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: These findings suggest that the effects of estradiol on seizure threshold and damage may be altered by sex - related differences in the hormonal environment .
### Output Text:
These findings suggest that the effects of <span class="chemical">estradiol</span> on seizure threshold and damage may be altered by sex - related differences in the hormonal environment . 