Between the first period of the study, from 1974 to 1989, and an eight-year follow-up period, from 1990 to 1998, soot concentrations declined 24 percent and mortality rates dropped 19 percent.Steubenville ???provided the benchmark that we compared everything else to,??? Dr. Dockery said, and the study was a crucial underpinning of the first federal regulations on soot, which went into effect in 19