Sun protection: The most useful and worst-used cosmetic

What it is

Sunscreen is the skincare product with the most evidence: it reduces sunburn, photo-ageing and, used regularly, the risk of some skin cancers. And it is the worst applied.

Many people associate it with 'the beach in summer', but UVA radiation passes through clouds and glass and is present all year. Most visible ageing of the face is due to the sun, and that can be prevented.

Two kinds of UV radiation

Both age and damage the skin, in different ways:

UVB (290–320 nm) → sunburn and skin cancer. SPF measures protection against UVB.
UVA (320–400 nm) → ageing (wrinkles, dark spots) without burning. SPF does NOT measure it.
SPF 30 blocks about 97% of UVB and SPF 50 about 98%: little difference on paper, but more margin when too little is applied.
UVA protection: in the EU, the 'UVA' logo inside a circle indicates it is at least one third of the SPF.

At the counter

When to recommend it

Every day, sun or cloud: UVA passes through clouds and glass
SPF 50+ with intense exposure or skin using photosensitising actives
Reapply every 2 hours of exposure and after water or sweating
Skin using retinol or acids: protection is essential

When not to

There is no sunscreen that lasts all day without reapplying

What to say at the counter. The line that convinces most: 'daily sunscreen is the best anti-ageing product there is, better than any serum.' And show the amount: for the face and neck, roughly two finger-lengths of product.

Warnings

Important TOO LITTLE: almost everyone applies much less than the 2 mg/cm² at which SPF is measured. Real protection drops a lot.
Important NOT REAPPLYING: with prolonged sun, water or sweat, protection disappears within a few hours. Reapply every 2 hours.
Caution THE UVA MYTH: SPF does not measure UVA. Look for the UVA logo (in the EU, inside a circle).
Caution PHOTOSENSITIVITY: with retinol, vitamin C or AHAs, sunscreen is essential.
Worth knowing THE ACNE MYTH: on acne-prone skin, choose non-comedogenic textures and cleanse well at night.

Pharmacokinetics

Absorption: There are organic filters (they absorb the radiation) and mineral filters (mainly reflecting and scattering).
Distribution: QUANTITY is critical: SPF is measured at 2 mg/cm², and most people apply much less, so the real protection drops a lot.
Metabolism: Filters are lost with sweat, water and rubbing: that is why it has to be reapplied.
Elimination: It is removed with the evening cleanse.

SPF 30 vs SPF 50+

Aspect	SPF 30	SPF 50+
UVB blocked (applied properly)	≈ 97%	≈ 98%
Margin if too little is applied	Smaller	Larger
When	Everyday exposure, applied properly	Intense exposure, fair skin or photosensitising actives
Reapplication	Just as necessary	Just as necessary

Because almost everyone applies too little, SPF 50+ gives a useful margin. But no number replaces applying enough and reapplying.

Self-assessment

Three questions. When you check your answers you will see the explanation for each one.

Why does the amount of sunscreen matter so much? Because it smells better Because SPF is measured at 2 mg/cm² and almost everyone applies much less, so real protection drops Because it looks better
What does SPF measure? All UV radiation How many hours it lasts Protection against UVB (not UVA)
How often should it be reapplied with prolonged exposure? Every 2 hours, and after water or sweating Once a day is enough Every 4–6 hours

Training content. It does not replace the summary of product characteristics or clinical judgement.

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