Scientists have confirmed a 2.5 million-square km hole in the low-level ozone layer over western China's Qinghai-Tibet Plateau.

The hole formed in December 2003 over the plateau, which stands at an average 4,000 meters above sea level, according to an article in the reputable Chinese science magazine "Scientific Report".

Experts from the Chinese Academy of Sciences and the China Meteorology Research Center have proved a significant decrease in total column ozone.

The article is based on comprehensive research and analysis of data from both ground monitoring and the Total Ozone Mapping Spectrometer, a satellite-borne instrument used to measure global ozone levels.

The scientists have been monitoring ozone changes over the plateau since a dramatic loss in upper-level ozone was recorded in summer 2003.

However, they are divided over whether another hole will form over the plateau (the other two are over the Antarctic and the Arctic).

The article says an area of 2.5 million square km air with a total ozone of less than 220 Dobson Units (DU) was found over the plateau from December 14 to 17, 2003, and hit a record low of 190 DU.

The international measuring system of Dobson Units prescribes 100 DU to equal a one-millimeter thick layer of pure ozone with conditions of one standard atmospheric pressure and a temperature of zero centigrade.

This is the first time that an ozone minimum-hole or extremely low ozone has been witnessed over the region, the article says.

However, experts believed that the cause of the hole over the plateau differs from that in the two poles, due to varied atmospheric movements.

"The decrease of ozone over the plateau was caused by airflow exchange in the sky. When low-ozone air currents in lower layer enter the upper air layer, the overall ozone density is reduced," the article says.

The ozone holes over the Poles were caused by the global "greenhouse effect".

Ozone is one of the gases forming the Earth's atmosphere and is the major shield against Ultraviolet-B (UVB) radiation, absorbing approximately 90 percent of solar UVB. Excessive exposure to UVB can cause skin cancer in humans and is a major contributor to glacial melting.