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Union University Department of Physics

The Science Guys



Science Guys > April 2004

April 2004

I have heard that UV light causes sunburn, but what is the difference in UVA, UVB, and UVC light?

If you don’t want to look like a cooked lobster next summer, you'd better put on some sunscreen lotion, because ultraviolet light (UV) from the sun damages the skin cells it hits. Why does UV light cause damage?

Light is both a wave and a particle. You can consider light a packet of energy called a photon (particle), or a time-varying electric and magnetic field (electromagnetic wave, EM). Although it seems contradictory that something can be a particle and wave at the same time, experiments and quantum mechanics assure us that this is so.

Physicists classify EM waves according to their wavelength, the shorter the wavelength the more energy. UV waves are shorter than visible light waves, so UV possesses more energy than regular light.

UV photons have the right energies to cause chemical changes. When UV light hits your skin, the DNA in your skin cells can undergo chemical change. These DNA lesions are called thymine dimers. Your body has several strategies to repair cell damage, but occasionally the repair is not done correctly and a mutation results. An accumulation of mutations can make the skin cell malignant; resulting in cancer. If the damage is too great, the cells just die. That's what happens when you suffer severe sunburns. The outer layer of skin dies and a new layer must grow back.

Not every UV photon causes chemical damage, and not all UV light reaches Earth. The ultraviolet region is commonly divided into UVA with wavelengths from 400 to 320 nanometers (a nanometer is a billionth of a meter); UVB from about 320 to 290 nanometers; and UVC from about 290 to 200 nanometers. By comparison, visible light has wavelengths from about 400 to 700 nm.

Since UVB and UVC waves have more energy, they have the greatest potential to damage the skin. However, we Earthlings are fortunate because atmospheric chemicals, especially ozone, absorb much of the UVB and UVC before reaching the Earth’s surface. However, some UV light does reach the surface and can cause skin damage. You should avoid excessive exposure to UV light, whether from the sun or a man-made source, to protect your skin. An estimated 90% of basal and squamous cell skin cancers and 65% of melanoma cancers result from UV exposure.

There are two methods of protection from UV. First, simply prevent long exposure. Stay out of the sun, or cover yourself when out in the sun. Second, use a sunscreen containing chemicals like PABA which can absorb UV light and convert it to heat. This shields the DNA in your cells and you don't burn. But don't stay in the dark, either! UV light synthesizes Vitamin D in your skin; children who don’t make enough vitamin D get rickets, so people need to get a little sun, or take a vitamin supplement.