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

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!

Shame, Ageism, and Sunscreen

Many of us just didn’t grow up with good sun protection education.

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.

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

Sunscreens and moisturizers with SPF are tested at a standardized density. That density is 2 milligrams of sunscreen per square centimeter of skin. If we want protection closer to what’s on the label, we should be using sunscreen or moisturizers with SPF at the density they’re tested at too.

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.

A photo showing 3 fingerlengths of a liquid, runny and milk appearing sunscreen. A photo showing those 3 fingerlengths of the milk appearing sunscreen collected in a syringe. It amounts to about 1.5 mL of sunscreen.

A photo showing 3 fingerlengths of a cream sunscreen. A photo showing those 3 fingerlengths of the cream sunscreen collected in a syringe. It amounts to about 3 mL of sunscreen.

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!

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

I often get emails like the following:

“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.

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

Recycling is one of the ways we can mitigate part of the impact we have on the environment. Reducing and reusing are the better options, but sometimes waste is unavoidable.

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:

For the list of Nordstrom stores:

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:


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:

For the list of Holt Renfrew stores:

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:

For the list of Maybelline drop-offs:


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

For more information:

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:


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:

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

To check if your country has a program visit or

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

Understanding Sun Protection Labels

SPF, PA, PPD, UVA, Stars…What does it all mean?

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.


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.

PRE 0.6 to 0.79PRE 0.8 to 0.89PRE 0.9 and over
POST 0.57 to 0.75* * ** * ** * *
POST 0.76 to 0.85* * ** * * ** * * *
POST 0.86 and over* * ** * * ** * * * *

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.