18703
Because there are so many small airways , their plugging is difficult to detect early , and this locus is known as a " silent zone . " In chronic obstructive airway diseases , plugging of small airways may persist and increase over time , finally resulting in severe airway obstruction .
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Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: Chemical 
Text: {0} 
Answer:
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