18700
Elevations in DNA damage levels were observed at the 6 and 48 h time points , while very low levels of DNA damage were observed at the 24 and 72 h time points .
 None
Please list all entity words in the text that fit the category.Output format is "type1: word1; type2: word2". 
Option: Chemical 
Text: {0} 
Answer:
 None