Polished concrete is concrete that has been treated with a chemical densifier and ground with progressively finer grinding tools to a smooth glossy surface, often to a highly reflective polish. Typically, concrete is not considered polished before 400 grit, and it is normally finished to either the 800, 1500, or 3000 grit level. Stains or dyes are often applied to enhance polished concrete, and there are other options including scoring, creating radial lines, grids, bands, borders or other creative designs. Polished concrete is considered a great design flooring option because it makes use of materials already present. Most modern buildings are built on a concrete slab so polishing the exposed concrete eliminates the energy and material consumed by applying a floor covering. Polished concrete floors are low maintenance, as they are more durable and easier to clean that most flooring options. Its relatively high coefficient of friction can make it non-slippery. Polished concrete reduces dust mite and allergen problems and does not support mold growth. Evidence suggests that using highly reflective polished concrete flooring reduces lighting needs and improves natural lighting. Polished concrete flooring is hard wearing and does not chip or dent like softer materials such as wood or tile. LEED 2009 standards contain guidelines for using polished concrete flooring to qualify for LEED credits.