In high humidity and water, hair can swell up to 15% in diameter, but only 2% in length. Sodium lauryl sulfate (in shampoo) causes hair to swell in a similar manner, but causes more damage because of its ability to de-grease. Avoid too-frequent wetting of hair or prolonged wetting, and always watch hair for signs of needing special treatment such as breakage.
Water causes hair to swell. Avoid too-frequent wettings and prolonged saturation.