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: Among 547 preterm infants of < or = 34 weeks gestation born between 1987 and 1991 , 8 children ( 1 . 46 % ) developed severe progressive and bilateral sensorineural hearing loss .
### Output Text:
Among 547 preterm infants of < or = 34 weeks gestation born between 1987 and 1991 , 8 children ( 1 . 46 % ) developed severe progressive and bilateral sensorineural hearing loss . 