Why Your Skin Has a Memory for Sunlight
A sunburn heals in a week. The damage that caused it does not necessarily go with it. Ultraviolet light injures DNA directly, and while the skin has remarkably good machinery for repairing that injury, the repair is not perfect and the errors accumulate. This is why sun damage appears decades after the exposure that caused it, why one side of a long-distance driver’s face ages faster than the other, and why the single most useful thing anyone can do for their skin is boring.
A sunburn heals in a week. The damage that caused it does not necessarily leave with it. The sun’s ultraviolet light damages the instructions inside your skin cells, and although your body is remarkably good at repairing that damage, the repair is not perfect. The mistakes build up. That is why sun damage shows up decades after the sunbathing that caused it, why one side of a long distance driver’s face ages faster than the other, and why the single most useful thing you can do for your skin is also the most boring.
Why Your Skin Has a Memory for Sunlight: the essentials
The short version
Damages DNA directly, causing the lesions behind sunburn and most skin cancers
They damage the instructions inside your cells directly. That is what causes sunburn and most skin cancers.
Penetrates deeper, generates reactive oxygen, degrades collagen and drives pigment
They go deeper, break down the fibres that keep skin firm, and set off brown patches.
Nucleotide excision repair, which finds and replaces damaged DNA segments
Your cells find the damaged section, cut it out, and rebuild it.
Repair is good, not perfect; unrepaired errors persist in the cell line
The repair is very good, but not perfect. What gets missed stays there and is passed on.
Photoageing: brown patches, broken vessels, rough spots, loss of firmness
Brown patches, tiny broken veins, rough scaly spots, and skin that has lost its firmness.
Reducing exposure at any age reduces further accumulation
Cutting down your sun exposure at any age slows down what gets added from now on.
Two kinds of light, two kinds of harm
Two kinds of light, two kinds of harm
Ultraviolet reaching the ground comes in two forms that matter here. UVB is shorter, mostly absorbed in the epidermis, and is directly absorbed by DNA, which is what makes it the principal cause of sunburn and of most skin cancers. UVA is longer, penetrates into the dermis, and does most of its damage indirectly, by generating reactive molecules that degrade collagen and stimulate pigment cells.
Two kinds of ultraviolet reach the ground and matter here. One is absorbed mostly in the top layer of skin, and it is taken up directly by the instructions inside your cells. That is what causes sunburn and most skin cancers. The other kind goes deeper, into the layer below, and does its damage in a roundabout way, by creating unstable particles that eat away at the fibres holding your skin firm and prod your pigment cells into action.
The practical difference is that one announces itself and the other does not. UVB gives you redness within hours, which is a warning. UVA gives you nothing at the time and a great deal later, which is why a person can be certain they never burn and still show substantial sun ageing.
The practical difference is that one warns you and the other does not. The burning kind turns you red within hours, and that redness is a warning. The deeper kind gives you nothing at the time and a great deal later. That is why someone can be quite certain they never burn and still have skin that shows years of sun.
UVA also passes through window glass, while UVB largely does not. This is the mechanism behind the well-documented cases of asymmetric facial ageing in people who spent years driving, with the window side markedly more damaged. It is the most vivid demonstration available that a dose you never noticed is still a dose.
The deeper kind also comes through window glass, while the burning kind mostly does not. This is why there are well documented cases of people who drove for a living having one side of the face noticeably more aged than the other. It is the clearest proof you will ever see that a dose you never felt is still a dose.
The repair crew, and its limits
The repair crew, and where it falls short
When ultraviolet light strikes DNA it can fuse adjacent bases together, distorting the strand. Cells detect this, cut out the damaged section and rebuild it from the intact opposite strand. The system is fast, constant and extraordinarily effective, and it is running in your skin every time you step outside.
When ultraviolet light hits the instructions inside a cell, it can fuse two neighbouring parts together and bend the strand out of shape. Your cells spot this, cut out the damaged piece, and rebuild it using the undamaged matching strand as a guide. The system is fast, always on, and remarkably good, and it is running in your skin every time you step outside.
It is not perfect. Some damage is missed, some is repaired incorrectly, and the cells carrying those errors continue to divide. The errors accumulate over a lifetime in a way that is invisible for decades and then is not. The rare inherited condition xeroderma pigmentosum, in which this repair pathway does not work, is the clearest evidence of how much work it is doing: affected people develop skin cancers in childhood.
It is not perfect. Some damage is missed. Some is repaired wrongly. The cells carrying those mistakes keep on dividing, and the mistakes build up over a lifetime in a way that stays invisible for decades and then does not. There is a rare inherited condition in which this repair system does not work at all, and children with it develop skin cancers very young. That is the clearest possible evidence of how much work the system is doing for the rest of us.
There is a second, quieter consequence. Ultraviolet exposure also temporarily suppresses local immune surveillance in the skin, which is part of how damaged cells escape being dealt with. This is one of the reasons sunburn is not simply a painful inconvenience.
There is a second, quieter effect. Sun exposure also turns down your skin’s local defences for a while, which is part of how damaged cells slip through without being dealt with. It is one of the reasons a sunburn is more than just a painful few days.
What accumulated damage looks like
What years of sun actually look like
It is easiest to see by comparison. Skin on the outer forearm and skin on the inner upper arm are the same age and, in most people, look nothing alike. The difference is not time. It is exposure.
The easiest way to see it is to compare. The skin on the outside of your forearm and the skin on the inside of your upper arm are exactly the same age, and on most people they look nothing alike. The difference is not time. It is sunlight.
The features are consistent: mottled brown pigmentation, fine broken vessels, a coarser and more leathery texture, deeper etched lines, loss of firmness, and rough scaly patches that represent damaged cells growing abnormally. Those last ones are actinic keratoses, and they matter more than the rest, because a proportion of them can progress to squamous cell carcinoma if left.
The signs are always much the same: patchy brown colouring, tiny broken veins, a rougher and more leathery feel, lines carved in more deeply, skin that has lost its firmness, and rough scaly patches. Those last ones matter more than the rest. They are damaged cells growing abnormally, and a proportion of them can turn into a skin cancer if left alone.
This is the point where cosmetic and medical dermatology stop being separate subjects. A person arriving to discuss brown patches on the chest is presenting a record of ultraviolet exposure, and the same exposure that produced the patches may have produced something that needs a diagnosis rather than a laser. Which is why a proper assessment looks at the whole field, not only at the thing being complained about.
This is the point where the cosmetic side and the medical side of skin care stop being separate subjects. Someone who comes in to talk about brown patches on the chest is showing us a record of their sun exposure. The same sun that made the patches may also have made something that needs a diagnosis rather than a laser. That is why a proper assessment looks at the whole area, not only at the thing you came in about.
The useful conclusion
The useful part
Accumulated damage is not reversible in the sense of being undone, but it is treatable, and further accumulation is entirely within your control. Both halves of that sentence matter.
Damage that has already built up cannot be wiped out, but it can be treated. And whether more gets added is entirely up to you. Both halves of that sentence matter.
Treatment can genuinely improve pigment, texture, vessels and firmness, and can remove or destroy precancerous lesions. What it cannot do is restore the DNA repair history of the tissue, which is why treating sun damage without addressing ongoing exposure produces a result that quietly undoes itself.
Treatment can genuinely improve brown patches, texture, broken veins and firmness, and it can remove or destroy the spots that could become cancers. What it cannot do is give the tissue back a clean history. That is why treating sun damage without changing what you do in the sun gives you a result that quietly undoes itself.
The protective measures are unglamorous and well established: shade at the strongest hours, clothing and a hat, broad-spectrum sunscreen applied in a quantity most people find surprising, and no sunbeds at any age for any reason. It is not an exciting conclusion. It is simply the one the biology supports.
The things that protect you are dull and well proven: stay in the shade during the strongest hours, wear clothing and a hat, use a broad spectrum sunscreen in a quantity most people find surprisingly large, and never use a sunbed, at any age, for any reason. It is not an exciting conclusion. It is just the one the science supports.
Questions patients ask
If I never burn, am I safe?
If I never burn, am I safe?
No. Burning is caused mainly by UVB, while much of the ageing change and pigment is driven by UVA, which produces no immediate signal. People who tan easily and never burn still accumulate damage, and still develop skin cancers.
No. Burning comes mainly from one kind of ray, while a lot of the ageing and the brown patches come from the other kind, which gives you no signal at the time. People who tan easily and never burn still build up damage, and still get skin cancers.
Is a base tan protective?
Does a base tan protect me?
A tan represents damage that has already occurred, and the protection it offers is very small, on the order of a low single-digit SPF. It is not a meaningful protective strategy and acquiring one causes the harm it claims to prevent.
A tan is damage that has already happened, and the protection it gives is tiny, roughly the same as the lowest sunscreen you can buy. It is not a real way to protect yourself, and getting one causes the very harm it claims to prevent.
Can sun damage be reversed?
Can sun damage be undone?
The visible consequences can often be substantially improved with treatment, and precancerous lesions can be removed. The underlying accumulated genetic damage is not reversed, which is why continued protection matters as much after treatment as before it.
The parts you can see can often be improved a lot with treatment, and spots that could turn into cancers can be removed. The underlying damage inside the cells is not undone, which is why protecting yourself matters just as much after treatment as before it.
Does window glass protect me?
Does window glass protect me?
Ordinary window glass blocks most UVB but transmits much UVA, which is the wavelength driving pigment and collagen change. Prolonged time beside a window or in a car delivers a real cumulative dose.
Ordinary window glass blocks most of the burning rays but lets a lot of the deeper ones through, and those are the ones behind brown patches and loss of firmness. Hours beside a window, or a lot of driving, adds up to a real dose.
At what age is it too late to start protecting my skin?
Is there an age where it is too late to start protecting my skin?
There is no such age. Reducing exposure reduces further accumulation at any stage, and the risk of new precancerous lesions and cancers falls with reduced ultraviolet exposure regardless of what came before.
No. Cutting your exposure slows down what gets added at any stage of life, and the chance of new precancerous spots and cancers falls when you get less sun, whatever came before.
Where to go next
Talk it through with a physician first.
Every plan starts with an assessment of your skin, your anatomy and what you actually want to change.
This page is general health information, not medical advice. It cannot tell you whether a treatment is right for you. Approved uses, risks and results differ from person to person, and product labelling changes. Ask us at Dermatology and Laser Centre about your own situation, or make an appointment, and seek urgent care for any serious symptom.
