The chemicals in hairspray vary by brand but usually include polymers, such as polyvinylpyrrolidine. Some also contain copolymers with maleic anhydride or copolymers with vinyl acetate. In other cases, hairspray is made using natural polymers.
Hairspray is basically made up of polymers, which are long chain-like molecules, and an evaporable solvent. When hairspray is applied to hair, it deposits an extremely stiff layer of polymer over the hair. The solvent prevents the polymer from becoming stiff until it evaporates, after which the polymer is allowed to stiffen.
When hairspray was first invented in the 1940s, the solvent was made up of chlorine, fluorine and carbon. This solvent was ideal for aerosol cans because it was nonflammable and nontoxic; however, it was later discovered that this combination causes destruction of the Earth's stratospheric ozone. This is when these ingredients were replaced by other solvents, including hydrocarbons and alcohols. These replacement solvents are flammable.
The chemical polyvinylpyrrolidine used in some hairspray products is the same that is used in plywood to glue layers of wood together. While polyvinylpyrrolidine is water soluble, most hairspray manufacturers also add polydimethylsiloxane, which is not water soluble, to make the hairspray hold the hair in place for longer periods of time.