KnowHow team explains: Hair is curly or straight, depending upon the number of disulphide bonds between the hair proteins found in the hair shaft. The greater the number of links, the curlier is the hair, and the fewer the links, the straighter the hair.

Hair is primarily composed of keratin, a protein that grows from sacs called follicles. Cells in the follicle generate keratin and various other proteins that become a part of the hair shaft. These proteins contain sulphur atoms and when two of these atoms pair up and bond, they form a disulphide bond. If the two sulphur atoms in the same protein are at a distance and join to form the disulphide bond, the protein will bend.

The amount of humidity in the air not only makes for what some call a “bad hair day”, but also alters the degree of curliness or straightness of hair. High humidity forces water back into the hair fibre, acts on its protein structure and forces the hair shaft to return to its original structure.

People can alter their hair to force it into a straight or curly state, but only on a temporary basis. When a straight-haired person gets a “permanent wave” — or “perm” — he or she is chemically forcing the making of strong disulphide bonds. The wave does not stay permanently because new hair, which is straight, grows in as the perm grows out. Similarly, those with curly hair may chemically alter their hair to give it a straighter appearance but this, too, is a temporary solution.