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In the summer, there is always an increase in advertisements and public awareness campaigns to get people to wear suntan lotion, to reduce the risk of sunburn (naturally).

Some of these advertisements often say that it is still possible to get a suntan while wearing suntan lotion, presumably to encourage those with higher resistances to the sunrays (or those that are consious of the way they look) to still where sun lotion.

How can this be true? If you are being protected from sunburn, surely you are also protected from suntans? My understanding is that a suntan is just underdeveloped sunburn - likewise, sunburn is over exposed suntan, both being caused by the same exposure to the same UV rays. Is this not correct? If it is, how can protection from sunburn not also protect you from sun tans?

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Not an answer, because I can't source the following. However; sun burns tend to be caused by UVB while tans are caused by UVA. Most sunscreen block UVB to a larger extent than UVA. Also the lasting tan will start before the visible immidiate tan, meaning that stretching the process out gives less chance of burns by overtanning. –  Taemyr Jul 28 at 9:22
    
@Taemyr: My first reaction to your claims is to be skeptical. It is a non-answer, and I should probably delete it. If you could find a definitive source, it would be much better. However, if you could find a notable claim, we can turn it into a new question. I'd be interested to hear if it was true. –  Oddthinking Jul 28 at 10:01
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@Oddthinking Well, a quick google seems to indicate that what I say about UVB being the cause of burns, and blocked by most screens agrees with wikipedia. Acording to Wikipedia both UVA and UVB causes tanning, so blocking UVB but not UVA will prevent burning without preventing tanning (or skin cancer). What I say about lasting tans starting earlier than immidiate tans seems to be completely false. tanningtraining.com/btc/ch4.html?prodid=2. –  Taemyr Jul 28 at 10:14
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Its worth noting that a suntan is not a "light" sunburn. A tan is your body attempting to protect itself from sunburn by increasing Melanin production (the oxidation of the existing Melanin also causes a darkening). Obviously they have a very similar cause and will generally both occure at the same time but they are not the same –  Richard Tingle Jul 28 at 14:04
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In the first instance; have a look at this wiki article; en.wikipedia.org/wiki/Sun_tanning In particular the third paragraph "Melanogenesis leads to delayed tanning, and first becomes visible about 72 hours after exposure. The tan that is created by an increased melanogenesis lasts much longer than the one that is caused by oxidation of existing melanin, and is also actually protective against UV skin damage and sunburn, rather than simply cosmetic. " –  Richard Tingle Jul 28 at 14:07

1 Answer 1

up vote 6 down vote accepted

Yes, if you use sunscreen. No, if you use sunblock.

Sunscreen absorbs an amount of radiation, but not all (sun protection factor is used to measure effectiveness):

Sunscreen [...] absorbs or reflects some of the sun's ultraviolet (UV) radiation on the skin exposed to sunlight and thus helps protect against sunburn [1].

Sunscreen combines organic and inorganic chemicals to filter the light from the sun so that less of it reaches the deeper layers of your skin. Like a screen door, some light penetrates, but not as much as if the door wasn't present [2].

Sunblock reflects light:

Sunblock, on the other hand, reflects or scatters the light away so that it doesn't reach the skin at all. The reflective particles in sunblocks usually consist of zinc oxide or titanium oxide [2].

Sunscreens absorb both UV-A (the radiation that can lead to cancer) and UV-B (the radiation that leads to sunburns) and transforms them into heat [2].


References:

  1. Wikipedia contributors, "Sunscreen," Wikipedia, The Free Encyclopedia, http://en.wikipedia.org/w/index.php?title=Sunscreen&oldid=618251463 (accessed July 28, 2014).
  2. Anne Marie Helmenstine, Ph.D. How Does Sunscreen Work? Available from http://chemistry.about.com/od/howthingsworkfaqs/f/sunscreen.htm (accessed 28.07.2014)
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Does this mean that if sunscreen is being applied properly, you can guarantee that you will not be sunburnt while also receiving a sun tan? Or can sunscreen applied properly still let in enough UV rays to cause sunburn? –  Dr R Dizzle Jul 28 at 12:40
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@DrRDizzle Applying sunscreen increases the time needed for suntan and sunburn to appear. –  Cornelius Jul 28 at 12:43
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@DrRDizzle Sunscreen weakens the sun radiation by a certain factor. This means that it both takes longer to get a sunburn and to get tanned. When you use sunscreen with a low protection factor, you might get some tan, but you also risk sunburn when you have a lot of exposure and sensitive skin. –  Philipp Jul 28 at 12:54
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Note that also sunblock doesn't block all sunlight, otherwise you wouldn't even be able to see your skins color. –  PlasmaHH Jul 28 at 15:07
    
For this answer to be true, doesn't this imply that sunblock effectively has an SPF of infinity? –  Michael Jul 28 at 18:17

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