Because health data were available only to 1998 and because of concerns about the quality of the early landfill data, 9,631 sites that closed before 1982 or opened after 1997 were excluded (allowing a 1-year lag period for the birth outcomes), as were landfill sites for which there were inadequate data. The remaining 9,565 sites included 774 sites for special (hazardous) waste, 7,803 for nonspecial waste, and 988 handling unknown types.