Tag: sunscreen

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

KindofStephen

You may have heard that: “SPF 30 absorbs 97% of the UV” or “SPF 50 absorbs 98% of the UV”. These numbers are from a math model and it’s quite simple!

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!

An Open Letter About Sunscreen Shaming

KindofStephen

I think a lot of us have forgotten that the bad effects caused by sun 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 photoaging are a relatively recent understanding and concern.

Sunscreens marketed as an appearance maintaining essential is arguably modern.

The first widely used “sunscreen” was Red Vet Pet. Used by American soldiers during WWII, 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 sunscreen was published in 1994 by Brian Diffey. And it wasn’t until 2011 that the US FDA harmonized and set down rules as to what sunscreens could be labelled as “Broad Spectrum”.

Japan’s cosmetic industry would adopt the UVA protection test, persistent pigment darkening, in 1996.

The European Cosmetics Trade Association (COLIPA) wouldn’t publish their standard for testing sunscreen for UVA protection until 2009.

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 is often 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.

And some people just don’t care about getting wrinkles or pigmentation.

There needs to be space in the beauty community for them as well.

Paperview: Sunscreen application to the face persists beyond 2 hours in indoor workers

KindofStephen

What happens to your sunscreen throughout a work day? I often get this question, especially from people who work inside for most of the day. A group of researchers at Mahidol University in Thailand did an experiment that may provide us with some guidance.

The researchers took 20 people (15 women) with mostly skin phototype III and up. Skin phototype III means that they tan, but sometimes get mild burns.

The participants were asked to apply 1 gram of sunscreen to their face. The sunscreen was mixed with a blue fluorescent dye that would glow under UV light. This glow allowed the researchers to see the sunscreen on the skin and note changes in its brightness throughout the day.

The people only wore the sunscreen and were asked not to reapply. They weren’t allowed to use makeup or other skincare. They were also allowed up to 1 hour outside. The temperature outside was between 23 and 35 degrees Celsius throughout the day and described as humid. The indoor condition was inside the air conditioned Siriraj Hospital.

Every 2 hours, the researchers took a photo of the people and measured the glow of the sunscreen under a UV light. They looked at the cheeks, forehead, nose, moustache area, and the chin. They used a Visia device to help make sure the photos were consistent.

Sunscreen brightness reduction every 2 hours by percent. The bars indicate the range in measurement values.

The researchers found that the fluorescent glow on the people’s faces decreased the most in the first 2 hours after applying the sunscreen. On average the areas of the face were 16.3% less bright.

Between 2 hours and 4 hours after application, the brightness decreased by a further 7.4 percentage points on average. Between 4 hours and 8 hours, there was an average 4.5 percentage point decrease in brightness.

At the end of the day, there was about a 30% decrease in brightness on average compared to just after applying the sunscreen.

A 30% decrease in brightness in this experiment doesn’t necessarily mean a 30% decrease in sunscreen on the skin. There are ways to model this more accurately, but they did not have the tools in this experiment.

So what does this mean for you? At the end of the day, you’ll still have to use your best judgement.

If you pigment easily, are very concerned about photoaging, or have a family history of skin cancer – I think the best recommendation is to be on the safe side and reapply at least once around 2 hours.

If don’t care that much, consider the opposite, about 70% of the glow from the sunscreen still remained after 8 hours.

In either case, that first application is important. I’d recommend choosing a sunscreen with a high SPF and UVA protection and aiming for a 2 mg/cm² layer. An easy way to make it more likely you’ve applied that amount is to apply your sunscreen in two layers.

Source: Rungananchai, C., Silpa-archa, N., Wongpraparut, C., Suiwongsa, B., Sangveraphunsiri, V., & Manuskiatti, W. (2018). Sunscreen Application to the Face Persists Beyond 2 Hours in Indoor Workers: An Open Label Trial. Journal of Dermatological Treatment, 1–14. doi: 10.1080/09546634.2018.1530440

Paperview: Evaluation of the protection of a broad-spectrum SPF50+ sunscreen against DNA damage

KindofStephen

Cyclobutane pyrimidine dimers (CPDs) are a form of DNA damage that is caused by UV exposure. CPDs interfere with base pairing during DNA replication – which can lead to mutations and cancer.

UVB radiation is directly absorbed by DNA. The energy causes changes in the bonding of pyrimidine structures found in DNA leading to CPDs and pyrimidine-pyrimidone (6-4) photoproducts.

UVA on the other hand is poorly absorbed by DNA, but was also found to cause CPD formation in human skin. CPDs were found to remain longer in the skin when there was UVA exposure, leading to speculation that UVA may also suppress a repair mechanism.

Our cells do have DNA repair capabilities, where damaged DNA is excised and replaced – but these processes can be overwhelmed by an accumulation of damage.

Experiments have measured the amount of CPD formation in human skin when exposed to UVB. One study found that CPDs were formed even when there was no visible sunburn (0.5 sunburn dose). They also found CPDs in both the epidermis and dermis and these levels were elevated for about 10 days as the skin sloughed off.

These two images from the paper show (A) skin that was not exposed to UVB and (B) skin that was exposed to UVB. The brown staining of the cells indicates presence of CPDs.

The amount of CPDs found in both the epidermis and dermis increased as UVB exposure increased.

A recent experiment performed by Pierre Fabre (manufacturers of Avène) looked at the effect sunscreen had on the  formation of CPDs in human skin after UV exposure.

14 volunteers applied a sunscreen to their forearm and were exposed to UVB and UVA on skin protected by the sunscreen and also on unprotected skin. The area covered in sunscreen received 15 times the dose of UV to cause sunburn, whereas the unprotected skin received 2 times the dose.

After this exposure, their skin was blistered by vacuum and the contents of the blister were examined for CPDs using two different methods: immunostaining and spectrometry (HPLC-MS).

They found that the unprotected skin after exposure to UV had an elevated ratio of CPDs to normal DNA bases (90 CPD to 106 DNA bases). In comparison, the skin protected with the sunscreen had an amount of CPDs similar to unexposed skin and statistically significantly less than the unprotected skin (P < 0.001) – even though the area received more UV exposure. The CPD to normal DNA base ratio was not reported for the sunscreen protected and unexposed skin.

The sunscreen was not named, but it is SPF 50+, broad spectrum, and contained; Tinosorb M and S, Iscotrizinol, Avobenzone, and the antioxidant bis-ethylhexyl-hydroxydimethoxy benzylmalonate.

Preventing the formation of CPDs from reducing UV exposure is the most well-researched option, but there are other newer methods that are emerging – some of which are already available on the market.

Photolyase is a DNA repair enzyme that can be activated by the absorption of a photon and transfer an electron to the CPD, this can separate the CPD back into two normal pyrimidine bases – with the right timing. In humans, the photolyase enzyme no longer works, but there is some evidence that topical application of photolyase may reduce the formation of CPDs. An experiment where photolyase encapsulated in liposomes combined with light exposure was applied to human skin reduced the formation of CPDs by 40%-45% after exposure to UVB.

You can watch a lecture given by Aziz Sanzar about photolyase and DNA repair below. He won the Nobel Prize in Chemistry in 2015 for his work along with his colleagues Tomas Lindahl and Paul Modrich.

S.K. Katiyar, M.S. Matsui, H. Mukhtar, Kinetics of UV light–induced cyclobutane pyrimidine dimers in human skin in vivo: An immunohistochemical analysis of both epidermis and dermis, Photochemistry and Photobiology (2002), DOI: 10.1562/0031-8655(2000)0720788KOULIC2.0.CO2
J. Gwendal, T. Douki, J. Le Digabel, et al, Evaluation of the protection of a broad-spectrum SPF50+ sunscreen against DNA damage, Journal of the American Academy of Dermatology (2018), DOI: 10.1016/j.jaad.2018.05.570

Should you avoid sunscreens with Avobenzone?

KindofStephen

I’ve received quite a few questions about the organic sunscreen chemical Avobenzone over the years and I wanted to shed some light on one of the most common concerns – its photodegradation in UV. These concerns are usually raised by websites that say things like, “Avobenzone degrades in the sun, resulting in the release of free radicals that may actually increase the risk for cancer.”

What these quotes often leave out is the context, which is important in understanding why Avobenzone is so commonly used in sunscreens and why it is effective.

Avobenzone or butyl methoxydibenzoyl methane is an organic sunscreen that absorbs in the UVA region and has global approval. Among the sunscreen chemicals available in the US it is the strongest and most effective UVA absorber. Avobenzone exists in two chemical forms when in solution, the enol form and the diketo (or keto) form.

When exposed to UV light some Avobenzone in the enol form can be changed into the keto form – however this is slowly reversed once Avobenzone is removed from UV light.

In its keto form Avobenzone is susceptible to photodegradation from UV light. The energy from UV light causes structural changes in the Avobenzone that can lead to breakdown products. In many cases, those breakdown products no longer effectively absorb UVA and UVB (some of them will absorb UVC). Some of these breakdown products are also thought to be irritants. The other concern is that some singlet oxygen can also be formed – a reactive oxygen species which can damage DNA and cells.

The above only relates to Avobenzone on its own though, the material that Avobenzone is dissolved into and other chemicals in the formula can change how easily Avobenzone photodegrades. Other modifications like encapsulating Avobenzone have also been tested, though the benefit is often reduced contact between Avobenzone and the skin – not photostability.

Photostabilizers generally work by absorbing energy from the Avobenzone before it becomes unstable and breaks and down. Effective photostabilizers will then be able to take this energy and dissipate it in safer forms, most often heat.

A company that produces Avobenzone, DSM Nutritional Products, performed a study testing different photostabilizers and their effect on Avobenzone’s phostability. The most commonly used and known photostabilizer of Avobenzone is the organic sunscreen chemical Octocrylene, but there are other photostabilizers that don’t act as sunscreens such as Polyester-8 and Polysilicone-15.

To perform the test, 4% Avobenzone and different photostabilizers were dissolved into a mixture of 70% ethanol, 15% caprylic/capric triglyceride, and 15% C12-15 alkyl benzoate. The solutions were placed on glass slides at a density of 2 mg/cm2 then exposed to 25 MED (Minimal Erythemal Dose, 1 MED defined by the US FDA as 200 Joules/Meter2) units of UV light. After exposure, the amount of Avobenzone remaining was determined.

What the researchers found was that the combination of 4% Avobenzone and 3-5% Octocrylene maintained 90% of the Avobenzone after 25 MEDs of UV light. Based on this, they tested different combinations of Octocrylene and other photostabilizers to see how well they stabilized Avobenzone.

They found that 3.6% Octocrylene with 4% Bis Ethylhexyloxyphenyl Methoxyphenol Triazine or 4% 4-Methylbenzylidene Camphor were able to completely stabilize the Avobenzone after 25 MED of UV.

There’s currently no global standard on photostability, different regions have their own standards. In the US as part of the Broad Spectrum test, sunscreens are pre-irradiated with 4 MED before testing.

Just like how some chemicals can increase the photostability of Avobenzone, others like Octinoxate (Octyl Methoxycinnamate) are known to speed up the photodegradation of Avobenzone – but this effect can be mitigated with formulations.. This paper is often misquoted to include Oxybenzone (2-Hydroxy-4-Methoxybenzophenone), often mischaracterized as not photostable, as a chemical that increases the photodegradation of Avobenzone, but it was included as an internal standard to allow comparison between samples – as it did not photodegrade in the experiment.

What matters when it comes to the protection offered by a sunscreen are the values and ratings determined from standardized tests like SPF, PPD, Broad Spectrum, etc and not the appearance of an ingredient on the INCI.

Basing assumptions on INCI is dangerous, as the only way to truly know is to test the products. An experiment on 6 different commercial sunscreens on their photostability highlights this. 4/6 of the organic sunscreens tested exhibited a decrease in photoprotection after UV exposure. Of the two photostable organic sunscreens one contained a combination of Avobenzone and 4-Methylbenzylidene Camphor and the other Octocrylene, Avobenzone, Mexoryl SX, and Titanium Dioxide. The one inorganic sunscreen tested was shown to be photostable after UV exposure.

Keep in mind, this study tested commercial sunscreens available in 2006, where photostability was a relatively newer concern for sunscreens and standards had not yet been defined. It was around this time that Neutrogena began marketing its Helioplex patent, a photostable combination of Avobenzone, Diethylhexyl 2,6-Naphthalate, and Oxybenzone. The Helioplex US patent was granted in 2002 and other patents for increasing photostability of Avobenzone are present as early as 1999, when the US FDA finalized the use of Avobenzone in sunscreens.

While we still do not have a global standard for photostability, the options for and knowledge to stabilize sunscreens has grown considerably. It also still very important to reapply your sunscreen throughout UV exposure, this compensates for any protection lost through photodegradation as well as physical changes in the film of sunscreen on the skin.

Source: C. Mendrok-Edinger, K. Smith, A Janssen, J. Vollhardt. The Quest for Avobenzone Stabilizers and Sunscreen Photostability, Cosmetics and Toiletries, http://www.cosmeticsandtoiletries.com/formulating/category/suncare/premium-the-quest-for-avobenzone-stabilizers-and-sunscreen-photostability-214405251.html

Edited on March 3rd, 2022 to include new research regarding the use of avobenzone and octinoxate together.

Visualizing how a daily sunscreen can protect the skin from UV damage

KindofStephen

Optical coherence tomography and reflectance confocal microscopy can be used to non-invasively to visualize deep into the skin. Using these techniques we can actually see changes in the structure of the skin and its cells.

This group of researchers with funding from La Roche Posay used the imaging techniques to compare the effect of UVB exposure on skin protected with a high SPF and UVAPF sunscreen and skin that wasn’t protected.

What they found was that doses of UVB that caused long-lasting erythema (redness) caused morphological changes in the skin. Changes observed were spongiosis (abnormal accumulation of fluid), microvesicles, sunburn cells, and blood vessel dilation. None of these were observed in skin that was protected by the sunscreen.

A minimal erythemal dose or MED is the amount of UV energy that causes long-lasting redness in the skin. Just 1 MED was enough to cause morphological changes and 2 caused significantly more. This also relates to SPF. An SPF of 2 would provide enough protection to protect an average population against 2 MEDs.

If reducing your risk of developing skin cancers and preventing photoaging are a goal of yours – this is a great reminder and justification to wear your sunscreen daily!

Antonio Gomes-Neto, Paula Aguilera, Leonor Prieto, Sophie Seité, Dominique Moyal, Cristina Carrera, Josep Malvehy, Susana Puig, Efficacy of a Daily Protective Moisturizer with High UVB and UVA Photoprotection in Decreasing Ultraviolet Damage: Evaluation by Reflectance Confocal Microscopy, Acta Dermato-Venereologica (2018), DOI: 10.2340/00015555-2736