Ok. So I’ve been wanting to write a post on sunscreen since before I even had a blog: for me it’s the most important part of skincare. It’s also one of the most difficult parts: there are so many aspects to factor in when using a sunscreen, and it’s hard to avoid all the sciencey theory bits.

WARNING: This post is going to be dense. Get some tea or something. If you would like to see this post full of cute things so you don’t get such a headache, click here.


Ok so most people know about SPF – basically the higher the better. SPF stands for Sun Protection Factor and it measures how long you can stay in the sun before you get burned. SPF 50 means it will take 50 times longer for you to burn than you would when wearing no sunscreen at all. Burning (and some forms of skin cancer) is caused by Ultra Violet B rays (UVB). However it’s not only UVB rays that you should worry about…

UVA rays, unlike UVB, don’t cause direct DNA damage. Instead they go deep into the skin and generate other chemicals, which in turn damage the DNA and can cause more deadly forms of skin cancer. UVA rays are also responsible for many signs of ageing because they damage collagen in the skin.

While SPF is pretty much universally used to indicate UVB protection, there are many different measurements of UVA protection – but relatively few sunscreens use any measurement of UVA protection at all. This is annoying because protection against UVA is so important especially as it is responsible for more forms of skin cancer than UVB. Also, UVA is a risk all year round – UVB rays change depending on cloud cover and time of the year, while UVA rays fluctuate with the time of year, they always pose a threat. Persistent Pigment Darkening (PPD) is my favourite measurement of UVA protection because it is nice and specific. A PPD of at least 10 is recommended for daily use. However as PPD is not used universally, this can be a difficult rule to follow. The Japanese system of UVA measurement is called Protection grade of uvA (PA) and its highest rating (PA+++) is only equivalent to a PPD of more than 8. This means I could buy two sunscreens, both SPF50 and PA+++ but be protected with a PPD of 9 by one, and 20 with the other. Not cool!

PPD vs. PA

Anyway then there is the Boots Star System used in the UK: this rating is connected to the ratio of UVA protection of a sunscreen to its UVA protection. The highest rating, 5 stars means there is a UVA/UVB ratio of more than 0.9:1 (ie. the sunscreen’s UVA absorption is the equivalent to at least 90% of the UVB absorption).

UVA/UVB Ratio 0.21-0.4 0.41-0.6 0.61-0.8 0.81-0.9 0.91+
Star Rating * ** *** **** *****

So these are the most popular UVA protection measurement systems, but there are others as well.

Physical and Chemical

So there are two types of protection in sunscreens. Physical protection is when the ingredients reflect the UV rays away from your skin, absorb them, and scatter them. Chemical protection is when the ingredients penetrate the skin and absorb the UV rays.


The information on the bottle-front rarely tells you all you need to know. First off the level of UVA protection is rarely advertised, secondly there could be issues of stability (we’ll get to that) that the bottle is not going to tell you about. So it’s best to look straight at the ingredients list.

There are numerous ingredients used in sunscreens to block/absorb UV rays, here’s a chart of the most common ones and their many names. On the right-hand side of the chart you can see the range each ingredient covers. An ideal sunscreen covers the entire spectrum of UVA and UVB light. Where possible I’ve listed how well the ingredients protect, but it’s difficult to find information on all of them. Click for a big size.

Ingredients - click for big

To make things difficult, ingredients and their allowed concentrations vary by country. To see a table of these differences, click here.

They also vary by brand. For example, both Mexoryl compounds are patented by L’Oreal, so you will only find them in L’Oreal-owned brands (but there are lots of those!).


Doesn’t that sound fun? So basically for some reason, some sunscreen chemicals start to degrade when they come in contact with UV light. It sounds weird that sunscreen ingredients break down when they come into contact with the thing that they are meant to be fighting. But they do. Ingredients which do this are called photounstable. As far as I’m aware there are a few ingredients which degrade in light, but many do so very minimally and so they’re still considered photostable.

The main time photostability is an issue is when using avobenzone. Avobenzone is really unstable so, even though when you first apply it you are getting great protection, its protection rapidly deteriorates over time. However it’s still a really popular ingredient in sunscreens because certain compounds can stabilise avobenzone – so you get great protection and no degradation, woo! Some stabilisers include: Octocrylene, Tinosorb S, Tinosorb M, and Mexoryl SX.

Full list of stabilisers:

  • 4-Methylbenzylidene camphor (USAN Enzacamene)
  • Octocrylene
  • Tinosorb S (USAN Bemotrizinol, INCI Bis-Ethylhexyloxyphenol Methoxyphenyl Triazine)
  • Tinosorb M (USAN Bisoctrizole, INCI Methylene Bis-Benzotriazolyl Tetramethylbutylphenol)
  • Butyloctyl Salicylate (Tradename HallBrite BHB – [1])
  • Hexadecyl Benzoate
  • Butyloctyl Benzoate
  • Mexoryl SX (USAN Ecamsule, INCI Terephthalylidene Dicamphor Sulfonic Acid)
  • Corapan TQ (INCI Diethylhexyl 2,6-Naphthalate)
  • Parsol SLX (INCI Polysilicone-15)
  • Oxynex ST (INCI Diethylhexyl Syringylidene Malonate)
  • Polycrylene (INCI Polyester-8)
  • SolaStay S1 (INCI Ethylhexyl Methoxycrylene
  • The thing is, as well as ingredients which stabilise avobenzone, there are also ingredients which destabilise it even more! Lame. So first of all octinoxate and avobenzone HATE each other (I’ve heard there was some kind of love-triangle feud). Octinoxate makes avobenzone seriously degrade. If they’re in the same product, usually the avobenzone will have been stabilised to the point that it’s not affected by the octinoxate. However if you’re using two sunscreens (or a make-up product with sunscreen in it), one with avobenzone and one with octinoxate, then as far as I know that’s not stable. The two products will act against each other and you won’t be getting the sun protection you think you’re getting.

    Avobenzone also has issues with titanium dioxide and zinc oxide (unless they’re in the same product and stabilised, as above). This clash can be bypassed by using ‘coated’ oxides in products, but it’s really difficult to tell if an ingredient is coated or not. The main time when this is an issue is with make-up being put on top of sunscreen. I’d say most foundations contain titanium dioxide – it provides the coverage of the foundation. Zinc oxide is a prominent ingredient in mineral make-up and in make-up with SPF.

    I haven’t been able to find a definite answer on how these products affect each other. Here are a couple of theories.

    1. Foundation does not usually contain a high amount of titanium dioxide (usually it’s near the bottom of the ingredients list) and so if you’re wearing a stabilised avobenzone sunscreen, then the negligible amount of titanium dioxide will not be a problem. This is not the same for foundations advertising SPF (using zinc oxide), or mineral foundations. Zinc oxide is an important ingredient in both these products and so there’s a significant amount of it in both of them. So, many say, this will will have an effect on the avobenzone.

    2. Zinc oxide and titanium dioxide in foundations are often ‘surface-treated’ for cosmetic purposes. The surface-treating has a shielding effect that reduces the interaction between these minerals and the avobenzone in sunscreen, so the avobenzone does not degrade.

    Both of these theories are reasonable but if you want to be absolutely certain you’re getting the protection you expect, you’re going to have to look for a new foundation or forego it altogether.

    Recommended Amount

    None of this matters at all if you’re not going to apply the right amount of sunscreen. Scientists estimate that not applying the correct amount of sunscreen reduces it’s effectiveness by at least the square root of the SPF (ie. SPF 50 reduced to about SPF 7).

    The problem is what on earth is the correct amount? I’ve heard measurements ranging up to a tablespoon (15ml) for just the face. That sounds ridiculous – and expensive – to me. In fact applying a tablespoon to the face would be totally pointless because at least half that amount would never absorb into the skin. It would provide sun protection in the same way a balaclava provides sun protection – physically concealing your entire face. If that’s what you want, then balaclavas are cheaper and actually look less ridiculous (who would have thought: a day when a balaclava was the stylish option…).

    Most people seem to agree it’s around 1 teaspoon (5ml) for the face and neck, or 1/2 or 1/3 of a teaspoon for just the face. I’d advise at least this much, and if you can wear more without looking weird then go for it. While wearing more sunscreen than recommended won’t give you more protection than that which is marked on the bottle, applying as much as you can gets you into a safe zone where you know you wearing the minimum at least.

    A lot of sunscreens make it difficult to apply the recommended amount. They can be too greasy, shiny, or white. As the sunscreen’s SPF and PPD goes up, usually its cosmetic elegance (how good it looks on your face) goes down. By all means buy a sunscreen with awesome protection, but if using the right amount makes you look like the tin man, then consider getting a lower SPF and PPD to strike a balance. It’s much better to be getting all the protection of SPF 30 than to be getting the square root of SPF 50.

    Consider this when using a powder sunscreen or a foundation with sunscreen in it as well. How much are you applying? Is it the right amount? I intend to write about this in more detail later, but if you are getting your sun protection from a make-up product, make sure you’re applying enough of it.

    Buying A Sunscreen

    So here are some tips for buying a sunscreen.

    • Check the ingredients. Most ingredients are online so you don’t have to spend hours memorising sunscreen compounds before you go to the store. Check which sunscreen filters the product is using to see if they are stable and how well they cover the whole spectrum.
    • Check other products you use: does your foundation have octinoxate in it? If so, make sure you choose a foundation that doesn’t have avobenzone in it.
    • Choose where you want to be on the protection vs. cosmetic elegance scale. Do you want super protection and are you ok with a bit of greasiness? Or would you rather something that looks nicer on skin and offers lower protection?
    • Check out sunscreens from different countries. The US does not offer great sunscreens, European sunscreens offer great protection but usually aren’t so cosmetically elegant, Japanese sunscreens look good but usually contain alcohol which can be extremely drying. Also, European sunscreens often seem to favour avobenzone as an ingredient whereas Japanese sunscreens don’t seem to use it much at all (they favour zinc oxide and octinoxate).
    • If you are going to wear sunscreen every day, buy yourself some vitamin D supplements. Using a sunscreen daily seriously compromises your levels of vitamin D, so make sure to take a supplement to stay balanced.
    • Before committing to wearing a sunscreen every day, please read up about controversy surrounding sunscreen ingredients – sorry I linked to wikipedia (the article is not particularly enthralling) but it has all the info in one place. It’s up to you to make an informed decision on if you want to wear sunscreen or not. For me, a pale redhead with a family history of skin cancer, I chose to wear sunscreen daily because the potential risks associated with sunscreen were less important to me than the certain risks of sun damage.
    • Remember that no sunscreen stops 100% of UV rays. If you want to go and meditate for 16 hours a day while trying to ‘find yourself’ in the Sahara desert, go ahead, but don’t expect to be totally sunvincinble (ahaha.).

    So I hope this post helps, I tried to make it as readable as possible but it’s difficult when it’s such a complicated subject. Likewise, I spent hours checking the facts but because research on sunscreen is relatively new, a lot of information is inconclusive. If you notice anything wrong in this post then please contact me.

    2 comments on “Sunscreen

    1. Thank you so much! I’ve been trying to figure out what all the UVA ratings mean for ages, it’s really nice to have them all together so I can start to understand what I’m wearing.

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