Day 20 of the chemistry advent timeline starts with Mario Molina, a Mexican chemist who contributed to the discovery that CFCs were depleting the ozone layer. This led to regulations against CFC use, and recent studies have shown that the ozone layer has consequently recovered to an extent.
Gertrude Elion helped develop the first successful antiviral drug and invented a number of other drugs for a variety of diseases and conditions. She was jointly awarded the Nobel Prize in Physiology or Medicine for her work.
Barbara Askins worked for NASA and invented a process that allowed underexposed negatives to be enhanced. The technique helped improve photos of space and also medical X-rays.