Destruction and Removal Efficiency (DRE) — a percentage that represents the number of molecules of a compound removed or destroyed in an incinerator relative to the number of molecules that entered the system. (For example, a DRE of 99.99 percent means that 9,999 molecules are destroyed for every 10,000 that enter. The percentage 99.99 is known as "four nines." For some pollutants, the Resource Conservation and Recovery Act (RCRA) removal requirement may be as stringent as "six nines.")