What a day in the sun does to you

News that scientists have found a rogue gene which causes the most deadly form of skin cancer, is just another reminder about how dangerous it is to sit in the sun all day without proper protection. If, as forecasters suggest, next weekend could be a hot one in London, just remember that sitting out in the garden all day is the main cause of wrinkles and premature ageing as well as skin cancer. But what actually happens to our bodies if we lie in the sun with no protection?

Mike Brown, scientific suncare adviser for Boots, explains the changes that take place hour by hour over one single day of unprotected sunbathing on a fairly hot day. It could affect the rest of your life...

9am

The sun is low in the sky so the burning effect is not great. However, UVA levels are already fairly high and the skin starts to absorb UVA immediately.

9.15am

UVA starts to generate free-radicals in the skin (highly reactive molecules which can damage skin tissue and DNA, the skin's genetic material). But at this stage, natural anti-oxidants in the skin help neutralise them. The initial direct damage to DNA starts to occur as the skin begins to absorb UVB. The skin begins to warm up and evaporation of moisture takes place, with the skin starting to become dry.

10am

UVB intensity is now increasing rapidly as the sun climbs higher in the sky. DNA is increasingly damaged and some skin cells may become lethally injured. Some cell death is now inevitable in the skin.

11am

The increasing levels of both UVA and UVB start to produce more free radicals which become increasingly difficult for the skin's natural anti-oxidants to cope with. These free radicals can start to affect many biological functions in the skin. The structural matrix of the skin, made up from collagen and elastin, is damaged by this attack. It is this matrix that gives our skin its natural elasticity, but over time the free radical damage slowly builds up and becomes visible as wrinkling and skin ageing.

By now, the skin will have already had enough exposure to ultraviolet light to have started the sunburn reaction. In a few hours, some slight pinkness will appear even if you get out of the sun immediately.

12pm

Damage to skin cells is now severe enough for the cells to begin releasing chemical messengers (cytokines and histamines) which start the body's damage defence mechanisms. These materials are the skin's way of "shouting for help" and the response is to trigger the skin's repair mechanisms and to increase blood flow to the skin to transport more oxygen and nutrients needed to repair the damage. Sufficient UV has now been absorbed to cause a pink sunburn - but since this is a delayed reaction, it will not occur for a few more hours yet. However, sunburn cannot now be avoided - even applying suncream now will not prevent it.

1pm

By now, the immune defence system has been damaged. This system is responsible for identifying and destroying UV-damaged cells - but at this point its ability to do this is reduced by half. As another measure, to try to prevent further damage, the skin responds to the UV by " switching on" the genetic pathways which lead to melanin production. Evidence of this is the start of a tan in four to five days' time.

A tan is an indication of damage; the body's last-ditch attempt to protect itself - not a healthy cosmetic sign. The amount of UV accumulated in the skin is sufficient to guarantee burning red skin and significant discomfort later in the day. However, because of the delay in the sunburn reaction, you will still not yet be aware of the pain to come.

2pm

UV levels are five times stronger than at 9am. The skin starts to take on a permanent warm glow and to become taut - the first signs of inflammation resulting from the UV damage - and it will now show the first signs of turning pink. This is the body's warning that things are becoming serious. Your ability to combat free radicals and the damage they cause is reduced. Premature skin ageing is accelerated and skin cancer risk is increased.

3pm

The sun may now be past its maximum burning strength but the skin will be visibly pink and hot from sunburn. DNA damage will now be severe, many skin cells having already been "killed". The damaged cells now begin to produce a protein called p53, the cell messenger which tells the skin cells to stop dividing and producing new cells until all the sun damage has been repaired. The more p53 is released, the less new cells the skin can produce.

4pm

The UV is still hammering the immune system, which is now likely to be down to a third of its normal activity. By evening, delayed effects of exposure mean that the skin is going to be very red and painful and you may have a fever. The combination of severe sunburn and immune system damage which has now occurred, means your risk of getting skin cancer in later life has probably just doubled.

5pm

The delayed sunburn is really kicking in and the skin will be very red and uncomfortable. The natural DNA repair mechanism has switched on and the skin does its best to repair the day's onslaught. However, DNA repair will probably not be finished even by the morning. So, any further sunbathing tomorrow will be on top of existing unrepaired damage.

7pm

Pain is severe and the skin very inflamed - possibly blistered. Nausea and fever may set in and you may need medical attention. Applying after-sun is important to stop the drying effects and help prevent peeling. But the DNA damage is there for good - it cannot be reversed.