fire-retardant vs noninflammable

fire-retardant

adj

  • Hard to ignite; not flammable. 

noun

  • A substance that makes an object resistant to burning. 

noninflammable

adj

  • Not inflammable. 

How often have the words fire-retardant and noninflammable occurred in a corpus of books? (source: Google Ngram Viewer )