Density altitude is often referred to as where the aircraft “feels like” it is performing at. DA degrades aircraft performance, and sometimes this comes as a surprise to the pilot.

"Pressure altitude is very predictable and constant," Feith said. "The only thing that changes is the aircraft's height above ground level because the terrain geography changes." ...

Atmospheric pressure decreases exponentially with altitude, causing a drop in blood oxygen saturation level to around 90% (73 hPa) in healthy passengers at cruising altitude, explain the researchers.