The ozone layer is the part of the Earth's atmosphere that has the most ozone in it.
It's the atmospheric layer within the stratosphere, extending from a height of c. 20 to c. 30 km (c. 12 to c. 18 mi). The ozone layer protects living things from the harmful radiation of the sun. It absorbs dangerous ultraviolet radiation from the sun and serves to maintain the temperature of the atmosphere.
A dobson unit is the most basic measure used in ozone research. One Dobson Unit (DU) is defined to be 0.01 mm thickness at STP (standard temperature and pressure). Ozone layer thickness is expressed in terms of Dobson Units, which measure what its physical thickness would be if compressed in the Earth's atmosphere. In those terms, it's very thin indeed. A normal range is 300 to 500 Dobson Units (3-5 mm), which translates to an eighth of an inch-basically two stacked pennies.
In space, it's best not to envision the ozone layer as a distinct, measurable band. Instead, think of it in terms of parts per million concentrations in the stratosphere (the layer six to 30 miles above the Earth's surface).
The unit is named after Gordon Dobson, one of the first scientists to investigate atmospheric ozone.
NASA uses a baseline value of 220 DU for ozone. This was chosen as the starting point for observations of the Antarctic ozone hole, since values of less than 220 Dobson units were not found before 1979. Also, from direct measurements over Antarctica, a column ozone level of less than 220 Dobson units is a result of the ozone loss from chlorine and bromine compounds.
Ozone depletion describes two related events observed since the late 1970s: a steady lowering of about four percent in the total amount of ozone in Earth's atmosphere (the ozone layer), and a much larger springtime decrease in stratospheric ozone around Earth's polar regions. The latter phenomenon is referred to as the ozone hole. There are also springtime polar tropospheric ozone depletion events in addition to these stratospheric events.
The main cause of ozone depletion and the ozone hole is manufactured chemicals, especially manufactured halocarbon refrigerants, solvents, propellants and foam-blowing agents (chlorofluorocarbons (CFCs), HCFCs, halons), referred to as ozone-depleting substances (ODS). These compounds are transported into the stratosphere by wind after being emitted from the surface, mixing much faster than the molecules can settle. Once in the stratosphere, they release halogen atoms through photodissociation, which catalyze the breakdown of ozone (O3) into oxygen (O2). Both types of ozone depletion were observed to increase as emissions of halocarbons increased.
Ozone depletion and the ozone hole have generated worldwide concern over increased cancer risks and other negative effects. The ozone layer prevents most harmful UVB wavelengths of ultraviolet light (UV light) from passing through the Earth's atmosphere. These wavelengths cause skin cancer, sunburn and cataracts, which were projected to increase dramatically as a result of thinning ozone, as well as harming plants and animals. These concerns led to the adoption of the Montreal Protocol in 1987, which bans the production of CFCs, halons and other ozone-depleting chemicals.