UVA, melanin and pigmentation: melasma, hyperpigmentation and skin tone

Most sun-protection advice is built around sunburn, which means it is built around UVB. For anyone whose main concern is pigmentation — melasma, dark marks left by acne, uneven tone — that advice is aimed at the wrong part of the spectrum. Pigmentary disorders are driven predominantly by UVA and visible light, and that single fact changes which sunscreen works, when you need it, and why a low UV Index is no reassurance.

Why pigmentation is a long-wavelength problem

Melanin production has more than one trigger, and the different triggers behave differently.

• UVB produces delayed tanning: DNA damage in keratinocytes signals melanocytes, new melanin is synthesised over days, and the result fades over weeks. This is the pathway most sun advice addresses.
• UVA produces immediate pigment darkening by oxidising melanin precursors that are already present — pigment appears within minutes, with no burning to warn you. It also reaches the dermis, where melasma's characteristic dermal melanin deposits and altered basement membrane sit.
• Visible light, especially the violet-blue near 415 nm, stimulates melanogenesis directly through opsin-type receptors in melanocytes. Crucially, this pathway is strongly skin-tone dependent.

The controlled comparison that made this concrete irradiated volunteers with visible light and with UVA-1, matched for dose. In Fitzpatrick IV–VI skin, visible light produced pigmentation that was darker and far more sustained than UVA-1 — still visible at two weeks, where the UVA-1-induced pigment had largely faded. In Fitzpatrick II skin, visible light produced essentially nothing.

That asymmetry is the reason generic sun advice serves darker skin poorly. The wavelength that matters most is the one that generic advice does not mention, and it matters most for the people the advice is least often written for.

What melanin actually protects against

It is worth being precise here, because both overstatement and dismissal cause harm.

Melanin is a genuinely effective broadband absorber. Measured across skin types, the epidermis of deeply pigmented skin transmits roughly 5–6% of incident UVB where lightly pigmented skin transmits closer to 30%. That corresponds to an intrinsic photoprotection of about SPF 13 versus about SPF 3. It is real, substantial, and reflected in the epidemiology: melanoma incidence is many times lower in Black populations than in white ones.

But "more protection" is not "enough protection", for three reasons.

1. SPF 13 is not SPF 50. Dermatological guidance recommends SPF 30+ for a reason. An intrinsic SPF of 13 delays damage; it does not prevent it, and it does nothing about cumulative dose over decades.
2. Outcomes are worse, not better. Melanoma is less common in skin of colour but is diagnosed at later stages and carries markedly lower five-year survival. Part of that is a diagnostic gap — including a long-standing under-representation of darker skin in dermatology training images — and part is that common subtypes in these populations, such as acral lentiginous melanoma on palms and soles, are largely not UV-driven, which makes "I'm protected" a dangerously incomplete belief.
3. Melanin does least where it is needed most. Melanin absorbs strongly in the UVB and progressively less at longer wavelengths. Against violet-blue visible light — the strongest driver of pigmentation in deeper tones — its shielding is weakest. And more melanocyte activity means a more reactive pigment system, so the same insult produces a more pronounced and longer-lasting mark.

Melasma: a UVA and visible-light disease

Melasma is chronic, symmetrical facial hyperpigmentation, strongly associated with pregnancy, oral contraceptives and hormonal changes, and disproportionately affecting women with Fitzpatrick III–V skin. Sun exposure is the dominant modifiable trigger, and the relevant exposure is long-wavelength.

Its behaviour follows directly from that:

• It flares in summer and relapses after treatment in a way that tracks cumulative UVA and visible light, not sunburn events.
• It does not respect glass. A commute or a desk by a window supplies plenty of UVA and all of the visible light — see does glass block UVA.
• It does not respect winter. UVA falls only modestly in the colder months, so a patient who stops sun protection in October keeps the driver running — see why UVA changes far less with the seasons.
• It does not respect a low UV Index. Every condition that lowers the UV Index suppresses UVB preferentially, leaving UVA proportionally higher.

Post-inflammatory hyperpigmentation — the dark marks left after acne, eczema or injury — follows a similar pattern. The mark itself is caused by the inflammation, but sun exposure during healing deepens it and extends how long it takes to fade, which is why sun protection is part of acne management rather than a separate concern.

Why tinted sunscreens outperform clear ones here

This is the most actionable finding in the field, and it is not intuitive: for pigmentation, a tinted sunscreen beats a clear one of the same SPF.

The reason is iron oxides. Conventional UV filters — organic absorbers, zinc oxide, titanium dioxide — are formulated to be cosmetically invisible, which means by construction they transmit visible light. Iron oxide pigments do not: they absorb across the visible range, including the 415 nm violet-blue that drives melanogenesis. The tint is not a cosmetic afterthought, it is the active visible-light filter.

Randomised comparisons in melasma patients have found tinted, iron-oxide-containing sunscreens producing significantly better pigmentation scores than clear sunscreens of equivalent SPF over 8–12 weeks, and lower relapse rates through summer after treatment. The difference is attributable to visible light, since the UV protection was matched.

A practical consequence worth stating: matching the tint to your skin tone is not vanity, it is compliance. A tinted sunscreen you will actually wear daily outperforms a better-rated one that leaves a grey cast and stays in the drawer.

What to look for on the label

• Real UVA protection, verified. The EU circled-UVA mark, PA+++ or PA++++, or four to five Boots stars. SPF alone tells you nothing here — the labelling article explains each system.
• Iron oxides in the ingredient list — often listed as CI 77491, CI 77492 and CI 77499 — if pigmentation is your concern.
• SPF 30–50 as the base, since UVA-PF is pegged to SPF in the EU system: a higher SPF raises the UVA floor along with it.
• Daily, year-round, indoors near windows. This is the part most people drop, and it is the part melasma responds to.

And check the number that matches the problem. If your concern is pigmentation, the sunburn-weighted UV Index is not the right gauge — the UVA Index is closer to the exposure actually driving it.