What happens to your DNA when you spend too much time in the sun?

UVB radiation, the short-wave portion of sunlight, is absorbed directly by your DNA. This causes neighbouring DNA building blocks to stick together, forming so-called dimers. After as little as one hour of exposure, this can occur extensively across the entire genome. UVA radiation, which makes up more than 90% of the sunlight that reaches Earth, works differently: it activates substances in skin cells that produce harmful oxygen radicals, which then damage DNA indirectly. UVA also forms dimers directly in living skin.

If a cell divides before that damage has been repaired, the copying process makes errors. This produces a recognisable mutation pattern that is specific to sunlight damage. That same pattern is found in skin tumours. Chronic, excessive exposure accumulates these mutations, and this is a primary cause of both common skin cancers (basal cell and squamous cell carcinoma) and the more dangerous melanoma. Skin cancer is therefore the most common cancer in the United States.

Fortunately, your body has built-in repair mechanisms that can detect and cut out UV damage. These systems work well with limited exposure, but become overwhelmed when the damage is too great or too repeated. DNA damage from the sun also plays a role in sunburn, in the temporary suppression of the local immune system in the skin, and in the tanning process itself.

If you use certain medications, pay extra attention. Some antibiotics (such as fluoroquinolones) and skin medications (such as psoralens) significantly amplify the damaging effect of UV on DNA. If you are regularly in the sun while using these agents, discuss this with your doctor or pharmacist.

There are cautious indications that antioxidants such as vitamin C, vitamin E and plant-based polyphenols might partially limit UV damage. However, that evidence is still weak and largely comes from animal studies. No firm conclusions for humans can be drawn from it.