— Submitted by Annelise Smithmier-Bohn, sixth grade, Jefferson Middle School

A. Ozone, a molecule containing three oxygen atoms, is a pollutant in the lower atmosphere. But at higher altitudes, ozone blocks deadly ultraviolet (UV) radiation from the sun.

Natural chemicals can deplete ozone, but the ozone hole above Antarctica is caused by the accumulation of chemicals called CFCs in the atmosphere, says Tony Wimmers, who tracks air pollution at UW-Madison's Space Science and Engineering Center.

CFCs were popular refrigerants because they are so stable, but when they reach the upper atmosphere, they break down into chemicals that destroy the protective ozone.

Scientists have worried about ozone destruction since the 1970s. The discovery of an ozone-free region (an "ozone hole") above Antarctica in the 1970s led to the 1987 Montreal Protocol, which halted the manufacture of many ozone-destroying chemicals. While a complete loss of ozone occurs only above Antarctica, ozone has declined around the globe, and increases in UV radiation may be contributing to more cases of skin cancer.

Despite Montreal, the biggest ozone hole occurred in 2006. "There are still lots of CFCs in the atmosphere," said Wimmers, and some can "remain for more than 60 years."

— Produced in cooperation with University Communications

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