Hair straighteners should only be used on dry hair , unless they are specially made with vents to let the moisture out . These sort of irons are usually called `` Wet Dry '' irons . There are three general kinds : curling irons , used to make the hair curly , straightening irons , commonly called straighteners or flat irons , used to straighten the hair , and crimping irons , used to create crimps of the desired size in the hair .