enhance vs impair

enhance

verb
  • To augment or make something greater. 

  • To take up contrast agent (for an organ, tissue, or lesion). 

  • To improve something by adding features. 

  • To be raised up; to grow larger. 

impair

verb
  • To weaken; to affect negatively; to have a diminishing effect on. 

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