Over 100 million people in the United States, nearly one-third of its population, live in metro areas that violate federal standards for daily soot pollution on at least some days, the U.S. EPA said this week. However, missing from the EPA’s list of officially sooty areas — and thus exempt from further enforcement actions to clean up the pollution — are at least five metropolitan areas that violated the annual standard for soot pollution and have been struggling with long-term soot problems, among them ultra-polluted Houston, Texas.