On April 6, DAL1439, a B737-939 had just departed KBOS and was climbing through FL200 when the crew contacted Boston Center and advised that they were descending to 12000. When the controller asked what was going on, the flight crew advised that they had lost pressurization and that the cockpit was on oxygen. They eventually declared an emergency and asked to go back to Boston. After some delaying vectors DAL1439 landed safely on runway 33l and was met by emergency crews who were there to check for any overheated brakes. The flight was cancelled.
https://www.flightradar24.com/data/flights/dl1439#39c734b0
