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Ozone levels over North America have bounced back to normal levels, according to a study published last month in Geophysical Research Letters. In 1992–1993, ozone levels reportedly dropped to levels up to 15% below what was considered normal—an occurrence that was largely attributed to the eruption of Mount Pinatubo. However, the journal also reports in another study that the overall ozone level continues to decline over populated areas at a rate of 7% in winter and 2.5% in summer on a timescale of decades.