UV-c is quickly absorbed by small amounts of ozone, so that none gets to the Earth's surface. Ozone is so effective at absorbing the extremely harmful UV-c that sunscreen manufacturers don't need to worry about these wavelengths. UV-b radiation levels are critical, since ozone doesn't fully absorb at these wavelengths. In this plot of ultraviolet flux versus wavelength for various altitudes, a large action- spectrum value implies the existence of significant biological damage. The screening of UV by ozone involves other factors, such as time of day and season.
Tropical regions have very intense exposure, while high-latitude regions have rather weak exposures.
Higher altitudes in the Rocky Mountains and the Himalayas have high exposures as well. In addition, clouds affect the amount of UV that is incident on the Earth's surface. Because ozone plays such a critical role in screening harmful ultraviolet radiation, it is important to understand the life cycle of ozone molecules. The molecule's life ends when it reacts with one of a variety of chemicals in the strato-sphere, such as chlorine, nitrogen, bromine, or hydrogen compounds. A typical reaction occurs when ClO reacts with an oxygen atom to form Cl and O 2 .