There is increasing evidence that hypochlorous acid (HOCl) can play a role in the protection of respiratory system epithelia against viral infection. Rhinovirus replication in primary cultured nasal epithelial cells was markedly inhibited by the exposure of infected cultures to HOCl, (5) suggesting that exogenous sources of HOCl may be able to intervene in the maturation of intracellular virions. Furthermore, inhibition of coronavirus infection within cultured epithelial cell lines has been shown coincident with intracellular production of HOCl.(6) Those each point to the possible operation of innate resistance mechanisms in mucosal epithelia mediated by HOCl, independent of the need for phagocytosis by neutrophils and tissue macrophages. Similar protective contributions by reactive oxygen species (ROS) have been proposed to occur in alveolar cells in the lower respiratory tract.(7)
A sprayable HOCl has been shown effective at inactivating 99.8% of SARS-CoV-2 in less than 1 minute in vitro in pre-clinical trials,(8) and the topical nasal application of HOCl-containing solutions has been proposed as a useful protective approach to avoiding common cold viruses.(5, 9) Inhalation of HOCl-containing aerosols was recently shown to be an effective therapeutic intervention in the treatment of COVID-19 virus infected patients, halting the progression of symptoms and speeding their return to normal.(10) In 2021 a clinical trial was conducted to test using inhaled HOCl as a prophylactic regimen for the protection of healthcare workers in a dedicated COVID-19 hospital. The results show a statistically significant reduction in COVID-19 conversion.(11)
Despite these advances and their obvious relevance to both prophylaxis and therapy opportunities in the ongoing pandemic of SARS-CoV-2 virus infections, little is known about the responsiveness of either the upper or lower respiratory tract to exposure to aerosolized HOCl in experimental animals or normal humans.
In our research described herein, none of the animals exposed to microaerosolized HOCl by inhalation for 4 hours showed any adverse signs in behavior, nor any gross or microscopic changes upon necropsy. It appears that even prolonged inhalation of HOCl generates neither acute nor lingering after-effects. Such findings satisfy the standard required by US federal regulatory agencies for determination of the acute toxicity of inhaled drugs or device solutions.
Acute exposure of ocular tissues of rabbits also proved entirely uneventful and without evidence of any irritant effects or adverse consequences. Human volunteers whose eyes were exposed to microaerosolized HOCl in the tent also reported no acute or subsequent eye irritation from the fog.
Of the 102 human subject exposures to aerosolized HOCl for 2 minutes, 7 (6.9%) reported minor side effects. All side effects disappeared within 60 seconds of exiting the tent, and all reports were trivial (e.g., "nose itched”). That reporting is contrasted with 12 reports from within the same remote cohort stating improved respiration, relief from nasal congestion, relief from a sore throat or bronchitis, description of an eased headache, and a reduction in bronchodilator use for asthma over subsequent days.
The amount of active chlorine declined markedly in aerosolized HOCl droplets expressed through the microaerosolizing nozzle, falling by more than 60% by the time they reached the inhalation point for seated subjects and the sample point for antimicrobial efficacy. Despite that reduction in concentration, the HOCl solutions used in these exposures were clearly antimicrobial and displayed high level inactivation of S. aureus and E. coli in laboratory suspension tests.
Some concentration attrition is inevitable in the administration of microaerosolized HOCl by these methods, regardless of whether the creation and dispersion of droplets is affected by pressurized air or via the ultrasonic oscillation of membranes typical of nebulizing devices.(12) The results of measurements of the antibacterial properties of the microaerosolized HOCl arriving at the surface of coupons placed within the enclosure and used to mimic essential worker exposure make it clear that, despite the attrition, there remains a high level of disinfecting power at the point of droplet deposition, again in accord with prior reports.(13, 14)
The dissipation of active Cl in air with distance from the aerosol source of HOCl has been reported previously in publications when establishing efficacy against norovirus or against several bacterial pathogens in vegetative and spore forms.(13, 14) How this comes about and what the reaction products are of the oxidative chlorine with air during transit remains to be determined.
A recent report on systemic administration of HOCl as a part of the therapeutic intervention in COVID-19 patients halted the progression of symptoms.(10) The authors attributed the success of their HOCl treatment of COVID-19 patients to its antiviral properties. Published evidence of the value of HOCl in vitro in protecting nasal mucosal epithelial cells against respiratory viral infections supports that interpretation.(5)
The changes experienced by healthcare workers who inhaled nebulized HOCl within the study conducted by Rafael et al. (11) indicate that there are systemic effects resulting from exposure to HOCl in this way, some of which are clearly beneficial and do not seem to be simply antimicrobial. Whether or not those are affected by HOCl directly or through reaction products arising at the point of mucosal contact remains to be determined.
There is a high likelihood that HOCl rapidly modifies a variety of constituents of both extracellular and intracellular fluids, with the interaction with taurine being one of the most prominent, resulting in the formation of N-chlorotaurine (NCT).(15, 16) Taurine is present in body fluids in amounts that can total 0.1% of total body weight and is particularly likely to be involved in the biological effects of HOCl exposure.(17) Previous work indicates that HOCl and NCT have anti-inflammatory properties (18, 19) and, while beyond the scope of this evaluation, it seems feasible that the anti-inflammatory properties of HOCl and NCT might also lead to reduced local inflammation, lower infectivity through reduced permeability, and perhaps lower morbidity and mortality overall.
Our recent observations on the efficacy of HOCl as an anti-inflammatory agent via inactivation of interleukin 6 (IL-6) [submitted for publication] suggest that it can intervene in the pathogenic pathways of COVID-19 beyond its antiviral efficacy, possibly by downregulating mediators of the cytokine storm that characterizes clinical deterioration in this disease.(20)
Our observations here are particularly relevant considering recent developments involving human exposure to HOCl. Solutions of HOCl have been shown to rapidly inactivate the SARS-CoV-2 virus in test systems comparable to those used in this study.(21) Based on those results, commercial products containing HOCl have been placed on the recommended pandemic coronavirus disinfectant list (List N) by the US Environmental Protection Agency.(22)
More broadly, we note that widespread, high-volume use of disinfectants has come about as a response to the current pandemic, and it is becoming increasingly apparent that many of these are inappropriate for such use patterns, never having been designed or intended for intensive applications around - and sometimes upon - humans.(1, 23) Toxicological evidence of serious untoward effects of repeated exposures to commonplace disinfectants, especially by inhalation of aerosols, are emerging.(2) Such findings have brought HOCl to the fore as a safe, yet highly effective, alternative to quaternary ammonium formulations, hypochlorite solutions, ozone, peroxide, and alcohols of various types.(24, 25)
We undertook this research because humans are being exposed to HOCl at an unprecedented rate, sometimes via aerosols that they are obliged to encounter as a condition of entry into public buildings to control the COVID-19 virus.(26) Some of these exposure patterns offer unproven benefits, but the overall safety profile of HOCl, recognition of its in vitro efficacy, and popular understanding of HOCl as a natural product of human cellular defense, have each encouraged these new patterns of use. Further expansion of HOCl as an environmental disinfectant is both appropriate and inevitable, and studies such as ours that begin to provide evidence about the benign consequences are timely. While further work is needed on longer term exposures, the data from our experiments to date have not raised issues of sufficient concern to affect popular use trends and should encourage further experimental evaluation.
Regardless of the possible mechanisms involved, the potential significance of the use of HOCl as a therapeutic, and possibly even a prophylactic tool, in the management of COVID-19 patients is substantial. Current approaches using monoclonal antibodies directed against the SARS-CoV-2 virus, or against IL-6, or against viral receptors on mucosal epithelial cells are costly, cumbersome, and scarce.(27, 28) The practical complications of the specificity, manufacture, transport, administration, and expense of such measures preclude their ever becoming available to the great majority of people around the world. In contrast, homogenous and stable forms of HOCl can now be produced in industrial quantities and readily distributed to local pharmacies and clinics without special requirements for transport or storage conditions.(29) Subsequent self-administration of HOCl via nebulizer as either prophylaxis or therapy could be, after further validation, a significant advance in patient care.
It is also worth noting that the specific amino acid residues in the SARS-CoV-2 spike protein against which the redox mechanism of action for HOCl is active are conserved in all variants evaluated to date. Each variant identified so far is likely to be similarly susceptible and that research is underway. The increasing morbidity and mortality seen with variant SARS-CoV-2 strains, therefore, shows that the availability of HOCl formulations suitable for systemic administration in humans, particularly through inhalation, presents an opportunity worthy of more extensive clinical evaluation.