SAN FRANCISCO — The banning of ozone-depleting chemicals hasn't yet caused detectable improvements in the Antarctic ozone hole, new research suggests. "Ozone is produced in the tropics, but it's transported by the winds from the tropics to the polar region," said Anne Douglass, a scientist with the Aura project at NASA's Goddard Space Flight Center in Greenbelt, Md. That transport "varies a little bit from year to year." The findings suggest that measuring the total size of the ozone hole says little about ozone depletion, and that it's misleading to use the hole's extent alone to measure environmental progress. Ozone is a molecule made up of three oxygen atoms, and the ozone layer, which stretches from heights of 12 to 19 miles (20 to 30 kilometers) above the Earth's surface, protects life on Earth by shielding it from ultraviolet (UV) radiation.