Harmful effects of excessive sun exposure

Time in the sun is good for you in moderation: it drives vitamin D synthesis and there is decent evidence for benefits to mood and sleep timing. Excess is another matter, and the harms extend well past sunburn. Here is what prolonged unprotected exposure does.

Skin damage (short-term)

How long it takes to burn depends on UV index and skin phototype; in strong sun a fair-skinned person can accumulate a burning dose in well under half an hour. The important thing is that the response is delayed: redness typically starts two to six hours after exposure and peaks between six and twenty-four hours. You cannot judge from how your skin feels at the time whether you have had too much. Severe cases blister, which is a second-degree burn, and warrant medical attention if extensive.

Skin damage (long-term)

Habitual exposure accelerates skin ageing: premature wrinkles, laxity, dull leathery texture, solar lentigines and mottled pigmentation. Skin also becomes thinner and bruises more easily, since UV degrades the dermal collagen and elastin that give it resilience. Over a lifetime, cumulative UV exposure is the principal modifiable risk factor for the three main skin cancers: basal cell carcinoma and squamous cell carcinoma, which are common and usually highly treatable when caught early, and melanoma, which is rarer but far more dangerous once it spreads.

Heat illness

Prolonged heat exposure carries risks separate from UV. The two are worth distinguishing, because the response differs. Heat exhaustion involves heavy sweating, weakness, cramps, nausea and headache with a broadly normal mental state; move to shade, cool down and rehydrate. Heat stroke means a core temperature above roughly 40 °C with confusion, disorientation or loss of consciousness, often with sweating having stopped; it is a medical emergency and can damage the brain and internal organs.

Warning signs to act on:

• Vomiting or nausea
• Throbbing headache
• Dizziness
• Weakness and muscle cramps
• Shallow breathing and rapid heartbeat
• Confusion, disorientation, or loss of consciousness

Dehydration

Sun and heat drive sweating, and without replacing the fluid the body's water and electrolyte balance shifts. Severe dehydration is dangerous and, untreated, can be fatal.

Common signs of dehydration include:

• Dark yellow urine, and less of it
• Dry mouth and swollen tongue
• Weakness and dizziness
• Increased thirst
• Inability to sweat
• Fainting and sluggishness
• Rapid heartbeat or palpitations

Drink regularly rather than waiting for thirst, and eat water-rich foods. In prolonged heavy sweating, replacing electrolytes matters as well as replacing water.

Hives or solar urticaria

Solar urticaria is a rare photodermatosis in which itchy weals or red patches appear within minutes of sun exposure. It is a mast-cell mediated reaction, quite distinct from sunburn in both speed and mechanism. It usually settles within a few hours of getting out of the light, though it can persist. It affects women more often than men, and extensive episodes can bring nausea, weakness and headache. It is also worth distinguishing from polymorphic light eruption, a commoner photosensitivity that appears hours to days after exposure.

None of this argues for avoiding the sun entirely. It argues for taking the dose seriously: broad-spectrum SPF 30 or higher, shade at peak hours, UPF clothing, and fluids. For how much sunscreen to use, see application techniques for sunscreens.