I attended a formaldehyde conference, the focus of which was on the many different test methods used to measure formaldehyde content and emissions, and I began thinking about the relative performance of plywood, particleboard and MDF.   The structure of plywood is very different to densified panels and this will affect its relative performance dependent on the test method; plywood does not normally have glue on its faces. Thus it will perform well in a test where the edges are sealed so the formaldehyde must escape through the faces. The gas analysis method is such a method; in fact the reference for plywood. The thickness and permeability of the outer veneers will control the formaldehyde emission rate.   Particleboard or MDF does have adhesive on its surface, mixed with the particles or fibres, but the surfaces tend to be densified by pressing and the higher density will lead to lower permeability and slower diffusion rates.   Panel structure will have slightly less influence in other tests where the edges are not sealed, like the Japanese desiccator and European flask methods. The perforator method is an extraction method where all the formaldehyde is supposed to be removed by boiling in toluene.   Although I know the typical adhesive content of densified and plywood panels, the methods of expressing the adhesive contents are different so a conversion is required.   The adhesive content of plywood is expressed as a glue-spread value used during manufacture (g/m?); for densified panel adhesive it is given as a percentage of dry wood content.   The glue-spread values for a plywood are dependent on the grade of panel, wood species and the veneer surface roughness.   A typical range of glue-spread values for a single glue line are 150-250g/m². To convert this to a percentage of dry wood we will have to make some assumptions about the solids-content of the glue and structure of the plywood. If the solids-content is 60% and the panel is 18mm five-ply, then the solid adhesive content as a percentage of dry wood is going to vary between about 4 and 5%, depending on the veneer density. A seven-ply version would have a range of approximately 5 to 7% because there are more glue lines.   This demonstrates that plywood generally has less adhesive as a percentage of wood weight than typical densified panels (8-14% depending on type and grade of panel), except OSB panels which have about 4%.