During the summer, the odors of unsecured rubbish and raw sewage combine in the moist heat. And, because Manhattan was designed without alleys to maximize living space, much of the city's trash ends up piled high on curbs or littered throughout. The pollution seeps down into New York's vast subway system. A study conducted by the nonprofit environmental group Natural Resources Defense Council found that four of the city's six most polluted streams were located in either Central Park or Hudson River Park. The majority of the city's waste is currently disposed of at two large landfill sites: one in Staten Island operated by Waste Management, the other in Otisville, N.Y., operated by Rosedale Landfill. Both of these landfills are considered major sources of air pollution, as they emit greenhouse gases like carbon dioxide and methane and release toxic substances into the soil and water table. Additionally, between 1800 and 1900, thousands of African slaves built many of Manhattan's streets with their own muddy hands. This practice ended when the city passed its first sanitation law in 1807. Since then, workers have been used to clean up after themselves. In fact, according to the Department of Sanitation, nearly a million people currently serve on city cleanup crews. However, even with this effort, some areas of the city remain unsanitary.