3908
Hydrogen sulfide , H2S , is a colorless gas with a strong odor that until recently was only considered to be a toxic environmental pollutant with little or no physiological significance .
 Chemical: Hydrogen sulfide; Chemical: H2S
Please list all entity words in the text that fit the category.Output format is "type1: word1; type2: word2". 
Option: Chemical 
Text: {0} 
Answer:
 Chemical: Hydrogen sulfide; Chemical: H2S