Two days after ProPublica published a first-of-its-kind analysis of industrial air pollution, the Environmental Protection Agency announced that its administrator, Michael S. Regan, would visit the communities featured in our reporting. During last week’s “Journey to Justice” trip across the South, Regan toured the Houston ship channel, the Louisiana community of Mossville and a stretch of land along the Mississippi River known as Cancer Alley — places that we identified as among the largest hot spots of toxic air pollution