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: From 1986 to February 1993 , 40 children aged 2 months to 18 years ( average age 10 . 4 + / - 5 . 8 years ) underwent heart transplantation .
### Output Text:
From 1986 to February 1993 , 40 children aged 2 months to 18 years ( average age 10 . 4 + / - 5 . 8 years ) underwent heart transplantation . 