after world war i, dry cleaners began using chlorinated solvents. these solvents were much less flammable than petroleum solvents and had improved cleaning power. by the mid-1930s, the dry cleaning industry had adopted tetrachloroethylene (perchloroethylene), widely called "perc," as the best solvent. it has excellent cleaning power and is stable, nonflammable, and gentle to most clothing. however, perc was also the first chemical to be classified as a carcinogen (a chemical that can cause cancer.) by the consumer product safety commission (a classification later withdrawn). in 1993, the california air resources board adopted regulations to reduce perc emissions from dry cleaning operations. the dry cleaning industry is in the process of replacing perc with other chemicals and/or methods.