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: After adjusting for potential confounders , we found that the risk of HIV seroconversion among participants who were daily smokers of crack cocaine increased over time ( period 1 : hazard ratio [ HR ] 1 . 03 , 95 % confidence interval [ CI ] 0 . 57 - 1 . 85 ; period 2 : HR 1 . 68 , 95 % CI 1 . 01 - 2 . 80 ; and period 3 : HR 2 . 74 , 95 % CI 1 . 06 - 7 . 11 ) .
### Output Text:
After adjusting for potential confounders , we found that the risk of HIV seroconversion among participants who were daily smokers of <span class="chemical">crack cocaine</span> increased over time ( period 1 : hazard ratio [ HR ] 1 . 03 , 95 % confidence interval [ CI ] 0 . 57 - 1 . 85 ; period 2 : HR 1 . 68 , 95 % CI 1 . 01 - 2 . 80 ; and period 3 : HR 2 . 74 , 95 % CI 1 . 06 - 7 . 11 ) . 