Hardboard (high-density fiberboard) is produced at higher pressure and density than MDF, typically without added resin binders, relying instead on the natural lignin in wood fibers for bonding. Density thresholds — generally above 0.8 g/cm³ for hardboard versus 0.5–0.8 g/cm³ for MDF — are the primary classification criterion. Mill technical data sheets specifying density and production method are essential for accurate tariff classification.

TSCA Title VI applies to composite wood panels that use formaldehyde-based resins. Hardboard produced without added resin (relying on lignin bonding) may fall outside the rule's scope, but importers must document the resin type and obtain a written statement from the manufacturer. When resin type is uncertain, treating the product as covered and obtaining TPC certification is the lower-risk approach.