Black lung is back and nobody knows exactly why. Black lung is a disease afflicting underground coal miners. They breath in coal dust and, over time, miners' lungs are so damaged they stop working. Laws passed in the 1960s helped cut down the number of miners with black lung by controlling the amount of dust in the air. But Carole Bass reports in an online publication that now those gains are starting to erode.Bass said federal officials have focused on several reasons for the resurgence of black lung disease among underground coal miners. First, the dust standards are still too lenient. There's too much dust in the air and the federal government has refused to lower the standards — to clean up the air in the mines. Also, miners are working longer hours. Instead of a 40 hour week, miners are underground sometimes 60 hours or more. They breath in more dust, and they have less time out of the mines for their lungs to clean themselves.Finally, researchers believe that as miners dig thinner seams of coal, they are cutting into more rock, mixing silica dust with coal dust. Silica is 20 times more harmful than coal dust and so miners are breathing more toxic air.