What is hypochlorous acid?

(It’s pretty much bleach.)

Hypochlorous acid is an ingredient that is growing in popularity in skincare. It’s marketed as a soothing ingredient and sometimes, often illegally, as a disinfectant, anti-inflammatory, acne treatment, or something that can “trigger an immune boost…repair and heal skin”.

But what is it? The chemical formula of hypochlorous acid is HClO. That means each molecule of hypochlorous acid is made up of one hydrogen atom (H) bonded to one chlorine atom (Cl) bonded to one oxygen atom (O).

In water, hypochlorous acid (HClO) can separate into a hydrogen cation, called a proton (H⁺), and a hypochlorite anion (ClO⁻). This is called dissociating. The reverse can happen too, a proton (H⁺) can join a hypochlorite anion (ClO⁻) and become hypochlorous acid (HClO).

The loss or gain of protons occurs simultaneously and reaches a balanced state called equilibrium – like a seesaw. In water at a pH of about 7.5, there will be about 50% hypochlorous acid (HClO) and 50% hypochlorite anion (ClO⁻). As the water becomes more acidic (lower pH), more hypochlorite anion becomes hypochlorous acid. As the water becomes more alkaline (higher pH), more hypochlorous acid (HClO) becomes hypochlorite anion (ClO⁻). Between pH 4.5 to 6.5¹ it exists mostly as hypochlorous acid.

Below a pH of about 5, poisonous chlorine gas begins to be released. Adding acids to these mixtures can be dangerous.

Both hypochlorous acid and hypochlorite anion are oxidizing agents. When they oxidize another molecule, they steal an electron from it. This oxidizing action can kill germs by denaturing or distorting the shape of the germ’s proteins. Hypochlorous acid is more microbicidal than the hypochlorite anion².

There are different ways hypochlorous acid is made. Saltwater electrolysis uses a solution of table salt (sodium chloride, NaCl). When dissolved in water (H₂O), sodium chloride (NaCl) dissociates into sodium cations (Na⁺) and chloride anions (Cl⁻)

When electricity is run through the solution, chlorine gas (Cl₂) is produced which reacts in the water and forms hypochlorous acid (HClO), which can dissociate to produce hypochlorite anion (ClO⁻). Sodium cations (Na⁺) can associate with the hypochlorite anion (ClO⁻) to give sodium hypochlorite (NaClO), but it stays dissociated when dissolved in water.

We have sodium (Na), chlorine (Cl), hydrogen (H), and oxygen (O) combined in different chemicals.

The equilibrium between hypochlorous acid (HClO) and hypochlorite anion (ClO⁻) in the water depends on the pH, as we learned.

Another way is much simpler, just add chlorine bleach also known as sodium hypochlorite (NaClO) to the water.

Sodium hypochlorite (NaClO) dissociates completely when dissolved into water. It forms a sodium cation (Na⁺) and a hypochlorite anion (ClO⁻).

Water is also constantly dissociating and associating, between H₂O and protons (H⁺) and hydroxides (OH⁻). Acidic water has lots of protons and alkaline water has lots of hydroxides.

A proton (H⁺) from water can associate with hypochlorite anion (ClO⁻) and form hypochlorous acid (HClO). A hydroxide (OH⁻) can steal a proton (H⁺) from hypochlorous acid (HClO) leaving hypochlorite anion (ClO⁻). The proportion of hypochlorite anions that become hypochlorous acid and vice versa depends on the pH of the water.

Regardless of the process: saltwater electrolysis, diluted chlorine bleach, or enzymatic action — we end up with an equilibrium of hypochlorous acid and hypochlorite anion in the water. Regardless of the source, the chemicals are the same.

Sodium hypochlorite (NaClO) and the hypochlorous acid (HClO) and hypochlorite anions (ClO⁻) it dissociates into have some uses in skincare.

Some dermatologists might recommend a patient with eczema to take a 5-to-10-minute bath in very dilute bleach (0.005%). It’s thought to help patients with eczema by reducing the levels of Staphylococcus aureus bacteria on the skin.

But the evidence is a bit contradictory, the recommended concentration of sodium hypochlorite in bleach baths was ineffective in reducing populations of Staphylococcus aureus and Staphylococcus epidermidis bacteria in lab tests³.

A systematic review comparing bleach baths and just water baths found that they both helped with eczema symptoms, with little difference between them.

There’s some experimental evidence that shows sodium hypochlorite solutions may be anti-inflammatory, but at higher concentrations than is often recommended for bleach baths, and usually in mice or cell studies.

Information from experiments about bleach baths is likely what was warped and inflated into the marketing of skincare sprays with hypochlorous acid.

Many brands also claim that their hypochlorous acid is at a special pH to prolong its stability or to increase its skincare benefits. The formulas are pH adjusted with either an acid like hydrochloric acid (HCl) or a base like sodium hydroxide (NaOH).

It’s uncommon to have other cosmetic ingredients in hypochlorous acid skincare formulas because they would be quickly oxidized. Each year a bottle of chlorine bleach is stored, its efficacy drops by about 20%.

We know that hypochlorous acid (HClO) can be changed into hypochlorite anion (ClO⁻) and vice versa. We know that the proportion of the two is dependent on the pH of the water. We know sodium hypochlorite (NaClO) dissociates into hypochlorite anion and hypochlorous acid when dissolved into water.

We also know that hypochlorous acid begins to release poisonous chlorine gas below a pH of about 5.

Hypochlorous acid skincare sprays don’t always make clear their concentration, so we often don’t know if they perform as an antibacterial or antimicrobial as claimed. Sanitizers, sterilants, and disinfectants, are also regulated by the FDA and EPA in the US.

The anti-inflammatory, skin healing, and skin soothing claims are considered drug claims in most countries and need to be supported with rigorous human evidence and regulatory approval.

If we knew the concentration, it’d be easy to make something similar with chlorine bleach and water. But don’t, releasing chlorine gas is an easy mistake to make.

Brands and their marketing might differentiate hypochlorous acid and diluted bleach in water, maybe because they can sell it at more than a 2,500,000% markup — but the chemistry tells the truth.

Think of it this way; whether you take 10 steps forward and 9 steps back, or 8 steps forward and 7 steps back…you end up in the same place. There’s multiple ways to make hypochlorous acid in water; dissolving chlorine gas into water, electrolyzing saltwater, or with the myeloperoxidase enzyme like our white blood cells — you end up with the same as dissolving chlorine bleach into water…some hypochlorous acid and some hypochlorite anion in water.

There’s a lot of disinformation from companies that sell hypochlorous acid and saltwater electrolyzing devices. Some even go so far to claim hypochlorous acid isn’t a chemical (it is). Creating hypochlorous acid through electrolysis has its uses, but mostly for industrial processes where adding chlorine bleach is cumbersome — for most of us at home it is a very expensive alternative. Being familiar with the chemistry and how sodium hypochlorite, hypochlorite anions, and hypochlorous acid are related can help you cut through the marketing muck.

If you understand why sodium hypochlorite and hypochlorite anions are essentially the same thing, you’ll also understand why that’s also true for sodium hyaluronate and hyaluronic acid! When you dissolve sodium hyaluronate into water, it dissociates into a sodium cation and a hyaluronate anion – a portion of which can become hyaluronic acid. The balance between hyaluronate anion and hyaluronic acid in the water depends on the pH.

Don’t mix acids or other cleaning chemicals into chlorine bleach, because you can make poisonous gases.

Further reading

Chemical names

  • Ions are a charged atom or molecule that has gained or lost electrons. Cations are positively charged ions. Anions are negatively charged ions.
  • Hypochlorous acid (HClO)
  • Hydrogen atom (H)
  • Hydroxide anion (OH⁻)
  • Oxygen atom (O)
  • Chlorine atom (Cl)
  • Proton (H⁺)
  • Hypochlorite anion (ClO⁻)
  • Sodium hypochlorite (NaClO)
  • Calcium hypochlorite (Ca(ClO)₂)
  • Sodium hydroxide (NaOH)

Thanks to Michelle Wong (Labmuffin Beauty Science) and James LaFortune for proof-reading.