18229
This research supports the hypothesis that endogenously produced reactive species , including acetaldehyde , are always present and constitute the majority of the observed biological effects following very low exposures to exogenous acetaldehyde .
 acetaldehyde, acetaldehyde
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: Chemical 
Text: {0} 
Answer:
 acetaldehyde, acetaldehyde