In a bid to clean up its act in time for the 2008 Olympics, China's choking capital city, which has seen traffic on its roads rising by 15% per year, has taken to planting grass on roofs to help filter the air. Most major capital cities rely on oases of green open spaces planted with trees and vegetation to preserve air quality. But Beijing is too crowded for any more green belts so officials are turning to rooftops instead. Last year they planted 10,000 square metres of evergreen grass on rooftop lawns and this year the target is 100,000. They're also intending to shift 200 of the worst polluting factories. During 2004 officials claimed to have reached their goal of 227 days with clean air, although many are sceptical about the standards used to make the claim.