5506
All the winter fractions increased within 1h the level of reactive oxygen species ( ROS ) , while only summer PM2 . 5 had this effect on A549 cells .
 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