[单项选择]
Lead Poisoning: A Silent Epidemic
How much lead is harmful Until the 1970s, exposure to lead was considered dangerous only at blood levels associated with extreme symptoms of poisoning: convulsions (惊厥), brains swelling, acute kidney disease, stomach pains and hallucinations (幻觉). But low -level lead poisoning can cause symptoms that, while subtle, are no less serious. As a result, the CDC has continually lowered the point at which patients should be treated for lead poisoning.
One of the most influential study of lead’ s low - level effects was published by Herbert Needleman, professor of psychiatry at the University of Pittsburgh Medical School. Testing first and second grade students in the Boston area, he found the lowest I. Q. , academic achievement, language skills and attention span among children with the highest tooth levels (起损害作用的量) of lead. "None of their lead levels exceed the CDC’s 1979 toxicity threshold. "Says Paul Mu
A. the amount of exposure was large.
B. it caused extreme symptoms of poisoning.
C. it caused symptoms whether serious or not.
D. poisoning had led to death among the poisone