Plywood, oriented strand board (OSB) and medium-density fiberboard (MDF) are more alike than they are different. They are all resource efficient choices. Wood not useful as dimensional lumber can be pressed and bonded with adhesives to form sheet goods for sheathing, furniture and flooring. Fiberboard can be made from plant fibers, recycled wood, or paper.
However, most conventional plywood, OSB and MDF are bonded with formaldehyde adhesives that off-gas and negatively affect indoor air quality. The Environmental Protection Agency has classified formaldehyde as a probable human carcinogen. Long term inhalation of the vapors can cause fatigue, respiratory illness, and allergic skin reactions.
The two most widely used formaldehyde adhesives are urea formaldehyde (UF), and phenol formaldehyde (PF). Most softwood plywoods used for outdoor and structural applications, like sheathing and floors, are bonded with the more expensive, water-resistant PF, which off-gasses at a much slower rate than UF. This means the vapors will persist longer, but at much lower concentrations, so most people consider them safer. Ironically enough, the hardwood plywoods most often used inside the home for cabinetry and paneling are bonded with UF, considered the more harmful of the two.
To avoid this harmful off-gassing altogether, request “formaldehyde free” manufactured wood products. If that is unavailable in your area, look for an “exterior glue” stamp on regular plywood. This indicates PF is the adhesive. If UF-bonded sheet goods can’t be avoided, sealing with a low-toxicity sealant will keep
