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 them⁴.

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 studies⁵.

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. Hypochlorous acid is unstable, it breaks down as it is exposed to light, air, and heat. It’s why EPA approved hypochlorous acid sanitizers recommend you use a “fresh” solution, some as soon as within 2 weeks. Hypochlorous acid sanitizers used in healthcare and sanitizers get around this stability issue by producing the hypochlorous acid on site – either with saltwater electrolysis, or by mixing sodium hypochlorite (NaClO) with an acid solution to release hypochlorous acid (HClO). Each year a bottle of chlorine bleach is stored, its efficacy drops by about 20%. When we dilute household bleach to use, its recommended to use it within 24 hours.

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 US⁶.

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.

So, is using hypochlorous acid as skincare harmful? At very low concentrations like 0.005%, almost definitely not — it might cause some irritation or dryness at worst. If you’ve swam in a chlorinated pool you’ve encountered a higher percent on your skin! A bottle of household chlorine bleach is between about 1% to 6% concentration for comparison. The concentration of hypochlorous acid in a stored bottle is also probably lower than indicated on the label, this is because of the hypochlorous acid breaking down over time as it encounters light, air, and heat. This might also mean a stored bottle of hypochlorous acid is no longer effective as a sanitizer.

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

• Mechanisms of actions of sodium hypochlorite in cleaning and disinfection processes
  
• Stability and bactericidal activity of chlorine solutions
  
• Efficacy and safety of sodium hypochlorite (bleach) baths in patients with moderate to severe atopic dermatitis in Malaysia
  
• Wikipedia: Chloralkali Process
  
• Wikipedia: Chemical Equilibrium
  
• Wikipedia: Myeloperoxidase
  
• Wikipedia: Dakin’s_solution

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.

"What is hypochlorous acid?" on KindofStephen.

While this experimental evidence is convincing, other experiments suggest stability. Two experiments⁵⋅⁷ with tretinoin encapsulated in microspheres (Retin-A Micro), found that about 95% of the tretinoin remained after 8 hours of mixing with benzoyl peroxide and exposure to non-UV light. When exposed to UV light, about 80% of the tretinoin remained after 6 hours⁷. An experiment⁸ with tretinoin that wasn’t encapsulated (Atralin Gel) also showed stability when mixed with 5% benzoyl peroxide. After 7 hours storage in an amber glass vial at 32ºC there was no degradation of the tretinoin.

Retin-A Micro’s encapsulation of tretinoin inside cross-polymer microspheres seems to have reduced the breakdown of it by benzoyl peroxide. This is most likely by physically separating the ingredients and reducing their ability to interact.

The second formula didn’t use encapsulation, but also had minimal-to-no degradation of the tretinoin. What stands out to me about the Atralin Gel is the butylated hydroxytoluene (BHT), an antioxidant commonly used in pharmaceuticals and food. Many other formulas of tretinoin have BHT, but it’s difficult to say whether they will also be stable when mixed with benzoyl peroxide. Only the Atralin Gel formula was tested.

These experiments show that the formulation of tretinoin largely determines how stable it is when mixed with benzoyl peroxide and exposed to light. There are differences between formulas available on the market, so a flat-out rule of “never mix” is likely false. This seems to be backed up by clinical reports of the combination of tretinoin and benzoyl peroxide being effective – as well as the experiments showing stability.

Twyneo is a new prescription that conveniently combines tretinoin encapsulated in silica core shells and benzoyl peroxide in the same formula. It has been shown to be stable⁹. Adapalene is an alternative retinoid that is stable against benzoyl peroxide. Differin and other brands of adapalene are available over the counter in the United States.

For people without access to these formulas, those with encapsulated tretinoin like Retin-A Micro and an antioxidant like in Atralin Gel may help put the combination of tretinoin and benzoyl peroxide back on the table.

Talk to your dermatologist or doctor!

Sources:

1. The Role of Benzoyl Peroxide in the New Treatment Paradigm for Acne
2. Retinoic acid cream (Airol cream) and benzoyl-peroxide in the treatment of acne vulgaris
3. Case-based experience with the simultaneous use of a fixed topical antibiotic/benzoyl peroxide combination and a topical retinoid in the optimization of acne management
4. The stability of tretinoin in tretinoin gel microsphere 0.1%
5. Chemical stability of adapalene and tretinoin when combined with benzoyl peroxide in presence and in absence of visible light and ultraviolet radiation
6. The effect of simulated solar UV irradiation on tretinoin in tretinoin gel microsphere 0.1% and tretinoin gel 0.025%
7. Absence of Degradation of Tretinoin When Benzoyl Peroxide is Combined with an Optimized Formulation of Tretinoin Gel (0.05%)
8. Twyneo® (Microencapsulated Benzoyl Peroxide 3%, Tretinoin 0.1%) Phase 3 Efficacy and Safety: Results From Two Randomized Controlled Clinical Trial

"Can you mix tretinoin and benzoyl peroxide?" on KindofStephen.

When examining the results of studies looking at the effect of visible light, like blue light, we need to be really focused on the context.

Studies that have shown a decrease in collagen, an increase in free radical production, or an increase in cell death…have been done on human skin cells in a petri dish.

Those results will probably not translate to our skin.

Our skin has more layers, including the epidermis. The epidermis contains a distribution of melanin. Melanin absorbs visible light like blue, green, and red light.

Almost none of the effects observed in human skin cells in a petri dish have been found in human skin.

The big exception is hyperpigmentation, which has been observed in people with deeper skin tones (Fitzpatrick phototype 3 and greater).

But there’s a big and bright caveat…

That caveat is irradiance. There’s a difference between a dimly lit light and a brightly lit one, but somehow many articles on the topic forget about this. I don’t think it’s much of a stretch to say that the sun is brighter than your phone.

One experiment that showed hyperpigmentation in people with deeper skin tones used a blue light dose of 99 joules per square centimeter of skin. That’s estimated to be about one-and-a-half to two-and-a-half hours of direct sunlight in the summer.

But the light from a screen? You’d need about 2000 hours to get that same exposure and that’s assuming you’re holding the screen close to your skin.

The brightest TV screens at a distance of 30 cm (about 12 inches) from your face are delivering about 1/200th (0.5%) of the blue light from the sun.

Irradiance also follows the inverse-square law, so doubling the distance from that bright TV screen to 60 cm means you’re getting 1/4th of the energy or about 1/800th (0.125%) of the blue light from the sun.

Unlike UV light, there’s also no standard for blue light protection like SPF. It’s difficult to compare products, ingredients, methods of protection, and their effectiveness.

Many of us are already using sunscreens, pigments, and antioxidants which may help.

For those with deeper skin tones who are concerned about hyperpigmentation, remember irradiance. Be sun-safe when you’re out on a bright summer day.

Watching a video or gaming?

You can just chill.

Sources and further reading:

DOI.org/10.1016/j.jdermsci.2018.04.018

DOI.org./10.1016/j.jdermsci.2018.11.011

"Does your skin need blue light protection from your devices?" on KindofStephen.

pH = -log₁₀[H₃O⁺ (aq)]

pH is equal to the negative logarithm (base 10) of the concentration of hydronium ions (H₃O⁺) in water.

Sometimes H3O⁺ is written as H⁺, in the case with pH – it’s the same thing.

H⁺ is called a hydron, a proton, a hydrogen cation, or a hydrogen ion.

In the case of pH, since we’re looking at aqueous solutions, we assume the H⁺ is bound to a water molecule (H₂O).

H₂O plus a H⁺ is H₃O⁺.

If we look at this part of the formula: [H₃O⁺ (aq)]

The (aq) tells us it is aqueous – it is dissolved in water.

The square brackets tells us it is a concentration – a type called molarity or molar concentration. Molarity or molar concentration is the number of moles of solute (the thing dissolved) per liter.

Moles can be confusing, but one way to think of it is like a dozen.

A dozen = 12

A mole = 6.022×10²³ (6.022 times 10 to the power of 23)

You can have a dozen of something or a mole of something. A dozen hydronium ions is 12. A mole of hydronium ions is 602200000000000000000000.

Let’s say we have a solution of sugar in water with a molarity of 1.

That means if we measured out 1 liter of that sugar solution, it would contain 1 mole (6.022×10²³) of sugar molecules.

What is the pH of a solution with a 1.0×10⁻¹² (1 times 10 to the negative power of 12) hydronium ion molarity?
Or, in other words, what is the pH of a solution with 0.000000000001 moles of hydronium ions per liter?

pH = -log₁₀[10⁻¹²] = (-1)(-12) = 12

What is the pH of a solution with a 10⁻³ hydronium ion molarity?

pH = -log₁₀[10⁻³] = (-1)(-3) = 3

What about a hydronium ion molarity of 10⁻¹⁵?

pH = -log₁₀[10⁻¹⁵] = (-1)(-15) = 15

That’s above 14!

What about a hydronium ion molarity of just 10?

pH = -log₁₀[10] = (-1)(1) = -1That’s below 0!

We’re not likely to bump into things with a pH higher than 14 or below 0, that’s why diagrams often end there. But they do exist!

A 37% concentration of hydrochloric acid has a pH that’s around -1.

A saturated solution of sodium hydroxide has a pH of about 15.In California at the Richmond Mine of the Iron Mountain the pH of the water has been measured to be as low as -3.6!

While this may seem pedantic, I think it’s important to understand what we’re discussing and educating others about – especially in the beauty science community.

For example, did you know the way acid and bases are often described in the beauty community is only one of the theories?

It’s called the Arrhenius theory of acid and bases, but there’s also the Brønsted-Lowry theory and the Lewis theory!

"pH isn’t a scale between 0 and 14: Many diagrams are just incomplete!" on KindofStephen.

The math model is:

1 – (1 ÷ SPF)

1 divided by the sunscreen’s SPF, subtracted from 1.

With an SPF 30:

1 – (1 ÷ 30) = 1 – (1/30 or 0.0333…) = 1 – 0.0333… = 0.9666…

The ellipses (…) means repeating, the 666 in the decimal number 0.9666 repeats forever.

For simplicity, we can round up 0.9666… to 0.97. We can then convert a decimal number to a percent by multiplying it by 100.

0.97 x 100 = 97%

What’s the basis of this math model? The SPF of our sunscreens are tested experimentally on real people. SPF is the ratio between the amount of UV the participants’ skin can be exposed to before sunburn with and without the sunscreen.

SPF can be affected by things that aren’t absorbing or reflecting UV – like antioxidants, anti-inflammatories, protection boosters, and an individual’s skin. We also know that not every wavelength of UV causes sunburn equally. The math model only accounts for the amount of UV the sunscreen passes through to the skin and the amount of UV it doesn’t.

That’s why these percentage protection numbers are a model, they’re a simplified representation. But models can be useful in understanding complicated things.

So let’s break down this model

1 – (1 ÷ SPF)

1 ÷ SPF represents the fraction of UV that the sunscreen lets through.

So in the model, an SPF 25 exposes the skin to 1 ÷ 25 or 1/25 or 0.04

To convert a decimal number into a percent we multiply by 100
0.04 x 100 gives us 4%.

If we want to know the fraction of UV that the sunscreen prevents from reaching the skin in this model, we subtract it from the total, which is 100%. 100% can be written as 1/1 or 1 or 25/25.

1 – (1 ÷ SPF)

With an SPF 25, we can write 1 as 25/25

1 – (1 ÷ 25) = 1 – (1/25) = 25/25 – (1/25) = 24/25 or 0.96

To convert a decimal number into a percent we multiply 0.96 by 100, which gives us 96%.

The model doesn’t account for how, or really what form of UV. Just the UV that causes sunburn – which SPF is a ratio of, and what is being allowed through and not let through.

1 ÷ SPF gives us the fraction of UV the sunscreen lets through.

1 – (1 ÷ SPF) gives us the fraction of UV that the sunscreen doesn’t let through.

The fraction of UV that is being let through and not being let through add up to all of the UV, 1 or 100%.

On the previous slides, we showed that an SPF 25 in the model lets through 4% and doesn’t let through 96% of the UV.

4% and 96% add up to 100%.

Let’s run through this for an SPF 60. Working it out with your calculator can make it easier to understand!

1 ÷ SPF gives us the fraction of UV the sunscreen lets through.

1 – (1 ÷ SPF) gives us the fraction of UV that the sunscreen doesn’t let through.

Since the SPF is 60, we can put that in

1 ÷ SPF gives us the fraction of UV the sunscreen lets through. We can write 1 ÷ 60

1 – (1 ÷ SPF) gives us the fraction of UV that the sunscreen doesn’t let through.

We can write 1 – (1 ÷ 60)

What fraction or percent of the UV does this model show an SPF 60 letting through and not letting through?

So the amount of UV that an SPF 60 lets through in this model is:

1 ÷ SPF, since SPF is 60, we write 1 ÷ 60

1 ÷ 60 can be written as 1/60. Enter that into a calculator and you get the decimal number, which is 0.01666… for simplicity, we can round that up to 0.0167. We multiply that by 100 to get a percent, 1.67%

The amount of UV that the SPF doesn’t let through is 1 – (1 ÷ SPF). We know 1 ÷ SPF is 1.67%, so 100% minus 1.67% gives us 98.33%

1 – (1 ÷ SPF) = 1 – (1/60) = 60/60 – (1/60) = 59/60 = 0.98333… = 98.333% rounded to 98.33%

We can check our work by seeing that 1.67% and 98.33% add up to 100%.

Sometimes the percentages don’t add up to exactly 100% – that’s usually because of how the decimal numbers were rounded.

The math here might look complicated, but it is just fractions.

If you know a quarter is 1/4 and can be written as 0.25 or 25%

That 4 quarters is equal to 1 and can be written as 4/4 or 100%

Then you can do this!

"Making sense of sunscreen protection percents. Where does “SPF 30 absorbs 97% of the UV” come from?" on KindofStephen.

I think a lot of us have forgotten that many of the bad effects caused by sun and UV exposure have only been recently well understood. While we’ve observed for a long time that sun exposure causes sunburn, the impact UVA has on skin’s appearance and photoageing are a relatively recent understanding and concern. Sunscreens marketed as an appearance maintaining essential are arguably modern.

The first widely used “sunscreen” was Red Vet Pet. Used by American soldiers during WW II, it was a by-product of oil refining with a strong red hue. In the later 1940s, pharmacist Benjamin Green would base his Coppertone product on it, but it was marketed to improve one’s ability to tan.

One of the first effective commercial sunscreens, Gletscher Crème, was introduced by Franz Greiter in 1946. Rudolf Schulze published the first method to measure sun protection in 1956. It’s estimated that Gletscher Crème only had a Schulze Factor of 2.

It wasn’t until 1974 that Schulze’s method would be adapted as the Sun Protection Factor and slowly start spreading around the world. In 1965, doctors J. Graham Smith and G. Rolland Finlayson presented their summary of the sun’s impact on skin, “The changes in human Caucasian skin commonly believed to be due to aging are primarily the effects of prolonged repeated damage to the skin from the sun”. There’s no discussion on the different effects caused by UVA and UVB.

One of the first standards to measure the UVA protection of a sunscreen was published in 1994 by Brian Diffey. It wasn’t until 2011 that the US FDA harmonized and set down rules as to what sunscreens could be labelled as “Broad Spectrum”.

While sunscreen use might reduce the risk of some skin cancers, it doesn’t reduce the risk of all of them. “Wear sunscreen to prevent skin cancer” messaging can be blunt and not inclusive. Dr. Adewole Adamson, a dermatologist, researcher and professor explains:

“In Blacks, melanoma usually develops in parts of the body that get less sun exposure, such as the palms of the hands and soles of the feet. These cancers are called ‘acral lentiginous melanomas,’ and sunscreen will do nothing to reduce the risk of these cancers…even among Whites, there is no relationship between sun exposure and the risk of acral lentiginous melanomas. Famously, Jamaican singer Bob Marley died of such a melanoma on his great toe, but sunscreen would not have helped.”

Sometimes we forget what it feels like to not know something – once we’ve learned it. A lot of the understanding of the sun’s effects and sunscreen protection labels are relatively modern. Not all of us had the opportunity to grow up in households or communities that were sun protection prescient. Not all of us knew the effects that prolonged sun exposure could have on our skin. Not all of us cared when we were younger.

To shame someone for not having consistently worn sunscreen throughout their life is to say that their skin – the interface of their body to the world – is irredeemable.

Would I prefer people to wear sunscreen more often? Yes, but you haven’t failed if you didn’t start wearing sunscreen when you were a child. Some people just don’t care about getting wrinkles or pigmentation. I think there needs to be space in the beauty community for them as well.

"Shame, Ageism, and Sunscreen" on KindofStephen.

Most of us don’t know the surface area of our skin and most of us don’t know the density of our sunscreens either. This has led to techniques and recommendations, like using 2 or 3 finger lengths of sunscreen, using 1/4 teaspoon of sunscreen, or applying our sunscreen twice. These techniques are all meant to encourage a more generous application of sunscreen, because when unprompted people tend to not apply enough.

With sunscreen, I think it is better to err on the side of applying too much rather than not enough. A higher density and thicker film of sunscreen generally means higher protection. It also makes sense to use more, because some of the sunscreen we apply will remain on our fingers, palms, or tools.

In some cases, I think these techniques might be leading people to use much more sunscreen than they might need. A sunscreen that might have been acceptable at a lower density might leave a strong cast, be too greasy, or pill (when a formula adheres to itself and rubs off the skin). This is especially relevant for people with deeper skin tones evaluating sunscreens for cast.

I’ve made a rough estimate of the surface area of my face, as well as the density (grams per millilitre) of two different sunscreens (IGTV: ‘How much sunscreen do you need?’). One is a cream and the other is a free-flowing milky texture. For both sunscreens, I need about 0.8 millilitres (mL) of sunscreen to get about the 2 milligrams of sunscreen per square centimeter of skin for my face. If I used a 1/4 teaspoon (1.23 mL), I’d be dispensing about half more than I need to protect my face.

With the cream sunscreen, applying 3 finger lengths of sunscreen dispensed about 3 mL of sunscreen. That’s almost 4 times more sunscreen than I might need. With the milk sunscreen, applying 3 finger lengths of sunscreen dispensed about 1.5 mL of sunscreen. That’s about 2 times more sunscreen than I might need. There’s also going to be differences in the thickness of our ‘lines’. That depends on things like how hard we squeeze, how slow we dispense, and the packaging.

If I apply the milky sunscreen with my palms and fingers the amount that ends up on my face might be close to the density I need. With 3 fingerlengths of the cream sunscreen, the finish is greasy and leaves a strong cast. The sunscreen is unusable for me with this much. But If I apply around 0.8 mL of the cream sunscreen, the finish is much nicer and I don’t notice a cast.

There’s no right or wrong method, they’re all just recommendations to encourage “proper” sunscreen use. Measuring your skin’s surface area, sunscreen density, and then each dose is going to be impractical for most people. But we might think about taking another look at sunscreens that may have been overused and left an unusable finish. They might not have at a density closer to the one it was tested at.

One of the ways we can encourage use of sunscreen is modeling realistic use. Many people can’t stand wearing heavier coats of sunscreen, and that’s OK. Some sun protection is better that none.

On the opposite end, brands need to take responsibility for their marketing, and show actual and proper use – especially since they’re the ones testing it at the right density. A smidge might look nice in an advertisment, but we all know it’s not enough and misleading. In the US, sunscreens are drugs not cosmetics – brands need to respect that and stop playing around.

Cheers to @FiddySnails for popularizing the 2 and 3 finger methods. They work for her because her 3 finger-lengths are not as chonky as mine. As well, when using 3 finger-lengths she uses a cushion puff to apply – which is absorbing some of the sunscreen.

If they made 1/6th teaspoons, I’d be golden. You don’t need to overthink this like I did! Our first application of sunscreen doesn’t need to be “perfect” if we reapply throughout a very UV-exposed day. All these techniques, besides precise measurements, are just estimates or rules of thumb – so some common sense and adjustments are sometimes needed!

"Sunscreen dosing. Teaspoons, shotglasses, and fingers…are we using too much?" on KindofStephen.

“I use X sunscreen, after 15 minutes, I use a foundation.

The ingredients of X sunscreen are Drometrizole Trisiloxane (Mexoryl XL) 7%, Bemotrizinol 5%, Octisalate 5%, Octocrylene 5%, Avobenzone 3%, Homosalate 2%, Ensulizole 0.5%

The makeup contains Octinoxate 6.0%, Titanium Dioxide 3.8%, Zinc Oxide 3.0%.

Will this destabilize the avobenzone or affect the SPF protection?”

Usually, my response is that it’s impossible to know just based on the ingredients alone.

We need to be able to measure the changes we’re interested in. In this case with an SPF test performed on humans and a photostability test.

This is the only answer that isn’t completely hypothetical.

That’s how I respond, and almost every time there is a follow up question:

“If I used this sunscreen without avobenzone, would that be better? Should I change the makeup I am using? Should I wait longer between applications? Would that be better?”

I think this anxiety is partly due to a growing amount of science-washing in the beauty community.

People, brands, and retailers sometimes describe skincare down to an unrealistic level of precision and accuracy. Using scientific terminology, biochemistry, and statistics in a way that almost becomes untruthful or irrelevant.

But what’s most important is the removal of context. Experiments often simplify reality to their most relevant parts, and their results shouldn’t necessarily be extrapolated.

Scientific literacy isn’t just about recognizing and understanding equations, keywords, or jargon.

It’s also the ability to recognize what is being discussed and what isn’t, how it fits into the larger context, and when to apply or not apply new information.

This is the same with medical literacy. Yes, we have access to more medical information than ever before, but we don’t necessarily have the experience, or critical skills to diagnose or treat ourselves.

To people who have anxiety about whether they’re getting the “most” out of their skincare products, be they sunscreen or otherwise, I think these two thoughts are important to keep in mind…

It is impossible to “optimize” or “maximize” something if you can’t measure it.

Some is better than none.

"Skincare Optimizing and Anxiety: Unrealistic expectations of perfection from imperfect information." on KindofStephen.

I’ve noticed a few people in my life prefer skincare and beauty products that either come in squeezable tubes or come in pump bottles. They’re often easier to dispense and use. Unfortunately, depending on where you live, your local home recycling may not accept these types of packaging – even if the materials themselves are recyclable. Many pumps contain a metal spring which means it is mixed material, in Toronto that means it goes into the garbage. Even if you take the extra step of removing the spring, the plastic pieces are too small to be accepted into Toronto’s recycling stream.

You should always check with your local recycling collection to see what types of material and products can or can’t be recycled from your home.

A recycling logo doesn’t necessarily mean you can put it in your recycling. Check on your city’s recycling website first. Don’t just wishfully recycle, that can cause clogs and contamination, creating even more waste.

For example in Toronto, New York, and Seattle stiff plastic bottles are recyclable, but plastic squeeze tubes are not accepted. However, Chicago will accept plastic squeeze tubes and all sunscreens — even sprays!

Don’t dismay, some stores will offer recycling for harder to recycle items like plastic bags and electronics.

TerraCycle has partnered with a few companies to provide a way to recycle most cosmetic packaging.

United States

In the US, all brands of beauty products and packaging can be dropped off at Deciem and Nordstrom stores.

For the list of Deciem stores:
https://bit.ly/3wnt20m

For the list of Nordstrom stores:
https://bit.ly/3gtvnQM

In the US, all brands of beauty products and packaging can be mailed in through a program sponsored by Garnier.

The minimum weight of each shipment must be 15 pounds.

To sign up for a shipping label:
https://bit.ly/3pQiSTu

Canada

In Canada, all brands of beauty products and packaging can be dropped off at Deciem and Holt Renfrew stores.

For the list of Deciem stores:
https://bit.ly/3glZVoA

For the list of Holt Renfrew stores:
https://bit.ly/3q0OpTb

United Kingdom

In the UK, all brands of beauty products and packaging can be dropped off at Deciem and Maybelline drop-offs.

For the list of Deciem stores:
https://bit.ly/2ToZAbR

For the list of Maybelline drop-offs:
https://bit.ly/3gxS8mv

Australia

In Australia, all brands of beauty products and packaging can be dropped off at David Jones locations.

For more information:
https://bit.ly/3iFmxSS

New Zealand

In New Zealand, all brands of beauty products and packaging can be mailed in through a program sponsored by Garnier.

To sign up for a shipping label:
https://bit.ly/2TBqvRJ

Brasil

In Brasil, all brands of beauty products and packaging can be dropped off at Natura and The Body Shop locations.

For the list of locations:
https://bit.ly/2TpdV8m

TerraCycle does offer other recycling partnerships in other countries that I haven’t listed here.

To check if your country has a program visit TerraCycle.com or https://bit.ly/3cGvRCa

Find your country, click through to ‘Free/National Recycling Programmes’ and browse through ‘All Recycling Programmes’.

"How to recycle cosmetics through TerraCycle in the US, Canada, the UK, Australia, New Zealand, and Brasil" on KindofStephen.

Sun Protection Factor (SPF)

The SPF number is a ratio of how much UV exposure your skin can be exposed to with the sunscreen on compared to without. The SPF test uses erythema or sunburn as a marker, which is mostly caused by UVB.

Hypothetically, an SPF 30 would mean you can be exposed to 30 times more sunburn causing UV compared to without the sunscreen on.

SPF 30 does not necessarily mean that you can stay out 30 times longer in the sun before sunburn. That is because the amount of UV coming from the sun is not always the same.

Broad Spectrum

Broad spectrum is a statement on how a sunscreen absorbs different wavelengths of UV light.

While the label requirements differ between regions, they all must meet a minimum critical wavelength of 370 nanometers.

That means when measured, at least 90% of the UV light the sunscreen absorbs is below the wavelength of 370 nanometers.

Broad spectrum is a signal that the sunscreen offers better UVA protection than a comparable sunscreen without the label.

UVA Circle

The UVA Circle symbol means that the sunscreen’s UVA protection is at least a third of the SPF.

If a sunscreen with the UVA Circle symbol has an SPF 30, you can expect a UVA 10 or higher.

The UVA Circle symbol does not tell you the exact UVA protection, just the minimum you might expect.

UVA Protection Factor

The UVA number (sometimes UVAPF number) is a description of how protective a sunscreen is against UVA radiation.

The UVA number is a ratio, like SPF. A UVA 30 means hypothetically you can be exposed to about 30 times more UVA with proper application of the sunscreen than without.

The UVA number can be determined in vitro (non-human testing) by measuring and comparing the amount of UVA light that passes through the sunscreen on textured plastic.

The UVA number can also be determined in vivo (human testing) with persistent pigment darkening or PPD.

Persistent Pigment Darkening (PPD)

The PPD number is a ratio of how much UV the skin can be exposed to before developing a long-lasting tan with the sunscreen on compared to without. Long lasting tans are mostly caused by UVA.

PPD is tested for on people with Fitzpatrick Skin Types 2 to 4 and mostly measures the effects of UVA.

Hypothetically, a PPD 10 would mean you could be exposed to 10 times the UVA with the sunscreen on, compared to without.

PPD results are sometimes labelled as UVA or UVAP. To find out, contact a brand’s customer service and ask which method they used to determine their UVA or UVAPF number.

PA

The PA system ranges from + to ++++ and is based on values from PPD. It was developed in Japan and used in countries like South Korea.

PA+ = PPD 2 to 4
PA++ = PPD 4 to 8
PA+++ = PPD 8 to 16
PA++++ = PPD 16 or greater

The PA system runs into issues when comparing sunscreens with higher PPD numbers. For example, a PPD 20 and PPD 25 sunscreen would both be labelled as PA++++.

Boots Star Rating

The Boots Star Rating is a UVA rating system used by the British retailer Boots.

It describes the ratio of the UVA to UVB absorption of a sunscreen and is tested in vitro (not on people.

The test compares the ratio before and after UV exposure, which accounts for the sunscreen’s photostability.

Getting what’s on the label

Most people don’t apply enough sunscreen when unprompted. To get close to the protection label on the sunscreen, the recommended amount is 2 milligrams per square centimeter of skin (2 mg / cm²).

There’s many different techniques to increase the amount of sunscreen you use. The one I prefer and do most often is to apply it twice, with an aim to reapply at least once during an outdoor day.

We don’t need to stress out too much about the exact amount we’re applying…more is generally better and reapplication can make up for a first application.

The best sunscreen is the one you enjoy using consistently, often, and liberally.

"Understanding Sun Protection Labels" on KindofStephen.