Mineral v chemical sunscreen: Which one should you be using?

BBC News

3 days ago

Some people are trading chemical sunscreens in favour of mineral versions because of fears over toxicity, pollution and effectiveness. Is there actually any difference?
Mineral sunscreen is having a moment. Amid concerns that so-called "chemical" sunscreens may be bad for our bodies, brains, and even coral reefs, mineral-based formulations have become the fastest-growing share of the global sunscreen market.
But debates over "chemical" versus "mineral" sunscreens are riddled with misconceptions. Many commonly repeated claims – such as mineral sunscreens not containing chemicals; that chemical sunscreens have been proven harmful; or that chemical sunscreens absorb UV, while mineral ones only reflect it – are misleading, even false.
The confusion begins with terminology. "Everything is a chemical," points out Brian Diffey, emeritus professor of photobiology in dermatological sciences at the UK's University of Newcastle and inventor of sunscreen's UVA star rating. What people call "chemical" filters are more accurately termed organic, since they contain carbon-hydrogen bonds, says Diffey. Inorganic filters (often called mineral), primarily titanium dioxide and zinc oxide, lack those bonds. All are chemicals.
Seeking to protect our skin and bodies from the Sun is not a new trend – nor are sunscreens, organic or inorganic. Ancient Mesopotamians used umbrellas; ancient Greeks, wide-brimmed hats. Along with various coverings, people applied concoctions to the body. In Africa, the use of ochre-based pastes, still used as sunscreen by people such as the Himba in Namibia, dates back at least 285,000 years, while the Roman writer Cornelius Celsus advised slathering the skin with olive oil.
It wasn't until the 19th Century, however, that scientists discovered ultra-violet radiation (UVR) – and realised that some ingredients, like quinine sulphate (derived from a tree bark), could absorb it. Scientists duly recommended it as a sunscreen. By 1930, researchers had found a number of other ingredients that absorbed UVR, including aesculin (from trees such as horse chestnut) and larch bark tannin. Though they wouldn't meet today's SPF standards, in terms of how they protected the skin, they all were organic ("chemical") sunscreens.
Later, dozens of other ingredients were added to this list – including those produced by mixing together different substances in a laboratory to induce a chemical reaction. Often referred to as "synthetic chemicals", these types of ingredients – including avobenzone, oxybenzone, octisalate and octinoxate – have been found to absorb UV rays far more effectively than their predecessors. Another type of sunscreen came to market, too: "mineral" sunscreens.
While they might seem more "natural", the titanium dioxide and zinc oxide in today's sunscreens are usually lab-produced.
The great deflection debate
At first, it was thought that organic sunscreens absorbed UVR, while inorganic sunscreens physically reflected and scattered UVR away from the skin – a belief that was perpetuated further in a 1970s United States Food and Drug Administration (FDA) monograph.
This idea is still commonly heard today, including from seemingly authoritative sources. It also is partly why inorganic sunscreens sometimes are also called "physical sunscreens", implying that they block out UV rays like an umbrella deflects raindrops.
"People say that mineral or inorganic sunscreens reflect ultraviolet radiation," says Antony Young, professor emeritus of experimental photobiology at King's College London and a lifelong researcher of sunscreen efficacy. "And that's not true."
In fact, modern titanium dioxide and zinc oxide only reflect or scatter 4-5% of the UV range, an authoritative, peer-reviewed 2015 study found. They absorb the other 95%.
Indeed, scientists have been aware that inorganic sunscreens absorb UV since the 1980s – so much so that the authors of the 2015 study already seemed exasperated with having to provide even further proof. Their study emphasised "yet again", they wrote, "that the true function of these insoluble 'physical' or 'mineral' UV filters is in fact identical to that of the soluble 'chemical' UV filters.
"These data indicate clearly that these filters act primarily as UV-absorbing materials, and not as UV-scattering or UV-reflecting materials."
They're not even actually "reflecting" that 5%, adds Diffey: "They scatter it." UV rays aren't bounced off the surface of the inorganic particles. Instead, he says, "the light rays go into the medium. They bounce around from the atoms or molecules. Some of them then will come back out again. And that's called scattering."
Meanwhile, many sunscreens, even some marketed as "mineral", use both organic and inorganic UV filters.
But in general, experts say, whether a UV filter works by absorbing, reflecting or scattering UVR doesn't really matter. The amount of heat generated in the skin by absorption is negligible – and a tiny fraction of the heat generated from the Sun's exposure itself.
Ultimately, says Mary Sommerlad, a consultant dermatologist based in London and British Skin Foundation spokesperson: "You don't need to decide whether you want your UV energy to be absorbed or reflected, because they're working in pretty much the same way." That is, by reducing how much UVR your skin absorbs to protect it from damage and risk of developing cancer.
Particles and solutions
If organic and inorganic sunscreens work so similarly, why do they feel different?
It comes down to solubility. Most organic filters are soluble, meaning their active ingredients can be dissolved in a medium like water or oil. Inorganic sunscreens are not: their particles remain intact. As a result, inorganic sunscreens can feel thicker and give a white cast, while organic filters can provide smoother, clearer formulations.
As chemistry advances have shrunk inorganic particle sizes down, the white-cast effect has decreased. These "nanoparticles" (less than 100nm in size) of titanium dioxide and zinc oxide have led to their own set of concerns around skin penetration. But even this minuscule particle size doesn't penetrate more than the stratum corneum – the outermost skin layer – thus preventing systemic absorption.
Most organic UV filters operate at the surface of the skin, too. Because sunburns develop at the upper layers of the skin, a UV filter must bind to the stratum corneum in order to work, say experts. Like inorganic sunscreens, therefore, organic sunscreens absorb the vast majority of UV at the skin's surface.
But it is true that some organic filters are systemically absorbed. "Some active ingredients will find their way through to the bloodstream," says Diffey. "Whether or not that's doing us any harm or not remains to be seen."
So far, there isn't good evidence that it is.
The vast majority of research finding risks of chemicals like oxybenzone has been performed on animals, using massive amounts. In one 2001 study that sparked concern about endocrine disruption, for example, baby rats were fed extremely large quantities of UV filters like oxybenzone for four days. Those that consumed oxybenzone had uteruses that were 23% larger than rats that didn't.
But when later researchers put these numbers into perspective, they found that – to reach the same systemic concentration of oxybenzone the rats had – a human would need to apply a 6% oxybenzone sunscreen every day… for 277 years.
Why are animals exposed to so much of a particular ingredient? Because it helps scientists determine the potential safety limit. "The reason for these studies is to determine how much is safe," says Michelle Wong, chemist and author of the book The Science of Beauty who frequently tackles sunscreen myths online. As a result, "they are always looking for an effect. They will generally use a large enough amount of the ingredient… to elicit some sort of effect.
"If they don't, then they don't know where the line is."
So far, the threshold at which the ingredients pose a risk seems to be many times higher than the quantity in which people are using them. One scientific review published earlier this year found no evidence that UV filters like avobenzone and homosalate can damage DNA or cause cancer in humans – and that blood levels of these chemicals from topical sunscreen are far below the amount at which they might have an effect.
In one 2004 study, for example, 32 people applied creams made up of 10% oxybenzone. Four hours after application, both men and women had slightly lower levels of testosterone. But after just four days of application, the differences between the appliers and the control group disappeared – leading the researchers to conclude that differences in the hormones weren't actually from the sunscreen itself.
Even so, because ingredients like avobenzone are absorbed into the bloodstream, out of caution regulators like the FDA have requested more safety data from manufacturers.
More like this:• Sunscreen: Are you using it correctly?• Sunscreens: Safe or toxic?• Why sunscreen is not enough to prevent sunburn
The effects of organic filters on the environment – particularly coral reefs – are a little more unclear. Studies that have raised concerns have mostly been lab-based experiments; real-world impacts may be different. One study, for example, found that while UV filters were detected in the seawater across 19 tourist hotspots in Hawaii, 12 locations showed less than 10 parts per trillion of oxybenzone – the equivalent of 10 drops in a water-filled football stadium. The area with the highest concentration, Waikiki Beach, had 136 parts per trillion.
All were at levels far below the concentration at which the lab-based studies found damage to coral reefs. However, in 2018 Hawaii made the move to ban the sale of sunscreens containing chemicals oxybenzone and octinoxate. "If you have places with a high load of tourists going in, it is not unreasonable to stay cautious and say, 'Yes, there may be additive effects'," marine scientist Jorg Wiedenmann said at the time.
Still, while much of the focus regarding coral toxicity has been on organic UV filters, inorganic UV filters may have an effect too. Meanwhile, some marine biologists point out that the far larger (and better-proven) threat to corals is climate change – and that the biggest bleaching events have been in places without tourists.
While scientists haven't yet proven any concrete, adverse effects to humans of using organic (or inorganic) sunscreens, aside from occasional side effects like allergic reactions, we can't say the same of excessive UV exposure. At worst, it can lead to skin cancer, the most common type of cancer in countries including the US and the UK. If it spreads, the deadliest type, melanoma, has only a 35% five-year survival rate.
This is why the best sunscreen, experts say, is one you are happy to use.
For some people, that is a sunscreen that is smoother, clearer and absorbs more quickly. For others, that might be a sunscreen that has fewer toxicology concerns, no matter how theoretical.
"SPF is SPF," says Young. "It doesn't really matter what the ingredients are."
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