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However , very few studies have evaluated whether chelation improves cognitive outcomes in Pb - exposed children , or whether these agents have adverse effects that may affect brain development in the absence of Pb exposure .
 Chemical: Pb; Chemical: Pb
Please list all entity words in the text that fit the category.Output format is "type1: word1; type2: word2". 
Option: Disease, Chemical 
Text: {0} 
Answer:
 Chemical: Pb; Chemical: Pb