When first invented, sunscreens were designed only to protect against sunburn. Sunscreens today must also protect users from discreet damage (you don’t have to experience a sunburn for UV radiation to damage your skin) that shows up years later as prematurely aged skin in the form of wrinkles and pigmentation, and in some cases, even cancer.
UV filters reduce the amount of UV light that reaches the skin and when developing a sunscreen, filters are selected based on what we expect out of the sunscreen in terms of performance and skin feel. UV filters are of two main types:
Organic filters. These are carbon-based and include avobenzone, octinoxate and diethylamino hydroxybenzoyl hexyl benzoate, among others.
Inorganic filters. These aren’t carbon based and include the different forms (coated vs. uncoated, nano vs. micro particle sizes) of zinc oxide and titanium dioxide.
While the chemical and natural terminology is commonly used to describe these different types of filters, this is incorrect and misleading as even inorganic, or mineral, filters are chemical filters produced by chemical means. It’s also incorrect to describe mineral filters as physical filters. There is a pervasive myth that chemical filters work by absorbing UV and converting it into heat, while physical filters work by scattering or reflecting UV. Mineral filters however, also work mostly by absorbing UV: they absorb around 95% of UV radiation and convert it to heat; only about 5% is scattered, and that too, towards your skin. What’s more? Insoluble, particulate “chemical” filters (e.g. tris-biphenyl triazine and methylene bis-benzotriazolyl tetramethylbutylphenol) work the same way. Luckily however, a high probability exists that UV scattered forward will encounter another filter particle and get converted to heat before it reaches your skin. Of the ways - absorption, scattering and reflection - that sunscreens can protect your skin from UV, absorption is best since it destroys UV and turns it into something less harmful.
So does zinc oxide really protect better than “chemical”, or more accurately, organic UV filters? Well, no. While zinc oxide does offer broad spectrum protection, which means that it protects you from both UVA and UVB light, this doesn’t tell you anything about the strength of its protection and zinc oxide isn’t a particularly strong UV filter. In other words, while zinc oxide can absorb multiple wavelengths of UV light, it can’t absorb a lot of any particular wavelength, which means that while it offers broad spectrum protection, this protection is relatively weak and a lot of zinc oxide is needed to create a high strength sunscreen. As a result, when zinc oxide is used alone, the sunscreen usually tends to have a heavy finish and a white cast that looks particularly terrible on darker skin tones. This makes people less likely to apply the required amount of sunscreen for adequate protection since protection really does depend upon the amount of sunscreen that you apply. Titanium dioxide (the less talked about but stronger mineral UV filter), on the other hand, is a stronger filter than zinc oxide but protects mostly in the UVA II/UVB region.
Mineral filters are also harder to stabilise into a sunscreen for uniform protection especially when uncoated - a form that’s usually leveraged by “natural” brands - leading to an ineffective and unstable sunscreen. Formulating a stable sunscreen with mineral filters that also feels good when applied is no monkey business. Nowadays we also have fantastic broad spectrum “chemical” or organic UV filters that have a low probability of irritating the skin and offer exceptional photo-stability and UV protection. They are also not easily absorbed by the skin, making them highly safe to use. Since we also have more variety when it comes to organic UV filters, we have more flexibility when it comes to formulating. Different chemical filters can be combined in different ways to optimise a sunscreen to be efficient and strong over a broad spectrum of wavelengths, all while also being pleasant to apply.
So which type of sunscreen is truly better? The answer is that there’s really no short answer. The entire formulation matters from the solvents used, to how the filters have been combined and stabilised. Assuming that you have the option to choose between two good, stable sunscreen formulas, a mineral one and an organic one, which one should you go for? It boils down to personal preference. Ultimately, the best sunscreen is a well formulated one that you’ll actually enjoy applying - generously.