Cosmetic ingredients, especially those that have a natural origin, can be prone to sensitivity in humans. The reaction can be very mild and almost unnoticeable to very severe requiring medical attention. I personally consider irritation and allergy potential as very closely related although there are differences between the two. In some cases a safe, harmless, non-irritating ingredient can cause an allergic reaction in some people while an ingredient which is a known irritant may not exhibit a reaction on everyone who comes in contact with that ingredient.
Once again the irritation potential for a cosmetic ingredient is graded on a numerical scale from 0 to 5 with 0 to 1 being non-irritants and 4 to 5 being much more severe irritants. The following is just a very partial list of cosmetic ingredients that have been rated for their irritation potential. Some of the results may be surprising to you while some of the others are just quite obvious in terms of their ability to cause irritation.