How SPF Changes With How Much Sunscreen You Use

We all know that we need to apply enough sunscreen… but do any of us apply the full amount evenly, every time? Here’s what happens to your UV protection (SPF) when you apply less than the amount you should…

1 What Is SPF? 2 Sunscreen is tested with more sunscreen than you probably use 3 What happens when you use less sunscreen (in theory)? 4 What actually happens when you use less sunscreen? 5 Why isn’t SPF tested using realistic amounts of sunscreen? 6 What does this mean for my sunscreen use? 6.1 References What Is SPF?

SPF stands for sun protection factor, and while you probably know that a larger number means more protection, most people don’t know what the number actually means.

The simple explanation of SPF is that it’s how many times more UV you can receive with the sunscreen on before you get sunburnt. For example, if your bare skin usually gets burned after 10 minutes, SPF 15 will let you stay in the sun for 150 minutes before you get burned (provided that the UV level stays constant, which probably won’t be the case in real life).

Sunscreen is tested with more sunscreen than you probably use

The amount used in the sunscreen studies to determine SPF is 2 milligrams of sunscreen per centimetre of skin (2 mg/cm²). 2 mg/cm² doesn’t sound like a lot, but it’s more than the majority of people normally use without measuring it out, even when they’re instructed to apply it liberally. It translates to 1/4 teaspoon for your face alone, and 35 mL for a full adult body, which is about half of a small tube of sunscreen (more than half a tube of Biore Watery Essence, for example). For specific areas of the body on an average adult, 2 mg/cm² means:

• 1/2 teaspoon on your face & neck combined, and each arm
• 1 teaspoon on each leg, the front of your torso and the back of your torso

using Imperial teaspoon measurements (a US teaspoon is slightly smaller). On average, people apply only a quarter to a half of the recommended amount (0.5 to 1.0 mg/cm²), and in most studies, it’s closer to 0.5 mg/cm², even with photosensitive people who are more conscious about sun damage than the average person.

What happens when you use less sunscreen (in theory)?

As you’d expect, less sunscreen means less protection. But how much less? I’ve got some good news!

In physics, you’d expect protection to decrease exponentially with less sunscreen, so applying half the amount of sunscreen would give you less than half the protection. This is based on a relationship called the Beer-Lambert law, and is illustrated on the graph below (source):

The graph shows the predicted relationship between the actual SPF and the amount of sunscreen you apply, for products with varying levels of protection (SPF 50, 30, 15, 8, 4 and 2). On the far right of the graph, you can see that 2.0 mg/cm² gives the measured protection for each product. As you’d expect, applying nothing (far left) gives negligible additional protection.

However, it’s the middle of the graph that’s disturbing. According to this theoretical relationship, reducing the applied amount by one-quarter will already halve your sun protection if you’re using SPF 30 or above. Applying half the recommended amount of sunscreen (1.0 mg/cm²) cannot give more than SPF 10, even with SPF 50 sunscreen! And as we know from earlier, most people apply less than this – we’re below SPF 3 for all products by 0.5 mg/cm².

(For those of you who are into math, the equation is SPFactual = SPF(amount applied/2).)

What actually happens when you use less sunscreen?

For years, most scientists thought using less sunscreen would decrease sun protection exponentially like in the graph above… and then some studies tested this theory out, and it seems like most sunscreens actually give a linear relationship, where half the sunscreen would actually give you around half the protection, like in the purple line below.

One study even found that there was a logarithmic relationship like in the blue line on the graph, where half the sunscreen would actually give you more than half of the full protection! There’s a brief summary of the relationships found in different studies in this review. In all of the studies, the protection achieved was much better than the amounts in the theoretical prediction. A good rule of thumb is that applying half of the recommended amount will give you around 1/3 to 1/2 of the labelled SPF.

The proposed reasoning for why the Beer-Lambert law fails is because it assumes an even surface, when skin is actually uneven. The reason for the different relationships found in different studies is probably because of the different products used.

Why isn’t SPF tested using realistic amounts of sunscreen?

The obvious way to rule out the confusion would be to test sunscreen using the amounts that people actually use. However, it turns out that 2 mg/cm² is needed to get reliable, reproducible results on different people, in different laboratories around the world. So while SPF isn’t very useful for working out how much longer you can stay out in the sun, it’s still useful to compare the relative protection abilities of different sunscreens.

What does this mean for my sunscreen use?

The fact that the SPF/amount relationship is closer to linear than exponential has a few interesting implications:

• Using a smaller amount of sunscreen is better than not wearing any sunscreen. While the old exponential model suggested that insufficient sunscreen wasn’t much better than no sunscreen, the actual linear relationship says otherwise. So even if you can’t manage to apply the right amount, it will still make a difference!

• Higher SPF sunscreen can be used to compensate for underapplication. A higher SPF sunscreen will always give more protection. According to the theoretical exponential relationship, the difference would be relatively small, but according to a linear relationship, the SPF will scale. This means that an SPF 30 product will always let in half as much sunburn-causing UV as an SPF 15 product. On the flip side, higher SPFs tend to result in greasier products and are more expensive which means you won’t want to apply as much, so go for the highest SPF sunscreen that you can, considering your budget and the formula.

As always, sunscreen isn’t the be-all and end-all of sun protection, or even the most important aspect. You should be avoiding sun exposure where possible, and using other types of protection as well (clothes, hats, sunglasses). You can’t underapply a shirt, and it you don’t need to remember to reapply it!

References

H Ou-Yang, J Stanfield, C Cole, Y Appa & D Rigel, High-SPF sunscreens (SPF ≥ 70) may provide ultraviolet protection above minimal recommended levels by adequately compensating for lower sunscreen user application amounts, J Am Acad Dermatol 2012, 67, 1220-1227.

B Petersen & HC Wulk, Application of sunscreen – theory and reality, Photodermatol Photoimmunol Photomed 2014, 30, 96-101.

U Osterwalder & B Herzog, The long way towards the ideal sunscreen – where we stand and what still needs to be done, Photochem Photobiol Sci 2010, 9, 470-481.

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