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Polybrominated diphenyl ethers ( PBDEs ) represent an important group of flame retardants extensively used , tonnage of which in the environment has been steadily increasing over the past 25 years .
 Polybrominated diphenyl ethers, PBDEs
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: Chemical 
Text: {0} 
Answer:
 Polybrominated diphenyl ethers, PBDEs