the corners of the base to provide a clearance for the manifold and tubes the length of each tube embedded within the foam is selected to provide a discharge of air at areas of the mattress that are likely in use to be most heavily loaded in the example shown in the drawing the tubes 13 14 which enter from the same end of the body of foam extend by different amounts into the body of foam so that their discharges are provided at the aforesaid heavily loaded regions furthermore the diameter of the tubing 13 at the end of the mattress which will support a user 39 s head is larger than that of the tubing 14 at the opposite end of the mattress since that end of the mattress is likely to be more heavily loaded than the opposite end the tubes 13 are 3 inches in diameter and the tubes 14 are 2 inches in diameter the upper surface of the body of foam which in use receives a patient is covered by a further relatively thin layer 17 of foam which is releasably secured to the main body of foam by providing along the edges on the opposed faces of the body of foam and the layer of foam strips of a material which provide an attachment connection when pressed together for example the strips may be made of velcro registered trade mark the foamed material of the layer 17 must be non toxic and also the layer must impede the flow of air therethrough the layer 17 can also be made.