There is exactly one skincare product with clinical evidence for prevention of visible ageing over the long term — not correction, prevention. It's sunscreen. The reference study (Hughes et al., JAMA Dermatology, 2021) showed that consistent SPF use in adults prevented and partially reversed markers of photo-ageing over 4.5 years. No serum has this level of longitudinal evidence.

UVA vs. UVB: The Distinction That Changes the Urgency

UVB: responsible for visible sunburn, erythema and skin cancer. Partially blocked by clouds, doesn't penetrate glass.

UVA: penetrates to the deep dermis, fragments collagen and elastin, activates metalloproteinases that degrade the extracellular matrix. Penetrates clouds, car windscreens and office windows. Causes no visible redness. Responsible for 80% of visible ageing accumulated over a lifetime.

The Quantity Mistake

SPF efficacy studies use 2mg/cm². For an average face, that's ¼ teaspoon. Applying half doesn't result in SPF25 — it can drop to SPF10 or lower. This is why reapplication throughout the day is necessary in direct exposure.

SPF Myths

I don't go outside, I don't need it — UVA penetrates windows.
Cloudy days don't need SPF — clouds only block 20% of UV radiation.
Dark skin doesn't need SPF — darker phototypes have greater natural protection but aren't immune to photodamage.