Although the earth’s ozone layer can block out most of the UV radiation from the sun, the small amount that penetrates through the atmosphere can still cause damage to people. UV radiation is actually a spectrum of light that can be further subdivided. The natural source of UV, which is the sun, emits UVA, UVB, and UVC, which all have different wavelength ranges. There are also human-made sources of UV light such as UV fluorescent lamps and “black lights”, which are lamps that emit long-wave UV radiation and a very limited amount of visible light. When we are outside in the sunlight, especially in the summer when it is more intense, we use sunscreens to protect ourselves from sunburn. Sunburn and some types of skin cancer can be caused by caused by overexposure to UVB radiation. Although UVC has a shorter wave-length and thus higher energy, it is mostly filtered out by the earth’s atmosphere. Both UVB and UVC can cause direct damage to DNA. UVA, on the other hand, does not cause direct damage to our DNA. However, it is still harmful because it can generate highly reactive chemicals that do cause damage to DNA.

Even though both UVA and UVB are harmful, only certain sunscreens on the market contain chemicals such as titanium dioxide, zinc oxide, and avobenzone that protect against both UVA and UVB. Zinc oxide produces the most complete protection. Most sunscreens only contain chemicals that shield UVB rays. Even wearing clothing does not pre-vent UVA from penetrating deep below the skin. A good rule of thumb is to remember that clothing that can prevent sunburn is preventing some UVB from causing damage, but UVA goes right through clothing, deep into the skin to cause damage to the DNA (Figure 14).