Naptha was used by the dry cleaning industry until the '60s, when it was discovered to be carcinogenic. Then perchlorethylene (sp?) or "perc" started to be used. Although perc was also carcinogenic, the theory was that the machines using it were designed to recover 100% of it, so the employees wouldn't be exposed to it. But the machines only did that when they were maintained properly; perc tended to degrade the rubber seals quickly; and the dry cleaning shop owners didn't want to spend the money to maintain their machines that well, and they leaked into the work area. So perc turned out to be carcinogenic "as applied" (you constitutional law guys will know what that means. Reading, Jay?)