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.

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.

"An Open Letter About Sunscreen Shaming" on KindofStephen.

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: Sunscreen application to the face persists beyond 2 hours in indoor workers" on KindofStephen.

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

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

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.

"Should you avoid sunscreens with Avobenzone?" on KindofStephen.

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

"Visualizing how a daily sunscreen can protect the skin from UV damage" on KindofStephen.

A group of researchers tested 32 commercial sunscreens for their ability to stain white and black 100% cotton.

Of the tested sunscreens; Alba Botanica Hawaiian SPF 50 Spray, L’Oreal Invisible Protect SPF 50, Solbar Thirty, and Aveeno Protect and Hydrate SPF 50 were among the most staining sunscreens.

The least staining sunscreens were; Cerave Baby, Solbar Zinc 38, Cerave Face SPF50, and Babyganics Mineral Based SPF 50

Using statistical analysis to group the sunscreens by sunscreen ingredients they created four distinct groups. Based on these groupings they tested 8 sunscreen ingredients; Avobenzone, Homosalate, Octinoxate, Octisalate, Octocrylene, Oxybenzone, Titanium Dioxide, and Zinc Oxide.

For white fabric; Avobenzone was a strong yellow stainer and so was Oxybenzone to a lesser extent. Titanium Dioxide and Zinc Oxide both left faint white stains.

For dark blue fabric; Avobenzone and Oxybenzone both left faint white staining, but Titanium Dioxide and Zinc Oxide left strong white stains.

The sunscreen ingredients were applied directly to the fabric, whereas in real-life it’s likely transferred to skin by friction and smearing throughout the day

A sunscreen that stains is by no means a reflection of its ability to protect your skin from UV. If reducing extrinsic photoaging is a goal, it’s important to use a sunscreen frequently. Often people are discouraged from using sunscreens because of the texture, scent, and in some cases staining of their clothes.

If staining is an issue I’ve had good luck with soaking it with 99% isopropyl alcohol and then a soak in sodium percarbonate (Oxiclean) or hydrogen peroxide.

Cornell also has a great stain guide for a myriad of stains.

Ginnetti M, Buhnerkempe M, Wilson M, The staining of clothing by
sunscreens: a pilot study, Journal of the American Academy of Dermatology (2018), doi: 10.1016/j.jaad.2018.02.022

"What’s causing sunscreen to stain clothing?" on KindofStephen.

I was recently sampled a bottle of the Anthelios Ultra-Fluid Lotion SPF 60 and loved the invisible finish as well as its Mexoryl SX and XL content. I ended up gifting it though, because its high price meant it would not be a product I’d likely to repurchase. I found myself rationing it and probably not using enough to get the protection on the label.

I wanted to see if there were other sunscreens in La Roche Posay’s Anthelios line that had a similar finish but was more affordable. Oddly though, the Canadian La Roche Posay website doesn’t list the ingredients for their sunscreens! So, I headed to my local Shopper’s Drug Mart and took some photos. I’ve transcribed the ingredients here for your reference as well

The Anthelios XL Melt-In Cream SPF 45 in 100 mL size is not on the Canadian La Roche Posay website, but was available in the Shopper Drug Mart when I visited. The photo I have here is old, the packaging has been updated to match the Anthelios XL Melt-In Cream SPF 60. I’m not sure if this means  it is being discontinued or not.

I’ll be posting a review of the products that I tried shortly, as I’m still in the process of testing one (The Anthelios Mineral Tinted Anti-Aging Primer SPF 50 for the curious!)

Mexoryl SX and XL are two patented sunscreens that are only used in the L’Oreal family of brands which includes La Roche Posay and Garnier Ombrelle. They are similar to Tinosorb S and M, but not the same. They tend to offer better UVA protection, as well as greater photostability, and less skin penetration. 

Anthelios Ultra-Fluid Lotion SPF 50 For Body, 125 mL

Active Ingredients

Homosalate 10%, Oxybenzone 6%, Octisalate 5%, Octocrylene 5%, Avobenzone 3%, Ecamsule (Mexoryl SX) 2%

Other Ingredients

Aqua, Cyclopentasiloxane, Alcohol Denat., Cyclohexasiloxane, Styrene/Acrylates Copolymer, Silica, Dicaprylyl Ether, PEG-30 Dipolyhydroxystearate, Dimethicone, Triethanolamine, Glycerin, Nylon-12, Polymethylsilsesquioxane, Caprylyl Glycol, Dicaprylyl Carbonate, Disodium EDTA, Disteardimonium Hectorite, Dodecene, Isostearyl Alcohol, Lauryl PEG/PPG-18/18 Methicone, PEG-8 Laurate, Phenoxyethanol, Poloxamer 407, Poly C10-30 Alkyl Acrylate, Tocopherol. (Code F.I.L.: C182364/1)

Anthelios Mineral Tinted Anti-Aging Primer SPF 50, 40 mL

Active Ingredients

Titanium Dioxide 25%

Non Medicinal Ingredients

Dimethicone, C12-15 Alkyl Benzoate, Dicaprylyl Ether, Dimethicone/Vinyl Dimethicone Crosspolymer, Talc, Triethylhexanoin, Isohexadecane, Styrene/Acrylates Copolymer, Hydrogenated Jojoba Oil, Aluminum Hydroxide, Stearic Acid, Aluminum Stearate, Alumina, Caprylic/Capric Triglyceride, Cassia Alata Leaf Extract, Diethylhexyl Syringylidenemalonate, Disodium Stearoyl Glutamate, CI 77491, CI 77492, CI 77499 / Iron Oxides, Laureth-4, Maltodextrin, PEG-8 Laurate, Polyhydroxystearic Acid, Silica Silylate, Tocopherol, Aqua. (Code F.I.L.: C179435/3)

Anthelios Dermo-Kids Lotion SPF 50, 150 mL

Active Ingredients

Titanium Dioxide 5.85%, Octisalate 5%, Drometrizole Trisiloxane (Mexoryl XL) 4.5%, Avobenzone 3%, Octocrylene 2.5%, Ecamsule (Mexoryl SX) 1.5%

Other

Aqua, C12-15 Alkyl Benzoate, Alcohol Denat., Caprylic/Capric Triglyceride, Isododecane, Propylene Glycol, Dimethicone, PEG-30 Dipolyhydroxystearate, Glycerin, Lauryl PEG/PPG-18/18 Methicone, Synthetic Wax, Ammonium Polyacryloyldimethyl Taurate, Caprylyl Glycol, Cellulose Gum, Dimethicone Crosspolymer, Dodecene, Glycine Soja Oil, Isostearyl Alcohol, Pentasodium Ethylenediamine Tetramethylene Phosphonate, Poloxamer 407, Silica, Tocopherol, Triethanolamine. (Code F.I.L.: C171811/1)

Anthelios Ultra-Fluid Lotion SPF 60, 50 mL

Active Ingredients

Homosalate 10%, Oxybenzone 6%, Octisalate 5%, Octocrylene 5%, Avobenzone 3%, Ecamsule (Mexoryl SX) 2%

Other

Aqua, Cyclopentasiloxane, Alcohol Denat., Cyclohexasiloxane, Styrene/Acrylates Copolymer, Silica, Dicaprylyl Ether, PEG-30 Dipolyhydroxystearate, Dimethicone, Triethanolamine, Glycerin, Nylon-12, Polymethylsilsesquioxane, Caprylyl Glycol, Dicaprylyl Carbonate, Disodium EDTA, Disteardimonium Hectorite, Dodecene, Isostearyl Alcohol, Lauryl PEG/PPG-18/18 Methicone, PEG-8 Laurate, Phenoxyethanol, Poloxamer 407, Poly C10-30 Alkyl Acrylate, Tocopherol. (Code F.I.L.: C182364/1)

Anthelios Targeted Protection Stick SPF 60, 9 g

Active Ingredients

Octocrylene 10%, Titanium Dioxide 6.25%, Avobenzone 3%, Drometrizole Trisiloxane (Mexoryl XL) 2%

Others

Ricinus Communis, Isopropyl Palmitate, Polyethylene, Isohexadecane, Ozokerite, Theobroma Cacao, Butyrospermum Parkii, Dimethicone, Glycine Soja, Tocopherol. (Code F.I.L. C24262/1C)

Anthelios XL Melt-In Cream SPF 60, 100 mL

Active Ingredients

Octocrylene 10%, Titanium Dioxide 4.15%, Avobenzone 3.5%, Drometrizole Trisiloxane (Mexoryl XL) 3%, Terephthalylidene Dicamphor Sulfonic Acid (Mexoryl SX) 3%

Others

Aqua, Propylene Glycol, Glycerin, Cyclopentasiloxane, Triethanolamine, Isopropyl Palmitate, Stearic Acid, VP/Eicosene Copolymer, Dimethicone, Acrylates/C10-30 Alkyl Acrylate Crosspolymer, Aluminum Hydroxide, Carbomer, Disodium EDTA, Glyceryl Stearate, Glycine Soja, Hydroxypropyl Methylcellulose, Methylparaben, PEG-100 Stearate, Phenoxyethanol, Propylparaben, Stearyl Alcohol, Tocopherol. (Code F.I.L.: C15709/2C)

Anthelios XL Melt-In Cream SPF 45, 100 mL

Active Ingredients

Octocrylene 10%, Avobenzone 3.5%, Titanium Dioxide 3.3%, Drometrizole Trisiloxane (Mexoryl XL) 3%, Terephthalylidene Dicamphor Sulfonic Acid (Mexoryl SX) 2%

Others

Aqua, Propylene Glycol, Cyclopentasiloxane, Glycerin, Isopropyl Palmitate, Triethanolamine, Stearic Acid, VP/Eicosene Copolymer, Dimethicone, PEG-100 Stearate, Glyceryl Stearate, Stearyl Alcohol, Phenoxyethanol, Aluminum Hydroxide, Acrylates/C10-30 Alkyl Acrylate Crosspolymer, Methylparaben, Carbomer, Hydroxypropyl Methylcellulose, Disodium EDTA, Glycine Soja, Tocopherol, Propylparaben. (Code F.IL.: K17514/3)

Anthelios Lightweight Lotion SPF 60, 100 mL

Active Ingredients

Homosalate 10%, Octocrylene 7%, Octisalate 5%, Avobenzone 4%, Drometrizole Trisiloxane (Mexoryl XL) 2.5%, Terephthalylidene Dicamphor Sulfonic Acid (Mexoryl SX) 0.5%

Others

Aqua, Glycerin, Alcohol Denat., Styrene/Acrylates Copolymer, Dimethicone, Propylene Glycol, PEG-100 Stearate, Glyceryl Stearate, Silica, Synthetic Wax, Phenoxyethanol, Sodium Polyacrylate, Triethanolamine, Stearic Acid, Caprylyl Glycol, Palmitic Acid, PEG-8 Laurate, Xanthan Gum, Tocopherol, Disodium EDTA. (Code F.I.L.: K158295/6)

Anthelios Lightweight Lotion SPF 30, 100 mL

Active Ingredients

Homosalate 10%, Octocrylene 5.5%, Octisalate 5%, Avobenzone 3%, Drometrizole Trisiloxane (Mexoryl XL) 2.5%, Terephthalylidene Dicamphor Sulfonic Acid (Mexoryl SX) 0.5%

Others

Aqua, Glycerin, Alcohol Denat., Styrene/Acrylates Copolymer, Dimethicone, Propylene Glycol, PEG-100 Stearate, Glyceryl Stearate, Silica, Synthetic Wax, Phenoxyethanol, Sodium Polyacrylate, Triethanolamine, Stearic Acid, Caprylyl Glycol, Palmitic Acid, PEG-8 Laurate, Xanthan Gum, Tocopherol, Disodium EDTA. (Code F.I.L.: K158303/4)

Anthelios Mineral Tinted Ultra-Fluid Lotion SPF 50, 50 mL

Active Ingredient

Titanium Dioxide 11%

Non Medicinal Ingredients

Aqua, Isododecane, C12-15 Alkyl Benzoate, Dimethicone, Undecane, Triethylhexanoin, Isohexadecane, Styrene/Acrylates Copolymer, Nylon-12, Caprylyl Methicone, Butyloctyl Salicylate, Phenethyl Benzoate, Silica, Tridecane, Dicaprylyl Carbonate, Dicaprylyl Ether, Talc, Dimethicone/PEG-10/15 Crosspolymer, Aluminum Stearate, Pentylene Glycol, Alumina, Aluminum Hydroxide, Benzoic Acid, C9-15 Fluoroalcohol Phosphate, Caprylyl Glycol, Cassia Alata Leaf Extract, Diethylhexyl Syringylidenemalonate, Disteardimonium Hectorite, CI 77491, CI 77492, CI 77499, Magnesium Sulfate, Maltodextrin, PEG-8 Laurate, PEG-9, PEG-9 Polydimethylsiloxyethyl Dimethicone, Phenoxyethanol, Polyhydroxystearic Acid, Propylene Carbonate, Propylene Glycol, Stearic Acid, Tocopherol. (Code F.I.L.: K50867/4)

Anthelios Mist SPF 50, 155 g

Active Ingredients

Homosalate 10%, Oxybenzone 6%, Octisalate 5%, Octocrylene 5%, Avobenzone 3%, Ecamsule (Mexoryl SX) 2%

Other

Butane, Aqua, Cyclopentasiloxane, Alcohol Denat., Cyclohexasiloxane, Styrene/Acrylates Copolymer, Silica, Dicaprylyl Ether, PEG-30 Dipolyhydroxystearate, Dimethicone, Caprylyl Glycol, Dicaprylyl Carbonate, Disodium EDTA, Disteardimonium Hectorite, Dodecene, Glycerin, Isostearyl Alcohol, Lauryl PEG/PPG-18/18 Methicone, Nylon-12, PEG-8 Laurate, Phenoxyethanol, Poloxamer 407, Poly C10-30 Alkyl Acrylate, Polymethylsilsesquioxane, Tocopherol, Triethanolamine. (Code F.I.L. C182096/1)

"Canadian La Roche Posay Anthelios Sunscreen Ingredients" on KindofStephen.

You’ll often find different rules and advice for using “physical” and “chemical” sunscreens. One dermatologist says that you need to apply less physical sunscreen compared to a chemical sunscreen. There’s also the belief that “physical” sunscreens provide protection instantly, don’t absorb into the skin, don’t degrade in the sun, and don’t need reapplication.

These are myths and are not backed by research or chemical knowledge. By following these rules (or myths) you’re not using your sunscreen to its greatest effect!

“Physical” vs. “Chemical”

Dividing sunscreens into “physical” and “chemical” isn’t the best way to do it. These two categories overlap completely. If we were to draw a Venn diagram of the two groups, it’d look like this

Chemicals are physical – they have a mass and take up space. On the other end, the “physical” sunscreens titanium dioxide and zinc oxide are chemicals, you can find the elements titanium and zinc on the periodic table.

It’s sometimes explained that titanium dioxide and zinc oxide are suspensions of particles, they don’t dissolve or form solutions like chemical sunscreens. This is true and their even distribution in the sunscreen formula and on the skin is very important – poor distribution can greatly reduce how much UV protection titanium dioxide or zinc oxide can provide on the skin.

However, there are caveats, sunscreens like Tinosorb M (INCI: Methylene Bis-Benzotriazolyl Tetramethylbutylphenol) also exist as particle suspensions – not solutions. Tinosorb M comes as a very fine suspension of particles in water. So, if you were to draw the line based on that you’d have to include Tinosorb M,  a “chemical” sunscreen with the “physical” sunscreens.

What does differentiate titanium dioxide and zinc oxide then? Well, they’re both metal oxides or metals combined with oxygen. Metal oxide sunscreen doesn’t have the same ring to it, but there is another way to describe them.

Inorganic vs. Organic

In marketing, organic is a label that describes how something is produced – often with a safe-list of chemical treatments and approved practices.

In chemistry, organic means the chemistry of compounds that contain carbon. Titanium dioxide and zinc oxide don’t contain carbon. They’re made up of metal and oxygen and classified as inorganic.

Marking the categories as organic and inorganic makes more sense because all of the sunscreen chemicals used contain carbon, except for titanium dioxide and zinc oxide.

Organic and inorganic is also a useful way to categorise sunscreens because the way that the carbon atoms are linked up in organic sunscreens is why they absorb UV energy. If you look at the chemical structure of an organic sunscreen like avobenzone you’ll see that they have single bonds alternated with double bonds.

This alternation or conjugation of the single and double bonds allows the molecule to absorb energy along the electromagnetic spectrum. The amount of conjugation determines which part of the electromagnetic spectrum they absorb, whether that be in the visible spectrum to produce a colour, or in the ultraviolet spectrum to protect our skin from UV.

Inorganic and organic neatly divide the two sunscreen types and are also descriptive. I know most companies won’t want to confuse their customers by labelling their 80% organic-certified sunscreen product with titanium dioxide as inorganic, but at least as sunscreen shoppers we can understand the difference!

Myths about using Inorganic vs Organic Sunscreens

“Inorganic sunscreen and organic sunscreens work differently”

Mostly Myth! It’s often said that inorganic sunscreens (titanium dioxide and zinc oxide) reflect UV off of the skin and organic sunscreens absorb UV and convert it into heat. In reality, for most of the UV spectrum they work very similarly.

Organic sunscreens absorb UV because of the way the bonds between their carbon molecules are arranged. The number of bonds between the carbon atoms in the sunscreen molecules and their conjugated arrangement give sunscreens their absorptive properties in the UV region of the electromagnetic spectrum. Remember that conjugated means alternating single and double bonds!

The energy from UV light promotes electrons in the conjugated carbon bonds of organic sunscreen molecules from a lower energy state to a higher energy excited state. The excited electrons in the bonds then relax or release the absorbed energy by stretching, vibrating, or bending – this turns that energy into heat.

In some cases, the organic sunscreen chemical can’t relax and release the absorbed energy by bending, stretching, or vibrating and the absorbed energy causes a change in its structure. This is what happens with avobenzone, it absorbs the UV energy and instead of relaxing, it changes its structure – and this new structure formed from avobenzone doesn’t absorb UV energy as well. As more and more avobenzone molecules’ structures change, the less UV energy is absorbed by the sunscreen formula. Some of the new structures formed from avobenzone are also more irritating and sensitising to the skin. Photo-stabilizers prevent this from happening by absorbing the energy from excited avobenzone and releasing it before its structure can change.

Inorganic sunscreens work very similarly – even though their structure is different from organic sunscreens. Metal oxides, like titanium dioxide and zinc oxide, have solid structures made of alternating sheets of metal and oxygen atoms. The principle behind the UV protection is exactly the same as organic sunscreens. Instead of the arrangement and amount of carbon bonds, the particle size of the titanium dioxide or zinc oxide determines which parts of the electromagnetic spectrum it absorbs.

There is a strong belief that these inorganic metal oxide sunscreens act by reflecting UV light instead of absorbing it, but this isn’t the complete story. UV light is divided into UVB and UVA. UVB is between 280 to 315 or 320 nm and UVA is between 315 or 320 to 400 nm. Inorganic sunscreens predominately absorb in the UVB spectrum and reflect in the long UVA (above 360 nm) and visible spectrum. Only about 5% of UVB light is reflected by inorganic sunscreens and the remainder gets absorbed and converted – just like organic sunscreens.

The results of a measurement show how much energy is reflected by different types and sizes of titanium dioxide. The horizontal scale represents the electromagnetic spectrum with my yellow highlight marking the UV spectrum. The vertical scale represents how much of the energy is being reflected, the higher up on the chart – the greater the amount of reflection.

Between 250 nm and 350 nm titanium dioxide reflects less than 10% of the energy. Between 350 nm and 400 nm there is more reflection depending on the form of titanium dioxide and the particle size. The anatase form of titanium dioxide exhibits more reflection than the rutile form of titanium dioxide. These forms have to do with the way the titanium and oxygen atoms are arranged in the titanium dioxide. Sunscreens often use rutile titanium dioxide because they are safer and less reactive.

The same is seen with zinc oxide, with most of the reflection being above 350 nm. The rest of the UV spectrum is absorbed.

The high reflection above the UV spectrum (above 400 nm) into the visible light region of the electromagnetic spectrum is what causes the whitening effect and flashback when using inorganic sunscreens.

“You can use less of an inorganic sunscreen compared to an organic sunscreen”

Myth! All sunscreens are tested at the same density, which is 2 milligrams of sunscreen per square centimetre. That applies to inorganic, organic, spray, stick, lotion, wipes, etc.

If you want to get as close as possible to the protection on the label of the sunscreen product, you need to apply it at the same density it was tested at.

“Inorganic sunscreens sit on the skin. Organic sunscreens absorb into the skin”

Myth! Think of it this way, if we want to protect ourselves from the rain we need to hold the umbrella above our heads. Sunscreens work the same way, you want them to absorb the energy before they can reach our skin cells, particularly the living cells. The most effective way for this to be done is to have them on the surface of the skin in a continuous and even layer.

Both organic and inorganic sunscreen particles can penetrate into the upper layers of the skin. If and how much they penetrate is dependent on properties like their particle or molecular size as well as the overall sunscreen formula. This isn’t a desired effect and formulators work to reduce the amount that penetrates. Modern organic sunscreens often have larger molecular sizes, chemical and physical properties, or even coatings which make it more difficult for them to penetrate past the surface of the skin.

Keep in mind that skin penetration doesn’t mean that it’s causing harm to our bodies. There has to be a biological mechanism for it cause an effect. There is a lot current and ongoing research into this area, but we don’t have any strong answers yet.

“Inorganic sunscreens provide protection right away. Organic sunscreens need to activate on the skin”

Myth! Organic sunscreens and inorganic sunscreens absorb UV due to their electronic properties. There’s no activation or chemical reaction that occurs on the skin with organic sunscreens to create photoprotection.

We know this is true because we can measure how much UV is absorbed by an organic sunscreen off of the skin, like on a piece of clear plastic. Organic sunscreens will also prevent UV colour changing bracelets, beads, or stickers from changing colour.

Both inorganic and organic sunscreens will provide UV protection as soon as they’re placed on the skin. The reason why a wait time is part of the application instructions is to allow the sunscreen formula time to dry and form a film on the skin. This makes it harder for it to be wiped off and it also means it can dry to as even of a film on the skin as possible.

The more evenly distributed the sunscreen is on the skin, the more even the coverage and the greater the average protection. If we take 10 umbrellas and hold them over one person, that one person may remain very dry during a downpour but everyone else will get soaked – if we distribute the umbrellas evenly more people will remain dry. Photoprotection works the same way, it’s measured as an average – you don’t want some areas of the skin with more sunscreen and greater coverage at the expense of other areas with less sunscreen and less coverage.

“Inorganic sunscreens don’t need to be reapplied”

Myth! All sunscreens should be reapplied if you want to maintain photoprotection throughout the day. While it’s true that titanium dioxide and zinc oxide don’t change structure under normal UV radiation, that’s true for many organic sunscreens and sunscreen formulas as well.

The reason why reapplication is recommended is because we often don’t apply enough in the first place and it’s constantly being removed from our skin. Reapplication helps ensure that we have a minimum density of 2 milligrammes per square centimetre of sunscreen on our skin and that we maintain that density throughout the day.

We may not be conscious of removing our sunscreen, but touching our skin, putting on and taking off clothing, using our phones, sweating, eating…all these things will remove some of the sunscreen from our skin. Think about how the coverage of a foundation or lipstick changes throughout the day.

There is no clear answer as to when you should reapply your sunscreen. We all do different things throughout the day in regards to our skin, so the amount of sunscreen removed from the skin will differ from person-to-person and day-to-day. That’s why it’s difficult to have a single rule that will apply to everyone. Conclusions from studies vary in their recommendations for when and how often to reapply.

What you choose to do is up to you, but you should take into account how much UV you’re exposed to, how much you expect to be exposed to, and your activities. You should think about reapplying your sunscreen before going for a jog outdoors. Work in an office? Maybe reapply before you leave the office. What’s clear though is that you should definitely reapply after sweating, swimming, bathing, and abrasion (like laying on sand) – even if you are using a water-resistant sunscreen.

In the UK many sunscreens are marketed as ‘once-a-day’, but health organisations recommend disregarding that and still reapplying throughout the day.

I hope this post has helped you understand why calling some sunscreens “physical” and others “chemical” isn’t as descriptive as it could be, as well as why inorganic and organic sunscreens should be used the same way. Sunscreen is an important part of a skincare routine, and there’s a lot of conflicting advice on how to best use it. Understanding some principles will help you make sense of what is good advice and poor advice when it comes to sunscreen.

I’d also like to thank my friend Jonathon Moir for his help in editing this article.

"“Physical” vs. “chemical” sunscreens and other sunscreen myths" on KindofStephen.

Sunscreens in a waxy and solid base are portable, easy to apply, and can offer good water-resistance. They’re a great way to protect the lips and the skin around the eyes, and are small enough to fit in your pocket.

They can keep sunscreen from running into your eyes!

Applied around your eyes stick or balm sunscreens can help prevent other sunscreens from migrating into your eyes and causing stinging or blurring.

Sticks or balms with higher melting points tend to be better at this task. Look for ones that are harder and don’t melt when you hold your finger to it.

No more stinging, squinting eyes!

Do I Need To Protect My Lips?

Yes! Just like the rest of your skin, your lips are susceptible to UV exposure. The lower lip especially is among the most exposed areas to UV on the face ¹.

Lips lack some of the natural photoprotection that the rest of the skin has. There’s less melanin, which acts as a natural sunscreen by converting UV energy into heat 2. Also sebaceous glands are absent. The sebum produced by these glands contain photoprotective antioxidants like Vitamin E ^3,4. The skin of your lips is also about a third thinner than the rest of your skin.

Like the rest of your skin, UV exposure increases your risk of developing skin cancer. Lip cancers represent 0.6% of skin cancer cases, and are most common in men over 50 ⁵. Thankfully, they have a very high cure rate (90-100%) ^6, 7. Oral melanoma is much more rare and vastly more lethal, and is also linked to UV exposure ⁸. If you do find new spots on your lips or inside your mouth – please get it checked!

Interestingly, one study found that minimum amount of UV energy to cause marked redness was 25% lower on the lips compared to back skin ⁹. The amount of skin reddening is used to determine SPF. What this could imply is that our lips have better protection against UV, from something other than skin thickness, antioxidants, and melanin content. However, only the upper lip was tested, the comparison of protected and unprotected skin was performed on the upper and lower lip, and the analysis was performed by different labs.

How much of the stick or balm do I need to apply?

SPF and UVA protection are tested at a global standard of 2 mg/cm².

This information isn’t too useful for the lips, their size varies greatly, and you can’t really measure the amount of product easily.

One would assume that sunscreen sticks are designed to supply the required 2 mg/cm², but unfortunately that is not the case.

A group of researchers studied the amount of sunscreen applied when using a stick or balm. They found that the median amount applied was only half of the required density ¹¹. 3 of the 28 participants applied close to the required amount, with 1 participant applying a whopping 2.5 mg/cm².

Based on this, if you want the amount of photoprotection labelled you’ll have to apply the sunscreen twice.

This applies to all types of sunscreen sticks or balms. How stiff or soft the sunscreen was didn’t affect how much was applied. Despite this, researchers recommended choosing a stiffer stick or balm. They suggest that softer and oilier products feel like they deposit more sunscreen, which isn’t true!

How often should I be re-applying?

Unfortunately there isn’t a convenient recommendation that’s based on a lot of strong scientific evidence, however at a minimum you should tryt to reapply the sunscreen after 15 minutes to 2 hours of cumulative UV exposure ¹².

Since sunscreen sticks or balms are easily wiped off, you should reapply after eating, drinking, wiping your mouth and anything else that may remove the product (like kissing!).

How to choose a sunscreen stick or balm

Look for a product with an SPF of 30 or greater. You want a product with full coverage of the UV spectrum, UVA and UVB. Most sunscreen stick or balms available in the US and Canada only provide strong UVB protection, this is especially true of products that are SPF 15.

In the US and Canada, look for products that have the “broad spectrum” labelling. While it’s only a relative assessment of the UVA protection, it’s the only information we have regarding the UVA protection. Canadians have access to products containing better UVA sunscreens like Tinosorb S and M, as well as Mexoryl SX and XL. La Roche Posay, Avene, and Vichy have these sunscreen chemicals in some of their formulations.

Other countries have different standards for UVA protection labelling. In the UK and Europe look for a high UVAPF, PPD, or the UVA circle logo. In Japan look for products with a PA rating of +++ or higher.

In terms of texture of the product, it doesn’t make a significant difference in how much is applied per swipe. However, products with a higher melting point (they’ll feel stiffer and don’t melt as easily when you touch them), may last longer on the skin as they’re less easily wiped off.

I personally switch between Bioderma’s Photerpès SPF 50+ with UVAPF 38, Avene’s Haute Protection SPF 30, and La Roche Posay’s Anthelios Targeted Protection Stick SPF 60.

Can you provide an overview of the study?

Assessment of thickness of photoprotective lipsticks and frequency of reapplication: results from a laboratory test and a field experiment

The two products used in the study were: Labello’s UV-Alpin SPF 30 Sun-Block and Garnier’s Delial Sun Stick SPF 16 (both discontinued).

The Labello product has a higher melting point than the Delial product, it feels firmer and less oily.

The study was performed in two parts:

The first experiment was performed in the laboratory. 25 students and 5 professors were asked to apply the sunscreen in front of a mirror, without instruction about how application relates to photoprotection. After the sunscreen was applied, the stick was weighed to see how much of the product was used. The mass of product used was divided by the surface area of their lips to calculate the density. The subjects’ lip area was calculated by having them kiss a piece of paper while wearing a colored lipstick. Each subject was asked to apply the sunscreen 10 times.

The second experiment was performed during a 6 day skiing trip with 18 students. For 3 days they applied one stick sunscreen, and for the remainder of the trip they applied the other stick sunscreen. The amount of times the sunscreen was applied was recorded. The difference in mass of the sunscreen sticks was divided by 3 (for each day it was applied) to get the average mass used per application. The average mass used per application was then divided by the surface area of their lips (again by having them kiss a piece of paper wearing lipstick).

In the lab, the median density of application was 0.98 mg/cm² for the Labello product and 0.86 mg/cm² for the Delial product. There wasn’t a statistically significant difference of density between the two products, so it seems the hardness of a product doesn’t make a large difference. Other factors like age, sex, skin type, or using lipstick didn’t affect the density of application either.

On the skiing trip, the median density of application was 1.58 mg/cm² for the Labello product and 1.76 mg/cm² for the Delial product. Participants applied the softer Delial product more frequently, but there still wasn’t a statistically significant difference of applied density between the two products. It’s important to note that the temperature was between -6C and 4C. The sunscreen sticks get harder when cold, which could make them transfer less product onto the lips per application. As well the colder, windy environment may have prompted participants to apply the product more frequently – not so much for photoprotection, but for protection from moisture loss.

"What’s the best way to use stick or balm sunscreens?" on KindofStephen.