Ultraviolet radiation comes in three forms, depending on the wavelength: UVa, UVb, and UVc. Earth's ozone layer blocks most—but not all—UVb and UVc light. UVa This radiation slips through ozone to reach the Earth's surface. But UVa causes relatively little damage (unless you sit in the sun too long without sunscreen or a hat). UVb More harmful than UVa, UVb triggers the growth of cancerous cells in the skin. UVc This is the shortest and most energetic form of ultraviolet radiation. That means it's also the most damaging. It doesn't reach the Earth's surface.

Sunlight—we couldn't live without it. The sun warms our planet, and sunlight is the primary source of energy for plants. They, in turn, give us the oxygen we breathe and the food we eat. Our bodies need at least some sunlight, moreover, to produce vitamin D. This nutrient helps the body absorb calcium, a key ingredient in bones and teeth.

Yet there's more to sunlight than what meets the eye. Visible light is only a sliver of a much broader band of energy. This bigger band is called the electromagnetic (E-M) spectrum. The E-M spectrum also includes radio waves, microwaves,
x-rays, and other forms of invisible radiation. You can find diagrams of all this at NASA's Electromagnetic Spectrum site.

Different parts of the spectrum have various wavelengths. In general, waves with shorter wavelengths can carry more energy than those with longer wavelengths. The light we can see falls in the middle of the E-M spectrum.

Source: Cooperative Program for Operational Meteorology, Education, and Training (COMET®) of the University Corporation for Atmospheric Research (UCAR), funded by the National Weather Service. ©2002 University Corporation for Atmospheric Research. All Rights Reserved.

One part of the spectrum can have a huge impact on human health. That's ultraviolet (UV) light. UV wavelengths are slightly shorter than those of visible light. So we can't see ultraviolet light. But we see what happens when we get too much of it—sunburn.

Sunburn is just the surface of UV-related health issues. Repeated overexposure to ultraviolet light can cause skin cancer. UV light can also damage your eyes. The retina, the eye's most light-sensitive part, is particularly susceptible. Evidence suggests that too much UV light also weakens the body's immune system. That would leave you less able to fight certain kinds of infections.

This satellite image of the sun shows ultraviolet light that our eyes can’t see. Source: NASA: Ultraviolet Waves

UV light can cause deeper problems too. DNA and other molecules that make up living tissue in our bodies absorb radiation. This causes some of the DNA to mutate, or change. Up to a point, enzymes within the body can repair the DNA. But if they become overwhelmed, the enzymes cannot do their job.

The more we are in the sun, the greater the amount of DNA mutation. That lessens the chances that enzymes will be able to repair all the damage. So the DNA gets disrupted, which can set off a chain reaction that results in the uncontrolled growth of cancer cells.

	Still want a suntan?
	E-M Spectrum, meet Roy G. Biv!