How mineral sunscreens actually work is not widely understood, and neither is what’s actually in them. We tested ten of these sunscreens in the lab and reviewed the ingredient lists of over 100 more to clarify what’s going on.

We verified that many products marketed as “mineral-only” rely on a broad palette of chemicals, not only to make them nicer to use but also to boost their sun protection factor (SPF) rating.

Mineral and chemical sunscreens

Most sunscreens contain just a handful of “active” ingredients: avobenzone, homosalate, octinoxate, octisalate, octocrylene, oxybenzone, or zinc oxide.

The active ingredients in sunscreens are generally divided into two main categories:

• inorganic ingredients, often described as “physical” or “mineral” – usually titanium dioxide or zinc oxide. These inorganic minerals are often reported to reflect UV rays. The reality is these compounds mostly absorb UV light

• organic ingredients, often called “chemicals” – such as octinoxate, avobenzone, and homosalate. These are organic molecules that strongly absorb UV light and do not break down to form harmful substances.

Sunscreens are often marketed as either “chemical” or “mineral”. Mineral sunscreens are commonly said to “reflect” UV light, but our measurements reinforce what sunscreen experts and formulators know: that both mineral and organic filters work mainly by absorbing UV radiation.

‘Mineral’ sunscreens have chemicals too

The goal of our study was simple: to understand how sunscreens achieve high SPF, and to understand the role of chemistry in sunscreens. We measured the absorption of different frequencies of UV light by pure zinc oxide as well as ten commercially available sunscreens.

We then identified 143 sunscreens listed on Australia’s Therapeutic Goods Adminstration register where zinc oxide or titanium dioxide were the only declared active ingredients. For each product, we also looked at the full list of ingredients – not just the “active” ones – scanning for any compounds known to absorb UV light.

Our measurements on zinc oxide showed strong UV absorption, as we expected from knowing its chemistry. This confirms that mineral sunscreens work via absorbing UV light, just like organic sunscreens.

When we looked at “mineral” sunscreens, we found some use additional UV-absorbing chemicals. We could see this because zinc oxide absorbs light fairly evenly across the UV spectrum, but organic chemicals tend to show “humps” in absorbance in particular frequency ranges.

Indeed, it is difficult to make a very high SPF sunscreen that is pleasant to use by only using mineral filters. The minerals would need to make up around 20% of the sunscreen by weight, but minerals are dense and can easily settle out of the mixture.

The formula would likely need to be thickened to stop the minerals from separating out. The mineral particles can also scatter visible light, leaving a “white cast”, which consumers also dislike.

A solution?

So how do you make a high SPF “mineral” sunscreen that’s lightweight and transparent?

One way is to use smaller mineral particles. These, however, increase the cost of the product and can pose other stability challenges.

Many brands use chemicals such as butyloctyl salicylate and ethylhexyl methoxycrylene to improve the sunscreen’s stability and keep the consistency smooth, but these molecules also strongly absorb UV light. However, these chemical UV filters do not have to be listed as “active ingredients” because their primary role is to serve another function.

In our test of ten products, we found two products claiming to be “mineral only” showed the telltale absorbance hump of organic chemical filters, due to the presence of butyloctyl salicylate and/or ethylhexyl methoxycrylene.

Indeed, we noticed something similar in our review of sunscreens on the therapeutic goods register. At least 48 of 143 sunscreens listing only mineral active ingredients (~34%) contained additional chemicals known to strongly absorb UV light.

We also compared two “zinc-only” children’s sunscreens with very different skin-feels: one is thick, the other silky.

Our in-lab analysis (and the ingredient lists) showed that the silky product included UV-absorbing molecules. These may help make it easier to spread on skin, reduce white cast, and improve film formation.