At this time of year, you’re quite possibly wearing sunscreen every day. Have you ever wondered how it was designed? How do manufacturers know it will protect you from the sun? And how is that SPF number actually calculated?
Cosmos wanted to find out.
What’s in sunscreens?
Sunscreens are medicines. Just like over-the-counter pharmaceuticals, vaccines, and prescription drugs, they have active ingredients (things that block out UV light), and excipients (things that give the sunscreen other properties, like consistency, water resistance, and scent).
Both the active ingredients and the excipients need the approval of the Therapeutic Goods Administration. The TGA has a list of both active ingredients and excipients that are permitted for use in sunscreen – each active ingredient has demonstrated that it can block out UV light, and all active ingredients and excipients have been shown to be safe to spread on skin.
Any product that calls itself an SPF 15+ (or higher) sunscreen has to contain at least one of these active ingredients in the right concentration, and it can’t have any ingredients not on the lists. This still gives manufacturers plenty of flexibility, however.
“Within that space, a formulator can choose any combination,” says Dr Fabrizio Spada, Scientific Affairs Manager at Sunsense.
Manufacturers can design sunscreens for many different therapeutic, functional, and cosmetic reasons. They can add compounds that have been shown to increase the efficacy of active ingredients, or the skin’s long-term protection. They can tune ingredients for skin sensitivity. Or they can add polymers, for water resistance, or remove water, to make the sunscreen into a more portable gel.
Spada says that when designing a new sunscreen, a formulator first chooses active ingredients suitable for the purpose, then solvents that will keep those active ingredients stable, before choosing additions that will affect the consistency and extra features of the sunscreen.
Much is made about the distinction between ‘mineral’ sunscreens – which have inorganic active ingredients, like zinc oxide and titanium dioxide, and ‘chemical’ sunscreens – which have organic (carbon-containing) active ingredients, like homosalate or octocrylene.
All of these ingredients will do an equally good job of blocking out UV rays. But each of them can provoke irritation or allergic reactions in some people, so it’s important to have a range of options for different people to use.
There’s also evidence that some active ingredients are dangerous for coral reefs – by inhibiting the organisms’ ability to absorb UV or toxic if accumulated – meaning that you may want to be more selective about the sunscreen you’re using if you’re going swimming. Non-nano, or micro, titanium dioxide and zinc oxide are believed to be reef-safe substances.
How are they tested?
Once a manufacturer has a formula they’re happy with, the sunscreen needs to go through standardised tests – both in people and in the lab.
First, the sunscreen’s effectiveness against UVB rays is tested on people’s skin for sunburn.
“A minimum of 10 and a maximum of 20 subjects are tested with the sunscreen for erythemal [sunburned red] response,” says Spada.
These people need to represent a range of different skin tones, from skin with lots of pigment to skin with very little pigment at all.
“There is a standardised way to lay the sunscreen on the skin – it’s two milligrams per square centimetre – and then they irradiate it with a solar simulator.”
This simulator produces a standardised amount of UVB light.
“The light is shone through a circular port, so it looks like a red dot, of a certain size. The moment you can see a clear circle with defined borders, so it’s not a blurry or irregular shape, that is considered a full erythemal response.”
If the light doesn’t produce a response, the dose of UVB is increased. The dose is increased until the skins’ response matches that of a control: a non-sunscreened patch of skin.
“They work their way up in terms of dose, [and] the higher the dose, the higher the SPF.”
Next, the sunscreen moves onto in vitro (lab-based) trials.
“[UVA] is more harmful to the skin than UVB because it penetrates deeper,” says Spada.
“So ethically, we don’t do it on people. We do it in vitro on a plate, but the in vivo [in life] data is sort of a reference to make sure that we are comparing apples and apples.”
With these tests passed, the sunscreen is permitted to call itself broad-spectrum SPF. But there’s an extra test needed to claim water resistance.
Water resistance testing “is similar to UVB, but now the subjects will go into a pool or a spa or a water reservoir,” says Spada.
“They will still measure the erythemal response similarly to the UVB, but after specific time points during the water immersion.”
This makes the testing process longer and more expensive, which is why not all sunscreens will carry the water-resistant tag.
Finally, the sunscreen needs to be tested for stability – to ensure it will still work after sitting in a bottle for six months.
Once the sunscreen has passed each of these tests and has made it onto shelves, all of this work still has to be documented.
“The TGA can come to the sponsor and ask for that evidence. If you don’t have it, that listing may be cancelled, you might get fined, or get sanctions,” says Spada.
What does SPF actually mean?
Sun protection factor – or SPF – is determined in these tests.
It’s calculated by dividing the amount of time it takes to redden protected skin, by the amount of time it takes to redden unprotected skin.
So an SPF of 30+ means that it takes at least 30 times longer to get sunburn while wearing it.
These calculations are nearly always under-predictions. To claim an SPF of 50+, for instance, manufacturers need to show that at least 60 times as much UV needs to get through the skin.
“That’s very important for multiple reasons,” says Spada.
“One, because you want to know that the cumulative damage from repeated exposures to UV is the lowest it can be.
“And also, we know that people under-apply.”
The Cancer Council recommends a teaspoon of sunscreen per limb, applied 20 minutes prior to being in the sun. This guideline is often not met. They also recommend reapplication every two hours, because sunscreen protection wanes, worn away by clothing and sweat.
Finally, it’s the law to feature this on every bottle of sunscreen, but it bears repeating: sunscreen should never be your only protection against skin cancer. When the UV index is above 3, you should also be gravitating to the shade, as well as wearing hats, sunglasses, and long-sleeved shirts.
Ellen Phiddian is a science journalist at Cosmos. She has a BSc (Honours) in chemistry and science communication, and an MSc in science communication, both from the Australian National University.
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