0.05 PA A 0 Air Foam A B D E Concentration p, is in lb/ft' A refrigerated food warehouse is insulated with a layer of urea- formaldehyde foam 18 in. thick. There is some residual formaldehyde in the foam and it has a concentration of 0.05 lb formaldehyde/ft of foam when the material is manufactured. Formaldehyde can diffuse into the air where workers will be exposed to the vapor. The outside of the foam is covered with an impermeable layer to prevent moisture migration, so the formaldehyde can only diffuse in one direction, which is into the warehouse. There is a system available that will remove 0.2 lb/hr of formaldehyde from the air during recirculation through the refrigeration system. It will return the air to the warehouse with essentially no formaldehyde, and you can assume the concentration of formaldehyde to be zero in the warehouse as long as the removal system is not overloaded. The concentration of formaldehyde in the foam may have the profile given by: A C D DE