Sunscreen filters explained: physical, chemical – and what makes modern filters better

Almost every explanation of sun cream starts with the same image: mineral filters sit on the skin like a mirror and reflect the rays, whilst chemical filters absorb them. Sounds logical. Except it isn’t true. In our skin analyses at alestetics®, sun protection is almost always a topic of discussion – and hardly any other subject is surrounded by so much half-truth. Let’s set the record straight.

UVA and UVB – two types of radiation, two effects

Before we look at filters, it is worth considering what they protect us from. Sunlight reaches the skin in two wavelength ranges that are relevant to us.

UVB is the shorter wavelength of radiation. It remains at the surface and is responsible for sunburn – a mnemonic is ‘B’ for ‘Burning’.

UVA penetrates deeper. It does not cause acute sunburn, but over the years it breaks down collagen and elastin and promotes the formation of pigmentation spots. It is therefore the driving force behind premature skin ageing. And it is persistent: UVA is present all year round, even when it is cloudy and through window glass.

For our work – pre-ageing – UVA is therefore the real enemy. If you only think about sunburn, you’re only half-protected. And this is precisely where the choice of sunscreen becomes important, as not all sunscreens provide the same level of protection against the deep UVA spectrum.

Physical or chemical – the mirror myth

Filters are divided into two categories. The mineral (i.e. inorganic) filters consist of zinc oxide or titanium dioxide. The organic filters – colloquially known as «chemical» filters – are carbon-based molecules such as avobenzone or the more modern ones, which we will come to shortly.

The common myth is that mineral filters reflect, whilst organic filters absorb. The measurements tell a different story. A Study in Photodermatology, Photoimmunology & Photomedicine shows that zinc oxide and titanium dioxide primarily protect the skin by absorbing UV radiation. Only a fraction of UV radiation is reflected – around four to five per cent, which is less than SPF 2. Both types therefore convert the majority of the radiation into a tiny amount of heat. The difference is smaller than the advertising would suggest.

The correct distinction is therefore not between physical and chemical, but between inorganic and organic.

Nevertheless, mineral filters do have their advantages. They take effect immediately after application, tend to remain on the surface and are considered particularly gentle on sensitive or irritated skin. Their disadvantage is that they can leave a whitish film, particularly on darker skin tones. And titanium dioxide on its own does not provide protection against the deep UVA spectrum – zinc oxide is needed for that.

A filter such as Tinosorb M illustrates just how much the old classification is beginning to waver: chemically synthesised, but formulated as microfine particles, so that it also scatters light. A hybrid that cannot be fully classified into either category.

What makes modern filters better

The most exciting developments in recent years have been in the field of new-generation organic filters. Names such as Tinosorb S and M, Uvinul A Plus, Uvinul T 150 and Mexoryl are appearing more and more frequently on the back of high-quality sun creams.

Their greatest advantage is their photostability. Older filters can degrade when exposed to sunlight – avobenzone, the classic UVA filter, breaks down without additional stabilisers and thus loses its protective effect. Modern filters remain stable for hours. They also cover a broader spectrum, including low-energy UVA, and can be formulated into lightweight textures without leaving a white film.

For you in Zurich, one point is particularly interesting. This modern generation of filters is approved in Switzerland and the EU, whilst the US has been stuck with an older set of filters for years – not because the new ones are unsafe, but because of a regulatory backlog at the US regulatory authority. A well-formulated sun cream from Swiss or European shelves is therefore technologically a step ahead of most US products.

Are chemical filters harmful?

We often hear this question, and it deserves an honest answer rather than a panic reaction.

The debate was sparked by a FDA study, published in the JAMA. Six older organic filters were detectable in the blood after application – at concentrations exceeding the threshold at which the US authorities require additional safety data, and this was already the case on the very first day. That sounds worrying, but for the time being it only tells us one thing: the substance is entering the body. It says nothing, however, about whether any actual harm is being caused.

If you look at the health data, the picture becomes less worrying. For oxybenzone, the most widely studied of these filters, a systematic review no documented negative effects on fertility, hormonal balance or child development. Some links, such as those relating to the thyroid, remain unclear and are being investigated further.

The EU is not waiting to see what happens, but is taking a precautionary approach to regulation: several older filters are now subject to stricter concentration limits, and in 2024 the European Commission raised concerns about octinoxate. Then there is the environmental aspect. Oxybenzone and octinoxate have been linked to coral bleaching and are banned in places such as Hawaii. It should be noted, however, that some of these studies used concentrations higher than those actually found in the sea.

Our conclusion: Modern filters sidestep this whole debate because they are more stable and offer a cleaner profile. You don’t have to choose between good protection and feeling comfortable. By far the worst option remains: no sun cream at all.

How to read a sun cream label

On the shelf, a few points can help you make your choice.

The SPF is less indicative than most people think. It relates exclusively to UVB rays, i.e. sunburn. The number says nothing about protection against UVA rays – and therefore against skin ageing.

There is a specific symbol for this. In Switzerland and the EU, a cream may only claim to offer UVA protection if this accounts for at least one third of the SPF. If it meets this requirement, it bears the «UVA» inside a circle. Look specifically for this symbol, not just for a high number.

You should be sceptical of claims such as «sunblock», «total protection» or «100 per cent». No sun cream blocks everything, and you always need to reapply it. Speaking of quantity: most people don’t apply enough – the stated SPF only applies if the cream is applied generously.

Which filter is right for your skin?

There’s no single ‘best’ sunscreen. What’s right for you depends on your skin type, how sensitive your skin is, and whether you’re using the cream every day at the office or need it for a day out by the lake.

That’s exactly what we look at in the free AURA 3D skin analysis. We analyse your skin right down to the deeper layers and recommend the right sun protection for you – honestly and independently, as we don’t sell the products we recommend ourselves. Book your free AURA 3D skin analysis in Zurich now.

And if you’re wondering which treatments are actually suitable for the summer, we’ve broken it down here – Can you have a HydraFacial in the summer?.