A researcher says lead in the environment could be a major【D1】______by young

A researcher says lead in the environment could be a major(21)cause of violence by young people. Doctor Herbert Needleman is a professor at the University of Pittsburgh School of Medicine in Pennsylvania and he(22)presented his findings at the yearly meeting of the American Association for the Advancement of Science. Doctor Needleman says the presence of lead in the brain changes the(23)neurons that control actions and that can cause a person to act in(24)antisocial and criminal ways.
    In the 1970s, Doctor Needleman found lower scores on(25)intelligence tests even in children who did not have such signs of lead poisoning. After that, lead was(26)removed from gasoline and paint in the United States. Yet many homes still have old lead paint. Lead was also used in older(27)water pipes. In fact, officials just announced stronger testing and reporting requirements as from next year for lead in American drinking water.
    The newest research shows that even very small amounts of lead in bones can affect brain development. A simple(28)blood test can measure lead except that an X-ray process is needed to measure levels in bone. In 2004, such tests were done on 190 young people who were(29)in jail and the findings showed that their(30)average levels were higher than normal. And, in 1998, three hundred children were studied and the test scores showed higher levels of aggression and learning problems in those with increased levels of lead. Yet these levels were still considered safe by the government.