紫外线主要来自太阳辐射，是太阳光谱中波幅较短的某一段的光线，人眼看不见。紫外线分三种， 有紫外线A， 紫外线B和紫外线C。据说紫外线C由于地球大气层的保护而无法到达地球表面， 可以互略不计，紫外线A和B可以用来杀菌消毒（晒太阳可以杀菌消毒即为一例），但是它们也可以对皮肤造成不可修复的伤害。
据统计，美国每年有200万人诊断出患有皮肤癌。每年新增的皮肤癌病例比乳腺癌，前列腺癌，肺癌和直肠癌加起来的总数都要多。在过去30年中，罹患皮肤癌的人数超过了所有其他癌症病人的总和，每5位美国人中就有一位会得皮肤癌。而在65岁之前，有40-50%的美国人会得皮肤癌。从1992年到2006年， 非黑色素瘤皮肤癌的病患增加了77% （关于皮肤癌的数据请参阅本文所附链接）。
那还得从40多年前的1970年说起。 当时我在安徽肥东县的老家务农。 老家属于水乡，以水稻为主要农作物，也种一些小麦和红薯。白居易说田家少闲月， 五月人倍忙， 说的是中国北方农村的情况。我们家乡最忙的月份却不是五月，而是阳历的3月和8月。3月是插秧的季节, 8月是收获水稻的季节。插秧时的3月白昼已经很长，但是天气还很冷，在田里干活时，无论男女老少，上身都穿着很厚的上衣， 甚至棉衣，腿上常常冻得通红 （因为在水田里干活， 大伙儿都只能穿短裤赤脚裸腿下地）。8月份则是另外一种景象：男人都光膀子干活（虽然大家仍然要赤脚下田，但其时天气暖和，泡在水里倒不那么难受）， 因此，说到紫外线， 最好以8月的大忙季节为例子。
等到带着草帽， 拿着镰刀下地时， 常常是天还没有亮，等走到地头， 才勉强看得见曙光，然后就开始了一天漫长的时光。从凌晨到过中午，至少有7-8个小时的时间，就那么面朝黄土（其实是面朝浅水）背朝天地割呀割。腰酸腿疼就不用说了，太阳的暴晒也很厉害。记得北方的田野里， 常常会有一些小树林， 或者几课大树，可以遮荫， 而我们家乡的水田里， 水田头， 却是一棵树苗都看不到，所以一点遮挡都没有。 男人统统光膀子，无一例外。在烈日的烘烤下，汗水流了一层又一层， 太阳又一层一层地把汗水晒干。待到晚上回家时， 毫不夸张地说，个个男人背上都是一层白花花的盐末子， 用手一划拉，盐末子就扑扑簌簌地往下掉。
收割的季节少说也得十几天，而暴晒的日子远不限于收割的季节。可以说， 从春耕大忙， 到8月收获， 再到种二季稻， 种小麦，红薯， 蔬菜， 甘蔗，等等，一直忙到开始结冰了， 才能闲下来。算起来， 除了下雨天， 几乎天天在地里忙， 那么长期的暴晒，把大多数人晒得黝黑黝黑。
但是， 就是这样暴晒， 也没听说谁被紫外线灼伤了，至于皮肤癌， 就更是闻所未闻。当然， 也有人被晒蜕皮，但蜕皮归蜕皮， 烧伤归烧伤， 蜕皮不等于烧伤。我自己也被晒得蜕过多次皮， 但从来就没有被烧伤过， 一次也没有。
这样看起来， 中国的紫外线， 准确地说，是中国40年前的紫外线， 要远比美国当今的紫外线温柔可爱。
何以见得？ 我几周前去加州， 在室外游泳池里只不过泡了2个小时，回到家里， 肩膀上和前胸通红一片， 又痒又疼， 显然是被烧伤了。
所以， 即便在国内不怕紫外线的人， 到了美国也不能不防。从前不怕太阳晒的人， 现在也不能不防。因为现在的工业发达， 保护地球的大气层可能已被破坏得太严重， 虽然各个家庭的小环境越来越宜居， 似乎整个地球这个大环境却越来越不宜居。还有一种可能性是，美国国土上空的大气层遭到破坏的程度，要比中国的更严重（这只是我的猜测，读者谅之）。
难怪美国的皮肤科大夫，都会谆谆告诫人们要使用防晒霜。纽约一位著名的皮肤科大夫Ellen Marmur在她关于皮肤护理的书中说，假如您任何护肤品都不愿意用，您至少应该使用防晒霜；假如您只能使用一种护肤品，其他概不使用，那这唯一的一款护肤品，应该是防晒霜，而不是其它。她的基本逻辑是，你如想呵护你的皮肤， 首先要保护它免受伤害，若等到太阳光已经对皮肤造成伤害之后，再去涂抹润肤霜，是舍近求远，事倍功半。
因此， 我要呼吁中国的女子， 护肤要趁早。不要等到紫外线已经给肌肤造成了不可挽回的伤害之后， 才想到护理肌肤。不要象一根蜡烛，为这个家庭烧尽了自己，到头来什么也没有落下。
What Is It?
Simply put, ultraviolet radiation (also known as UV radiation or ultraviolet rays) is a form of energy traveling through space.
Some of the most frequently recognized types of energy are heat and light. These, along with others, can be classified as a phenomenon known as electromagnetic radiation. Other types of electromagnetic radiation are gamma rays, X-rays, visible light, infrared rays, and radio waves. The progression of electromagnetic radiation through space can be visualized in different ways. Some experiments suggest that these rays travel in the form of waves. A physicist can actually measure the length of those waves (simply called their wavelength ). It turns out that a smaller wavelength means more energy. At other times, it is more plausible to describe electromagnetic radiation as being contained and traveling in little packets, calledphotons.
The distinguishing factor among the different types ofelectromagnetic radiation is their energy content. Ultraviolet radiation is more energetic than visible radiationand therefore has a shorter wavelength. To be more specific: Ultraviolet rays have a wavelength between approximately 100 nanometers and 400 nanometerswhereas visible radiation includes wavelengthsbetween 400 and 780 nanometers.
Where does it come from?
The sun is a major source of ultraviolet rays. Though the sun emits all of the different kinds of electromagnetic radiation, 99% of its rays are in the form of visible light, ultraviolet rays, and infrared rays (also known as heat). Man-made lamps can also emit UV radiation, and are often used for experimental purposes.
What does it do?
Light enables us to see, and heat keeps us from being cold. However, ultraviolet rays often carry the unfortunate circumstance of containing too much energy. For example, infrared rays create heat in much the same way as rubbing your hands together does. The energy contained in the infrared rays causes the molecules of the substance it hits to vibrate back and forth. However, the energy contained in ultraviolet rays is higher, so instead of just causing the molecules to shake, it actually can knock electrons away from the atoms, or causes molecules to split. This results in a change in the chemical structure of the molecule. This change is especially detrimental to living organisms, as it can cause cell damage and deformities by actually mutating its genetic code.
What stops it?
Ultraviolet rays can be subdivided into three different wavelength bands - UV-A, UV-B, and UV-C. This is simply a convenient way of classifying the rays based on the amount of energy they contain and their effects on biological matter. UV-C is most energetic and most harmful; UV-A is least energetic and least harmful.
Luckily,UV-C rays do not reach the earth's surface because of the ozone layer. When UV-C rays meet the ozone molecules at high layers of the atmosphere, the energy inherent in them is enough to break apart the bond of the molecule and absorb the energy. Therefore, no UV-C rays from the sun ever come into contact with life on earth, though man-produced UV-C rays can be a hazard in certain professions, such as welders.
UV-B rays have a lower energy level and a longerwavelength than UV-C. As their energy is often not sufficient to split an ozone molecule, some of them extend down to the earth's surface. UV-A rays do not have enough energy to break apart the bonds of the ozone, so UV-A radiation passes the earth's atmosphere almost unfiltered. As both UV-B and UV-A rays can be detrimental to our health, it is important that we protect ourselves. This can be done through a variety of ways. The most obvious is to reduce the amount of time one spends in the sun, particularly between the hours of 11 am and 3 pm, when the sun is at its highest in the sky. However, especially during the summer holidays, this does not always work out. More ways to protect ourselves can be found here.
Variability of UV
UV levels are not constant over the course of a day, or even over the course of a year. An obvious factor is the position of the sun in the sky. At noon, for example, theelectromagnetic waves emitted from the sun travel a much shorter path through the earth's atmosphere then they would at, say, 5 pm, and thus noon-time intensity is stronger. A second important parameter determining UV at the ground is the amount of ozone present in thestratosphere. Low ozone correlates with much UV. However, there are many other features of the environment that contribute to UV radiation variability. Most important are clouds. On cloudy days, UV levels are usually lower than during clear skies as clouds can deflect rays up into space. Clouds can, however, also lead to increased UV levels. This happens, for example, when the sun is not obscured by clouds but clouds in the vicinity of the sun reflect additional radiation to the ground. So a general rule is not to feel save from UV radiation just because it's cloudy!
The amounts of UV one is exposed to also varies with altitude. As a rule of thumb, UV levels increase about 4% for every 1,000 foot gain in altitude. This increase has nothing to do with being closer to the sun - any elevation you might gain would be miniscule in comparison to the distance from the earth to the sun, and so would have an insignificant outcome on UV levels. Instead, the increase is the result of a thinner atmosphere with a smaller number of molecules being present to absorb or scatter UV. Examples of such molecules are tropospheric ozone (commonly associated with smog) and aerosols, molecules that remain suspended in the air. Aerosols can be a multitude of substances - dust, soot, sulfates, etc. These aerosolsabsorb and scatter UV rays, and so cut down on the ultimate UV irradiance.
Other factors that have an influence on UV levels are the physical features of the land - sand, snow, and water all tend to reflect UV rays. This phenomenon is called albedo. Some of the ultraviolet rays reflected off the ground encounter scattering by air molecules, aerosols or clouds back down to the earth, thus increasing the totalirradiance. When there is snow on the ground the amount of time it takes for sunburn to occur is therefore significantly reduced.
Also, the closer one is to the equator, the more ultraviolet rays one is exposed to. This can be explained by the fact that the sun is usually higher at the sky at low latitudes. In addition, the ozone layer is thinner at the equator as it is over, for example the United States or Europe, and this also contributes to more UV.
Since the 1980s, polar regions are affected by the ozone hole. Under the ozone hole, biologically relevant UV levels are 2-3 times as high as they were before. Learn, based on real data, how UV levels are affected by the ozone hole bygoing to the experiments page! Here you can compare UV radiation measured by the NSF network in Antarctica with satellite ozone data.
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Ways to Protect Yourself
How to protect your skin
Sun burning is not a pleasant experience. Fortunately, we have ways to prevent it. Avoiding the sun between the hours of 11 am and 3 pm during day light saving time can reduce your UV exposure by more than 50%. If you have to be outside during these hours, seek shade, wear T-shirts on the beach, hats with a broad brim, and sunglasses, and apply sunscreen of a Sun Protection Factor of at least 15 to all parts of your body that are not protected otherwise.
How do sunscreens work?
A sunscreen works in one of two ways; it either scatters oncoming UV rays away from the skin (physical sunscreen) or it absorbs the UV rays before they can reach the skin (chemical sunscreen). Both methods are effective, though chemical sunscreens tend to only block UV-B, which is mostly responsible for sunburns. Sunscreens that block out both UV-A and UV-B rays, with an SPF of at least 15 (SPF 30 is better) are preferable. They should especially be applied between the hours of 11 and 3, when the amount of UV rays hitting the earth is generally the greatest.
In order to allow the sunscreen to achieve its maximum protection, the sunscreen should be applied about 20 minutes before you head out into the sun. If your intended activity includes swimming or other water-related events, a waterproof sunscreen is best. It should be noted, however, that no sunscreen is completely water or sweat proof. Therefore, more sunscreen needs to be applied after any activity in which these factors are involved.It should not be assumed,however, that applying sunscreen allows you to safely stay out longer in the sun. Sunburn is the body's natural warning sign - eliminating the warning sign does not mean that you have eliminated all of the danger.
What is SPF?
SPF stands for Sun Protection Factor. It is the rating that the U.S. Food and Drug Administration (FDA) puts on sunscreens that describes how much they extend the time you can stay in the sun without getting sunburned. For example, if you can normally stay outside for about 15 minutes before you start to burn, then applying a sunscreen with an SPF of 20 allows you to stay out for 20 times 15 minutes before burning - or rather, 300 minutes. (Of course, that's assuming you apply the right amount and none of it comes off during your day.)
How to protect your eyes
The protection of the eyes from UV rays is actually very simple. Sunglasses with at least 99% UV-A and UV-Bblockage do an effective screening job, especially if they wrap around the head. This prevents UV rays from entering from the side. Check the label of your sun glasses when you buy new ones! Does it say that it blocks UV? Older glasses often did not absorb enough UV. If this is the case they may be doing more harm than good. The dark tint of the sunglasses causes the pupils to widen in order to let in more light. If sunglasses are only dark in the visible but not in the UV, actually more UV could penetrate the eyes.
I want a tan, not a sunburn!
First keep in mind that there is no scientific evidence that a tan does any good for your skin. If you still can't withstand the appeal of a bronze-colored appearance, sunbathe responsibly and always avoid over-exposure. Spend small amounts of time in the sun over a period of several days, to give your skin time to start its own built-in protection against UV rays, for example the production of a substance called "melanin". Never risk a sunburn. In addition to itching and skin damage that sunburn creates, sunburned skin will soon shed, and with it goes much of the bronze tan.