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Memory foam is polyurethane with additional chemicals increasing its viscosity and density. It is often referred to as visco-elastic polyurethane foam. In some formulations, it is firmer when cooler. Higher density memory foam reacts to body heat, allowing it to mold to a warm human body in a few minutes. A lower density memory foam is pressure-sensitive and moulds quickly to the shape of the body. A memory foam mattress is usually denser than other foam mattresses. This makes it more supportive but also heavier. It is often seen as a good compromise between the comfort of a soft mattress and the solidness of a firm one. Memory foam mattresses often sell for more than traditional mattresses. Memory foam has an open cell structure that reacts to body heat and weight by 'moulding' to your body, helping relieve pressure points, preventing pressure sores, etc. Most memory foam has the same basic chemical composition, however the density and layer thickness of the foam means different mattresses feel very different. A high density mattress will have better compression ratings over the life of the bedding. The lower density will suffer from slightly shorter life due to the compression that takes place after repeated use. Cell structures vary from very open, to almost close cell. The tighter the cell structure, the less airflow through the foam. Memory foam has been known to be very hot to sleep for some. "Breathable" visco-elastic foam will have a more open cell structure, allowing higher airflow, better recovery, and lower odor retention at packaging. The cell structure has little to do with the slower or faster recovery