Down is PURE natural. Down is a by-product(side products) of poultry industry, containing at least 80%   protein, so down is natural products and can be natural degradation. Down is sustainable. It's longterm demand for goose and duck meat , and down is the most comfortable and warm natural insulation material, down products have a stable market demand, so down and feather production will be permanent and sustainable  . Down have strong resilience  , high fluffy. Polyester Fiber, cotton, wool, silk and other linear fibers have poor pressure resistance, easy to knot, material degradation fast; Down look like three-dimensional structure and rich in non-flowing air,   will not knot after perennial use,   its function can keep longer. Down have lower flammable. Down is animal protein and does not flame  below 100 degrees Celsius temperatures, so it can pass the flammability test, but other materials, especially man-made material, must do fire retardant treatment if need pass . Like California in USA, the duvet with down fill don't need do  flammability test.   Down has good moisture absorption and humidity elimination function. The three-dimensional structure of down have large  surface area , so the moisture will release soon after   absorbing water  , in addition, Down contains  some  oil, the oil itself have good moisture absorption and humidity elimination.