The environment repercussions of the olympic effect are also mixed. On the one hand, since the sydney game ("the green games"), participating communities have had active recycling programs, emphasized renewable energy resources, and built or extended mass transit systems, like salt lake city's 21-kilometer Trax light rail system. Some, like sydney and beijing (2008) have made notable efforts to clean up their municipal environments. Sydney actually built most of its olympic venue over a reclaimed toxic waste dump that had once blighted city. To clear its polluted air, Beijing has converted much of its energy production from coal to natural gas, and implemented new smokestack and vehicle emission guidelines. Medical researchers estimate that the benefit to Beijing's resident's health will amount to many billions of dollars over the coming decades. On the other hand, the game themselves result in great consumption of fossil fuels and strain on waste-disposal systems. The winter game in particular are often accompanied by acceleration human development of important wildlife habitat.