In the sentence extracted from biomedical literature, identify all the chemical entities. The required answer format is the same sentence with HTML <span> tags to mark up specific entities. 
### Entity Markup Guides:
Use <span class="chemical"> to denote a chemical.

### Input Text: Treatment has been continued in 3 individuals for 6 - 13 months with persistence of the pressor effect , although there appears to have been some decrease in the degree of response with time .
### Output Text:
Treatment has been continued in 3 individuals for 6 - 13 months with persistence of the pressor effect , although there appears to have been some decrease in the degree of response with time . 