Since 2005, some 10,000 large industrial plants in the EU have been required to buy and sell permits to release carbon dioxide into the atmosphere. A so-called 'emissions trading scheme' enables companies that exceed individual CO2 emissions targets to buy allowances from 'greener' ones to help reach the EU's targets under the Kyoto Protocol. However, pollution credits were grossly overallocated by several countries during the initial implementation phase, forcing down carbon prices and undermining the scheme's credibility, which has prompted the EU to consider toughening up the system.