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: Although MHb formers are generally applied as treatments for CN poisoning , it has been proposed that a stable , long - acting MHb former could serve as a CN pretreatment .
### Output Text:
Although MHb formers are generally applied as treatments for CN poisoning , it has been proposed that a stable , long - acting MHb former could serve as a CN pretreatment . 