SPF explained by a formulator: the UV filter guide you actually need

The sunscreen aisle is overwhelming. I say that as a formulator who works with ingredients every day. There are dozens of UV filters, each with a different mechanism, stability profile, and skin tolerance record — and most brands aren't going to explain any of it to you.

So here's what you're actually looking at.

Chemical, mineral, and hybrid: the three categories

Chemical filters absorb UV radiation and convert it into heat, which is then released from the skin. They're what make most SPFs feel lightweight and invisible on the skin. The downside: some of the older chemical filters degrade in sunlight (photounstability — a real problem in a product designed for sun use), some are known allergens, and some bind to the skin's TRP receptors, the nerve channels responsible for heat, stinging, and itch. That's why certain SPFs make your face feel like it's warming up.

Mineral filters — zinc oxide and titanium dioxide — work differently. They sit on the skin and physically block UV, absorbing the majority and scattering some. No TRP binding, no reported allergic reactions in the clinical literature. Full stop. The default choice for reactive, rosacea-prone, or post-procedure skin. The trade-off is texture — they can feel heavy and leave a white cast, though modern formulations have improved significantly.

Hybrid formulas use mineral filters as the base but layer in chemical filters to improve texture, coverage, or photostability. Most of the SPFs dermatologists actually recommend fall into this category.

What SPF actually means (and what it doesn't)

SPF measures UVB protection only. The rays that burn. The number tells you how much longer you can stay in the sun before burning relative to bare skin — SPF 30 means 30 times longer, SPF 50 means 50 times longer. In theory.

In practice, that calculation assumes perfect, even application at the correct amount — and most people apply far less than they should. The real difference between SPF 30 (97% UVB blocked) and SPF 50 (98%) is smaller than the numbers suggest, but SPF 50 gives you a useful buffer when application is imperfect. Reapplying every two hours is non-negotiable regardless of the number on the bottle.

What SPF doesn't tell you anything about: UVA protection. UVA is what causes long-term damage, photoageing, and contributes to skin cancer. In the UK and EU, look for "broad spectrum" on the label, or the UVA circle logo — which means the UVA protection is at least a third of the SPF value. Without that, you're missing a significant part of the picture.

The filter-by-filter breakdown

Here's what you're looking at when you read an SPF INCI list. Trade names first, then the INCI name — the one that's actually on the ingredients label.

Tinosorb (BASF) Tinosorb S: Bis-Ethylhexyloxyphenol Methoxyphenyl Triazine Tinosorb M: Methylene Bis-Benzotriazolyl Tetramethylbutylphenol

New generation. Both are broad spectrum, highly photostable, and don't bind to TRP receptors — so no flushing or stinging on reactive skin. These are what you want to see if you have sensitive or rosacea-prone skin.

Mexoryl (L'Oréal) Mexoryl SX: Terephthalylidene Dicamphor Sulfonic Acid Mexoryl XL: Drometrizole Trisiloxane

Also new generation, though L'Oréal-patented so you'll mainly find them in La Roche-Posay and Vichy. SX is UVA-focused; XL covers both. Photostable and well tolerated.

Avobenzone (Butyl Methoxydibenzoylmethane)

The most common UVA filter globally — in theory, a decent one. The problem is genuine photounstability. UV exposure causes it to break down via keto-enol tautomerism, generating reactive species as it goes. It needs to be stabilised with other filters (Tinosorb, octocrylene) to work properly. A formula with avobenzone and no stabiliser is worth avoiding. A well-formulated one with stabilisers is fine.

Octinoxate (Ethylhexyl Methoxycinnamate)

One of the most widely used UVB filters globally, mostly because it's cheap and easy to formulate with. Photounstable and associated with some irritation reports. Banned in Hawaii alongside oxybenzone for reef toxicity — the evidence on marine impact was substantial enough to trigger legislation.

Octocrylene

Used as a UVB filter and commonly added to stabilise avobenzone. The catch: it degrades into benzophenone over time, which is a known irritant and allergen. A growing list of contact allergy reports, particularly in older or poorly stored products.

Homosalate

Extremely common UVB filter — in more products than most people realise. The European Commission has recommended lowering its maximum permitted concentration due to endocrine disruption concerns in published literature. Still fully approved, but worth knowing about.

Oxybenzone (Benzophenone-3)

The one to actually avoid. The most frequently reported contact and photocontact allergen of any UV filter. If you've had a sunscreen reaction and couldn't identify why, check for this first. The EU has lowered permitted concentrations due to safety concerns.

It also has the strongest evidence for coral reef damage alongside octinoxate. Studies have found it induces coral bleaching, damages coral DNA, and causes lethal deformities at concentrations measured in real ocean water near swimming areas. Hawaii banned it in 2018.

Zinc oxide / titanium dioxide

No reported allergic or photoallergic reactions in clinical literature. The safe default for reactive, rosacea-prone, eczema, or post-procedure skin. Zinc oxide provides full broad spectrum coverage; titanium dioxide is stronger on UVB and short UVA. Both have come a long way in texture — tinted versions work well if the white cast has been the barrier.

A note on "reef safe"

It sounds reassuring. It means nothing. There's no legal definition, no regulated standard — any brand can put it on any product. If you actually want an SPF that's less harmful in the water, check the INCI list and avoid Benzophenone-3 and Ethylhexyl Methoxycinnamate. That's the evidence-based version of reef-conscious SPF.

What I personally use

After years of buying dud sunscreens that either irritated my skin or sat on it like a film, I now swear by Ultrasun. Specifically the face SPF 30 in the tube. The tan activator SPF for body is great too. I'm not associated with them at all — I just genuinely love the texture and the fact that they use Tinosorb filters. My skin actually looks better after a day wearing it than it does without.

Micaela, Founder & Formulator / Neighbourhood Botanicals