What does Isaiah 2:22 mean? Stop trusting in mere humans,
  who have but a breath in their nostrils.
  Why hold them in esteem?
  (Isaiah 2:22, NIV)


All humans breathe with nostrils. So, should we distrust all people?
The question is not whether or not to trust people, but whether or not to put your trust in people.  Obviously, people are quite capable of telling the truth.  However, trusting in people to meet your needs is where the problem comes in.  People are mortal and finite with limited power.  Indeed, they have "only a breath in their nostrils".  This may be suggesting that everyone is living breath to breath.  If you put your trust in your parents to provide for you or get you out of trouble, you should be aware that there is no guarantee that they will be alive tomorrow or will have the power to help you.

God, however, is infinite and self-existent.  He will be around tomorrow and has all power to meet your needs whenever and however He chooses.  Thus, it is prudent and wise to look to God and trust Him for your needs and life--not a fellow human being whose life is but a vapor that appears for a little while and then is gone.