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There are 3 types of U.V rays. UVA, UVB and UVC. These rays are based on how they penetrate the skin.

Atmospheric Physics
Electromagnetic radiation coming from the sun travels at 300,000 km/s. This radiation comprises wavelengths that vary from the very short (gamma rays and X-rays) to the very long (microwaves). The visible spectrum contains wavelengths between about 0.4 and 0.7 µm (400 and 700 nanometers, or nm).
Radiation at wavelengths less than about 400 nm is ultraviolet and accounts for about 7% of total solar radiation. When discussing ozone, we are concerned with radiation in the ultraviolet region of the electromagnetic spectrum. In addition to gamma rays and X-rays, which are absorbed high in the atmosphere, ultraviolet (uv) radiation in the atmosphere is divided into three spectra: uva , uvb, and uvc.

Uva falls right below visible light, with wavelengths that vary from 320 to 400 nm. Although it is not absorbed by ozone, uva is the least energetic and least damaging to the skin as presented in sun light as it only makes up between 2 to 8% of natural sunlight

Uvb radiation, which ranges in wavelength from 280 to 320 nm, is more energetic than uva and thought to be harmful to the biosphere. Fortunately, it exists in lesser amounts and is largely absorbed by ozone. It makes up 92 to 98% of natural sunlight.

Uvc, at 200 to 280 nm, which is the most energetic and most damaging but least prevalent of the uv radiation types, is totally absorbed by ozone and normal diatomic oxygen high in the atmosphere.

Think of it this way, UVA is the Ageing ray, and is used predominately in tanning beds and sun lamps. You don't burn in UVA which is why tanning salons claim it is safe. But this is misinformation on the part of the Bed operator trying to sell you minutes. UVA will penetrate into the lower levels of the skin, where the skin is being created. This damage to the newly forming skin cells increases the chance of skin cancer, wrinkling, freckling, aging, and liver spots ( which have nothing to do with your liver, it is an accumulation of melanin which is the skins way of creating its own sun protection like a sun umbrella)

UVB is the Burning ray. It penetrates the upper layers of the skin and destroys the already matured skin cells. Over exposure will cause painfull sunburns that damage the skin and also can promote skin cancer. This is why sunscreen is necessary when you go outside at any time of day or any time of year.

UVC is the Cancer ray. Although it is completly absorbed by the ozone layer, because holes in the ozone are appearing over inhabitated areas, countries like Australia and New Zealand are seeing increased amounts of skin cancer in a broader spectrum of the population. Slip, Slop, Slap is a Government advertising campaign in these countries. Slip on a shirt, Slap on a hat, Slop on some sunscreen as the little kiwi says.

Incidental sun, such as walking to the store, working in the backyard, or sitting by a window, can add up to exposure similar to two weeks on a tropical holiday per year. Look at the left hand, (or right hand if in Europe)of someone who has commuted to work for several years. The hand that faces the window will have more freckling and be more wrinkled than the less exposed hand. This is why sunscreen should be worn everyday and every season. It should be no less than an S.p.f 15.
(sun protection factor) You really don't need more than a S.P.F of 30 as protection only goes up by 1/2 of a percent after 35. S.P.F 15 protects from about 91%, 25 from about 93% and 30 from about 95%.

The numbers are how much more exposure you can have to UVB without burning. Lets say you have fair skin, (which produces less melanin, a pigment found between the layers of the skin, produced by melanocytes, which pump out this ink like liquid to prevent further damage), red hair, blue eyes, a type 1 on the skin fairness level. If, without sunscreen, you can only stay in the sun for 15 minutes before you start to burn, then an S.P.F of 15 will give 15 times that amount or 3 hours and 15 mins. Some people belive a darker skin will provide a natural S.P.F, which is true to some degree, but even a very dark african-american skin, a 5 on the skin fairness scale, only has a natural UV protection of about a 2. Sunscreen is still a requirement for every skin. But , sunscreen needs to be reapplied every 2 hours to maintain this level. Reapplication will not increase the time. Also proper application is necessary. You should use one to two shot glasses of lotion per person. It must be applied 20 minutes before exposure as it needs to be absorbed by the skin to provide maximum protection.
Some sunscreens are chemically based, like paba, or parsol 1789. they work by absorbing the uv radiation and breaking it down. The chemical protection will weaken after 2 hours, even more often when sweating, swimming or rubbing with a towel. It will lose its ability to protect, which is why reapplication is required. Sunscreen manufacturers can no longer claim a product to be water proof as this is has been disproven, only a water resistant claim is allowed. Some lotions act like mirrors reflecting back the suns rays. These are known as Titanium dioxide and zinc oxide. You might remember those paste sticks that you ran over your nose and ears and were white and chalky. Now because of new technology, they are micronized, pulverized so small it is transparent. Because these are literally crushed rocks, they do not wear off and are sweat and water resistent, they are also less likely to cause allergic reactions. They do not require reapplication. They also have the added benefit of protecting against UVA. So when purchasing a product for Sun protection, make sure it has a UVA and UVB protection factor.

Some studies have shown that a lack of Vitamin D, which is a byproduct of sun exposure, can increase the chance of Multiple sclerosis, Breast cancer and Rickets. Vitamin D, also known as the sunshine vitamin, is used by the body to make calcium usable by the body. This is why it is added to milk and other fortified foods. Some experts believe 10 to 15 mins of unprotected sun exposure will help give the body extra Vit. D. Some areas of the world that receive less sun, tend to have higher rates of these diseases, like Canada, We have what is known as D-deficient winters, which can last from Sept to March, when we get less than 8 to 10 hours of sun per day. S.A.D, Seasonal Affective Disorder, has also been attributed to a lack of sunshine. The symptoms of S.A.D, also known as the Winter Blues, can be relieved with a Light Box. These devices mimic natural light without the damage to relieve the depression, moodiness, and anxiety associated with S.A.D.

As with anything in life, moderation is key. Too much sun can cause wrinkles, sagging skin, excessive aging, freckles and skin cancer. Not enough sun, and there is a higher risk of breast cancer, M.S. and the winter blues.
A little sun goes a long way

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how can tanning kill you please give me some answers pleases
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