may also have proprietary designs that they have had third-party tested and approved, provided that the material used in the field configuration can be demonstrated to meet the minimum requirements of Type X drywall and that sufficient layers and thicknesses are used. Type X drywall In the Type X gypsum board, special glass fibers are intermixed with the gypsum to reinforce the core of the panels. These fibers reduce the size of the cracks that form as the water is driven off, thereby extending the length of time the gypsum panels resist fire without failure. Type C drywall Type C gypsum panels provide stronger fire resistance than Type X. The core of Type C panels contains a higher density of glass fibers. The core of Type C panels also contains vermiculite which acts as a shrinkage-compensating additive that expands when exposed to elevated temperatures of a fire. This expansion occurs at roughly the same temperature as the calcination of the gypsum in the core, allowing the core of the Type C panels to remain dimensionally stable in a fire. North American market North America is one of the largest gypsum board users in the world, with a total wallboard plant capacity of per year, roughly half of the worldwide annual production capacity of . Moreover, the homebuilding and remodeling markets in North America in the late 1990s and early 2000s increased demand. The gypsum board market was one of the biggest beneficiaries of the housing boom as "an average new American home contains more than 7.31 metric tons of gypsum." The introduction in March 2005 of the Clean Air Interstate Rule by the United States Environmental Protection Agency requires fossil-fuel power plants to "cut sulfur dioxide emissions by 73%" by 2018. The Clean Air Interstate Rule also requested that the power plants install new scrubbers (industrial pollution control devices) to remove sulfur dioxide present in the output waste gas. Scrubbers use the technique of flue-gas desulfurization (FGD), which produces synthetic gypsum as a usable by-product. In response to the new supply of this raw material, the gypsum board market was predicted to shift significantly. However, issues such as mercury release during calcining need to be resolved. Types available in Canada and the United States Regular white board, from thickness Fire-resistant ("Type X"), different thicknesses and multiple layers of wallboard provide increased fire rating based on the time a specific wall