SPF 101


As a skincare obsessive convincing the world that they need to wear SPF is a life mission, and a mountain I am prepared to die on. Perhaps it’s been over-complicated, perhaps it’s because the market is saturated with so many products people just don’t know how or what to choose, but whenever I get into conversations about SPF people are confused, have been given dodgy pseudoscientific advice, or have just given up all together. My answer is always ‘please just wear a high factor dedicated SPF everyday’, but if I were told that myself I wouldn’t take my word for it. For me, what makes me change my routine or habits is truly understanding what am I doing and why I should be doing it. With that in mind, settle down for a comprehensive class on SPF.

What exactly am I protecting myself from?

By wearing sunscreen you are protecting yourself from ultraviolet radiation. UV radiation is part of the light spectrum that reaches the earth from the sun. The spectrum contains three wavelengths that are shorter than visible light, making them invisible to the naked eye, UVA, UVB, and UVC. UVC is absorbed by the ozone layer so we don’t need to worry about that for now.


UVB has the shortest wavelength of the three and so can only penetrate the superficial layers of the skin. Because of this, it causes visible damage: redness and burning. Up to 95% of UV exposure is UVA. UVA has the longest wavelength of the UVs and so can penetrate deeper into the skin, which is why it’s the type of UV associated with tanning and photoaging (aka: wrinkling). To make it easy to remember: UVAging, UVBurning.


Your skin colour and it’s sensitivity to the sun will affect how easily you burn, but remember that burning is not the only type of sun damage and anyone, of any skin colour, can develop skin cancer and related damage caused by UV rays. You are doing yourself a favour by protecting yourself from it.


What is SPF?

SPF stands for Sun Protection Factor. The factor is it’s ability to deflect UVB(urning) rays. This protection factor is calculated by dividing the shortest amount of exposure to UV radiation with protection that produces reddening of the skin, by the shortest amount of exposure to UV radiation without protection that produces reddening of the skin.


So, that means if you wear SPF15 you can stay in the sun 15 times longer than when you’re wearing no SPF right? In theory, yes. In practice, it’s more complicated than that. The numbers used to calculate the factor in a lab are all static, but in reality the environment you’re in is constantly changing. The sun gives off varying amounts of UVB at any one time, and sunscreen absorbs and degrades at different rates depending on how it was applied, whether you’ve been in water, how much you’re sweating, and so on. Meaning SPF15 = 15 times longer sun exposure is probably significantly overestimating the level of protection.


Another popular way to look at SPF is via the amount of UVB rays blocked from hitting the skin, but again, it’s a bit more complicated than that. Your skin has a natural correction and protection enzyme system (remember enzymes? Those catalysts from high school science?) that helps to repair UVB damage as it happens, but it can only do this at a certain rate. It’s easily overwhelmed by a sudden influx of UVB damage which causes sunburn. What’s important, and what’s causing the damage, isn’t how much UVB is blocked out but how much UVB is actually coming into contact with the skin. The less UVB damage happening at any one time, the greater the reparative enzyme effect on the damage that’s occurred.


It’s important to remember that a higher SPF number does not mean a higher level of protection. SPF100 is not double the protection of SPF50. In fact in most countries regulations now prevent suncare manufacturers from advertising SPF at a higher number than ‘50+’ as the increase in protection above this is negligible. If we think of the SPF number in terms of how much UVB is coming into contact with the skin, SPF10 allows ~10% of UVB through, SPF15 allows ~6.6%, SPF30 allows ~3.3%, and SPF50 allows ~2%.


What’s the difference between chemical and physical sunscreens?

Physical filters block, deflect, and scatter UV rays. They are considered the most photostable filter and are better for sensitive skin as they cause less irritation. However, they are the culprit behind the ‘white cast’ we all hate. The two main physical sunscreen ingredients are titanium dioxide and zinc oxide, if you have a particular dislike of the white cast you want to avoid products that contain those ingredients.


Chemical filters ‘absorb’ UV rays, preventing them from penetrating the skin. They are photostable, but less so than physical filters, and they can cause irritation to those with sensitive skin. However, they usually leave no white cast. There is some concern over the toxicity of chemical filters but the most common ones, mexoryl SX, avobenzone, and it’s stabiliser octisalate, are known to be safe. If you find you’ve been sensitive to chemical filters in the past you may want to check if that product contained para-aminobenzic acid (PABA), and if it did to give another product without it a try. It was used in early chemical sunscreen formulas but has largely been phased out for less irritating ingredients.


Often SPF50+ products contain a mixture of physical and chemical filters in order to achieve such a high factor. Taking a look at the ingredients list is a good habit to get into as other ingredients that are not physical and chemical filters can skew the SPF rating, particularly in products claiming to be ‘natural’ or ‘organic’. Antioxidant ingredients (like green and white tea extracts) allow light into the skin and help repair the damage after it’s occurred, and anti-inflammatory ingredients (like aloe vera) reduce the redness caused by burning but do not prevent or repair the damage. Yet in some countries these ingredients serve as a ‘booster’ to the SPF rating and so people think they are more protected than they are. An actual UVB filter, like the physical and chemical ones listed, should be one of, if not the first ingredient on the list.


Can I go out wearing SPF15, which allows some UV rays into the skin, and get a safe tan?

No. There is no such thing as a ‘safe tan’. As soon as your skin changes colour, even slightly, you’ve done some damage. A tan is injury to skin cells DNA, which is why I find it particularly perverse that culturally having a tan is seen as a ‘healthy glow’. The darkening of the skin, the tan, is its attempt to prevent further damage.


If the SPF number represents UVB protection, what about UVA?

Unlike for UVB there is no universal standard for measuring UVA protection. On some sunscreens, particularly those from Asian brands, you’ll see a PA, or Photo-Aging Scale. It’s represented by ‘+’ signs, and the more + signs the greater the protection with four being the current maximum. From European brands you may see a PPD (Persistent Pigmentation Darkening) number: the higher the number, the higher the protection. The best way to ensure adequate UVA protection is to look for the term ‘broad spectrum’. If your product is missing those two magic words it’s probably also missing UVA protection. (As an aside, zinc oxide and avobenzone offer the best UVA protection of their filter types.)


If I wear an SPF15 moisturiser and an SPF15 sunscreen am I protected to SPF30?

No. Layering doesn’t work, SPF does not accumulate. Different products have different ingredients and these ingredients interfere with each other. Some degrade and destabilize each other, and so SPF15 + SPF15 does not equal SPF30.


The highest factor product can also be affected by this ingredient interaction. For instance, if you wear an SPF10 moisturiser and then put on an SPF30 sunscreen you’re getting SPF30 protection, the highest factor protection, right? Not necessarily. If an ingredient in the moisturiser degrades an ingredient in the sunscreen it’s going to decrease the level of protection. The best way to avoid these issues is to have one dedicated SPF product and apply that properly.


So how do I apply it?

The general consensus on the amount of SPF to apply is 2mg of sunscreen per cm2 of skin (2mg/cm2). This is roughly the equivalent of ¼ teaspoon for your face, two teaspoons for your neck and chest, two for your back, one or two per arm, and two or three per leg, depending on your height.


Dedicated SPF products are closer to makeup than skincare products in that they are supposed to sit on top of the skin as an even unbroken layer; they are not supposed to be absorbed. It’s tempting to rub or buff sunscreen into the skin but this can break the intended layer of SPF and therefore decrease protection. Improper application = less protection. There’s an argument here for always using a higher SPF number in case of poor application: if you’re not applying it properly you’re not getting the full protection factor, meaning you could be applying SPF15 but only really getting SPF5.


Proper application is often hindered by not using a dedicated sunscreen. Your foundation may be marked SPF30 but I guarantee you’re not applying ¼ teaspoon of it, unless cakey is the look you’re going for. We also rub in moisturisers and makeup products which reduces protection, and as aforementioned, the different various ingredients can interact with each other in ways we don’t want.


A sunscreen is always the last product applied in a skin care routine, and the general advice is that it needs to be reapplied every 2 hours, more often if you’re in contact with water. This presents an obvious problem if you’re wearing foundation and/or concealer, and unfortunately there’s no magic solution. Some people recommend transparent spray SPFs to use over the top of makeup. I personally don’t wear foundation and have found an easy reapplication during the day, that doesn’t take up much room in your bag, can be done using stick SPFs.


What about vitamin D?

We need vitamin D and we need the sun in order for our bodies to produce it, so does covering ourselves in SPF cause a vitamin D deficiency? In theory yes, but the research suggests otherwise. Most people don’t apply sunscreen well enough to prevent the skin from producing vitamin D entirely, and even SPF50+ let’s some of the sun's rays into the skin. There are too many variables (vitamin D is also in foods, and exposure depends on where you live and what season it is) for a recommended amount of sun exposure for adequate vitamin D levels to be given. However, it is recognised that the time it takes for the body to produce sufficient vitamin D is less than the time it takes to burn, so being outside for few minutes without SPF protection is enough. It’s also clear that it’s a bit more complicated than sun exposure = adequate vitamin D levels as shown in a famous study of Hawaiian surfers in which half, despite being out in the sun a lot more than the average population, were vitamin D deficient.


This is complicated.

I know. Most things in life are. But the only thing you need to know for the exam is this: wearing a high factor dedicated SPF product every day is good for your skin.