How does keratin in a leave-in spray actually help hair that feels rough or looks dull? — "Dr.S Keratin" Spray, 150 ml

Keratin is the main structural protein of hair. In a leave-in spray, it does not repair hair from within like living tissue, but it can coat damaged areas along the cuticle where the outer layer no longer lies flat. This coating may reduce the feeling of brittleness and make strands easier to comb and style. The effect is most noticeable on porous, heat-exposed, or chemically processed hair — for example after bleaching, coloring, or frequent hot-tool use — where the cuticle has been lifted or worn down. The spray format distributes a light layer through the mid-lengths and ends, which are typically the oldest and most damaged parts of the hair. Unlike heavier oils or masks, it adds softness and shine without a dense or greasy finish, so it can be used on days when you want a neater look without weighing the hair down.