Hair is curly or straight, depending upon the number of disulfide bonds between hair proteins found in the hair shaft.
The greater the number of links, the curlier the hair, and the fewer the number of links, the straighter the hair.
Hair is primarily composed of keratin, a protein, which grows from a sac called the follicle. Cells in the hair follicle generate keratin, and various other proteins, which become a part of the hair shaft. These proteins contain sulfur atoms, and when two of these sulfur atoms pair up and bond, they form a disulfide bond. If the two sulfur atoms in the same protein are at a distance, and join to form the disulfide bond, the protein will bend.
The amount of humidity in the air not only makes for what some label a "bad hair day," but alters the degree of curliness or of straightness of hair. This occurs when high humidity forces water back into the hair fiber, acts on its protein structure, and forces the hair shaft to return to its original structure.
People can temporarily alter their hair to force it into a straight state, or a curly state, but only on a temporary, not on a permanent, basis. When a straight-haired person gets a "permanent wave," known as a "perm,"he or she is chemically forcing the making of strong disulfide bonds. The wave does not stay permanently, because new hair, which is straight, grows in as the "perm" grows out. People with curly hair may chemically alter their hair to give it a straighter appearance, but this, too, is also a temporary solution to a permanent "problem," with the same end result.