What UV light does to skin

UV light causes everything from sunburn to skin cancer and the wrinkles and sagging that come with age. But what exactly does it do in our bodies to wreak all that havoc?
Ultraviolet (UV) light is just a higher energy version of the light we see (visible light). And it's that higher energy that makes the UV in sunlight damaging to our cells and tissues.
To do any damage, UV light has to be absorbed. This happens at the molecular level. And it happens one electron at a time.
When a single electron absorbs a photon of UV light, that electron goes into a higher energy state. An excited electron like that makes a molecule behave in different ways — sticking to things it shouldn't stick to, changing shape, and generally messing with normal healthy cell business.
Luckily we evolved in a world that's saturated with UV light, so our cells have built-in repair kits for the damage UV radiation can do. But if the damage outweighs our capacity to repair, or our repair kits themselves get damaged, it's hello liver spots, cataracts and — way too often — skin cancer.
There are two kinds of UV radiation in sunlight — UVA and the higher energy UVB. And the trouble starts when they're absorbed by our more important molecules — DNA, RNA or proteins.

Sunburn: When UV meets RNA

The pain. The heat. The redness. The judgemental looks from everyone who thought to dress appropriately.
They're all part of the sunburn experience. And — judgemental looks aside — they are all caused by UVB radiation.
Sunburn is the body's way of healing from the damage caused by UV radiation. It's your standard inflammation to clear away damaged cells, plus a little pain.
Like all inflammations, it involves a bunch of messenger chemicals (cytokines) signalling for more blood to be delivered (causing redness and heat) and more white blood cells to clean up the mess. And the whole thing can be kicked off by UV damage to a single molecule — a small bit of RNA called U1.
When a photon of UVB hits U1 RNA in the nucleus of a cell, it changes the RNA's structure. That tiny tweak is enough to kick off the whole heat-redness sunburn cytokine festival.
The pain part of sunburn is down to one particular messenger molecule, CXCL5. It attracts white blood cells into the skin's dermal layer, activating pain fibres there.
So it's CXCL5 that makes us sensitive to pain for a couple of days after the event, which has made it a 'molecule of interest' for pharmaceutical companies looking for new pain-relief drugs.

Cataracts: UVA makes proteins sticky

The cloudy lens that makes it hard for cataract sufferers to see can also be caused by UV light.
The lens in your eye is normally clear, because the cells that make it up don't have a nucleus or other lumps to interfere with light — they mostly contain proteins, neatly aligned to let light through. But in 2014, researchers showed UVA light can trigger a chain reaction inside the lens that makes the proteins clump together, causing the cloudy look.
Our eyes have a special set of proteins, called chaperones, that work against the clumping, but if the clumping outperforms the chaperones, cloudy cataracts are the result.