Sunscreen ingredients explained — what the filters on the label actually do.

Sunscreen is the single most evidence-backed topical intervention for preventing visible skin ageing. The evidence for this is not contested. What is less clear to most people is what the ingredients on the label actually do — and why different formulations behave so differently on skin.

UV radiation and why filtering it matters

Ultraviolet radiation from the sun reaches the skin in two clinically relevant wavelengths.

UVB (280–315 nm) causes sunburn and directly damages DNA in keratinocytes. Its intensity varies with season, latitude, time of day, and altitude. SPF — the Sun Protection Factor — measures protection against UVB specifically.

UVA (315–400 nm) penetrates deeper into the dermis, reaching the fibroblasts that produce collagen and elastin. It generates reactive oxygen species that degrade collagen, and it is the primary driver of photoageing — the fine lines, laxity, and pigmentation changes associated with sun exposure. UVA intensity is more constant throughout the day and year than UVB.

A sunscreen that protects against UVB but not UVA prevents burning while still allowing the chronic, cumulative damage that produces visible ageing. Broad-spectrum protection covering both wavelengths is not optional.

How chemical (organic) filters work

Chemical UV filters are organic molecules that absorb UV energy at specific wavelengths and release it as heat. They are the active ingredients in most lightweight, cosmetically elegant sunscreen formulations.

Common chemical filters include:

Avobenzone — absorbs across the full UVA spectrum. It is photounstable on its own, meaning it degrades under UV exposure. Photostabilisers such as octocrylene or Tinosorb S are added to counteract this.

Octinoxate and octisalate — primarily UVB filters. Octinoxate is under regulatory scrutiny in some markets for potential endocrine activity.

Homosalate — a UVB filter, often used in combination with other filters to broaden coverage.

Tinosorb S and Tinosorb M (available in the EU and Australia, not approved in the US) — photostable broad-spectrum filters with strong UVA and UVB coverage. Their availability significantly improves the quality of sunscreens in markets outside the US.

Mexoryl SX and XL (widely available in Europe and Australia) — strong UVA filters, photostable.

The advantage of chemical filters is texture: they tend to be lightweight, transparent, and easier to formulate into cosmetically acceptable products.

How physical (mineral/inorganic) filters work

Physical UV filters — zinc oxide and titanium dioxide — sit on the surface of the skin and scatter or reflect UV radiation. Modern understanding also attributes an absorption component to both.

Zinc oxide provides broad-spectrum coverage across UVB and the full UVA range. It is photostable. Its main drawback is cosmetic: it tends to leave a white or greyish cast, which becomes more apparent on darker skin tones at higher concentrations. Micronised forms reduce this significantly.

Titanium dioxide primarily covers UVB and short-wave UVA. It is more cosmetically elegant than zinc oxide — less white cast — but provides narrower UVA coverage. Many mineral formulations combine both.

Mineral filters are generally better tolerated by sensitive, rosacea-prone, and reactive skin. They are also the preferred option during pregnancy, when the precautionary principle suggests avoiding ingredients with uncertain systemic absorption profiles.

What SPF actually means

SPF measures how long it takes for UVB radiation to cause a perceptible reddening of protected skin compared to unprotected skin. SPF 30 means it takes thirty times as long to burn.

SPF 30 filters approximately 97% of UVB radiation; SPF 50 filters approximately 98%. The difference is real but narrow. The more meaningful distinction is SPF 30 versus no SPF — not SPF 30 versus SPF 50.

SPF is not additive across products. SPF 30 moisturiser plus SPF 30 foundation does not equal SPF 60.

The amount of sunscreen applied determines the SPF achieved. Clinical testing uses 2 mg per square centimetre — approximately a quarter of a teaspoon for the face. Most people apply significantly less. Under-application is the most common reason sunscreen provides less protection than the label states.

The PA rating system

The PA system — used in Japan, South Korea, and increasingly on Australian products — rates UVA protection on a graded scale from PA+ to PA++++. For meaningful broad-spectrum protection, PA+++ or PA++++ alongside an adequate SPF covers both UV wavelengths.

Products without a PA rating often use broad-spectrum labelling, which in most jurisdictions requires meeting a minimum UVA protection threshold but does not specify the level.

Choosing the right formulation

The best sunscreen is the one used daily, in adequate quantity, and reapplied after swimming or significant sweating.

Chemical filters are generally better for oily skin, darker skin tones, and layering under makeup.

Mineral filters are generally better for sensitive, reactive, or rosacea-prone skin, and during pregnancy.

Hybrid formulations combining chemical and mineral filters are common in Australian and Asian markets. They often offer the cosmetic elegance of chemical filters with the photostability and broad UVA coverage of mineral ones.

Reapplication every two hours in direct sun is not excessive — it is the condition under which SPF testing is conducted. A product applied once in the morning does not provide full protection through an afternoon outdoors.