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: The average FEV1 increases during the 6 - hour observation period were 18 % for ipratropium and 8 % for theophylline .
### Output Text:
The average FEV1 increases during the 6 - hour observation period were 18 % for <span class="chemical">ipratropium</span> and 8 % for <span class="chemical">theophylline</span> . 