1578
We investigated the possibility of using this Ca ( 2 + ) signal as a biomarker for a reaction to chemical irritants of normal human keratinocytes ( NHK ) in submerged primary cell culture .
 Ca ( 2 + )
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: Chemical 
Text: {0} 
Answer:
 Ca ( 2 + )