 Plywood for indoor use generally uses the less expensive urea-formaldehyde glue, which has limited water resistance, while outdoor and marine-grade plywood are designed to withstand moisture, and use a water-resistant phenol-formaldehyde glue to prevent delamination and to retain strength in high humidity.The adhesives used in plywood have become a point of concern.Both urea formaldehyde and phenol formaldehyde are carcinogenic in very high concentrations.