The CHS is one of the largest and most detailed studies of the long-term effects of air pollution on children's health in the United States of America. Since 1993, the CHS has enrolled five distinct cohorts, comprising more than 12,000 school children from 16 communities across Southern California. The first two cohorts included over 900 tenth-grade students (Cohort A) and 900 seventh-grade students (Cohort B) recruited in 1993 and followed up until 1995 and 1998, respectively. Cohort C included around 1,800 fourth graders, aged 9 - 10 years, who were followed up from 1993 until their high school graduation in 2001. In 1996, another cohort (Cohort D) of around 2,080 fourth graders was recruited and followed up through high school graduation in 2004. In 2002-2003, the fifth cohort (Cohort E) of 5,600 children from kindergarten and first grade, aged 5 - 7 years, was recruited from 13 Southern California communities. Participants from Cohort E are actively being followed up as part of the NIH Environmental Influences on Child Health Outcomes (ECHO) program.