SPF Ratings – what they really mean!

The SPF (or Sun Protection Factor) can be found printed on the bottle or packaging of all reputable sunscreens Australia wide. The most common are SPF30+ and SPF50+. There is also an SPF15+ but this is becoming a lot less common.

Before we delve deeper into SPF ratings and what they truly mean, as well as how sunscreen really works, you first need a basic understanding of UVR (Ultraviolet Radiation).

What is Ultraviolet Radiation?

Ultraviolet radiation (UVR) is defined as the portion of the electromagnetic spectrum between 100 nanometres (nm) and 400nm. UVR is classified by wavelength into three regions:

• UVA – Ultraviolet radiation in the range 315nm to 400nm is thought to contribute to premature and accelerated ageing and wrinkling of the skin, and has recently been implicated as a cause of skin cancer.
• UVB – Ultraviolet radiation in the range 280nm to 315nm is more dangerous than UVA and has been implicated as the major cause of skin cancers, sunburning and cataracts.
• UVC – Ultraviolet radiation in the range 100nm to 280nm is extremely dangerous but does not reach the earth’s surface due to absorption in the atmosphere by ozone.

SPF ratings found on sunscreen refer to UVB (erythemal UV) only – the type of UV capable of burning. The SPF rating has little relevance to UVA or UVC protection – and we’ll discuss why shortly.

What does the SPF indicate?

Now, you might be thinking that SPF indicates the amount of time you can stay in the sun before getting burned. This is the most common explanation and what the vast majority of people believe it means.

For example, if it normally takes your skin 5 minutes to burn, then an SPF30+ would protect you for 30 times longer than that – up to about 150 minutes (or just over 2 hours). An SPF50+ would provide 50 times longer protection – up to about 250 minutes (or approx. 4~ hours protection).

But if you thought that – you’d be wrong! It’s actually a myth. The SPF rating has nothing to do with how long you can remain in the sun before getting burnt.

The SPF rating, in fact, is a measure of the ‘fraction’ of sunburn-producing UV rays (UVB or erythemal UV) that reach the skin when sunscreen is applied at the correct dose of 2mg per cm2.

For an SPF30+, this would mean that 1/30th of the erythemal UV gets through (reaches the skin); while SPF50+ means only 1/50th gets through.

So SPF50+ provides almost twice the amount of protection.

Keep in mind that the protection you get is only true if sunscreen is applied properly at the correct dosage. Many people don’t realise this, but to achieve the SPF rating stated on the bottle, you need to apply at least 2mg of sunscreen per cm2.

If you don’t apply this amount, then the protection you get could be significantly less. For example, applying an SPF50+ sunscreen incorrectly could result in a protection level closer to an SPF30+ or worse!

Where did the myth come from?

Based on our research, it appears to be an attempt to simplify the SPF rating in a way that the average human can understand it. It’s just a little easier to say “you can stay in the sun 30 times longer” then try and explain that SPF30+ provides 1/30th of the protection against erythemal UV, assuming you apply it correctly.

Whether that’s actually true or not, we don’t really know.

We couldn’t find anything else that suggests where it came from or why. If you think you’ve got the answer, please let us know – we’d love to hear it and will update the article accordingly!

What is erythemal UV?

Erythemal UV refers to a specific range of UVR that is capable of causing erythema (redness), or in other words, a sunburn. It’s another way of referring to UVB.

Erythemal UV (UVB) changes throughout the day. Generally speaking, the UV will be at its highest between 10am and 3pm; however, this does change from day to day, month to month, season to season. In the cooler months, UV is generally lower, whereas in the summer months, UV will be higher.

Here is Melbourne on Jan 1st, 2024.

Here is Melbourne on June 1st, 2024.

On January 1st, you would certainly need sunscreen to prevent burning. On June 1st, however, you’d be able to remain outdoors without having to worry about “sunburn”.

Now, we say “sunburn” because there is more to worry about than just UVB, and we touched on this at the beginning of the article when talking about UVR.

What about UVA and UVC?

As mentioned earlier, SPF ratings refer to UVB (erythemal UV) only – the type of UV capable of burning. The SPF rating has little relevance to UVA or UVC protection.

UVC

Although it is the most harmful to us, it’s also the least penetrating! In fact, UVC is blocked by the earths’ atmosphere / ozone layer, so it’s really of no concern to us. No sunscreen we are aware of provides protection against UVC because it’s not relevant.

If a sunscreen claims to protect you against UVC, it’s likely to be a marketing gimmick of sorts – because they are claiming to protect you from something that you don’t actually need protection from.

UVA

UVA on the other hand – this is where things get interesting, and the introduction of Broad Spectrum sunscreen comes into play.

UVA is capable of penetrating clouds, penetrating much deeper into our skin, and will even penetrate glass such as car and home windows. Its effects on the skin are not immediate and will take years to show, but essentially, it’s responsible for accelerated skin ageing and more recently, has also been implicated in causing skin cancer.

This is why we will often tell you to wear sunscreen all year round – even if the sun isn’t visible. It’s one of the best defences you have against premature / accelerated ageing – and skin cancer!

To illustrate this point, take a look at the photo below.

This clearly shows how UVA can affect the skin over a long period of time. After 28 years of driving trucks, the left side of his face (exposed to the driver side window) shows significantly greater signs of ageing than the right-hand side of his face.

Broad Spectrum protection

Because SPF ratings only refer to UVB protection – we need something to protect us from UVA as well. This is where Broad Spectrum sunscreen comes in.

Sunscreen that protects against both UVA and UVB will make note of this on the packaging – either by stating UVA/UVB protection, or by simply stating “Broad Spectrum”.

Although it’s fairly standard today for all sunscreens to be Broad Spectrum, there are still a few that don’t offer Broad Spectrum protection. We would suggest and highly recommend avoiding any sunscreen that isn’t Broad Spectrum – especially since UVA has been implicated in skin cancer.

So there you have it. The true meaning of SPF, how it works, and the difference between UVR types. With this knowledge, hopefully you can better protect yourself from the damaging effects UVR can have on the skin – from severe burns, to premature and accelerated ageing and skin cancer.