Regional air pollution regulators have again cited TXI Riverside Cement for allowing plumes of potentially harmful dust to escape its century-old plant north of Riverside. "Riverside Cement will continue to get notices of violations until they solve their dust problem," Sam Atwood, spokesman for South Coast Air Quality Management District, said Saturday. A recent air district investigation found that dust from the plant contains hexavalent chromium, a cancer-causing substance that is a byproduct of cement-making. The plant, located just south of the border between Riverside and San Bernardino counties, was cited for producing dust clouds longer than 100 feet and...