MDF is basically compressed sawdust (with a few chemicals added in for good measure)
If you
MUST use MDF - use the Red coloured MDF as its Fire Retardant. Remember - Wood is an insulator and will contain heat, as opposed to Alu/Steel which are radiators and disipate any heat. Usually why PCs are made in steel/alu cases :P
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http://en.wikipedia.org/wiki/Medium-density_fiberboard