As norovirus surged across the U.S. last winter, the only thing more horrifying than descriptions of the highly contagious illness—violent projectile vomiting!—was learning that nothing seemed to kill the microbe that causes it. Hand sanitizers made with alcohol are useless. Water needs to be above 150 degrees Fahrenheit to kill the virus, which is too hot for handwashing. Rubbing with soapy water and rinsing can physically remove the virus from your hands and send it down the drain but won’t effectively kill it. Bleach dismantles norovirus, but you can’t spray bleach on skin or food or many other things, and norovirus can live on surfaces for weeks.
During the early days of the COVID pandemic, however, I had learned about a disinfecting agent called hypochlorous acid, or HOCl. My dad, a now retired otolaryngologist, had been wondering whether there was something he might put up patients’ noses—and his own—to reduce viral load and decrease the chance of COVID infection without, of course, irritating the mucosa or otherwise doing harm. He was imagining a preventive tool, another layer of protection for health-care workers in addition to masks and face shields.
Hypochlorous acid is a weak acid with a pH slightly below neutral. It should not be confused with sodium hypochlorite (NaClO), the main active ingredient in household bleach products, even though they both involve chlorine. Chemically, they are not the same. Sodium hypochlorite is a strong base with a pH of 11 to 13, and when added to water for consumer products it can be irritating and toxic. Hypochlorous acid, in contrast, is safe on skin.
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All mammals naturally make hypochlorous acid to fight infection. When you cut yourself, for instance, white blood cells known as neutrophils go to the site of injury, capturing any invading pathogens. Once the pathogen is engulfed, the cell releases biocides, including hypochlorous acid, a powerful oxidant that kills invading microbes within milliseconds by tearing apart their cell membranes and breaking strands of their DNA.
Hypochlorous acid is a well-studied disinfectant that appears to be extremely effective and safe—so why isn’t it a household name?
The synthetic form of hypochlorous acid destroys a broad spectrum of harmful microbes—including highly resistant spores and viruses such as norovirus. Like most disinfectants, it kills pathogens by penetrating their cell walls. But compared with bleach, hypochlorous acid has been shown to be more than 100 times more effective at much lower concentrations, and it works much faster.
Hypochlorous acid isn’t new. It’s listed as one of the World Health Organization’s essential medicines and is approved by the U.S. Food and Drug Administration for use on food products and in certain clinical applications. It’s increasingly used in industrial and commercial settings, such as water-treatment plants, hospitals and nursing homes. It doesn’t irritate the skin, eyes or lungs. In fact, optometrists use it to clean eyes before procedures, and people have been treating wounds with it for more than a century. It breaks down quickly, doesn’t produce toxic waste, and isn’t harmful to animals or the environment. The U.S. Environmental Protection Agency lists it as a surface disinfectant for the COVID-causing virus SARS-CoV-2.
Hypochlorous acid is a well-studied disinfectant that appears to be extremely effective and safe—so why isn’t it a household name?
Scientists have known about the powers of hypochlorous acid for nearly 200 years. In 1834 French chemist Antoine-Jérôme Balard made hypochlorous acid when he added a dilute mix of mercury oxide in water to chlorine gas. Later in the 19th century, English chemist and physicist Michael Faraday developed a technique for synthesizing HOCl from salt and water via a process called electrochemical activation.
Before the advent of antibiotics, hypochlorous acid was a go-to disinfectant. It was used as a wound sanitizer during World War I. The authors of a 1915 article in the British Medical Journal set out to investigate antiseptics that could be used to dress wounds in the field. They compared the efficacy of sodium hypochlorite (bleach) with that of hypochlorous acid and “found that hypochlorous acid is a more potent germicide than its salts.” They “accordingly devised a method in which the free acid is employed as the antiseptic agent.”
For all its benefits, hypochlorous acid solution has one major weakness: it’s highly unstable. It remains stable only in a solution with a pH between about 4 and 6. The solution is still made using salt, water and electricity through the process of electrolysis. Within minutes of exposure to light or air hypochlorous acid starts to deteriorate back into salt water, making it useless as a disinfectant. If the solution were to get too acidic, it would start converting into chlorine gas. If it were to get too alkaline, it would gain a higher percentage of hypochlorite. This lack of shelf stability is the biggest reason hypochlorous acid sprays never became a staple of the cleaning-products aisle.
For decades hypochlorous acid lingered in the background, used as a disinfectant in specific industrial and commercial contexts that could justify a pricey, on-site manufacturing process to create products on demand. But COVID accelerated the need for different methods of disinfection that would be safe, effective and easy to use in a wide range of environments. According to an article in the magazine Health Facilities Management, during the pandemic “many countries introduced continuous HOCl misting and fogging tunnels for entry and exit corridors at mass transit facilities.” Since then, use of HOCl in places such as kitchens, gyms, nursing homes and medical offices has been rising significantly.
Hypochlorous acid consumer products are now proliferating, thanks to the development of new manufacturing processes that reportedly make an extended shelf life possible while keeping costs low. The more reputable of these companies claim their products are effective within two years of the manufacturing date stamped on the bottle if stored correctly (ideally at room temperature, away from sunlight).
Most common are surface sanitizers sold by the bottle and marketed as all-purpose disinfectants for your home, although pure hypochlorous acid isn’t really a cleaner—it’s not meant to get rid of grime and grease. Like all disinfectants, once hypochlorous acid is applied, it must be left to sit for a period of time. But unlike some germicides that require up to 10 minutes to kill harmful stuff, hypochlorous acid requires only one minute. You don’t have to wipe it up, either, but because it doesn’t dry quickly, I found it was easier to do so on hard surfaces such as counters.
A frustrating thing about the finicky nature of hypochlorous acid is that you can’t really decant it from its original bottle into a smaller one without potentially affecting its quality and longevity. When I needed hypochlorous acid that was suitable for air travel, I had to buy a two-ounce bottle of Magic Molecule, an FDA–cleared product launched in 2023. These bottles are conveniently sized but don’t last long, and not being able to refill them results in significant plastic waste.
A customer was sprayed with fogged hypochlorous acid solution at a pub entrance in Tokyo in May 2020 during the COVID pandemic. Fogging certain surfaces may be useful; fogging people probably isn’t.
Kyodo News via Getty Images
Other companies have taken a different approach to the shelf-life problem. Force of Nature, for example, sells countertop electrolysis machines for home use. The idea is that you can make as much disinfectant as you need for a week or two, as often as you want, using salt tablets you buy from the company. The process takes about eight minutes. Force of Nature also includes vinegar in its formulation, which gives the product cleansing abilities that the company recommends for use on hard surfaces or carpets. Other businesses sell devices that let you add your own salt. In online forums dedicated to fans of hypochlorous acid, members discuss how they use these devices. Some use pH test strips to make sure each batch of hypochlorous acid is within the correct range. Some people, however, are skeptical that at-home machines can consistently make pure HOCl.
Last December, troubled by Reddit posters’ descriptions of suffering with norovirus, I bought a range of products from Briotech, a company based in Washington State that has been around for years and has coordinated its research with the University of Washington. It sells “pure” hypochlorous acid, as well as some skin-care products. It claims its proprietary manufacturing processes extend the shelf life of its HOCl to up to two years, although the company recommends that you use its products within six months of opening the bottle.
Briotech sells different concentrations and formulations, including a “skin renew serum” at 0.018 percent concentration (or 180 parts per million) and a stronger gel for taking care of body piercings. Magic Molecule calls its hypochlorous acid an “antimicrobial skin cleanser” under the umbrella of “wound care,” marketing it as a treatment for acne, eczema, rashes, bug bites, and other concerns. It’s currently sold online and, as of this year, at the beauty-supply shop Ulta. When I went to Ulta to buy Magic Molecule, however, the person I asked for help had no idea what I was talking about. “What is this stuff?” she asked when she found the bottles, encased in slick, vague packaging. The label proclaims it “The Solution for All Skin Types” and encourages you to use it as often as needed. If I didn’t already know about hypochlorous acid, my skepticism radar would have been on highest alert.
Lots of things made by the body are manufactured and sold as serums, pills and powders. I might have dismissed hypochlorous acid as just another snake oil.
But it’s not just the beauty industry showing new interest in HOCl. Research into medical uses for hypochlorous acid has expanded as well. Before the pandemic, it was known that low levels of hypochlorous acid showed some promise in reducing the symptoms of viral and bacterial infections in nasal epithelial cells, but it was unclear how well people would tolerate HOCl administered straight up the nose as an irrigation or a spray. COVID led to some novel investigations.
At one hospital in Reading, Pa., for example, 74 COVID-positive patients, all of whom were unvaccinated, completed an experimental course of treatment that involved using a neti pot to rinse their nose with a hypochlorous acid solution for 10 days. Participants used Vashe Wound Solution, a hypochlorous acid that is safely used to treat wounds on skin or eyes and in the mouth. Although the author of the study acknowledged several limitations (for example, its small sample size and the lack of follow-up swabbing), it’s notable that none of the participants, who started the treatment within 72 hours of testing positive, required mechanical ventilation in the hospital. The reported adverse reactions were mild—a sensation of nasal burning, a nosebleed that stopped on its own—and the researcher suggested this application would be safe and effective, albeit one that requires more investigation.
Other studies have since shown hypochlorous acid to be effective in reducing symptoms in a range of upper respiratory infections—and, more important, that it does not cause adverse effects. In Europe, Sentinox, an over-the-counter nasal spray containing a low concentration of HOCl (0.005 percent), is already certified as a medical device to reduce the risk of infection from viruses and bacteria, including SARS-CoV-2, by lowering the microbial load in the nose. In a randomized, controlled trial published in 2022, researchers used Sentinox on people with COVID and reported good outcomes with no evidence of safety concerns. “These promising results,” the authors wrote, “support future larger-scale clinical studies in order to assess whether the [Sentinox] spray is also effective in the primary prevention of both symptomatic and asymptomatic SARS-CoV-2, influenza, RSV [respiratory syncytial virus], and other acute respiratory infections in the at-risk population.” More work is needed to explore whether HOCl might have the potential to stop the spread of viruses from person to person.
Over the winter, my relationship with hypochlorous acid was like the gag in the 2002 movie My Big Fat Greek Wedding where the grandfather sprays Windex on everything as if it’s a panacea. I sprayed down my phone case, sink faucets, toothbrush bristles and car steering wheel. I spritzed my face, hands and water bottle multiple times during workouts at the gym. After returning from the grocery store, I showered my bananas, limes, avocados and leafy herbs in the stuff. At a restaurant, I watched a server deliver my drink by holding the rim of my glass, so out came the bottle. I refrained from spraying my friend’s toddler as I anxiously tracked his germy behavior while he moved across a carpeted airport floor.
When my seatmate on a crowded train coughed the entire ride, I lifted my mask and sprayed HOCl directly up my nose; for good measure, I also soaked my eyes.
Here’s what happened: I got sick with type A influenza. My husband got it first, and I didn’t try to avoid the inevitable. Just after recovering from the flu, I picked up COVID at a large family gathering. Given the nature of airborne respiratory viruses, these events didn’t sour me on HOCl. I was diligent about spritzing myself and objects of potential exposure during travel, but it’s not like I was excusing myself from dinner conversations to take a huff of the stuff. Without constant hypervigilance and social isolation, your time comes when your times comes. I can’t live in a fog of aerosolized HOCl even if part of me wants to.
As of this writing, I have not been sickened by norovirus, and I’d like to believe my judicious use of HOCl has something to do with that. If more people were aware of this molecule, maybe they would swap their Purell bottles and Clorox bleach for a more effective, safer option. (One product called a “norovirus cleanup kit” contains hypochlorous acid.)
One thing I hope we’ve learned from the early days of the COVID pandemic is that stopping the spread of infectious illnesses requires a collective effort. Hypochlorous acid has been shown to work against avian influenza. If bird flu becomes the next pandemic, HOCl could be one potentially effective mode of virus control that’s easily available and cheap to access. Fogging machines could be used to clean surfaces and objects in medical settings, for example. (Fogging people with hypochlorous acid, which was done in Japan in 2020, for instance, is not known to be effective.)
But something about hypochlorous acid as a new product at a beauty store makes me uneasy. Although some of the products sold by recently established companies have been cleared by the FDA, many are not regulated. Notably, few of these products are specifically marketed as hand sanitizers, at least in the U.S. (A U.K. company does make a hypochlorous acid sanitizing hand gel approved by European regulatory agencies.) But if the efficacy of the product depends on its long-term stability, how much can you trust a bottle that’s lived in your car for six months?
So-called miracle products abound on the Internet. Lots of things made naturally by the body are manufactured, bottled and sold as serums, pills and powders. Hypochlorous acid sprays now show up in my social feeds, promoted by influencers gushing about their skin-rejuvenating properties. Some of the products I’ve seen use specialty salts (truly unnecessary) and charge significantly more for the purported luxury. If I hadn’t first encountered this disinfectant in academic literature, I might have scrolled right past these ads, dismissing hypochlorous acid as just another snake oil sold to exploit people’s fears.
Hypochlorous acid might go through rigorous regulatory channels if it’s pursued as an intranasal spray that prevents infection by killing viruses before they get into the lungs. Until then, I’ll be discreet any time I spray hypochlorous acid up my nose, not because I’m worried this off-label use is harming me—the biggest risk is that I’m irrigating with very expensive saline—but because I think back to President Donald Trump’s infamously cringey April 2020 press conference where he suggested possible COVID treatments. “And then I see the disinfectant where it knocks [SARS-CoV-2] out in a minute, one minute,” Trump said. “Is there a way we can do something like that by injection inside or almost a cleaning, because you see it gets in the lungs, and it does a tremendous number on the lungs.”
Trump was rightly skewered by experts (and many others) for promoting dangerous advice. It goes without saying that bleach, the disinfectant in question, should never be injected into your body. But behind Trump’s misinterpretation of whatever medical information had been shared with him prior to that press conference was the seed of an idea: What if a disinfectant could do a type of cleaning, as it were, knocking out virus particles in less than a minute? With norovirus still circulating and the possibility of a bird flu spillover, the potential uses of hypochlorous acid might be worth a closer look.
