填空题

A researcher says lead in the environment could be a major cause of violence by young people. Doctor Herbert Needleman is a professor at the University of Pittsburgh School of Medicine in Pennsylvania. He says the (36) of lead in the brain changes the neurons that control actions and that can cause a person to act in antisocial and (37) ways.
Lead is a metal that is (38) dangerous to babies and young children. They can get it into their bodies by breathing or (39) lead dust, or by eating soil or pieces of lead paint. Children with high levels of lead can (40) brain and nervous system damage, learning (41) , slow growth, headaches and hearing loss.
In the 19th century, Doctor Needleman found lower scores on (42) tests even in children who did not have such signs of lead (43) . After that, lead was removed from gasoline and paint in the United States. Yet many homes still have old lead paint. Lead was also used in older water pipes. In fact, (44) .
The newest research shows that even very small amounts of lead in bones can affect brain development. (45) . In 2008, such tests were done on 190 young people who were in jail and the findings showed that their average levels were higher than normal. And, in 2002, (46) . Yet these levels were still considered safe by the government.