenic, irritant free, eyelid and lip safe, safe for teens and safe for babies [17] . Antioxidants are molecules which slow down or prevent oxidation of other molecules. The mechanism of action of antioxidants are as follows: [18] 1. Removal of radical intermediate species 2. Blocking secondary production of toxic metabolites and inflammatory mediators 3. Converting free radicals into less toxic compounds 4. Blocking chain propagation of secondary radicals 5. Repairing molecular injury 6. Enhancing the endogenous antioxidant system function of exogenous antioxidants 7. Inhibiting other oxidation reactions by being oxidised themselves As such, there are two classes of antioxidantsenzymatic and non-enzymatic antioxidants. Enzymatic antioxidants include superoxide dismutases, catalases, glutathione system, thioredoxin system. Non-enzymatic antioxidants include ascorbic acid (vitamin C), glutathione and thiocyanate, tocopherols and tocotrienols (Vitamin E), beta-carotene (Carotenoidsprovitamin A).
The Antioxidant Defence System (ADS) is what protects the body from oxidative damage. The presence of the ADS alone makes HOCl safe for use in humans and other species that make use of this kind of system.

The Skin Microbiome
The skin is the human body's largest organ and is colonised by a plethora of microorganisms. This colonisation is highly dependent on topographical location, host, and environmental factors [19] . The skin is the body's primary defence mechanism against microbial infection, acting as a physical barrier. The skin on the face is particularly susceptible to colonisation by foreign microbes, due to frequent face touching and deposits of airborne microbes (droplets) on the skin and mucosa of the face. Many common skin disorders are postulated (although having not been scientifically proven in most cases) to have an underlying microbial cause because clinical improvement is seen with the use of antimicrobial treatments. However, there are certain skin disorders, such as acne vulgaris, with definitive causative microorganisms (e.g., Cutibacterium acnes (formally Propionibacterium acnes) and Staphylococcus epidermidis) [20] . The presence of Staphylococcus aureus on the skin and scalp in itself is a cause of many infections, such as boils, folliculitis, impetigo, and cellulitis. Additionally, S. aureus is highly prevalent in cases of atopic dermatitis (eczema) [21] . S. aureus, in addition with group A streptococcus (S. pyogenes), and other microorganisms, can cause a severe polymicrobial infectionnecrotising fasciitis. As such, microorganisms on the face may pose a threat of a range of skin infections ranging from minor easy to treat infections to severe life-threatening infections [22] . The variety of bacteria which form the microbiome of the face may create biofilms. This begins with microorganisms forming a microcolony and surface colonisation. Biofilms provide advantages to microorganisms including: protection from host defences, differential gene expression, and increased resistance to antimicrobials Epithelial biofilms are implicated in a number of dermatological conditions, such as chronic wounds, atopic dermatitis, candidiasis, and acne vulgaris. These biofilms pose a challenge, as there are limitations for their detection, and their resistance to conventional antimicrobial therapy due to persistence and their chronic nature [23,24] .

The Face as a Harbour for Respiratory Infections
The entry points to the nose, mouth and eyes are key regions which predispose infections of the upper respiratory tract. Microorganisms proliferate on the surface of the skin, and constant touching of the face, nose, mouth, and eyes results in an increase in presence of the number and variety of microorganisms present. The major causative agents for upper respiratory tract infections (URTIs) are viral in nature, although not limited to viruses alone [25] .  1. At an individual level: "viral dose in inoculum is related to severity of disease (dosedependent relationship)" 2. At a cluster level: "Severity of disease is related to transmission potential" leading to clusters of mild cases and clusters of severe cases. 3. At a community level: "In certain contexts, chains of severe cases can build up through intensive transmission with high inoculum to severe local outbreaks, which can result in large-scale intensive epidemics, while this is less likely in other contexts"

Microorganims involved in the pathogenesis of upper respiratory tract infections include
This theory plays out in practice on three levels: [26] 1. Individual level: A person infected with a small dose viral inoculum will on average develop milder disease than a person infected with a high viral inoculum and vice versa. This is independent of other well known risk factors for severity of disease, mainly old age, and comorbidities, such as diabetes. 2. Cluster level: A person with asymptomatic infection or mild disease, will on average spread lower dose of the virus, and is less likely to transmit disease; and when the person transmits, the newly infected person is more likely to have a mild disease compared to a person infected by a severely ill person, who spreads on average higher doses of the virus. This causes clusters and chains of milder cases or more severe cases.
3. Community level: In certain contexts, such as dense urban centres with a moderate climate, during the season that people live mostly indoors, the potential for intensive transmission and explosive outbreaks is higher than in rural areas, or in regions with a hot and humid climate where people live mostly outdoors. Hence, a cascade of intensive transmission is more likely in certain contexts than others.
This model is based on other pathogens which show dose-dependent severity, such as:

-Influenza virus -Coronavirus (seen with MERS and HCoV-229E) -Human immunodeficiency virus (HIV) -Measles -Mycobacterium tuberculosis (TB) -Streptococcus pneumoniae
It is of note that the inoculum dose-dependent hypothesis needs further investigation in order to be well established, and medical experts need to pay close attention to these factors [26] .

Dermatological Benefits of Hypochlorous Acid
As the use of alcohol-based sanitisers has increased drastically since the onset of the Covid-19 pandemic, resulting in an increase in the incidence of contact dermatitis and allergic contact dermatitis associated with the use of alcohol-based sanitisers [27] . The incidence of allergic contact dermatitis increased with use of other non-alcohol-based hand sanitisers (such as chlorhexidine and quaternary ammonium compounds). Another concern with the use of alcohol-based sanitisers is the depletion of the lipid barrier with repeated exposure, resulting in increased risk of destruction or changes of the skin flora and colonisation of pathogens [28] . Additionally, the use of personal protective equipment (PPE) has shown an increase in dermatological conditions such as allergic dermatitis, irritant dermatitis, friction blisters, contact urticaria, acne, and infections [29] . HOCl is a possible solution to the aforementioned conditions, while possessing other benefits associated with dermatological applications. In a mouse model, anti-inflammatory properties were demonstrated on sensitised mice, whereby HOCl demonstrated effective reduction in an inflammatory response quantified by reduced secretion of inflammatory cytokines. Associated with this was a reduction in itch and scratching behaviour [30] . The antimicrobial and anti-inflammatory (immunomodulatory) properties of HOCl correlate with a clinical improvement in a variety of cutaneous disordersincluding atopic dermatitis, seborrheic dermatitis, diabetic ulcers, pruritis, and acne vulgaris. Other clinical benefits are demonstrated with the promotion of wound healing and reduction or prevention of scar formation [31] .

Potential Benefit of HOCl on the Face and Skin
Due to the high efficacy of HOCl and the inherent safety profile, it offers very good potential for use for face as well as nasal and oral sanitisation. HOCl acts as secondary reactive oxygen species when produced endogenously and this is the way it carries out its mechanism of action. Due to its own endogenous nature, HOCl possesses an inherently good safety profile as demonstrated in numerous international studies [32][33][34][35] .
Testing of antimicrobial efficacy of HOCl has been sponsored by Aquaox LLC, Loxahatchee, Florida, on multiple pathogens following the USP <51> testing protocol [36] . Relevant pathogens tested include Candida albicans, S. aureus, and S. epidermidis demonstrated a reduction of >99.9999% with 15 seconds of contact. P. aeruginosa showed a reduction of >99.9999% with 60 seconds of contact and MRSA had a reduction of >99.999% with 60 seconds of contact. The 2009 Pandemic strain of H1N1 had a >99.963% reduction with 5 minutes of contact, and M. bovis demonstrated a >99.99% reduction with 10 minutes of contact. From the above results it is clear that Hypochlorous Acid (HOCl) will offer an additional defence mechanism to prevent microbial infection.
Due to the wide antimicrobial efficacy, and the lack of toxicity, HOCl has been described as having potential pharmaceutical applications in the control of soft tissue infections [37] .

Discussion and Conclusion
The published literature indicates that HOCl is highly efficacious against a wide range of microorganisms. This, alongside knowledge of the mechanism of action provides insight into the antimicrobial properties which may bypass the potential of microbial resistance against HOCl. The range of microorganisms killed by HOCl are all significant microbes which cause dermatological or respiratory tract infections. As such, it can be deduced that use of HOCl as a facial and face mask sanitiser would decrease incidence of the skin conditions mentioned. The safety of HOCl to mammalian cells has been demonstrated even the most sensitive cells.
As HOCl with a concentration of 200 ppm (0.2%) and 210 ppm (0.21%) has been proven safe using Vero cells, it can be concluded that HOCl is a safe to use as a facial sanitiser [35] . In addition to the benefits of preventing upper respiratory tract infections, HOCl has further benefits with the potential to treat a range of dermatological disorders, such as acne vulgaris, seborrheic dermatitis, and atopic dermatitis. Furthermore, a range of cutaneous conditions associated with the use of PPE and alcohol-based sanitisers can see a benefit from the use of HOCl. As such, the combined benefits of HOCl use further potentiates the use of the compound as a facial sanitiser. HOCl has a proven efficacy against biofilms, and as such, the presence of epithelial biofilms would be eradicated, alongside conditions associated with them [38,39] . Use of a facial sanitiser, in addition to the arsenal of hand sanitiser and masks, further improves protection against and prevention of the spread of SARS-CoV-2. The inoculum dose-dependent relationship with severity of infection postulation brings to consideration that any reduction of SARS-CoV-2 numbers on the face and mucosal surfaces will be beneficial with reduction of both risk of infection and severity of ongoing infection. The potential administration forms of HOCl for the purpose of a facial sanitisation could be by the use of a spray due to ease of application, as well as the ability to use the spray for sanitisation of masks and hands as well. Alternative methods of sanitisation could be by use of a gel formulation. This method would result in increased dermatological benefits, however, ease of use and sanitisation of masks would not be possible with this method. Thus, there are great benefits with the use of HOCl as a facial sanitiser, and this provides the another arm of tackling URTIs and COVID-19.

Data Availability
All article and book references are available online and can easily be found using their respective DOI references. Reference 35 (Kabamba & Malatji, 2020) is available on request. Reference 36 is unavailable to readers as it contains confidential information.

Conflicts of Interest
Dr. Avis Aman Nowbuth is an employee of BSafe HOCl , whereby he researches possible uses, benefits, and safety aspects of hypochlorous acid. His affiliation does not influence the quality and integrity of the data in this review article. Dr. Josh Barrie Armstrong was contracted to assist and review the integrity of the data by BSafe HOCl , in order to verify the data was unbiased and accurate.

Funding
Funding for publication was provided by BSafe HOCl (Pty.). The funder had no influence in the writing, editing, approval or decision to publish this manuscript.

Acknowledgements
A preprint has previously been published. [40]