Manhattan's population has grown in tandem with its size. Battery Park City, the island's most recent addition, was built on top of rubbish and debris from the first World Trade Center's construction in 1973. The site was previously home to Hudson River landfill sites that were abandoned when they reached capacity. The presence of garbage on top of garbage led to public fears about contamination of the surrounding area. In response, Congress passed the Clean Air Act in 1972, which led to the regulation of toxic pollutants such as lead and benzene. The act also prompted the development of recycling programs for discarded materials. Battery Park City is now considered one of the city's most sustainable developments due to its emphasis on reuse and recycling. Although the majority of the landfill site has been converted into a park, some pollution remains. The site is known for having high levels of lead and other toxic metals not commonly found in soil. Children who play in the park are at risk of lead poisoning because it is easily absorbed by young bodies. However, studies have shown that the level of lead in blood samples from children under six years old living in Battery Park City is lower than those living in other parts of Manhattan or Brooklyn.