The ozone hole over Antarctica is the biggest it’s ever been, 11 million square miles, or three times the size of the U.S., scientists at NASA said on Friday. Synthetic compounds in refrigerants, aerosol sprays, and foam-blowing agents cause depletion of the ozone layer. The amount of such chemicals in the atmosphere is leveling off, thanks to the 1987 Montreal Protocol, which halted the production of most of them, but the effects of past production are only now just being felt and experts say it could be another 20 years before ozone levels recover noticeably.