Particleboard and medium-density fiberboard (MDF) are almost always made with a urea formaldehyde (UF) binder. This is one of the largest in-home sources of formaldehyde gas—a known human carcinogen. These products can off-gas for 5 years or more into the living space. UF-based particleboard and MDF are typically used for cabinet boxes, substrates for countertops, shelving, and stair treads. If possible, these materials should be avoided. Any UF-based materials used in a building should be sealed with a low-toxic, low-permeability coating.
A few formaldehyde-free particleboard and MDF products are available, including some made with straw instead of wood fiber. These are made with a urethane-type (MDI) resin. Be aware that MDI is highly toxic before it cures, so its use increases the health risk to factory workers if a manufacturer doesn’t have good safety standards. Once cured, MDI-based wood panel products are very stable, without measurable off-gassing.