The ozone layer absorbs most of the biologically damaging ultraviolet radiation from the Sun, allowing only a small amount to pass. Through UV absorption, it creates a source of heat that defines the temperature characteristics of Earth's stratosphere. Left unfiltered, certain frequencies of UV radiation would more easily penetrate the protective coverings of organisms, causing severe damage to DNA molecules.
Ozone molecules consist of three oxygen atoms bonded together and make up only a trace amount of the total oxygen molecules in the air; there are approximately three ozone molecules for every 10 million air molecules. This scarcity stems from the fact that ozone molecules are highly reactive. They are present in another layer of the atmosphere closer to the surface called the troposphere, and their high reactivity causes these molecules to have a toxic effect on life-forms with which they come into contact.
The high reactivity has another consequence: the ozone layer is thinned by reactions with chlorofluorocarbons, molecules that were once widely used as aerosol propellants. As humans release these chemicals into the air, the molecules rise to the ozone layer and convert the oxygen in ozone into more stable forms, allowing more UV radiation to pass through the Earth's atmosphere.
Experts commonly classify UV radiation into two spectra: UVA and UVB. UVB radiation causes sunburns and certain types of cancers such as basal cell and squamous cell carcinoma, while UVA radiation causes melanoma skin cancer and premature aging.