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However , induction of oxidative stress related pathways was not found after silica NP exposure for 24h in gene array studies conducted in A549 cells at a relatively low NP concentration ( EC20 ) .
 Chemical: silica
Please list all entity words in the text that fit the category.Output format is "type1: word1; type2: word2". 
Option: Chemical 
Text: {0} 
Answer:
 Chemical: silica