24944
HepG2 cell cultures were used to assess the following parameters of toxicity : cellular viability , intracellular levels of reactive oxygen species ( ROS ) , mitochondrial membrane potential and initiation of apoptosis .
 Chemical: oxygen
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: oxygen