The term MATTRESS FILLER can be broken down into two crossword solutions: AIR (3 letters) and KAPOK (5 letters). AIR is a common term for the mixture of gases that make up the Earth's atmosphere, which is composed of nitrogen, oxygen, and other trace gases. KAPOK is a type of fiber derived from the seed pods of the KAPOK tree, which is native to tropical regions of the Americas and Africa. It is often used as a natural filling material in mattresses, pillows, and other soft furnishings.