Ever wondered what the SPF number on your sunscreen actually means? The higher the number, the better the protection, right? Well, yes and no. To understand SPF, and to help you choose the best sunscreen for your needs, it’s good to first have a bit of an understanding of the different types of UV radiation we’re exposed to, and how they affect our skin.

Solar radiation comprises everything from very low energy infrared and radio waves, right up to high energy x-rays and gamma rays, and nestled somewhere in the middle, just beyond the purple end of visible light, is the ultraviolet (UV) region.


UV radiation can be divided into the following:

UVC: This is the highest energy UV radiation, but it’s absorbed by the ozone layer and doesn’t reach the Earth’s surface, so unless you’re planning a trip above the stratosphere, let’s not worry about it¹.

UVB: High energy UV which causes sunburn. Penetrates the epidermis where it causes DNA damage and has a strong carcinogenic effect. Definitely not something to ignore¹.

UVA: Lower energy UV, but it penetrates deeper into the skin (to the dermis), triggering the formation of reactive oxygen species (radicals), causing degradation of collagen fibres which leads to photoageing and contributing to some types of skin cancers¹. Yep, that’s a concern, too.


A sunscreen’s SPF (Sun Protection Factor) tells you how much longer you can stay in the sun without getting burned, compared to unprotected skin. Let’s say it takes 10 minutes of sun exposure without sunscreen for your skin to start to redden and burn. This is known as the minimal erythemal dose. Sunscreen extends the time it takes for skin to start burning by reducing the amount of UVB radiation that reaches the skin. The same skin with an SPF 50 sunscreen, applied evenly and liberally, should take approximately 50 times longer to reach that minimal erythemal dose and start to burn. That’s 500 minutes, or just over 8 hours!²

So, SPF = time to burn with sunscreen / time to burn without sunscreen.

But wait. Remember that sunburn is mainly caused by UVB radiation, so SPF doesn’t tell you anything about how much protection you’re getting from UVA radiation. We know UVA radiation is responsible for the majority of premature ageing and has been linked to cancer, so it’s definitely something we want protection from. In fact, recent changes to the sunscreen regulations in Australia recognised this, and have raised the threshold of UVA protection that a sunscreen must meet in order to qualify as ‘Broad Spectrum’². To make sure your sunscreen is protecting you from both UVA and UVB radiation, look for ‘Broad Spectrum’ on the label.

But that’s not the end of the SPF story. If we can theoretically get up to 8 hours protection from an SPF 50/50+ sunscreen, why do sunscreen labels tell us we need to reapply every few hours? Look out for the next part of SPF explained to learn about some of the things that affect how much protection you’re getting, and why it’s so important to follow the directions on the label and reapply frequently.


Above,  I talked about the different types of UV radiation, what they do to your skin, and how SPF is calculated. But how much can we rely on the label SPF? The fact is, calculations of SPF can only ever serve as a guide because in reality it depends on so many variables, like your skin type, geographical location and the weather conditions on the day.

How much is enough?

When SPF is measured experimentally, it’s applied to the skin at a specific dosage of 2 mg/cm2 (that’s about 7 teaspoons for the average adult)³, but studies show most people use much less than this. If you’re using less than the recommended amount of sunscreen, you’re going to fall short of the SPF on the label.

It’s all in the melanin

How long it takes for someone to burn without sunscreen depends on skin type. Melanin is the natural pigment contained in your skin cells and is the primary determinant of skin colour⁵. The more you have, the darker your skin. It also absorbs UV light, acting as a natural sunscreen, which is why people with darker skin tones take longer to burn, and have lower rates of skin cancer. That means someone with very pale skin might start to burn after only 8 minutes instead of 10 without any protection, and an SPF 50 sunscreen (properly applied) could only push that time out to a little over 6.5 hours, instead of 8.3 hours.

UV index

To complicate things further, the amount of UV radiation hitting you on a particular day varies greatly with the time of day, the season, and how much ozone is in the atmosphere. A UV index of less than 3 means the UV is so low you can safely stay outside for short periods with minimal protection. On the other hand, if the UV index is above 10, you’ll be experiencing the sun’s full fury, and even with sun protection you should seek shade or stay indoors. The important thing to remember is that UV levels have no link to temperature, so even on a cold, cloudy day you could be taking UV damage⁶. Make sure when you’re checking the weather forecast for the day you also pay attention to the UV forecast.

The SPF on the label tells you the maximum UVB protection it provides under ideal conditions, but it’s up to you to make sure you’re applying enough and taking other precautions to minimise UV damage, particularly during the summer months when Australian UV levels can soar. Avoid prolonged sun exposure and wear protective clothing, hats and eyewear to further reduce risk.


Have you ever wondered how SPF is tested? Is there some kind of computer algorithm or powerful laboratory instrument that figures it all out? Do we slather sunscreen on a bunch of lab rats and let them run about on the beach in tiny hats and thongs? As much fun as that sounds, SPF is actually tested on humans, but it’s done inside a testing facility, not anywhere outdoors.

We talked about some of the variables that can affect the amount of UV we’re exposed to on a particular day, and so, how long it takes us to burn. Basically, there’s a bunch of things that affect UV levels and since good science is all about reducing and controlling the variables, it’s impossible to test SPF using the sun as the source of UV.

SPF testing

To ensure the test is accurate and repeatable a solar simulator is used, which mimics the sun’s UV radiation, but delivers a controlled dose of UV.

The test is conducted on volunteers (at least ten for a particular product). Test sites are drawn up on the volunteers’ backs, and the same amount of sunscreen is applied to each site (exactly 2 milligrams per centimetre squared), then the sites are exposed to UV radiation. The dose of UV radiation is increased incrementally from one site to the next, by about 25%, until all the sites have been exposed to different levels of UV. The volunteers then go home and return the following day to have their responses measured, which basically means an analyst determines which sites have been protected by the sunscreen, and which have begun to turn red. This is done in a ‘blind’ manner, so the person measuring the results is not the same person that applied the product, and they don’t know anything about what the product is, or the test design (the sites are randomised to remove the chance of bias). By noting which sites show signs of burning (erythema), the analysts can work out how much UV radiation was needed to produce a response, and compare this to the dose needed for unprotected skin.

The calculation is simple: SPF = time taken (or dose) to burn with sunscreen / time taken (or dose) to burn without sunscreen.

UVA testing

So that’s SPF, or UVB protection, but what about UVA? UVA exposure doesn’t produce a clear reddening response like sunburn, so it can’t be measured in the same way as UVB. Instead, it’s measured using a method which replaces human skin with plastic plates, and instead of a solar simulator they’re placed inside a laboratory instrument called a UV spectophotometer. The instrument measures how much UVA radiation passes through the film of sunscreen, which gives an idea how much UVA would reach our skin in the real world. The values are plugged into an equation, which gives a result called the UVAPF, or the ‘UVA Protection Factor.’

In the past, a sunscreen could claim ‘Broad Spectrum’ if the amount of UVA passing through that film was no more than 1% (for a 20µm thick layer) or 10% (for a 8µm thick layer), no matter the SPF. But with increasing awareness of the dangers of UVA exposure, the regulations have changed to place more importance on UVA protection. Since 2012, the UVAPF must be at least one third of the SPF, and all primary sunscreens must offer broad spectrum protection¹⁰. This way, sunscreen manufacturers can’t simply pump up the SPF without also raising the UVA protection.