Sun “light” is actually a form of electromagnetic radiation. There are three types of electromagnetic radiation that are emitted by the sun that make it through the atmosphere and reach us; ultraviolet radiation, visible light and infrared radiation. It is the ultraviolet radiation that makes us feel warm when we are out in the sun but it is also the form of sun’s radiation that is believed to cause skin cancer. On the scale of electromagnetic waves, visible light is only a very narrow band of waves in the middle between ultraviolet and infrared radiation. If you think about visible light as being the color red at the low frequency end and going up to the color violet at the high frequency end, ultraviolet is the next higher up form of radiation from the visible color violet and infrared is the band of waves just down from the visible color red. You can’t see either ultraviolet or infrared radiation. Except for rare cases, visible light and infrared radiation from the sun are not dangerous to your health. Ultraviolet radiation however can be very dangerous to your skin and is a significant contributor to skin cancer.
There are 3 types of ultraviolet radiation: UVA (tanning rays). UVB (sunburn rays) and UVC (dangerous cancer causing rays). UVA is the shortest frequency and longest wavelength of the three and is not filtered out by the earth’s atmosphere at all. Since it is not filter out UVA is pretty much the same intensity from sunrise to sunset. Glass windows do not block UVA. It used to be thought that UVA was not a danger as far as skin cancer is concerned but recent studies indicate that the cumulative effect can be just as dangerous as UVB and UVC. UVB is partially filtered by the earth’s ozone layer and is at its greatest intensity at noon. It is currently the most dangerous form of ultraviolet radiation from the sun and can burn, wrinkle, age your skin and cause skin cancer. UVB and UVC cannot go through glass. UVC is currently completely blocked out in the atmosphere by the earth’s ozone layer. But as the ozone layer recedes there is a danger that more and more UVC will not be filtered out and contribute to the spread of skin cancer.
Several government agencies and weather reporting services have developed a UV Index to help people avoid the dangers of UV radiation. The UV index is a number from 0 to 11 or more that is a daily forecast of the predicted risk if you are out in the sun too much. A UV index of 2 or less means the risk is low, wear sunglasses if it is bright out and cover up if out for more than an hour. If the UV index is over 6 many weather reporting agencies will issue a UV Warning asking that you actively reduce your exposure to sun and stay indoors between 11 AM and 4 PM. In most northern climates, the UV index rarely gets to 6.
The skins reaction to UV radiation varies widely from person to person. Heredity, skin coloration and exposure to industrial chemicals are all considered to be contributing factors. However, there is little doubt that the more time you spend out in the sun the greater is your danger of getting skin cancer.