A day after the U.S. Centers for Disease Control and Prevention cut in half the threshold for determining lead exposure in the nation's children, pediatricians faced the task of identifying new cases from thousands of their old files. The recommendation from the CDC recognizes what doctors have long believed: that any amount of lead can be harmful. And they expressed satisfaction that the level was lowered. But the new guidance will likely pose new logistical and financial challenges for doctors and public health officials. He said lead is a concern for children through age 5, so doctors will need to consider looking at records for all in that age range.