This year's Antarctic ozone hole is the biggest ever, U.S. government scientists said Thursday, according to AP. The so-called hole is a region where there is severe depletion of the layer of ozone _ a form of oxygen _ in the upper atmosphere that protects life on Earth by blocking ultraviolet rays from the sun. Scientists say human-produced gases such as bromine and chlorine damage the layer causing the hole. That is the reason many compounds such as spray-can propellants have been banned in recent years. «From September 21 to 30, the average area of the ozone hole was the largest ever observed, at 10.6 million square miles (27.4 million square kilometers),» said Paul Newman, atmospheric scientist at NASA's Goddard Space Flight Center. That is larger than the area of North America. In addition satellite measurements observed a low reading of 85 Dobson units of ozone on Oct. 8. That is down from a thickness of 300 Dobson units in July. The ozone hole is considered to be the area with total column ozone below 220 Dobson Units. A reading of 100 Dobson Units means that if all the ozone in the air above a point were brought down to sea-level pressure and cooled to freezing it would form a layer 1 centimeter thick. A reading of 250 Dobson Units translates to a layer about 1 inch (2.54 centimeters) thick. In a critical layer of air between eight miles (13 kilometers) and 13 miles (21 kilometers) above the surface the measurement was only 1.2 Dobson unit, down from 125 in July. «These numbers mean the ozone is virtually gone in this layer of the atmosphere,» said David Hofmann, director of the Global Monitoring Division at the National Oceanic and Atmospheric Administration's Earth System Research Laboratory. «The depleted layer has an unusual vertical extent this year, so it appears that the 2006 ozone hole will go down as a record-setter.» The size and thickness of the ozone hole varies from year to year, becoming larger when temperatures are lower. Because of international agreements banning ozone-depleting substances researchers calculated that these chemicals peaked in Antarctica in 2001 and have been declining. However, many of them have extremely long lifetimes once released into the air. While there are year-to-year variations, scientists expect a slow recovery of the ozone layer by the year 2065.
