17703
The close overlap between the present study ' s estimate of the PHC5 and a field - derived threshold concentration suggests that the population - level SSD approach provides a reasonable level of protection for species richness in the natural environment .
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Given options, please tell me the categories of all the listed entity words.Output format is "type1: word1; type2: word2". 
Option: Chemical 
Text: {0} 
 Entities:  
Answer:
 None