At what point does UV radiation cause permanent damage?

Question

I know that UV radiation damages the skins causing it to first tan and later burn. This damage is cumulative and can lead to wrinkles, cancer, and other health problems. I find it hard to believe that just stepping outside for 5 minutes would be enough to cause permanent damage so at what point is this damage become permanent? When you burn? After you've become heavily tanned? or is tanning itself proof of permanent sun damage?

Answer 1

There are specific enzymes that can repair the damage to the DNA.  The enzymes aren't perfect so any exposure could cause some damage even though the risk isn't very great for low exposures.  High rate of exposure can overwhelm the enzymes and kill skin cells.  The amount varies by person and in my opinion it would be about the point where the skin burns that you get anything that should be considered significant permanent damage.  It is cumulative so it matters if you do it often or just occasionally.  

Answer 2

UV radiation does damage the skin - but it does so by damaging the DNA of your skin cells.

If your skin is exposed to slight UV, it responds by tanning - making more melanin (in cells called melanocytes). Melanin absorbs UV, and therefore protects the DNA of other cells from damage.

If you receive a greater UV dose, it might damage enough of the DNA in enough of the cells to make them actually die. The natural response of cells to DNA damage is to try and repair the damage, but too much damage signals the cell to kill itself in order to avoid becoming cancerous. If enough of the cells die, it causes an inflammation - and this is sunburn. So any sunburn is a sign that some cells have been damaged enough to die: you could consider this "permanent" damage.

Some other effects of this are an increased aging of the skin: this is caused because cells which die off and are replaced by other cells have effectively aged faster than they otherwise would have (the number of times cells can divide and still retain their function is limited - so more divisions means less function).

Cancer, as you say, is another possible effect. If a cell's DNA is damaged, but it *doesn't* die (for example, if the damage was to the genes controlling the cell's suicide mechanism) - then it might start to divide uncontrollably, and this is a cancer.