nerve gas vs tear gas

nerve gas

noun
  • A toxic chemical compound and banned weapon of mass destruction which, when inhaled or ingested, impairs nervous system functions, typically causing extreme physical distress and death by asphyxiation. 

tear gas

noun
  • Any lachrymatory, non-lethal chemical compound that causes the eyes to sting and water and/or irritates the respiratory system, mostly used for controlling crowds during riots or as self-defense. 

verb
  • To use tear gas. 

How often have the words nerve gas and tear gas occurred in a corpus of books? (source: Google Ngram Viewer )