Much of the furniture we enjoy every day is slowly “off-gassing” toxins to the air. These wood products traditionally rely on formaldehyde as an integral part of the bonding system. Formaldehyde is a colourless, flammable, strong-smelling chemical known to be a respiratory irritant and carcinogen. The problem is amplified with urea-formaldehyde when humidity and temperature are high, as the adhesive can break down and continually emit formaldehyde.
This study presented in the Proceedings of the International Convention of Society of Wood Science and Technology and United Nations Economic Commission for Europe (Paper WS-23) provides more information on the specific chemistry.